NAVAL POSTGRADUATE SCHOOL
Monterey, California
THESIS
DESIGN OF AN ELF/VLF SATELLITE FOR UNDER THE ICE
SUBMARINE COMMUNICATIONS
by
Gary C. Thompson
September 1988
Thesis Advisor:
Approved
Richard C. Olsen
for public release; distribution is unlimited
T2A2386
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Design of an ELF/VLF
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Satellite for
|
Under
Project
No
'
the Ice
|
Tisk
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Woric Unit Accession
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Submarine
Su
Communications.
!
12 Personal Authors)
Gary C. Thompson
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From
September 1988
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Master's Thesis
.
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Supplementary Notation The views expressed in this thesis are those of the author and
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Subject Terms (continue on reverse
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if
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do not
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ELF, ELF/VLF Transmitter, Tether, Tethered Antennas, Tethered
Subgroup
5 Page
Satellites,
Tethers in Space
1
9
Abstract (continue on reverse
if
necessary and identify by block number
satellite system for ELF/VLF
and under the polar ice. By using the
the tether cable can produce sens of kilowatts
This thesis proposes the design of a space based tethered antenna
communications with submarines
dynamo
effect of a
moving wire
in the far northern latitudes,
in a
(geo)magnetic
field,
of its own radiation power. The transmitted signal of lKHz-3KHz will use whistler mode propagation to
couple to the earth's field lines and follow them down to the surface. The signal can penetrate 100m of
seawater, and ice of unlimited thickness. A constellation of 12 satellites will provide 75% duty cycle
coverage for each submarine operating area of over four million square kilometers. Issues examined
are: tether electrodynamics and mechanics, debris survivability, ionospheric radio and plasma physics,
plasma contactors, satellite and constellation design concepts, cost analysis, and military mission needs
analysis.
20
21
Distribution/Availability of Abstract
[X]
unclassified/unlimited
I
I
same
22a Name of Responsible Individual
as report
R. C. Olsen
DD FORM
1473, 84
MAR
83
APR
Abstract Security Classification
Unclassified
LL
edition
22b Telephone (Include Area code)
(408) 646-2019
security
may be used until exhausted
All other editions are obsolete
22c Office Symbol
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classification of this page
Unclassified
Approved
Design of an
for
public
ELF/VLF
distribution
release;
Satellite for
Under
unlimited
the Ice
Submarine
Communications
by
Gary C. Thompson
Lieutenant, United
States
Navy
B.A. Physics, The Ohio State University, 1980
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN SYSTEMS TECHNOLOGY
(SPACE SYSTEMS OPERATIONS)
from the
NAVAL POSTGRADUATE SCHOOL
September
1988
ABSTRACT
This thesis proposes the design of a space based tethered antenna
system for
latitudes,
ELF/VLF communications
and under the polar
a (geo)magnetic field,
radiation power.
ice.
By
using the
dynamo
effect of a
signal of
lKHz-3KHz
signal can penetrate
100m
down
75%
in
own
mode
to the surface.
and ice of unlimited thickness.
of seawater,
constellation of 12 satellites will provide
its
will use whistler
propagation to couple to the earth's field lines and follow them
The
moving wire
the tether cable can produce tens of kilowatts of
The transmitted
satellite
with submarines in the far northern
A
duty cycle coverage for each
submarine operating area of over four million square kilometers. Issues examined
are:
tether electrodynamics and mechanics,
radio and plasma physics,
plasma contactors,
debris survivability,
satellite
ionospheric
and constellation design
concepts, cost analysis, and military mission needs analysis.
THZ2&
TABLE OF CONTENTS
I
.
IT.
INTRODUCTION
1
HISTORY AND BACKGROUND
5
A.
PRESENT COMMUNICATIONS CAPABILITIES
Electromagnetic Transmission Properties
of Seawater
5
2.
Present Communications Networks
7
B.
SUBMARINE COMMUNICATION ALTERNATIVES
C.
STRATEGIC CONSIDERATIONS FOR SATELLITE
SURVIVAL
D.
E.
8
10
IDENTIFICATION OF A MILITARY MISSION
NEEDS REQUIREMENT
12
1.
The Problem
12
2.
The Solution
12
TETHERS
13
1.
Tether Fundamentals
13
2
Tether Programs
16
Tether's Future
18
.
3.
F.
5
1.
THE SPACEBASED ELF/VLF TRANSMITTER AND
MISSION REQUIREMENTS
18
III.
IV.
TETHER ELECTRODYNAMICS
MOTION INDUCED ELECTROMOTIVE FORCE
20
B.
MAKING CONTACT WITH THE PLASMA
22
C.
DRAG AND DECAY
25
D.
RESISTANCE AND IMPEDANCE LOSSES
26
E.
ALTERNATING POWER AND MODULATION
28
F.
ALTITUDE AND INCLINATION EFFECTS
31
THE IONOSPHERE AND BEAM PROPAGATION
33
A.
THE IONOSPHERE
33
B.
THE GEOMAGNETIC FIELD
38
C.
THE WAVE PROPAGATION MODEL
1.
V.
20
A.
The Coupling Model
40
44
2.
The Whistler Waveguide Transmission Model.. 48
3.
The Uncoupling or Reradiation Model
49
4.
Primary Coverage Area
53
5.
Illuminated Footprint Power Density
54
6.
Received Signal Voltage Level
56
7.
Sweep Rate and Swath Coverage
57
D.
NOISE AND INTERFERENCE
58
E.
SELF-POWERED GENERATION CAPABILITIES
59
TETHER MECHANICS
64
A.
ORBITAL DEBRIS AND SEVERING
64
B.
TETHER STRENGTH
66
C.
TETHER MASS
66
VI.
D.
TETHER BOWING
68
E.
SATELLITE MASS
69
F.
TETHER DEPLOYMENT AND RETRIEVAL
70
SUBCOM:
A.
B.
VII.
VIII.
THE PROGRAM
72
THE SATELLITE
72
1.
Description
72
2.
Operation
75
3.
Trade-off Analysis
79
THE CONSTELLATION
81
1.
Description
81
2.
Operation
82
3.
Trade-off Analysis
83
ESTIMATED PROGRAM COSTS
84
CONCLUSION
88
A.
SUMMARY
88
B.
WHAT STILL NEEDS TO BE DONE?
91
APPENDIX: FIGURES
94
LIST OF REFERENCES
114
BIBLIOGRAPPHY
118
INITIAL DISTRIBUTION LIST
120
vi
ACKNOWLEDGEMENTS
would like
The author
his time,
ideas,
to give thanks to
and clarifying
Gnanalingam for his meticulous
to
Denis Donohue
Mr.
tracing.
for
Thanks are also
engineers out
Dr.
explanations,
attention and
insight,
his valuable information
due to
all
the
Melody,
would
Ideas
do
but grow upon each other.
also like to
give special thanks to my
for her support and unending
intensive period.
and
thesis in many
ways via their own diligent reseach publications.
I
and
on ray
scientists
there who contributed to this
not sprout in a vacuum,
Olsen for
to Professor
patience
during
wife,
this
INTRODUCTION
I.
Almost
one-half
of America's nuclear strategic warhead
arsenal is carried aboard nuclear powered ballistic
submarines (SSBN's).
are
triad,
ocean,
These forces,
missile
as one leg of the nuclear
by mission concealed beneath the surface of the
deployed to all areas of the world. The
strength
of
this strategic arm lies in its ability to hide in the depths
of the world's oceans, denying an enemy total neutralization
of
U.S.
nuclear
allowing the U.S.
forces
in
a surprise first strike,
an assured survivable
retaliatory
The key to submarine survivability is stealth.
thus
force.
[Ref.
1].
Contributory factors to stealth include the vastness
of
world's oceans in which to operate and hide within,
and
the
the increasing opaqueness of seawater to the electromagnetic
spectrum
with
detectability.
order
to
carry
increasing
depth,
affording
reduced
Submarine commanders must avoid detection in
out their mission and be effective,
but in
order to utilize their powerful ballistic missiles they must
maintain a critical communications link
Command
via
emergency
presently
the
with
National
Authorities (NCA) for positive release instructions
action
increases
messages
their
Whether
or not
process,
just to passively
the
(EAMs),
vulnerability
submarine radiates in
process
a
to
that
detection.
a communication
monitor transmissions
increases
the
vessel's
effectively
security
operational
Seawater
problems.
electromagnetic
underwater
cloaks
process that occurs for both transmission
signals,
and reception.
above
reduce the opacity, the ship must put an antenna
water,
on the
or just below the
water,
proceedure
that
performance
and
confines
places
submarine
the
increased detectability by others.
At
other
the
transmitters
authorizing
nuclear
methods
platforms,
an
aggressor
realm
of
messages
action
Although
multiple
there
are
transmitters,
the system is essentially for
and is not projected
after a nuclear exchange,
by
operational
a
2].
emergency
of
release.
frequencies,
and
peacetime use only
[Ref.
transmission,
of
into
of the communications links are the
end
initiating
several
the
water's surface, a
submarine's
the
a
To
to survive
intact
or even limited tactical targeting
intent
weakening
on
our
command,
communication, and control networks (C cubed).
The
problem
national
that exists,
defense
vulnerable
attack
to
escalations,
command
and
at
the
is
that at one end of a vital
the
link
from
other
all
end
transmitters
levels
the
of
are
offensive
receivers
are
vulnerable to detection and further prosecution while in the
act of trying to receive their own command instructions.
If
this communications link is severed, or if the
is
localized
and
receptor
attacked, then a significant portion of U.S.
strategic forces will have
been
lost
for
each
ballistic
submarine that is unable to respond as directed.
following
The
thesis
long wire
gradient,
operational
increase
degrees
strategic
and
advances
communicating with submarines
70-80
spacebased, gravity-
satellite that would
systems
technology
in
and
space environment suggest new methods of
of
existing systems,
a
antenna
security
Recent
survivability.
understandings
proposes
tethered
that
would
be
superior
to
including the ability to communicate above
latitude
and
through
the polar ice cap to
submarines lurking beneath. A constellation of Extremely Low
Frequency / Very Low Frequency (ELF/VLF)
proves
to
be
cost
a
effective,
orbiting
relatively
antennas
risk
low
technology, that could be put into operation expeditiously.
This
thesis
operations,
lower
examines
the principles of tethered space
of electromagnetic propagation in the upper
ionosphere
from
low
earth
orbit,
and
and
suggests a
constellation)
design
that would satisfy the identified security problems.
It also
possible operational satellite
(and
recommends that an experimental satellite be deployed first,
to
test new ideas and collect data, before committing to an
operational system.
Chapter
II will
trace the history and background of the
present submarine communications network and its
mission
requirements
identified
for future operational security.
Then
it will discuss basic tether fundamentals and how a tethered
antenna
can
meet
the
nation's
security
requirements.
Chapter III will examine tether electrodynamics,
space
physics
tethered
proposed
system
operations,
including
and Chapter V tether mechanics.
environment,
Chapters VI, VII and VIII,
respectively, will
satellite
program
an experimental
prove concept feasibility.
Chapter IV
concept
costs,
satellite
and
and
to
look
at
the
constellation
future studies,
gather
data
and
HISTORY AND BACKGROUND
II.
A.
PRESENT COMMON I CAT IONS CAPABILITIES
1.
Electromagnetic Transmission Properties of Seawater
Due to the opacity of seawater across
electromagnetic spectrum, there are only two
windows
most
the
of
communications
which submarines may communicate from below the
in
Outside
surface of the water.
of these
two
windows,
all
communications techniques require the exposing of an antenna
above the water. Raising of such an antenna puts the crew at
grave
creates
present
but
motion
its
wake
feather
a
that
emphasize
the
use
low
of
water
at
great
security,
networks
communications
profile
antennas,
Although long wire antennas can
submerged.
the
seen
operational
to
critical
future
and
through
easily
is
Because of this risk
distances.
the
Not only does the antenna provide
risk of discovery.
a radar cross section,
be
preferably
trailed
on
they are clearly discernible from an airplane
surface,
or satellite.
But,
putting
antenna below
an
the
surface
available
immediately
restricts
the
communicate.
Of the two
usable communications frequencies,
frequencies
other
one is in the lower RF,
and
spectrum
223-239].
[Ref.
3:
pp.
the
option is a future possibility,
The lower RF window
that
is
The
is
in
to
the visible
blue-green
visible
and will be mentioned later.
not
opaque
to
seawater
is
used presently for submarine communications,
and is divided
into three adjacent bands: Low Frequency (LF) from 30khz
300khz,
Frequency
Low
Very
to
(VLF) from 3khz to 30khz,
and
Extremely Low Frequencies (ELF) from lOhz to 3000hz [Ref.
P.
4:
21].
Low frequencies (LF) use an exposed antenna that has
a
high
degree
of
detectability.
Signals
very
at
low
can penetrate no more than about 30 feet
frequencies
(VLF)
of seawater.
This forces the submarine to
antenna
that
surface.
In either case,
must
lie
trail
lengthy
a
or just under the
the surface,
on
the trailed antenna can
broach
or
affect surface water patterns, and increases the risk of the
submarine's discovery [Ref.'s
restrictions
submarine
on
and the operation of
its
7:
p.
acoustic
counter
is
unreeled
near
frequencies
an extremely low data rate,
to a sufficient
operate
detection
surface
the
depth
where
(ELF),
though
allowing
do penetrate seawater down
submarines
can
more
safely
with reduced operational security problems [Ref.
49-51].
pp.
iv,
so
under-ice
Sea-ice is essentially transparent
operational
communication
regulated by how close the skipper
underside
vessel.
also
are
33].
Extremely low
only
There
6].
maneuverability, depth,
own
equipment when its antenna
[Ref.
and
5
speeds,
of
the
icepack,
and
In the rare event that a
wishes
depth
to
get
7:
to
ELF,
is
only
to
the
the depth capacity of his
submarine
does
broadcast
receive
(vs
can
submerged
a radiating antenna
only),
triangulated
rapidly
be
ELF/VLF
antenna
antenna) is not directional,
[Ref.
3:
233- 239,
pp.
used
(if
as
The existing
worldwide
and would
hard
be
ground
dispersed,
and
network
VLF
(original)
station
transmitters,
to
locate
of
by
is
redundant,
operated continuously in
still
susceptibility
the development of mobile VLF transmitters,
in 1973,
the
in
EC-130 Hercules aircraft (soon to be replaced by a
of
new 707-320B derivative Boeing airframe).
transmitters,
called TACAMO,
These airborne VLF
are more survivable than
their
relatives because of their mobility, but they
based
still transmit a signal which puts the submarine at risk
receiving
reception,
but
end,
now
mobile
this
risk
at
allowing a greater submarine depth for
utilizing
(again) vulnerably exposed.
The
to
VLF signal has a shallow penetration depth. A
the
receive;
follow on ELF system was developed to reduce
[Ref.
ground
2:
pp.
stations
that
are
48-49].
VLF network reduced the vulnerabilities
of the fixed base international
and
of
ground stations to attack and jamming led to
foreign
ground
consists
augmented
This system is reliable,
several simultaneous modes. However, the
the
a
253],
additional LF stations.
widely
form
while
transmission
a
Present Communications Networks
2.
fixed
above the surface
geo-located,
and
VLF
transmission
systems,
the newly built ELF system was to have further improved
allowing
upon the mobile VLF system by
installations
foreign
than
environmental
and
effectiveness
has
and hence
[Ref.
political
but
to
due
operational
interference,
[Ref.
secure
more
is
8],
compromised
been
to
This ELF system is
at a greater depth.
EAMs
their
receive
based in Michigan and Wisconsin,
submarines
the
7:
iii].
p.
The
present system is considered nonsurvivable in a nuclear war.
Sabotage,
malfunction,
disable it permanently.
"bellringer",
i.e.,
a single nuclear
or
Its purpose is now
if the signal
can
as
a
the worst is
lost,
is
strike
serve
to
assumed to have happened w.r.t. national security [Ref. 9].
B.
SUBMARINE COMMUNICATION ALTERNATIVES
There are a number of
communications
HF,
VHF,
UHF,
and
high
frequency
EHF
bands
are
primarily
communications traffic and secure voice/data
Navy
EHF
Satellite
satellite
radio
currently used by submarines. These
systems
Communication
for
basic
(and NESP,
Program).
All
the
present
satellite communication systems require a submarine to raise
an antenna mast.
There
are
alternative communication capabilities that
can be developed in the near future. At present,
investigated possibility is
Laser
Communications
called
action
one heavily
for
Submarine
Satellite. Designed to operate in the
blue-green spectrum, this laser
emergency
SLCSat,
messages
satellite
would
downlink
from the President to submarine
operating areas.
viewing
Submerged
program
This
substantial
has
technical momentum.
qualified
space
submarines
would
have
upward
sensors to receive these signals through the water.
reliability
is
scientific
It is also still in development,
speculative.
prototype
first
The
and the
operational
to be an expensive program, with cost
deployment
for deployment
expectations
a
still
driving
effectiveness
and
transmitter of sufficient power and
projected
is
and
laser
satellite is well over ten years away,
system
validity
Realistic
timing.
are well beyond the year 2000,
and its capability to transmit through sea-ice is also being
questioned [Ref.
1:
p.
45].
Other futuristic ideas
Sea
bottom
landline
crisscrossed with
times
subs
worthy of mention include:
communication
sub
sono nodes. Here,
operating
receivers,
sonobouys
acoustic
can be
and
areas
and
status
predetermined
either physically or
reports.
(2),
Hydro-
sonobouy fields are deployed in
with
equipped
hydrophone
are relays,
At
cables.
are required to "plug in",
through coupling, to pick up
acoustic
(1),
plug-ins, whereby the ocean bottom is
acoustic
radio
antennas,
transmitters.
The
converting the RF signal into a coded
signal at some noninterfering audio frequency that
received
intermittently
and
decoded
by
a
local
submarine,
by a distant sub at convergence zones.
Towed submarine radio bouys
optic wires of great length.
connected
by
severable
or
(3),
fiber
A
expands
alternative
final
tendered by M.D. Grossi in
ULF/ELF antenna [Ref.
upon
an idea originally
considering
1972,
orbiting
an
This thesis expands on that idea,
10].
a communications satellite that broadcasts in the
proposing
ELF/VLF band. Common colloquialism
between
division
boundary
ELF
has
and
but the expected
broadcast window for this system would be between
providing
3khz,
exact
the
blurred
VLF,
the higher data rate of VLF. High transmission power,
transmission
arriving
operating
cost,
early
will
areas
increase
power
the
This approach is also expected to
at the receiver.
have low risk
short
and focussed propagation paths to
path length,
submarine
the
and
lkhz
the penetration depth advantage of ELF and
and
competitive
prototyping,
rapid
technology,
deployment,
affordable
replacement
and
sacrifice.
C.
STRATEGIC CONSIDERATIONS FOR SATELLITE SURVIVAL
An
orbiting
corresponding
functions.
ground
station
by
satellite
A
in
several
of
survivable
more
that
prenuclear
the
single
a
carries
a
similar
a ground station could
saboteur,
or
tactical
A satellite is not likely to
hostilities.
following
vulnerability of satellites.
out
than
expensive national resource, as
is an
are ant i -satellite weapons.
attacked
is
a low level conflict,
destroyed
easily be
strike.
In
satellite
factors
be
In nuclear exchanges,
may
determine
the
Moving the battle into space crosses a threshold that is
more serious than attacking equivalent hardware on the
much
We have not yet fought in space. Once that
ground.
it will be difficult to retreat.
is crossed,
boundary-
Satellites are
considered national resources, and the loss of a space based
national
(security)
resource
would
retaliatory response than the
loss
draw
of
a
a
much harsher
ground
station.
Satellites are much harder to replace and provide advantages
that
stations
ground
attacking
cannot.
based
space
a
attacker
An
delay
would
asset much longer than an earth
based installation because of
possibly
this
very dramatic
retaliatory response.
are hundreds of orbiting satellites in space that
There
enemy
have been identified and cataloged, but an
and purpose
know the mission
The very number of satellites
level
security:
of
provides
orbit
in
will
not
every satellite.
of each and
its
own
which satellites should one prosecute,
and of course, did one get them all?
the
Finally,
national
also
agressor's own anti-satellite weapons are
resources.
military
considerable
would
He
resources,
that
have
renewable, to destroy a sufficient number of
to
expend
rapidly
not
are
satellites
at
once.
Thus
a
satellite
increases
its
threshold
of
has
at
survivability:
permanently
moving
11
four advantages that
least
the
tactical
the
battle
/
strategic
to
a
new
national
frontier, the response to the loss of a
camouflage
numbers,
by
and
resource,
the resource threshold of the
attacker who must ponder the commitment of
his
own
scarce
national resources.
D.
IDENTIFICATION OF A MILITARY MISSION NEEDS REQUIREMENT
The Problem
1.
military
The
requirements
procurement
defines
systems.
It
industry
strict
has
aquisition
the
for
and
hardware. Before any requests for proposals
of
there must be a mission needs
can be distributed,
that
defense
guidelines
and
shortcomings
the
has
become
analysis
problems with existing
or
apparent
that
the
present
ELF
communication system, and the previous systems superseded by
are
ELF,
vulnerable
at
both the transmission end and the
receiving end. The only two ELF transmission
vulnerable to attack
areas.
and are limited
The present system is not
locations
are
in power and coverage
capable
of
covering
the
polar areas, or of reasonable data rates.
2.
The Solution
What is required is a system that is less vulnerable
to
being
receiver,
including
put
out of action, provides greater power to the
and covers more of the submarine operating
below
the
polar
ice
submarine operational flexibility.
and
inexpensive
enough
to
be
12
cap.
areas,
It should increase
It should
sacrificed
be
redundant
and
replaced.
Solutions
mission
this
for
involve
need
satellite
transmitters which can offer greater survivability.
system
would
Such
a
increase the operational effectiveness of our
stategic nuclear forces by ensuring
emergency
that
action
messages get transmitted with a higher degree of reliability
and survivability in a nuclear exchange or crisis.
E.
TETHERS
1.
Tether Fundamentals
Tethering,
Tsioklovskii
as
geostationary
beyond
weightlessness
altitude,
force.
would
one
1960
In
a
experience
Russian
dropped down until it
opposite
the
In
engineer,
("upward")
touched
the
from
direction
the
a cable would be deployed with a ballast mass to
satellite,
earth
the
deployed
cable
satellite maintains a center
geosynchronus
practical
in
the
subsatellite
such
mass,
gravity
that
space
were
shuttle
as in Colombo's
100km
13
that
the
remains
in
the idea gained a more
concepts
more
as
using
research tether platform,
a
of
In the 1970' s
orbit.
aspect
particularly
tethering
to
suggested that a massive satellite be "anchored"
in space and a cable be
offset
tower
space
and "inverted gravity" farther out,
at geo,
centrifugal
Artsutanov,
earth.
possible
a
by
If an equatorial tower were built to extend
weightlessness.
i.e.,
1895
in
described
first
was
concept,
as a
below
developed,
orbiter
as
a
(1974) concept of
(or
above)
the
atmospheric
conduct
to
shuttle
experiments.
[Ref.
magnetospheric
and
11].
can be best described
Basic tether fundamentals
quoting from the introduction of "Tethered Satellite
(TSS)
Science
Core
by
Equipment",
C.
Bonifazi [Ref.
and by referencing Figure 2.1 in the Appendix
by-
System
12],
at the end of
the thesis:
The principle by which the system works is quite
simple and can be explained with reference to Figure 2.1
showing
the stabilizing forces acting on tethered
masses. An elementary tether system has "dumbbell"
form
with two masses connected by the tether. The top mass
experiences a larger centrifugal than gravitational
force,
being higher than the orbit of the center of
gravity, whereas the reverse occurs at the bottom mass.
Displacing the system from the local vertical generates
restoring forces at each mass,
tending to return the
system to local vertical. The system will remain aligned
with the local vertical or "gravity gradient" vector.
The center of mass, halfway between equal masses, is
in
free fall,
but the end masses are not. The top mass
travels too fast for its altitude, thus giving rise to
the excess centrifugal acceleration felt as tension in
the tether, with the inverse occurring in the lower
mass.
The masses experience this tension as artificial
gravity.
.
.
.
.
.
this proposal the tether is an antenna tensioned
In
by artificial
length
with
gravity,
the
also a conductor,
cutting
the
perpendicularly,
moving dynamo),
stabilized
and
local vertical.
and
it
then
a
in
is
magnetic
earth's
we
have
and a generated
14
along
its
entire
If the vertical tether is
in
low
equatorial
field
lines
effect
electromotive
orbit
almost
a generator (a
force
along
the
In the moving reference frame of the wire
wire.
electric
is an
velocity
field
vector
perpendicular
to
both
and the geomagnetic field vector,
field vector is directed
along
the
and this
generated
The
wire.
there
orbital
the
electric field results in an emf in the wire, making one end
of the tether positive and the other end negative.
Electrons
collected at the positive end will be pumped to the opposite
end
via this emf boost, producing a tether current.
contactors at each end can be designed to
more
Plasma
efficiently-
exchange electrons with the surrounding plasma than the bare
wire
ends
thus increasing the level of current.
can,
load is inserted in the wire,
be harnessed for work.
then the flowing
If a
current
can
Work comes at a cost however, because
the power extracted across the voltage drop comes out of the
momentum
angular
of
the
system causes electromagnetic
system
drops
and
it
Removing
system.
which
drag
a lower orbit.
into
work from the
decelerates
the
This decay will
continue until atmospheric drag becomes the predominant drag
force,
and rapidly destroys the system.
concept
The
can
also be reversed.
If a current is
pumped through the wire in the opposite direction
normally
source),
generated
self
then
the
direction
system
geomagnetic field and boosted
extracting
power
lower orbits,
out
of
and pumping in
accelerated
is
to
the
a
15
higher
tether
power
from
its
(from a separate power
within
orbit.
the
Thus,
drags the system to
boosts
the
system
to
higher
orbits.
If
that
should
load
be
transmission
a
antenna, then by alternating between normal drag
boost
powered
at ELF cycles,
modes,
modes
and
one can obtain an ELF
gravity
gradient
stabilized and altitude controllable. The next two
chapters
antenna
radiating
is
and the space environment
properties
tether
examine
will
that
orbit
in
more closely.
2.
Tether Programs
history of tether programs, and related antenna
The
studies, goes back a short time, with only
related
experiments.
spacecraft
coupled
In
late
1966,
Atlas-Agena
the
and
a
directly
the Gemini XI and XII
exhausted
D
few
stage
were
in the first tethered application experiments.
Two
modes of operation were examined. One mode explored inducing
angular momentum into the tethered system via
gradient
trans lational
and the other mode studied the stationary gravity
thrusting,
motion
the
of
system.
Both
experiments
successful and verified analytical assumptions.
In 1971,
[Ref.
were
11].
the 0V1-21 satellite experiments (NASC-117)
showed that straight-forward transmitters were not effective
at driving electrical
frequencies
(
varied wildly.
between
the
400hz
better
tether
at
ELF/VLF
result
of
the
coupling
and the surrounding conducting plasma
13].
connect
antennas
because antenna impedances
14.5khz)
This problem was a
antenna
environment [Ref.
to
dipole
-
A solution to this problem
the
ends
16
would
be
of the tether antenna to the
immediate environment through the use of plasma
or better yet,
use
to
modulated
currents,
naturally
the
contactors,
occurring
tether
at the appropriate frequency (ELF),
to
drive the antenna.
The United States
tethered
significant
experiment
conducted a
and Japan
experiments
rocket
the
in
called
early
Charge
2
series of
1980'
s.
A
[Ref.'s 14 and 15]
studied the effects of a 200 meter tether wire as an antenna
in the VLF bands with electron
beam
emissions,
then
and
again when the tethered system's bodies (mother and daughter
satellites) were charged to high voltages.
November,
In
neutralization
Also
plasma.
MAIMIK
1985,
beam interaction with
electron
examined
tether
field,
one
is
around
electron
by
major
funded
program
beam
the near
in
1991 as a shuttle orbiter payload.
tali an -American
dynamics
orbit,
modified
called Tethered Satellite System One (TSS-1), to
launched
I
and
ionospheric
the
in
[Ref.'s 16 and 17].
There
future,
joint
environment
how the non-neutralized plasma
was
wake behind a space vehicle is
emissions.
launched to study
was
plasma
vehicles
charged
of
a
and
project
electrodynamics.
that
will
examine
examine
interactions
tether
With the shuttle at a 200km
one test will deploy a subsatellite upward on a
to
be
It is a
30km
with the earth's magnetic
energy generation, and thrust production. Mesospheric
studies of this
kind
are
virtually
17
impossible
by
other
techniques.
proposed (but unfunded) mission will lower a
A
down
subsatellite
atmosphere.
[Ref.
tether
100km
a
to
study
upper
the
12].
Tether's Future
3.
tethers
While
are
(historical experiments are few,
science
the
limited),
publishing
related
concept
applications
present
and
space
and
new
journals
ideas on how to use them.
new
many
relatively
a
are
This thesis
proposes a very basic use of the tether, as an antenna,
but
there are some very novel and ingenious proposals suggesting
new uses. Some of these are: a power generation system using
dynamo
the
technique;
station applications such as
space
microgravity experiments; gravity gradient fuel (or liquids)
transfer
space;
in
conservation
or garbage;
micro-g
and the transfer
bodies for various purposes,
[Ref.
F.
materials
processing;
of angular momentum when deorbiting spacecraft
angular
of
momentum
between
including Mars space operations
18].
THE SPACEBASED ELF/VLF TRANSMITTER AND MISSION
REQUIREMENTS
This thesis
is proposing that
transmitting antenna
system,
an
ELF/VLF,
be placed in orbit.
identified basic concepts, this
satellite
spacebased
Using well
system
will
be
a constellation of gravity gradient stabilized
composed
of
antennas,
each antenna several kilometers in length
18
and
in
complementary orbits, but with propagation paths that permit
communications
with submerged submarines in those operating
areas which are
under
poorly
near
and
the
covered
at
present,
transmitters to space we increase the
message
get
will
survivability,
particularly
polar ice cap. By moving the critical
because
out
reliability,
multiple
the
that
likelihood
increased
of
transmitter
satellite redundancy,
power reception density increases, and coverage patterns.
Using
gravity
a
gradient
approach
for
antenna
construction provides a stable platform with a constant
known
orientation.
between
its
natural
driving
By
current
antenna
the
and
alternately
and the powered state
state
(with the use of plasma contactors) we obtain an effectively
radiating
antenna system.
As will be shown in
Chapter
IV,
by using the properties of the earth's geomagnetic field, and
plasma
physics
in
the
we
ionosphere,
can
"focus"
propagation paths directly to the areas of desired
our
coverage
(increasing the received signal strength and the penetration
depth
of
security
the
signal),
by limiting
interception.
electrodynamics
In the
will
thus further increasing operational
and controlling
areas of reception /
next chapter
the
physics of
studied
and
Chapter
be
examine the near-earth space environment.
19
tether
IV
will
III.
A.
TETHER ELE CTRODYN AM ICS
MOTION INDUCED ELECTROMOTIVE FOKCE
earth
radial
orientation, that cuts the earth's magnetic lines of
force,
conductive
A
orbit
in
with
an
develop a voltage potential across its ends.
will
the
wire
tether
strength
velocity
vector,
tether length,
vector,
"1"
"B"
If "v"
geomagnetic
the
the tether direction vector,
"I" the tether current,
'"X"
a cross
is
field
"L" the
product,
and "." the dot product, then the electric field is (v x B),
the
voltage is
associated
(v X B
.
1)*L,
and the Lorentz
force is (I X B)
Opposite ends of an
accumulate
opposite
insulated
tether
will
induced
emf.
conducting
based
charges
the
on
Current will attempt to flow through the tether, and the end
"electrodes", drawing from the
The
ionospheric
electrostatic
cable,
plasma
fields
available
itself
is
will slightly reduce
end
reservoirs
motion.
the
of
or
sinks,
plasma.
conductor,
so
between the ends, and external to the
the accumulated charge that the
moving conductor emf boost created.
each
electron
a
tether
The
act
plasma
as
sheaths at
either
charge
depending on the orbital direction of
The ionospheric plasma allows for the return current
path to be completed, supporting a continuous
through the wire,
and into the plasma.
20
[Ref.
current
19:
p.
3].
flow
we
If
assume
v
to
is
east
be
(the tether orbital
velocity and direction), and B north, then (v X B) is up.
If
the end electrodes are inefficient in exchanging charge with
the surrounding plasma, there will be minimal
induced
the
in
insulated
current
develop large voltage potentials with respect to
If the electrodes
more
local
(commonly called plasma contactors) can be
efficient
current
their
in
significant current can be passed
the
into
will
the
with positive at the top and negative at the bottom.
plasma,
made
flow
tether wire, and the ends
coupling
through
the
so that
tether
and
plasma (with an insignificant voltage drop across
the connection junction) then the tether ends will float
the local plasma potentials.
(v
X
B
will
L)
at
The entire open circuit voltage
be across the tether and any loads in
series with the tether.
Due to the
properties
breakdown
higher
positively
plasmas.
if
charged
plasmas
Therefore,
a
they
insulators,
of
voltages
they
than
load
negatively
by
should
majority
tether
the
of
respect to the surrounding plasma.
much
length
by
charged
placed
be
negative end of the tether (the bottom), because
leave the
have
surrounded
are
the
at
that would
negative
In the same vein,
with
if the
tether is to be used as a thruster, by reversing the current
flow and overcoming
source
end.
should
also
Figures 3.1 and
the
be
emf,
then
inserted
the
electrical
power
at the bottom (negative)
3.2 diagram the tether potentials
21
in
both the generator and thruster modes.
tether
is
deployed upwards, and the load or power supply is
at the bottom.
Typical voltages that might be induced
20km long tether range from
on
1500 to 4500
by
a
depending
volts,
at which the field lines are crossed.
angle
the
the
In both figures,
[Ref.
20].
B.
MAKING CONTACT WITH THE PLASMA
plasma contactor needs to fulfill several performance
A
system
In order to make the
criteria.
efficient,
and
the
path impedance low, the plasma contactor should have
return
a low resistance to current flow.
power
consumption,
It
have
should
minimal
and it should be capable of electron (or
switching
between
ion)
emission as well as collection (for
the
generator and thruster modes). A general implementation
of a contactor can be visualized as a balloon.
surface
area
efficient,
so
Unfortunately,
limits
are
it
as a
is
an
effective
restricted
through
thermionic
a
collector.
current
charge
emission
one
could
emit
and electron guns.
available
with
this
but plasma impedance and filament energy losses are
significant [Ref. 21]. The most
uses
contact
microampere levels. To improve
to
Higher positive current is definitely
option,
electron
positive charge collector, the
upon simple collection of positive
electrons
The
and the method is mass and energy
great,
is
device
called
a
effective
Hollow
Cathode
method
to
to
produce
date
an
expanding cloud of
highly
conductive
plasma
The
plasma.
cloud is then the (enhanced) collecting surface (Figure 3.3,
Ref.
The
22).
current
expands
cloud
flow
balances
random
ambient
current sense: they can be
placed
either
on
and driven in both directions [Ref.
tether,
end
of
the
Figure 3.4
23].
schematic diagram of an electrodynamic tether
is a
22]
thermal
ionospheric
Hollow cathodes can be operated in either
electron density.
[Ref.
the electron
until
the
system interacting with the ionospheric plasma.
hollow
A
cathode
(Figure
Ref.
3.5,
22)
consists of a
narrow tube with a gas expellant orifice plate
and
(or toroidal)
tube
near
out
of
To make the hollow cathode operate,
the
orifice,
pressure in the hollow cathode.
the
anode
is
Figure 3.6
ionizing
biased
[Ref.
in operation.
22]
by
producing
discharge
ignition
emission
electron
which
allows the heater to be eventually
accelerates,
separates,
slight
section of a hollow cathode
The thermionic electron
collisions
up
hundred volts positive.
several
is a cross
building
The heater is energized and
is
have
flows
/ ion pairs.
bombarding the insert heat it further. This
cathode
end
anode is positioned just off of the end of the
the cathode.
gas is ejected
one
on
insert with heater at the other end. A disk
cathode
a
heating
Ions
causes
self sustaining and
turned
off.
The
anode
and collects the respective charges
from the hollow cathode plasma discharge.
23
It is the charged
downstream of
pair production process that forms the plasma
the hollow cathode.
[Ref.
22].
An electrode placed downstream
assembly
electrons
collect
can
hollow
the
of
cathode
from the hollow
ions
or
cathode plasma plume,
depending
positive
Thus the plasma contactor can be used
or negative.
as either an electron or
electrode is
on
emitter.
ion
the
plasma will be collected by the hollow cathode
biased
plasma.
would
collected
be
changing
by
Thus,
when
from
space
the
collector, drawing in charges.
plasma
the
Charges intercepted by
the cloud are directed via coulomb forces towards the
at
the
of the tether,
end
is
polarity on the assembly,
the
either electrons or ions can be emitted to form
cloud
it
If the cathode were biased to to emit
emit ions.
to
electrons, then ions
is
downstream
the
If
bias
then the electrons in the
instead,
a plasma,
whether
anode
to form the tether current.
The
Ring-Cusp Ion Source and the Closed-Drift Ion Source are two
newer devices
that
hollow
are
derivatives
cathode
higher efficiencies, but higher complexity.
Some
increased,
studies
that
indicate
impedance
production
[Ref.
current
(the
This
24].
efficiencies,
tether current levels.
total
total
as
with
22].
current
is
at fixed potentials, the plasma cloud contracts,
increasing the emitter voltage drop
path
[Ref.
and
increasing
return
that
power
implies
and current gain,
Current gain is
tether current
24
the
drop at higher
ratio
of
the
that flows through the
contactor system into the plasma, e.g.,
electron collection)
to the emitted ion current (which determines the energy
mass
expended).
cathode heating and a constant, but very slow,
gas
for
plasma.
a
and
The only other expenditures are for initial
It
emission
of
suggested then that high tether
is
current demands would be more effectively met with
multiple
plasma contactors (on separate cables) than one large plasma
contactor.
DRAG AND DECAY
C.
When
the
tether
direction
of motion.
velocity vector
(da/dt)
=
(3.
in
electrodynamic drag force,
"m"
satellite
"w"
P = (F
kg,
km/day,
1800kg,
.
v)
an
the
where
the
and a and w are for an altitude
19:
is
is the
"F"
orbital
of
B)*L.
[Ref.
orbit
is the mass of the
is
X
F = (I
and "a" is the orbital height in km.
rad/sec,
-
in
power,
opposes
decay time to fall out of
6*24) (2F/mw)
system
that
This drag force is opposite the tether
and is of magnitude Force
The associated
3].
electrical
generated
also
electrodynamic power involved is
The
p.
generating
is
electrodynamic force is
tethered
motion in
If L = 50km,
m
then
1000km,
the naturally powered tether operating 100% of the time will
decay
out
of
orbit
in
under
two
Obviously there must be compensating
system
days [Ref.
reboost
to
10:
p.
keep
3].
the
operational for any length of time. Atmospheric drag
becomes significant when below altitudes of 250km.
25
The
more
active the system (drag and boost), the less important short
aero-drag
term
aero-drag
except for
will be,
forces
so
severe that assymetrical loading occurs on the lower mass.
D.
RESISTANCE AND IMPEDANCE LOSSES
equivalent
An
circuit
for
a
tether
model
three resistors in series with a battery ("Vbat").
includes
resistance
The resistances include the tether cable
typically
km),
current
the
largest
("Rt",
loss in the system at 5-30 ohms per
ionospheric resistance ("Rion", the
return
path
and
included effective plasma contactor resistance, typically 250-100
ohms
total),
and the load impedance ("Zl", which is
the work load or energy storage
drop
across
voltage drops and is equal to
in equatorial orbits.
flow
("I") depends
where
I
is the
supply
=
(
total
(v X B
primarily
Vbat) /(Rt+Rion+Zl
1)*L,
.
voltage
or Vbat = vBL
is the induced voltage.
"Vbat"'
)
on
the
Current
load impedance Zl,
Power available for the load
.
standard (I**2)*Z1. Let "Vrev" be the
reverse
power
voltage necessary to drive the same current level in
the reverse direction (i.e.,
reboost).
voltage
The
load).
the entire system is the sum of the individual
Then Vrev = (2*Vbat
as in thruster
-
I*Z1).
operations,
or
Note that the reverse
must be twice the self generated voltage to produce
the same current level in the opposite direction.
for balanced
antenna radiation,
26
That means
internal power expenditure
will be
twice the
self generated
the broadcast period.
[Ref.
10:
power expenditure during
5-8].
pp.
The return current path through the plasma is a
and
to follow the field
lines,
discontinuities
turbulence,
or
results in a random
between
the
are
there
are
plasma
motion
electron
walk
that
completes
two plasma cloud plumes.
has highly variable,
that
if
density-
may
be
causing electrons to join a new field line. This
disrupted,
and
dependent
current
a
impedance
nonlinear,
loop
This return path also
properties
upon current densities and oscillation
frequencies (among many other unidentified processes)
13].
complex
poorly understood process. Although electrons are bound
[Ref.
For practical purposes, most present discussions assume
infinite charge sinks at both ends,
With
drops.
proper
plasma
impedance can be brought fairly
tether
deployment,
deployer
drum
(inductance)
any
will
tether
have
on the system,
and
ignore
contactors,
a
low.
ionosphere
the return path
Additionally,
cable
residual
after
left
on the
impedance
effect
still
and induce greater losses than
if it were completely deployed.
The
largest
loss
the tether cable.
to
total
of power is due to the resistance of
The percentage of tether cable
resistance
system resistance is also the percentage of power
that is wasted
as heat
in
the
tether
conductor.
Tether
temperature depends on solar conditions, materials, current,
and orientation.
There is a finite limit on the steady state
27
carrying
current
capacity
determined
limit is
increases
in
energy
thermal
allowable
maximum
tether's
dissipation
temperature and its heat
and this current
of the tether,
the
by
be
balanced by
solar
radiation outward against any influx radiation (i.e.,
at
and reflected solar). Any power saved
watts/sq.m,
1400
reduction
through
in
tether
unit
available for the load. Primary
resistance
is
made
control
of
load
in-orbit
power comes by controlling the load resistance.
load resistance, more current can flow,
(or
Any
characteristics.
must
input
By reducing
providing more power
radiated power if the load is an antenna). Any solution
must use the lowest possible resistance per unit length that
weight will allow. Larger diameter
better,
and
can
bear
more
conduct
cables
current
tension, but their cost is in
increased weight.
E.
ALTERNATING POWER AND MODULATION
the natural self-generator
The normal mode of operation,
mode
(
expense
self -powered
of
direction,
down).
energy
)
orbital
,
produces
an
power
at
eastward
pumping
of
current
operation
in
will
the
increases
reverse
flow down,
i.e.,
orbital
direction.
electrons
If these alternate modes of operation are cycled at
frequencies,
the
orbital
the natural tether current will be up (electrons
Internally powered current
up.
For
energy.
The thruster mode
by
electrical
ELF
with the generator and thruster current levels
28
but in opposite directions, then the tether can be
the same,
used as an ELF antenna and
constant
phases.
The internal
transmission
and
system
the
will
remain
power
power
isotope,
can
solar
be
batteries
cells,
the
in
for
opposite
the
solar power is insufficient,
generated
a
and / or
battery
solar power and battery are both
thruster.
battery).
drain will
inadequate
to
reinjected
into
the
orbit
from
after the broadcast,
10:
p.
The
occur.
match
If
If
the
power during radiation, then orbital energy
will decrease, manifesting itself as orbit decay.
[Ref.
duty
half phase during this mode comes
from onboard energy sources (solar
reboost,
"on"
supplying power for radiation in every other half
phase of the frequency cycle, and then as
be
nuclear,
it is performing alternately as a natural self
mode,
generator,
self
of
and chemical.
When the tethered antenna functions
power
a
supply needed for thrust
recharged by solar cells, or other methods such as
cycle
at
frequency cycle there are two
one
In
and each power source will furnish power for one
phases,
those
altitude.
10].
with
powered
onboard
Energy can
electrical
power
systems
The broadcast period will be no longer
than ten minutes.
When the ELF transmitter is in the "off" duty cycle mode,
continuous
DC
power
supplied from the available solar (or
other non-battery onboard power systems)
reboost
the
tethered
satellite
29
system,
can
be
used
if necessary,
to
and
If reboost is
recharge the batteries.
system may need to be designed to perform in
operational
drag
cycle,
depending
manner,
means
accepting
system.
variable
apogees that can be recorrected by reboost,
orbit flexibility.
further for
this
boost
The
/
the required duty
on
to save mass in the power supply
alternative
this
then
necessary,
can be dedicated to battery rejuvenation.
power
solar
not
accept
To
perigees
or even
and
altered
There would be no effect on
the orbital plane inclination other than the effect
of
the
equatorial bulge rotating the line of nodes. The system will
remain in the off cycle for 90% of its orbital period.
oscillating
ELF
This
signal can be modulated to carry
Standard methods using amplitude,
information.
phase modulation do not work well on a signal
frequency.
better
A
modulation (PPM).
technique
is
In this method,
to
frequency,
of
delayed
in
time,
while
one side of the waveform's
the
other
advanced
phase of the wave
maintains a constant time interval, being the sync [Ref.
P. 12].
By
digitally encoding
signal has only two states,
bit
windows,
flexibility
the
data,
increasing
data
30
rates
10:
such that the PPM
error codes can be
and reliability.
low
use pulse position
rise time (either the natural or powered pulse) is
or
such
or
included
for
in
greater
F.
ALTITUDE AND INCLINATION EFFECTS
All the considerations so far have been
for
earth
low
orbits (LEO) in the equatorial plane (inclination "i" of 0).
In a pure polar orbit (inclination of 90),
tether
no
the vertical wire
runs mostly parallel to the field lines,
induced
voltage.
emf
There
would
so there is
voltage,
some
be
because the magnetic poles are displaced roughly 11
however,
degrees away from the geographic poles. The magnitude of the
equatorial
induced emf is a cosine function of inclination.
in a polar orbit the minimum voltage would be
Thus,
satellite track
passed
over
the
magnetic
if the
poles.
In
an
equatorial orbit voltage variations will swing from the full
to the cosine of (0,+-ll degrees) times vBL,
vBL,
vBL.
An orbit with
generated
=
i
60 degrees will result
capability,
emf
power.
At 80
50%
degrees
of
75%
loss
.
the
maximum
chapter.
latitude
the
equator will
be
used.
system
be examined in the next
The special case of the satellite
will
self
03*vBL.
The case of tether self generated power when the
crossing
in
At 66.7 degrees the power loss
inclination,
generated equatorial power is
is not
less
A 50% power loss would be experienced at
an inclination of 45 degrees.
is 84%.
or .98 of
and a corresponding reduction in
current flow. A 50% reduction in current is a
available
in
at
its
maximum
This is the orbital segment when
self generated power will be utilized to
provide
transmission power for downlink communications.
31
radiation
Changes
First
there
strength
the tether altitude results in two effects.
in
is
decrease
the
The second cause is smaller,
velocity
geomagnetic
the
in
and
due
is
to
the
field)
at
increased
combined
effect is
an
induced voltage
inverse
3.5
power
of
the
orbit
orbital
altitudes.
The
that varies as the
radius.
With
voltage
to
current
proportional to current, and power proportional
7
reduced
of the tether relative to the earth's surface (and
geomagnetic
squared,
field
as the inverse cube of the increased orbit radius.
the power available is proportional to the inverse
power of the orbit radius.
at 500km as 100%,
If we take the power
available
the power available at 1000km would be 61%
(one earth radius is 6370km).
[Ref.
32
25:
p.
2].
THE IONOSPHERE AND BEAH PROPAGATION
IV.
A.
THE IONOSPHERE
ionosphere
The
is
atmosphere of ionized gases and
sin
electric charges that is broken up into descriptive
starting
regions,
extend
50km
at
or layers,
extending
and
are called
to the 'topside'
the
D,
600km
to
ionosphere merges eventually with the
Electron density w.r.t.
1000km.
the
Figure
centimeter.
concentration
night,
as
range of 300 to 700km,
significantly
Iceland.
This
frequently
communications
The altitude of
with
is
in
electrons
of
above
cubic
per
the
altitude
concentration
with the seasons,
The topside
.
altitude reaches a peak in
illustrates
4. 1
and
layer
F
magnetosphere,
10E6
function
a
Electron
latitudes.
roughly
at
layer,
F2
the
and
200-400km)
approx.
These
layers,
The D layer ranges from
ionosphere.
(F2
regions
600km.
F
E,
the E layer from 85 to 140km,
50 to 85km,
extends
above
to
for
electron
the
mid
varies between day and
In the altitude
and solar activity.
vary
electron concentration does not
altitude in the northern latitudes near
the
the
region
which
forthcoming
will
be
development
mentioned
of
a
case model centered on this geographic area.
500km
will
be
used
altitude for the satellite antenna.
33
as
a
good
average
The presence of electric charges in the upper atmosphere
affects
transmission of radio waves (via wave-particle
the
interactions), by attenuating the signal or
reflecting
it.
radio frequency (RF) energy encounters free electrons,
When
energy
some of the
electrons
wave
the
of
is
transferred
to
the
the form of oscillations at the RF frequency.
in
These oscillating electrons then reradiate the same RF wave,
restoring the
RF
signal.
neutral
the
however,
If,
gas
density is high, the oscillating electrons will collide with
Energy is lost to the neutral atoms
neutral particles.
the
form
in the
of
energy
electron
that
neutral atmospheric molecules.
is
the
precisely
Attenuation
factor.
the
due to
altitude,
and
relatively small in the E and F layers.
As
one
approaches
the
layer from either above or
F2
below, the electron density increases,
velocity
of
the
frequency),
and
refraction.
Thus,
the
available
The lower the frequency,
attenuation
neutral collisions decreases with increasing
is
the
at the D layer because of the high density of
happens
greater
reducing
energy,
can be reradiated at the original RF
This attenuation in signal strength is
frequency.
what
thermal
smaller
medium.
produce
the
electromagnetic
decreasing
the
refractive
(for
effective
a
phase
constant
index
of
the electron concentration,
index
For specific frequencies,
zero
wave
the
larger
increasing the
for
that
ionospheric
certain electron densities
indices of refraction. A wave transiting high
34
Higher
indices and reaching a zero index will be reflected.
frequencies will reflect at higher electron densities.
maximum
a maximum frequency that can be reflected,
signals
above
pass through without reflection.
will
incidence other than normal, the effect is
reflective.
than
Since
electron concentration is at the F2 layer, there is
which
RF
At angles of
refractive
more
electron density required to reflect
The
the the angled wave front is then less. Decreasing the angle
of incidence increases the maximum (critical) frequency that
can be reflected along that path.
critical
the
frequency,
frequency
the lower the
The smaller the
signal.
will
reflected.
that
altitude
angle
will
of incidence,
altitude at which it will be refracted.
This
The lower the
reflect
the
the lower the
how
is
long
systems work, by bouncing signals off
communications
range
below
frequencies
All
be
density layers in the ionosphere.
In general,
regular
frequency
both
under normal operating circumstances
communications
refraction.
an
signal
RF
(i.e.,
low
of
(such as ELF or VLF) will be strongly affected by
attenuation in
mechanism
systems)
There
which
the
is,
is
D
layer,
however,
effective
at
and
by
another
ELF,
and
reflection
or
propagation
that
is the
whistler mode.
An audio amplifier system hooked up to an elevated
long
wire antenna will receive RF atmospheric noises in the audio
frequency range.
Often these noises will resemble a falling
35
pitch tone,
the
hence
or whistle,
name
whistlers.
It
is
known that whistlers emanate from lightning flashes near the
earth's surface.
The low
penetrates
upper
magnetic
the
field
hemisphere, to a magnetic conjugate
is due to the
behavior
with
varies
velocity,
arrival
mechanism is believed to be a
propagation
in
believed
along
and
to
exist
to
ion
[Ref.
attenuation
will
line
paths
be
are
ranging from
between
and
100
26].
for a whistler waveguide path is
rate
27:
p.
10-38].
Figure
4.2
calculated ELF ionosopheric penetration losses
earth's
the
wave
Whistler
cyclotron whistlers that may extend out to
the earth's ambient day and night
the
field
guiding
different
bounce
that
paths
natural
These
that transport the signal,
shown in Figure 4.2 [Ref.
the
a
Several
transported.
several earth radii.
The
resulting
geomagnetic field lines.
the
subprotonic whistler
1000km,
waveguide
magnetoplasma.
a
front normals within 20 degrees of
coupled
lower
a
transport
magnetosphere act as tubes,
waveguides in the
signal
natural
have
The
time.
particular characteristics of electromagnetic VLF
the
from
wave
"whistler"
The
point.
to a
opposite
the
to
it
frequencies
Lower
delayed
a
couples
way in which the index of refraction
frequency.
thus
then
and
following
line,
energy
electromagnetic
frequency
ionosphere
surface.
The
graph
ionospheres,
is
plots
through
through
to
representative of the
losses that would occur at high geomagnetic latitudes and is
36
for plane waves incident on the ionosphere
parallel to the geomagnetic field.
direction
a
in
These losses are not per
path length traveled, but per trip through the ionosphere.
Figure 4.3
[Ref.
27:
p.
10-27]
rates for propagation in the Earth
They
show
the
shows
-
2000
and
5
attenuation
waveguide.
daytime and nighttime attenuation rates in
decibles per million meters of path length
between
the
Ionosphere
hertz.
These
for
frequencies
two graphs are for the
earth ionosphere-waveguide trap; for signals
following
the
earth's curvature, bouncing between the ground and the lower
ionosphere.
/
For 2000 hertz (daytime) the attenuation is 30db
A nighttime 2000hz signal is extrapolated out
megameter.
Daytime
to 15db / Mm.
attenuation
losses
twice
are
the
nighttime attenuation losses.
Note
that both Figures 4.2 and 4.3 are sensitive to the
time of day,
frequencies.
and both show increased attenuation
in penetrating the ionosphere from space than from
the
signal
in
higher
at
can also be seen that smaller losses occur
It
the
bouncing
earth-ionosphere waveguide. The losses
from space penetration also occur only once. Losses
in
the
ground waveguide are dependent upon distance. A frequency of
lkhz-3khz would be attenuated by 10-15db in the daytime, but
much
less
than 5db at night in penetrating the ionosphere.
This frequency range also appears to be the
ELF
communication
ionosphere,
using whistler
upper effective
that can easily penetrate the
frequency
propagation along
37
field
lines
(during
daylight
the
the
attenuation
be no greater than
penetration
net
The
and an additional
15db,
the ground hop portion.
B.
hours).
an ELF signal from a satellite should then
for
[Ref.
30db / Mm
for
27].
THE GEOMAGNETIC FIELD
The
magnetic
earth's
field
modeled after a
be
can
simple dipole magnet located in the center of the earth, but
approximately
tilted away from the rotational axis by
degrees.
The
geomagnetic
degrees North,
geographic
and
north
pole
degrees
70.9
coordinate system.
West,
accurate
Specific
bring
listed
The field strength
is
the
field
for
can
be
10
Adjustments
2.5
percent
strengths
computed
at
from
in the appropriate bibliographic listings.
over the earth,
depicted in Figure 4.4 [Ref.
constant
within
to
it
equations
various altitudes and zenith angles
formulas
to
The dipole model is
out to several earth radii.
to the dipole model can
accuracy.
relative
Future references will round
these values to 79N and 71W (289E).
percent
11.5
located at 78.8
is
28].
at an altitude of 500km,
In this chart,
lines of
magnetic field strength are ploted on a map of the
earth's surface, with lines of latitude and longitude marked
appropriately.
The strength
of
the
geomagnetic
field
is
important if the tethered satellite is to be evaluated as an
electrodynamic generator or propulsive unit.
38
Geographic
latitude
can
be
converted
latitude either by referencing maps printed
coordinates,
directly
formulas.
geomagnetic map of
Greenland
called the GIUK
Iceland
the
Iceland
-
Figure
world,
-
gap)
4.5
[Ref.
circa.
examination
geographic
examined,
is
in
one
would
from
to
60
70
geographic.
that
The
model.
entire
The
west).
degrees north geographic.
starting
around
gap
Ice pack
degrees
75+
chart shows that the corresponding
The
geomagnetic latitude
Iceland
for
is
70
degrees
north
Note that with a magnetic latitude of 70 degrees
magnetic.
Iceland's magnetic latitude differs by
its geographic position.
range
note
location of Iceland is roughly 65 degrees north,
operations would be possible
north,
a
the
and will be the
communications
this
and 340 degrees east (20 degrees
north
is
If
United Kingdom expanse (here after
This happens to be a submarine transit area,
ranges
29]
1960.
roughly centered in the middle of this waterway.
is
area of
geomagnetic
calculating those magnetic coordinates
by
or
from
to geomagnetic
in
previously
The
5
60 to 70 degrees north is shifted by
of
degrees from
geographic
quoted
5
degrees to
become 65 to 75 degrees north magnetic.
Once
the
magnetic
latitude is known, the magnetic dip
angle can also be calculated.
angle
that
degrees,
the
the local geomagnetic field line makes with the
earth's surface.
90
The magnetic dip angle is
and
The magnetic dip at the magnetic
at
the
geomagnetic
39
equator
poles
is
it is zero
degrees.
In between,
approximation
dipole
the
using
an approximate value can be
to
applied
surface.
Using Iceland at a magnetic
degrees
north,
calculated
earth's
the
latitude
70
of
(ML)
and ignoring local
then with substitutions,
inconsistencies, the solution equation is:
ArcTan [2(sin 70)/(cos 70)] = 80 degrees.
the range of 65
Again,
produces
dip
horizontal.
angle
to
ranges
degrees
75
of
This dip angle is
the
magnetic
angle
for whistler mode
arriving in the GIUK gap vicinity. The southern
propagation
edge of ice pack operations would
On
degrees.
north
to 82 degrees from the
77
have
a dip angle
of
85
the other side of the northern hemisphere,
the Bering Sea and
northward,
the
geomagnetic
sphere
at
is
northerly of the geographic sphere. Hence, magnetic
rotated
latitudes are several degrees south of
geographic
latitudes,
degrees less than
at
and
the
dip
same
angles
their
are
geographic
corresponding
also
several
latitude
near
Iceland.
C.
THE WAVE PROPAGATION MODEL
The
purpose of the proposed communications system is to
communicate with submarines in their operating areas.
areas
ice.
These
will be in the northern latitude waters and under the
In order to establish the validitity of the concept
40
of
using
whistler
communications
will
be
mode
propagation
method,
constructed,
techniques
this
and
model
communications link to the GIUK gap.
selected
because:
transit
area,
(3)
Except for the Hudson Bay
dip
pack
magnetic
As will be explained later,
increases
angles)
the power available for the receiver on the ground.
the
most
is
Greenland
-
located in a higher
angle.
lower magnetic latitudes (with lower dip
is
a
been
has
the GIUK is the northern-most operations area
magnetic
latitude
simulate
area
The northern ice
(2)
the geomagnetic sphere,
w.r.t.
will
This
The GIUK gap is where US and foreign
(1)
subs both patrol and transit,
relatively nearby,
the
as
geometric and mathematical model
a
difficult
scenario
in
The GIUK
which to establish a
whistler communications link.
constructing
In
techniques
accepted.
should
a
several
model,
be applied for
a
model
methodological
to
be
easily
Five concepts were applied to this model:
1.
Generalizations - keep things simple and use
approximate values where ever possible. Simple models
have longer lifetimes, are more flexible, easier to
change, and are comprehendible.
2.
Reasonableness - use concepts and steps that are
intuitively obvious, that are reasonable and
acceptable.
3.
Conservatism - use proven and logical steps
that are widely approved of.
41
Pessimism - lean toward conditions that would hinder
success or completion. By stressing the model and
proving capabilities in worst case situations, then
success is ensured in normal expectations.
Reproducibility - if the reader can immediately
duplicate the model in his mind, and it seems
logical, then it is probably true and applicable.
The model that follows will be reasonable,
towards
leaning
a
worst
conclusions drawn here would be
compared
to reality.
conservative
radiated
the
power
tracing,
and
demonstrate
signal
the
side lobes,
path
and
Much technical research is being done in ray
still
minimum
needs
to be done, but this model will
expected capabilities
proof
and
concept.
The following are the initial assumptions:
1.
any
when
density on the earth's surface,
received signal level, main beam footprint,
coverage area.
answers
and
Examined will be the antenna radiated
power that is coupled to the field lines,
loss,
simple,
case situation, hoping that
The tethered antenna has a length between
1
and
20km.
2.
The coupling angle between the field line and
the wave front normal is less than or equal to
20 degrees.
3.
The magnetic dip angle at Iceland is 80 degrees.
4.
The ground dip angle of 80 degrees is
extended into space, so that a satellite will
also see the same dip angle on its antenna.
42
of
5.
The magnetic pole, the satellite antenna, and
the receiver will all be in the same plane
defined by a common magnetic meridian.
6.
The wavelength will be between 100 and 300km,
for a frequency between 1000 and 3000hz.
7.
The satellite will be in an orbit between 200
and 1000km (average of 500km), because it must
remain in an environment of high electron
density in order to utilize self powered
properties.
8.
The efficiency of the antenna as a radiator
will not be considered. Only the actual radiated
power will be considered, not how much power is
required to pump the antenna to produce the
radiated energy.
9.
Within the coupling angle of 20 degrees, 100
percent coupling is assumed. This is reasonable
because although the nearest field line within 20
degrees may not couple all of the available
energy, the next field line will absorb a
percentage of the remainder, and so forth at
greater distances until essentially all of the
available energy has been coupled.
10.
The whistler propagation mode is a reversible
process, with the 20 degree coupling cone operating
along the entire field line and at both ends. A
signal can couple and uncouple at either end, within
20 degrees of the field line.
11.
Any radiation produced by non-symetrical return
currents through the ionospheric plasma will be
neglected.
12.
The orbit will have an inclination of 65 degrees
to bring the satellite over the Icelandic operation
area.
An equatorial orbit cannot be used because at
this low altitude the field lines the antenna would
couple to (in the whistler propagation model) would
intercept the Earth well south of Iceland. Going
higher than 1000km reduces the electron density,
which is necessary for powering the tether as a
generator.
43
The Coupling Model
1.
First,
the power available
antenna
in
topside
the
the
by
It is not entirely clear if a
field line must be determined.
10-20km
captured
to be
of the ionosphere can be
addressed as a dipole antenna. There is much work being done
to evaluate these radiation patterns, hence a limiting
will
considered. A
be
wave dipole antenna typically
long
has a radiation pattern similar to
transmission
ends
with
donut,
a
maximum
efficiency in a direction perpendicular to the
antenna axis (around the equator),
of the
case
of
the
Figure
axis.
minimum
and
efficiency
illustrates
4.6
the
a typical dipole antenna.
The
gain of a perfect dipole is 1.64 when the antenna's axis
is
gain
radiation
pattern
for
parallel to the RF wave front.
its
axis (off the ends)
the antenna can
depends
on
how
plane
equatorial
is zero.
transmit
many
The gain of the antenna along
to
a
degrees
Thus,
the amount of power
specific
away
point
from
the
in
space
antenna's
the point lies. A dipole shorter than the
optimum length will have reduced gain.
To approximate
meter)
antenna,
that
a
the power
point in space
density (watts per square
would receive from a dipole
one must first determine how much power that
point
would receive if it were the same distance away from a point
source
antenna
radiator,
with
a spherical
(
isotropically
expanding) wave front. Multiply the energy density
distance
times
1.5
at
that
for the approximate maximum gain of an
44
inefficient dipole antenna along its equator.
represents
maximum
the
received
this
at
orientation.
power
density
distance,
This
product
can
ever be
of
antenna
that
irrespective
To take into account antenna orientation w.r.t.
the same selected point,
square the cosine of
the
included
angle formed by a line joining the center of the antenna and
the
point,
and the antenna's bisecting plane (the plane of
Multiply this value by the
its equator).
previous
product.
An example follows.
When the antenna
all wave front normals
transmits,
within 20 degrees of the intersecting magnetic
will couple,
field
a
laying
line
first considered,
to
and Figure
the
antenna's
axis)
is
4.7 illustrates the situation.
the coupling angle for the field lines is 20 degrees,
only the energy inside of a cone emanating from
of
lines
the antenna's maximum gain plane
in
(therefore also perpendicular
Since
field
according to whistler mode physics. The case of
the
antenna
that
center
the
has a half angle width of 20 degrees
(which is a total of 40 degrees) will properly couple to the
field.
surface
The area that the base of the cone inscribes
of
the
isotropic
sphere,
surface area of that sphere,
on a one watt transmitter.
maximum
power
that
on
coupled
can
45
be
to
the
the total
the
is the
field,
The ratio is roughly 4.5%,
or 13.5db below the total transmitted power
the
by
and times a gain of 1.5,
maximum radiated power that can be
based
divided
"Pt".
transferred
This
into
is
the
link in this worst case evaluation.
communications
"Pa"
is
the maximum power available to couple to the field lines.
Pa
Pa
=
=
.
is
closely
relationship
to
the
5db
13.
that can be transferred into the
The probable power
lines
field
045 * Pt
Pt -
related to the dip angle, and its
pattern of the antenna: a
gain
dipole antenna pointing directly at the earth.
wave,
dip angle were
degrees,
perpendicular
antenna's wave front normals), then
signal energy of
case
interest,
end
the
If the
lines
maximum
045*Pt would be coupled.
the dip angle
the
of
.
magnetic
the
at
field
were
antenna's axis (and aligned with the
the
to
the
that
so
long
equator.
antenna
transmission abilities.
This would be the
In the geographic area of
is closer to 80 degrees,
where
it
available
is
the
efficient in its
less
If the dip angle
near
were
90
degrees,
no available energy to couple to the field
there
would
lines.
Along the antenna's axis, the effect of the dip angle
be
on power density can be
function.
of
80
degrees
available
lost.
estimated
with
squared
cosine
a
Therefore at 80 degrees of dip, the cosine squared
"Pc"
is
power to
is
the
close
to
couple,
.03,
or
3%
an additional
coupled
power (for
angle in this case).
46
of the maximum
15db
an
of
power
80 degree dip
Pc
=
.
03 * Pa
Pc = Pa
-
15db
The total net signal power that can be coupled to the
field lines over Iceland is now .00135*Pt.
summary
The net power
is:
Pc = .00135 * Pt
Pc = Pt - 28. 5db
Figure 4.8
crosscut
antenna.
The
slice
shows the dip angle relationships
area
the
of
of
The view is looking east.
northern
edge
illumination
North
degree
-
80)
Figure
4.9
views
the
looking down on the earth from above.
not
to
eventually
the
40
degree
meridian,
a
cone
4.
top
of
the
It shows the
If
it
were
intercept the ground, the energy inside
10
continue
would
decreasing in intensity as
Figure
and
+ 20],
pattern as it spreads in a northerly direction.
energy.
left.
with the lower ionosphere and the earth's
angle
surface (flat earth).
antenna,
the
is to
the cone of illumination makes a 30
of
degree angle from the antenna axis [(90
60
and a
below the
views
looking south.
the
field
outward,
lines
gradually
absorbed
the
the antenna along the magnetic
Here the ideal dipole
47
pattern
can
be
and the relationship of the coupling area with the
seen
low gain area along the antenna axis.
2.
The Whistler Waveguide Transmission Model
The dip angle at high latitudes is so close
the
to
vertical that the satellite is almost overhead the receiver.
The
beam
path
distance
traveled is only slightly greater
than the satellite's altitude. From Figure 4.2 we
conservative
could
be
obviously
attenuation
several
much
db
factor
overly
Nighttime
pessimistic.
favorable
more
for
this
communication, with a loss of less than 5db. A
another 97% reduction of the coupled power.
loss up to this point is .995%,
obtain
of 15db for daytime.
or 43.5db.
The
type
a
This
is
of
15db loss is
total
net
This is the power
ratio available to "uncouple" from the field line wave guide
and
"reradiate" to the earth's surface for reception at the
termination point of the ray trace.
The power summary is
as
follows ("Pu" is the uncoupled power in the lower ionosphere
for reradiation to the earth's surface):
Pu =
.
03 * Pc
Pu = Pc - 15db
Pu =
.
00004
*
Pt
Pu = Pt - 43 5db
.
48
The Uncoupling or Reradiation Model
3.
process of absorbing or releasing energy in the
The
whistler waveguide tubes occurs within the ionosphere,
but
low enough so that the normal attenuation of the D layer for
frequencies
below
critical plasma frequencies and the
the
gyro frequencies does not occur.
where
energy
absorbed
is
ionosphere
lower
The
into
whistler
the
electromagnetic disturbances that originate
(such
from sources at the
whistler
other
release
mode
the
on
end
area
field
the
of
can
as
the field line will propagate outward
path and process,
degrees
of
a
point
any
so
specific
surface
the
line
Therefore,
from
to
uncoupling
from the field lines. From
be
to
illuminated
be
and an angle of 20 degrees,
thus
within
the
on
field
considered
can
roughly
the
same
20
whistler transport mode is reversible in
The
cone.
The
due to
The power uncoupled from
(the bottom of the F layer).
degree
ground
line.
treated
be
multiple point wave front sources at an altitude of
150km
is
from
and where energy can be (re) released
lightning),
as
mode
added
an
can
150km,
by
within
earth
couple
to
area
this
reradiation
20
it.
is
energy
an altitude of 150km
extra
50km
radius
in
to both sides of the regular whistler
footprint that would be formed as the beam transited through
the the ionosphere, down
of
the
primary
geometrically.
beam
to the Earth's surface.
footprint
Rephrasing: the total size
49
The
size
can again be approximated
of
the
primary
beam
will
greater
50km
be
beam
illuminated
in
than the directly
radius
due
radius
the
to
uncoupling
reradiation effect. An even larger secondary footprint
and
will
be discussed later.
referencing Figure 4.10,
By
to
multiplying
primary
the
west) of
the
satellite's
the tangent of 20 degrees
ground
the
footprint
can
evaluated by
be
altitude in kilometers, times
(.36),
the multipoint reradiation.
to
the radius width (east
footprint
and then adding
50km for
The reradiation width
the
(vs
added
is
ionospheric footprint)
mainly for model simplicity, but also because of
steep
the
inclination of the rays that are still going to be following
their
original direction. Additionally, the ELF signal that
penetrates to the ground will enter an
waveguide
it radially away.
propagation
earth
-
This Earth
ionosphere waveguide
-
create
will
a
the
primary
reaches the ground,
that
the
refraction.
signal
This ground
secondary footprint around a
weak
satellite primary footprint. Refractive
spread
is
method ground based ELF systems utilize. Figure
4.3 shows the attenuation rates for that system.
hopping
ionosphere
traps a portion of the signal and disperses
that
in
the
effects
ionosphere
will
also
before it
and the reradiation submodel allows
for
A summary of the Primary Lobe Width (PLW)
is as follows:
50
Primary Lobe Width
=
Altitude( 36) (2
100 = width km
310km wide
=
PLW @ 500km
=
460km
PLW @ 800km
=
670km
PLW @ 1000km
=
820km
A secondary lobe can readily
alternate
from
+
)
.
PLW @ 300km of altitude
expected
be
paths that previously
have found. This is analogous to the side lobes
they
antennas,
system
refraction,
secondary lobe can also
eventually
50%
of
typical
reflection,
consist
of
all
etc...).
the
energy
wider
-
ionosphere wave guide. A secondary
than
the
6db down)
primary
footprint
footprint
(at one-forth the
untypical in communications systems.
is not
Given that the wavelength is on the order of 100km, this
only
Again,
few
wavelengths
a 6db loss
kilometers
of
wider
would
travel
the earth
from a ground based ELF system.
only
from
occur
in
penetrates
be
to the ground,
trapped
in
the
is
the primary beam width.
than
-
a
few
hundred
ionosphere waveguide
This same philosophy will be
applied to the secondary lobe diameter.
will
The
that
penetrates to the surface, but which gets caught
in the earth
power,
form
there because of inefficiencies of the
are
(aperture,
to
uncoupled energy may
and is not
Earth
-
The
radiation
that
immediately absorbed,
Ionosphere
waveguide,
expanding out to one and a half times the primary footprint.
Beyond
this
distance,
it
will be assumed that the signal
51
strength will have been attenuated below usable levels to be
Summary of the Secondary Lobe Width (SLW):
received.
Secondary Lobe Width
=
(1.5)PLW
SLW @ 300km antenna altitude =
460km wide
SLW @ 500km
=
690km
SLW @ 800km
=
1000km
SLW @ 1000km =
1230km
height,
The
or
north-south
width km
=
length
of
the
footprint,
can be evaluated while looking at Figure 4.8.
southern
extent
of
the
footprint
will
be
satellite's nadir, with the northern extent defined
leading
edge
Reradiation will
side of this dimension.
of 30 degrees
radius.
The
near
the
by
the
slant ray departing the antenna at 30 degrees
from the vertical.
each
beam
(.58),
add
another
50km
to
The altitude times the Tangent
plus 50km,
primary
the
is
footprint
Summary:
Primary Lobe Height (PLH) = Alt.
PLH @ 300km altitude
(
-
1
.
155 +100km = height km
)
450km high
PLH @
500km =
680km
PLH @
800km
-
1020km
PLH @ 1000km
=
12500km
52
secondary
The
footprint
height
will use the same
logic as the secondary footprint width:
Secondary Lobe Height (SLH)= (1.5)PLH = height km
SLH @ 300km =
675km high
SLH @ 500km
=
1020km
SLH 8 800km
=
1530km
SLH @ 1000km
=
1875km
Figure 4.11 shows the footprint as it would probably
be on the earth,
function
antenna
of
illuminated
altitude
(h).
on the earth from the
with
ellipse,
and roughly defines its
minor
the
axis
dimensions
beam
orientation,
larger
however,
but curves away from the satellite.
footprint
is
the
primary.
same
as
The
an
but
50
earth is not flat
The
effect
on
shining a flashlight at a
tangent near the perimeter of a basketball.
light becomes elliptical,
is
The secondary footprint
percent
the
lobe
oriented east-west and the
will also be an ellipse with the same
the
a
The primary footprint
main
major axis (35% longer) north-south.
than
as
The
circle
with a tear drop affect.
of
The major
axis in this case becomes even more elongated.
4.
Primary Coverage Area
The
area
of
the primary footprint is approximated
by the area of an ellipse.
footprint
It is
expected that the
sizes are conservative by design,
53
derived
and that actual
will be larger. Based on the above major and
coverage areas
minor axis, the following minimum square meter areas
should
be expected for a given altitude:
Primary Footprint Hot-Spot Size:
300km altitude
110,000 square km
:
@ 500km
240,000 sq. km
800km
520,000 sq. km
1000km
800, 000 sq.
km
Secondary Footprint "Warm-Spot" Size:
@ 300km
240, 000 sq.
km
500km
550, 000 sq.
km
800km
1,
1000km
Now
determined,
is
that
1,800,000 sq. km
the size of the signal footprint has been
and the coupled signal power delivered
known,
determined.
on
the
proposed
Total transmitted
earth in the main beam is .004%
By
energy.
(.24 X 10E6),
for a
1
watt
attenuation
power
of
the
.
10E-16 watts
If the
54
(based
original
radiated
00004Pt and dividing
for an altitude of 500km,
transmitter.
that
The power that reaches the
taking a simple ratio of
magnitude solution is
to
signal energy density can be
1000km of travel) was 43.5db.
it by
km
Illuminated Footprint Power Density
5.
area
200, 000 sq.
the order of
per sq. meter (-160db),
antenna were
radiating
10,000 watts, then the surface
the order of
If
watts
10E-12
is the
"Pi"
energy
density should be on
(picowatts)
incident
power
per
square meter.
uncoupled
(the
power
spread over the illuminated footprint) then:
Pi =
.
00004
Pt
*
Pi = Ft - 160db
This value can be compared to the attenuation factor
present
for
ground
based
ELF
From
systems.
previously
referenced figures it was noted that the signal loss
earth-ionosphere
pure
per 1000km (daytime).
typical.
wide
6000km,
The signal loss due to the
would
be
coupling
-
150
original intensity.
be
of
180db,
ground
or
path
or
job.
same
link.
ELF
would
be
attenuation
loss
10E-15 to 10E-18 below
based
would
Of
continuously.
satellite could
It
and
as powerful
can
cover
a
expected that a
is
downlink as
could
it may take a
to
do
the
ground station has essentially
the
course,
a ground system
transmitter
less than optimum,
At
unlimited energy resources,
area,
based
Other losses to be added to this
ground station 1000 times, or more,
same
30db
losses and spherical spreading as the radiated
100 times as powerful as a spaced
the
a
-
more,
wave expands out in all directions. At best,
form
for
would be around 25
world
For a
propagation paths
system,
alone
waveguide
much
55
energy
to
much
10,000
a
larger
watt
receiving
trailed behind
antenna being
a
1
ground station
Megawatt
a submarine in the arctic,
It would also
could.
as
be more
survivable in performing that mission.
actual
The
power received by an antenna trailed at
depth by a submarine will be less than the surface
Skin
power.
depth
dependent
to 1/e,
depth
refers
seawater).
of
incident
medium
and
36
ice has a skin depth in the thousands
(essentially
transparent
compared
when
to
A submarine can receive a signal just as well at
a depth of 100 meters as it can
bottom
frequency
For seawater, the skin depth is between
or 37%.
meters
a
in which signal intensity has been reduced
and 100 meters at ELF;
of
to
a
very
thick
at
ice formation.
for seawater:
the
At a depth of two
times the skin depth, the transmission factor
depth squared,
below
meters
100
the
is
skin
14%.
Received Signal Voltage Level
6.
The power (P) that a dipole antenna picks up can
be
converted to a voltage signal level (E) from (P = E**2/377).
The
free space watts per sq. meter Poynting vector produces
meter signal strength in an electric
a volts per sq.
This
conversion
is
for
ideal circumstances,
down will allow for margin in the model.
magnitude
of
10E-12
but rounding
power
incident
watts / sq. m (for a lOkw transmitter)
will produce 20 microvolts
length.
A
field.
per
An ELF antenna 1km long,
signal levels in the tens of
meter
of
antenna
should be able to generate
millivolts,
56
trailed
well
within
the
capabilities of present day technology.
of
trailed
the
antenna
to
Increasing the depth
several multiples of the skin
depth still allows millivolt signal levels.
Sweep Rate and Swath Coverage
7.
The orbital period for the satellite is
around
1000km of altitude.
Though the satellite can not
physically
orbit along the 65 degree north line of latitude,
while
speed
transiting
approximated by dividing
this
area
the
the
of
circumference
rate
400km
of
per
sizes.
restrictive
4
minutes
Unless
beam.
communications
of
footprint
actual tests can prove a longer
for this program.
within
satellite,
90 minutes.
highly predictable and regular.
desired,
cover
a
interval
However,
coverage interval is the rapid revisit
prove
could
offsetting the short
time
for
the
same
The overhead times are also
If a
50%
window
time
is
constellation of 12 satellites would be necessary
each
transmitter.
pass,
for a sweep
using the conservative
communications time, this short time
to
earth
Two and one half minutes of that time would be within
primary
the
can be
the
of
This is based on near circular
minute.
At 1000km of altitude,
should be possible
time
its ground
world
(40,000km) by the orbital period of 100 minutes,
orbits.
100
varying from 90 minutes at 300km to 105 minutes at
minutes,
On
area
of
operation,
based
on
a
lOkw
the northern most extent of each satellite
60 degrees of east - west coverage should be possible,
30 degrees either side of the northern limit.
57
At 65
degrees
north,
degree
one
east
degrees of swath is 2800km.
as
hot
the
divided into
spot
2
height.
west is about 47km wide; sixty
-
The swath height still
remains
A constellation of 24 satellites
orbital planes could cover both GIUK and
Bering Sea (or Straits) 50% of the time,
the
for a swath also 60
degrees of longitude wide.
D.
NOISE AND INTERFERENCE
Noise
levels
the
in
ELF ranges are relatively higher
than in other communication
advantages.
delivered
but
bands,
has
its
own
whistler mode field lines: reciprocal
the
along
hemisphere
noise transmitted from the opposite
complimentary
line's
ELF
There are two continuous sources of noises, both
position);
(the
charged
and
field
particle
oscillations along the field lines several earth radii away.
For
complimentary
southern
electromagnetic
hemisphere
has
little
noise
sources,
the
Complimentary
activity.
positions are in the open ocean just north of the antarctic.
Though this area is meteorologically very active, due to the
lack
of
land
atmosphere,
to
there
induce
is
vertical
apparently
disturbances
little
in
the
lightning.
The
farther from the equator one travels the less the electrical
atmospheric
emissions.
The motions of electrons and protons
along the field lines produces a continuous
which
increases in the lower frequencies.
that studies be dedicated
evaluate
to
58
broadband
hiss
It is recommended
high
latitude
ELF
noise,
how
and
communications.
it
[Ref.
might
27:
interfere
Most disturbances that
affect
ELF
systems
with
disrupt
communications
the
Sudden
that
variability
the
Ionospheric
Sudden
Enhancement
Enhancement
The
lower
interference from SID's.
transport
signal as
of
Some
(SID's).
Sudden Phase
Sudden
Signals,
Atmospherics (thunderstorms), and Polar Cap
of
Absorptions.
slightly
Disturbances
can have interfering effects are:
Anomalies,
the
systems
ionosphere that can be caused by ionospheric storms
and other
SID's
mode
because the propagation mode and
less
wave guide paths insulate ELF from much of
in
whistler
10-20 to 10-63].
pp.
mode
different
may
the
The variances of
produce
elements
delayed times
frequency,
of
[Ref.
30].
pulse
the
the
energy
less
paths
the
in
stretching of the
arrives
signal
at
Reflection of the signal
from the opposite hemisphere will produce ghost
signals
at
much reduced intensity levels.
E.
SELF-POWERED GENERATION CAPABILITIES
Driving the tethered antenna in a semi self-powered mode
has
several
advantages.
pumping electrons alternately
By
between two ionospheric charge sinks
(shells
at
different
altitudes) the efficiency of the antenna within a conducting
plasma
increases,
controllable.
kinetic
energy
and
antenna
Additionally,
into
impedances
the conversion
are
of
more
momentum
electromotive force on alternate half
59
cycles in the transmission
power
critical
supply
efficiently
phase,
conserves
mass during peak transmission power
Depleted energy storage systems can be recharged by
demands.
or other low power
solar panels,
duty
continuous
systems,
during the off duty cycles.
The maximum self generated power ability of a tether can
be easily
calculated,
the
explained,
as
will
induced
the
be
shown.
voltage is a cross product of
velocity (v) and field strength (B), dotted
direction
times
(1),
its
length
vector
is
perpendicular
containing the the field line. With
solutuion
the
is
to
down.
is
perpendicular
a straight forward v*B*L.
7.2 km/sec.
because
the
meridian plane
the
value for the orbital velocity minus the
velocity
tether
the
to
An orbit with zero
(L).
inclination produces the the maximum potential
velocity
previously
As
vectors,
A conservative
geomagnetic
field
The tether direction will always be
Field strengths vary from .20 to .50 Gauss at 500km of
altitude over various parts of the
Earth.
Field
strengths
vary more by latitude than by altitude.
As the inclination increases,
decreases
the self generated voltage
because of the cross product. The tether velocity
vector is not perpendicular to the field lines it
the
tether
(still at low
cuts.
As
reaches the most northern latitude of its orbit
inclination),
the
voltage
level
because the cross product is again perpendicular.
60
increases
the
As
voltage
inclination
increases
further
because
the
of
increases,
dot
The
product.
magnetic
so it is no longer perpendicular
dip
the
to
angle
tether
When the satellite is at its far northern latitude is
axis.
also when the antenna needs the maximum power
transmit
ELF
its
to
magnetic
The
field
strength,
at
over the Icelandic area is read as .42 Gauss (4200
altitude,
Tesla)
available
The dot product acts through the
signal.
cosine of the dip angle.
from
Figure
4.4.
With a dip angle of 80 degrees, a
tether of 1km will produce
*
maximum
the
,
produced at the northern extreme begins to decrease
cos (80)]
=
50
[(7200) * (.42 * 10E-4) * (1000)
volts
kilometer.
per
conducting 10 amps will produce
5
tether
10km
A
kw of power, over Iceland.
A 20km tether can produce lOkw here.
It
must
generation
levels.
the
noted
be
levels
are
here
tether
that
self
Tether power is only injected into one
phase cycle,
powered
same as the transmission power
the
half
of
and comes from orbital kinetic energy,
but
-
internal power supplies inject an equal and opposite current
into the
during
tether
phase
alternate
the
cycle.
The
internal voltage supplies must be twice the tether generated
voltage in order to overcome the tether potential,
produce
the
same
Current levels need to be the same
reduce
signal
and still
current level in the opposite direction.
distortion
in
both
directions
and impedance effects.
voltage for the same current is twice the
61
power,
to
Twice the
but
over
only half of the cycle.
of
X
watts
for a desired transmission power
the tether should generate voltage and current
watts,
for X watts,
So,
the internal power supplies should
generate
cycle time within the alternating cycle that the
"resting"
getting
and
be self powered,
"microboost"
its
tether
The operational
.
antenna efficiencies are greatly
increased
100% on board power 100% of the time).
losses of the cable permitting, transient bursts
Ohmic
of higher currents
be necessary
may
at times.
A temporary
to 20 amps in a 10km tether increases tether power
increase
(and transmission power if internal supplies can handle
surge)
is
By allowing the tether to
but necessary.
cycle is complex,
(v.s.,
X
continuously, but 2X watts intermittently for the 50%
to
lOkw.
A
tether
20km
can
the
be boosted to 20kw.
Sustained high power level durations are a function of cable
temperature and heat dissipation capability
Fortunately,
total
configuration
is
transmission
only
location,
current
over
as
when
the
function
the
cable.
in
a
normal
normal
of
By playing
time,
and
effective footprint can be
transmission
levels)
footprint is over the receiver
power
a
broader
a
created. By boosting the
tether
of
on the order of minutes.
with the transmission power
footprint
time
target,
power
while
and
(by
boosting
the secondary
then
reducing
primary hot spot is over the receiver, the
communication window is much expanded
temperatures can be tailored.
and
tolerable
cable
Tailored power boosting should
62
be
able
increase
to
the
duty
the
cycle coverage of an
operational area to 75%, up from the nominal non-boosted 50%
coverage.
the peak self powered current
If
limits
the
of
on board
exceeds
supplies, that
are
the
current
necessary to
reverse drive the cable antenna on the opposite phase,
the
electromotive
electromotive
drag
reboosted
If the fixed,
cycle.
limited in capacity,
system
is
systems,
the
and
boost,
will have to be
will
available
then
possibly
to
used
recharge
abound
degraded
throughout
to
reduced,
At
night,
this system.
for
power
duty
the
the
will
be
Obviously,
Another notable
timing
of
the
the attenuation level is drastically
but the satellite is in shadow
its solar cells.
reboost
performance
trade-off an operator must consider is
broadcast.
off
the energy storage
experienced on the next transmission duty cycle.
trade-offs
the
The orbit
system's
the
then
power generation sources are
energy
and the
not
than
orbit will decay.
during
on board,
greater
be
and cannot utilize
Solar panels are available in the daylight
production,
course,
of
attenuation factors are much higher.
63
but
the
path
loss
A.
ORBITAL DEBRIS AND SEVERING
earth space environment is increasingly being
near
The
filled with objects from man's activities
formed satellites comprise a mass spectrum
to
These
kilograms.
in
largely
are
from
micrograms
result
the
increases
object
the
as
size
rocket
of
explosions and collisions. The population density
objects
Aside
space.
satellites in orbit, unintentionally
intentional
the
from
of debris
Below
decreases.
diameters of 1mm, the micrometeoroid population exceeds
made
man
particulate debris. Over time the debris population is
increasing,
due
to
hypervelocity
collisions
continuing addition of more material from space
particle
Debris
density
is
sufficient to be of concern in
designing the tethered antenna cable.
size
(mass)
[Ref.
31:
1995
space
p.
can
per year per square
particle
A particle of
sever a small diameter cable.
diameter,
the lines show
the
meter
The vertical axis is flux,
area.
The
in centimeters.
the
cumulative
horizontal
for
axis
flux
for
is
all
debris
The graph is
of NASA Johnson Space Center.
64
the
impacts
The graph is log-log and
greater than or equal to the selected diameter.
courtesy of D.J. Kessler,
enough
Figure 5.1
359] graphs the projected debris flux
environment.
the
and
operations.
As can be seen from Fig.
5.1,
the probability of
increases dramatically at small diameters,
minimum
diameter
should
cable
meter
area
selected
be
satisfactory lifetime before it is
severed.
hit
to survive a
A
square
one
has a probability of roughly 3X10E-3 (per year)
of being hit by particles larger than 1mm,
of 10E-4 from those larger
Deployment
tether
a
indicating that a
System
than 3mm.
(SEDS)
and a probability
The
Expendable
Small
[Ref.
report to NASA
32]
argues that a tether can be cut by all particles larger than
one
third
-
particle
of
sever
can
tether
the
diameter.
a 3mm cable,
A 3mm diameter
(lcm)
cable.
area,
per 1km of length,
Therefore,
a
cable
has
a
cross
of three square meters;
sectional
a lcm cable
The probability that a
of 1km length has ten square meters.
10km
long cable would be severed in ten years would be:
cable
-
90%;
10mm
-
10%.
Another
pessimistic view,
(steel
and
report
and the lcm cable would be
[Ref.
33]
takes
a
much
100
more
and with much more durable cable materials
aluminum).
In that paper,
the authors believe a
lcm diameter electrical cable (with a steal
length
3mm
The expected maximum lifetime for a
3mm cable would be 11 years,
years.
1mm
and a 3mm particle a 10mm
core)
of
10km
will have a 95% probability of surviving five years.
By comparing
these two evaluations it can be seen that
for
a long tether to just survive debris collisions from five to
ten
years,
it must have a minimum diameter of between
65
.6cm
Determining
and 1cm (including the insulation).
diameter
servicable
very
is
minimum
a
important because doubling a
cable's diameter will quadruple its mass.
B.
TETHEE STRENGTH
The diameter of a cable,
in addition to its composition,
largely determines its strength. A
quite
be
will
cable
great
of
In the tether concept,
massive.
length
the tether
must not only support (i.e., be tensioned by) the end masses
which are under opposite acceleration forces, but
mass
the
of
also increase radially away
gravity
The
mass.
appreciable
with
tether's mass,
[Ref.
to
end
distributed
length.
C.
long
the
tidal
tethers.
masses
the
of
become
can
Temporarily disregarding a
a tether 10-20km long
.
center
system's
forces
will
have
tensioning
Olg exerted on it from the end masses
Worst case analysis would add total
34].
the
from
gradient
of just under
forces
also
whose acceleration tensioning forces
tether,
without
considering
tether
the
mass
tether's
gravity gradient accelerations across its total
Actual loads will be discussed shortly.
TETHER MASS
Tether mass becomes significant as
are used.
Tether cables of
2
to
66
3
diameters
over
3mm
millimeters have typically
averaged
about
to 8.5 kilograms per kilometer [Ref.
7
These have been low stress kevlar wrapped wires.
diameter
the
to
under
just
enough
easily
A
be
drum
/
cubic
two
around two meters
a
tether
could
but the volume of the deployer mechanism
meter in diameter and one
use
thick
suggests a tether
conducting
10km
and spool would be at least
could
conductor
a
The volume that this 10km cable displaces
be 1000kg.
is one cubic meter,
would
and
to pass several tens of kilowatts,
of about 100 kg/km.
mass
allowing for high
and
1cm,
tension materials that weigh more
35].
Increasing
meters.
in diameter,
half meter long.
-
spool
The
with the drum one
spool one meter long.
A
cable
20km
This is a very
manageable size for a satellite and its launch vehicle.
This estimate for tether
by
study
10km
-
Dr.
mass
aligns
20kw tether motor
/
generator. His
continuous
a
125kw.
His tether mass,
highly
is
tether
mass
cable
1200kg.
relationship
rated
supply
for
Although this system
it allows a seven degree
because
of
less
than
ideal
relationships, which may not be acceptable
when using the tether wire as an antenna. Figure
the
is
a peak power capacity of
including the argon gas
efficient electrically,
bowing of the
structural
has
and
20kw,
the hollow cathode assembly,
is
a
system
reference
uses a ten amp tether current through a 6.5mm wire,
at
with
closely
McCoy in which he outlines requirements for a
between
5.2
plots
the maximum desired tether power
67
capacities and corresponding minimum required
for
a
stable tether configuration.
tether
to be used for a tether as a motor / generator,
be
mass,
This chart is designed
but
can
it
used as a guideline to provide upper and lower bounds on
system considerations for a tether as an antenna. Note
driving
expectation,
ten
permitting).
D.
and that increasing tether current
is not only reasonable,
amps
[Ref.
above
well
but desirable (technology
23].
TETHER BOWING
The
between
stretched
tether
oscillations
experiences
due
end
masses,
electromotive
two
the
forces
to
exerted upon
of
and
from
forces
powered
self
electrical boosting. The dynamo effect
of
force
impressed upon
oscillations
motor,
is
the
the
system.
similar
to
largest
transient
The
that
of
operation
the
wire,
that
forces
as
may
or
a
be
induce
a vibrating string.
steady state forces impress a bowing effect into the
geometry.
it.
translational and longitudinal forces,
satellite
generator
or
masses
end
motion
These forces can be due to dissimilar
the
that
long tether at lOkw is a very conservative
10km
a
The
tether
The degree of bowing is proportional to the power
that is being produced or pumped by the tether, and
mass of the system relative to the tether.
68
to
the
End masses that are small relative to
tether
the
mass
be pulled together under high load conditions when the
will
tether tends to bow the most.
system
Increasing
the
mass
of
the
provides more inertia to resist the bowing tendency.
Additionally,
increasing the mass of
system
the
increases
total system momentum, providing greater resistance to orbit
decay
during
power production. Of course,
high
more mass that must be reboosted, but 80 to
duty
idle
cycle
time
is
before
the
of
available for the reboost. For a
given tether power, greater mass allows
reboosts
it is also
percent
90
satellite
pair
more
time
between
jeopardy
in
is
of
terminally decaying out of orbit.
E.
SATELLITE MASS
Tether mass is not the primary driving force determining
what
the
total
system mass will be, but it can be used to
help establish minimum
there
are
weight
the total
energy
much
storage
stable
relationships.
mass
allowance
devices.
like
However,
fuel,
electronics,
to
both ends,
behave as a vibrating string,
and not have an
excessive
lateral
displacement
length),
which can pull the mother
much
less
69
and
a simple relationship can
help define a first guess satellite system weight.
tether
Surely
more important considerations that go into
than
-
For
the
firmly attached at
deflection
the
angle
(a
tether's total
daughter
end
masses
together, then the tether should have a mass of no more than
percent
ten
system
total
the
of
may
(and
have
well
but it should
both
that the maximum imbalance should be no more
split
tether
the
(with
system mass).
the
as
With a 30/60 split,
one
total
satellite
apportioned
system
mass
mass
determining
guess
first
a
derived,
is
between the tether and two end mass satellites.
tethered satellite, and a 12,000kg
also
obviously
is
10,000kg
of
With the broad generalizations made here,
are
30/60
a
and only three times as
Using this analogy,
massive as the tether.
than
other ten percent of the
as much as the other end mass,
half
divided
but it is reasonable to suppose
masses,
end
much
not be
Ideally this mass should be evenly
less than this.
mass
be in order to have sufficient on
to
board peak power capabilities),
between
The satellite's
mass.
nine times the tether
nontether mass can be more than
reasonable
factors
-
The
tether cable
the
-
10km
20km tethered satellite
possibilities.
are
an 8000kg
primary
mass
mass
and
the
onboard power supply system mass.
F.
TETHEE DEPLOYMENT AND RETRIEVAL
The deployment of a tether is a fairly easy
and
stable
process, mainly entailing providing an initial outbound kick
along
the
resistive
local
friction
vertical,
forces
and
to
70
applying
then
keep
the
unreeling
varying
cable
aligned along the vertical. Growing gravity gradient
on
the tether accelerate
have been developed
maintain
very
to
determine
uncoiling
the
on
its deployment.
complicated
matter,
proper
the
wire [Ref.
and
an
forces
Rate control laws
tension
to
Retrieval is a
36].
inherently
unstable
operation.
When a tether is retracted,
conserved,
and if it is retracted too fast it could spin the
tether
and
subsatellite
its
subsatellite.
around
tether,
prevent
and
mother satellite
retrieving
the
[Ref.
37].
maintain
flipping
its
the
However,
tension
around
By pulling in or feeding out the tether
oscillations can be dampened [Ref.
moments,
will be
deployed in
except for
The
next
it will
active
of
not be
at
proper
In general
the tether
retracted
oscillation dampening.
chapter will put together all of the elements
that have been covered,
concept.
but
orbit
purposes
the
38].
purposes of this design,
and for the
practice,
transverse
be used to stabilize oscillatory motions of the
can
tether.
on
the
same effect of
using conserved momentum and translating it into
motion
is
Small thrusters on the mother satellite can be
used near the end of the retrieval to
the
momentum
angular
into the proposed tethered satellite
The last chapters will analyze program costs, draw
conclusions,
and
make recommendations for further study and
research.
71
VI.
A.
SUBCQM: THE PROGRAM
THE SATELLITE
Description
1.
SUBCOM the satellite will be
combination
satellite
single
unit,
kilograms
with
(4000
a
is
launched
mass
of
between
power
production.
Once
orbit (60 to 80 degrees),
500km
around
1000km),
two
the mother
in
8000
tether
i.e.,
circular,
a
-
daughter satellite will separate
along
tether
the
local
vertical.
The
as
tether
into
two
The daughter satellite will
be
of
the
two
This acceleration must be slowed by a frictional
deployment
program.
The center of mass of the
altitude.
be approximately 10km long, with possible
will
ranges of between
controlled
of
the mother satellite descends. Gravity
dual satellite system will remain at the original
The
and
be connected by a tether on a drum reel
will
upward
satellites.
length
high inclination
gradient forces will accelerate the separation
drag
12,000
and
and at an appropriate altitude
within the mother satellite.
unreeled
orbit as a
(with a flexibility of choice between 300 and
subsatellites,
subsatellites
daughter
-
into
The total mass will be dependent
pounds).
final design capabilities,
upon
mother
a
that
5
and 20 kilometers.
deployment,
the
tether
At
the
antenna
end
will
gravity gradient stabilized, vertical orientation.
72
of
the
be in a
The
mass of
the tether will be on
the
daughter mass relationship (relative to total
will
range
from
subsatellite
45%: 45%
to
60%: 30%,
weighing equal to, or more
subsatellite.
With
probable
forces exerted across
end
order
and about 10% of the total system mass.
1000kg,
masses
of
total
The mother-
system
the
mass)
mother
with
the
than,
the daughter
gravity
differential
the entire tether length of
9000kg,
of
.
Olg,
cable must support is 900kg, plus the apparent tether
Therefore,
design
cable
must
consider
also
(
mass.
materials
construction that will allow a cable diameter of
to support weights of 1000kg
and
apparent mass that the tether
lg weights).
.
6
to 1.0cm
tether
The
and
will
insulated against electrical leakage to the local
be
plasma.
subsatellite
mother
The
will be nearest the earth
when the system is properly oriented. Being the most massive
of the subsatellite pair (4500 to 6000kg),
most
of
main satellite systems.
the
it
will
on the mother subsatellite that are unique to this
satellite
are
communications,
oriented
as
follows:
telemetry,
solar panels
and
data
ground
earth
10
internal
kilowatts
sun
each
and
three
axis
attitude
batteries
capable
of
delivering
stored
of
station
storage;
relay
kilowatts;
of
type
internal power generation,
capable of twice the tether generated voltage
than
contain
The systems installed
and
no
less
stabilization;
at
least
20
for 10 minutes; high amperage
energy
73
capacity hollow cathode and pressurized gas tanks to
the
hollow
cathode;
supply
deployer mechanism; and high
tether
capacity digital switching device that can handle 10
kilowatts,
switch
and
to
20
up to 6000 times per second for
at
at least 10 minutes; an intrasatellite communications system
so
mother
the
that
with
communicate
each
daughter
and
other
(via
subsatellites
transceivers on each subsatellite or digitally
HF
signal
thecable);
in
pairs;
encoding
an
an intersatellite communications
system so each satellite pair
satellite
can
small UHF antennas and
can
an apogee kick
manuvers or orbit decay control.
communicate
with
other
motor (AKM) for emergency
The term AKM is used in the
generic sense,
regardless of where in the orbit it is fired.
The AKM would
primarily be used in case
reboost
not
is
shortages,
auxiliary
tether
electrical
of
extremely high inclination, or a
orbit situation. Also
the
because
successful
short
term power
highly
decayed
on board the mother satellite are all
subsystems
necessary
for
maintenance
systems
not
unique
operation of all satellites;
to
and
this
satellite, but common to all.
The daughter subsatellite will be the smaller of the
two (3000 to 4500kg),
and
at the
higher altitude.
It will
replicate some systems onboard the mother subsatellite on
smaller
will
be:
system;
scale.
one
a
The installed daughter subsatellite systems
axis rotational
solar cells
stability
attitude
control
one-third the capacity of the mother
74
subsatellite
capacity
batteries
panels;
of the main system;
hollow
cathode
and
one-third
supply;
an
identical
intrasatellite transceiver system; and a small backup
communications
system.
capacities must be
in
order
to
the
an identical amperage capacity
bottle
gas
with
also
earth
Some of the systems' components and
distributed between
subsatellites
both
distribute the total mass and redundancy. The
daughter subsatellite does not need much station keeping
attitude
the tether. Lateral
longitudinal
and
subsatellite
mother
or
because of the stabilizing nature of
capabilities
will
positioning
translate to the
by
the
daughter along
dampened cable angles and radial positions.
Operation
2.
Operation
the
of
satellite
stations will uplink via UHF all satellite control
and
data that the satellite
is the
be
the
either
will
downlinking
satellite
minutes
out
of
transmitting
The data relay
uplink
channel,
uplink
transmissions.
only
during
100
minute
During
the
the
on duty cycle,
or a
northern
than
orbital period. Normal
satellite transmission duty cycles will be 10%
off.
can
burst
an interval that lasts no more
a
back
a real time
transmit
will
most orbital segments,
10
be
submarine operating areas.
store and dump technique from
The
commands
data relay information. The data relay information
the
down to
Ground
simple.
is
on
and
90%
transmission power will be
provided by tether self powered electrodynamics for one half
75
cycle of the frequency transmitted,
and onboard systems will
provide the power for the other one half cycle.
A
digital
switching
direction
between
the
twice the
rate of
the
system
opposite
switch
will
current
polarity power systems at
being
frequency
transmitted.
Intelligence (the data being relayed) will be transmitted by
digitally
delaying
polarity
the
switch
differentiating between a time sync pulse and
frequency
phase
switch,
By
time.
received
the
a primitive two state encoding can
used to transmit data at a very slow
rate,
present
the signal will not be
a bellringer.
role as
Thus,
truly modulated, but be digital pulse
fulfilling
positioning
the
about
a
carrier frequency that will be between lkhz and 3khz.
The on board power will come from sun oriented solar
panels,
and any other internal supplies that may
batteries,
be installed to boost power levels (such as
dynamic
nuclear
for lOOkw or higher levels).
and
batteries
The size of
depend on trade of f s
primary power source for long
minimum
they
satellite
RTG's
even
or
generators if a massive system is designed
should
internal
be
.
term
panels
If solar cells are the
operation,
sufficient
power
solar
the
needs,
then
plus
the
an
batteries.
orbital
Power
period
recharging
for
transmission
because during the on cycle,
half
76
batteries
the
a
satellite
transmission power level (for 10 minutes) spread across
half
at
provide permanent
to
one
the transmission
are
necessary
transmission
power
must
come
level.
Ideally,
large
enough
from
when sunlit.
only
option
internal
sources
completely
to
to
have
have
If transmission is on the dark side,
batteries
is
or fuel cells.
then
then
power,
solar
half an orbit (at least half the orbit would be
in the previous
10 minute broadcast,
recharging time.
sun
the
If batteries are
sunlit) to replace all the power removed from the
of
panels
solar
supply this power requirement
used to match the self generated tether
cells
a very high energy-
at
it would be optimum
light,
then
batteries
about 35 to 40
minutes
If the broadcast is being done in full
battery
later
recharging
may
not
be
necessary.
To keep the orbit
battery,
(solar,
generated power.
conservation
of
fuel
This system
to
be
decaying,
whatever)
at
expended
not
does
energy applies.
90 minutes (35 minutes
batteries
from
cells,
create
is
being
minutes.
10
tether power comes at the expense of orbital
in
in
the
internal
power
supplies
can
the satellite will exit
slightly
lower
orbit.
deposited
in
Self generated
energy,
which
The orbit decays
and
is
order of milliseconds, during the on
duty cycle powered transmission
power,
the
power,
power in the opposite
turn is redeposited from internal
phase from which it is extracted.
reboosted all
self
Solar power collected over
worst)
in
power
internal
must match
In
phase
10
minutes.
If
not match the self generated
the
this
77
of
broadcast
case
phase
at
a
solar power must be
routed not only to the batteries to recharge them, but
to
also
the tether for continuous DC reboost during the off duty
The system is
cycle.
its
in
anywhere
function
can
and
range,
flexible
operating
between
300
altitude
and 1000
kilometers. Higher altitudes allow more tolerance for
decay
monitoring,
atmospheric
less
permit
also
altitudes
and
Higher
and
longer
footprints
larger
orbit
drag.
illumination windows.
Changing the orbit altitude is an easy
time
can be utilized.
is
same
the
Circularizing a slightly
closely monitored.
If
elliptical
orbit
and drag phases must be
boost
but
process,
process.
simple electrical boost or drag
not of the essence,
is
If an emergency exists and there
is
not
time or energy for an electrical reboost, then the emergency
AKM
the mother subsatellite must be used.
on
tether and daughter subsatellite
must
process
still
tether tension
is
ever
not
daughter velocity
follow,
The tensioned
though
the
done at a rate slow enough so that
be
condition may result,
the
will
lost,
possibly
or
snapping
else
the
an
unstable
cable
when
becomes out of phase with the mother
velocity.
Changing
nodes)
and
the orbit plane
inclination
are
(rotation of
the line of
simple
but require
not
so
standard propulsion packages on the mother subsatellite.
proceedures would
satellites
except
be
the
same
as
utilized
on
The
standard
the corrections should be slow enough to
78
allow the
daughter
subsatellite
remain
to
stable
a
in
position relative to the cable and the mother subsatellite.
Trade-Off Analysis
3.
Orbit
power.
At
inclination
low
affects
transmission
radiated
maximum
inclinations,
self powered tether
voltages are restricted by the cross product,
and at
inclinations
(dip
the
dot
product
dominates
higher
angle).
Inclination also affects reboost ability, because continuous
power pumped into the tether for reboost will be working
DC
against a field vector other than perpendicular.
polar orbit,
effective
pure
and an
operational
dictate
The necessity of
higher inclinations.
than
transmitting to
hemisphere
a
Reboosting at lower inclinations will be
AKM will be needed.
more
In
electrical reboost will not be possible,
areas
high
in
the
northern
that the system should be designed for
minimum reboost by increasing solar power and battery power.
Inclination indirectly
affects
the
coupled
inclination
angles,
in
the
because
at
high
power
northern
operating areas, the magnetic dip angles are also very high.
High
dip
angles
mean
less
of
efficiently coupled from the dipole
lines.
It
is
the transmitted energy is
antenna
to
the
field
the coupled energy which propagates down the
field line to the earth's surface.
Altitude
as
does
not affect the satellite performance
long as it remains within
window.
Higher
the
300
to
1000km
altitude
altitudes will produce a larger footprint,
79
time
longer illumination
slightly
(window),
swath
wider
width and height, but lower signal density.
The design
capacity
needs
power
between
ratio
solar and battery
be closely evaluated so that an optimum
to
configuration can be obtained. As discussed
is
a minimum solar requirement.
The advantage to increasing
the solar capacity is less reliance on battery use.
solar
arrays
were
enough
large
there
earlier,
to
supply
If
the
maximum
the
continuous internal transmission power requirement (sunlit),
then minimal recharging would be required and
the
risk
of
needing a reboost is slight. By increasing battery size, the
transmission power can be temporarily boosted above
maximum
the self generated power to
satellite's orbit.
and
solar
put
back
arrays
increased,
have
also
will
This
power.
increase
into
and
safety
efforts are reduced.
panels
all
effect
radiated
extracted
batteries. By maximizing both the
the
the
signal
of boosting the
The excess energy can later be
battery
margins
capacity,
are
increased,
flexibility
is
and management
Higher transient power levels are also
possible on a temporary basis.
solar
the
the
have
initial cost/kw ratios.
Energy
specific
storage systems
and
volume/kw,
and
These ratios need to be compared
to
mass/kw,
the operational requirements and the cost budget.
80
B.
THE CONSTELLATION
Description
1.
With
a
maximum coverage window of
to 10 minutes,
5
depending on the altitude of the transmitter and
the
orbital period of 100 minutes,
geographic
spot
estimations)
there
same orbital plane,
For
coverage
the
gap
coverage
75%
would
need
(+-
one
equally distanced apart,
operational
area
to
is
their
own
be
course,
the
would
require
if larger gaps
satellites
could
If more
broadcast too
satellites
plane and inclination is necessary.
orbital
Three geographic areas (GIUK, Bering,
Islands)
swath
for each
simultaneously, then another constellation of 12
in
to
due
25%
to be 12 satellites in the
of 1500km by 3000km in the northern latitudes.
than
one
for
satellite is 90 minutes, plus or
one
and
minus a few minutes.
area
close
how
system compares to the conservative model, and an
real
and
Queen
Elizabeth
satellites in three planes. Of
36
in coverage can be tolerated,
and
if
themselves occasionally into
propel
other orbits for nonsimultaneous coverage,
then
many
less
satellites would be required.
Ground station
point
of development.
communications
assets
support
exist
program at present, with the
command
and control center.
is hard to
conceivable
It is
to
evaluate at this
that
support
addition
of
this
sufficient
satellite
manpower
and
a
It is also possible that a full
program of 36 satellites that are
81
constantly
decaying
and
reboosting would involve
requiring
support,
considerable managerial effort to
dedicated
ground
The final
stations.
satellite product will determine the ground station demands,
obviously.
Operation
2.
satellites
sequential
As
coverage
desired
pass over the
responsibilities
communications
the
area,
will be handed off just as with earthbound
In order to simplify instructions to
mobile car phones.
constellation,
orbiting
the same orbit plane
of
dual-satellite
each
the
combination
should be in communications contact with each satellite just
preceding
following
and
it
it.
station command instruction can be
satellite,
and
have
it
a ground
any
relayed
satellites.
As
one
satellite
swath
it
can
signal
area,
In this manner,
passed
to
to
up
one
other
the
all
moves off-station from the
following
the
satellite
to
commence broadcasting.
Ground
station
coordination
maintain
to
the
constellation's integrity will be significant. Unequal boost
and
drag factors will not only disrupt the common broadcast
altitude of
intervals,
orbital
the
constellation,
because
periods.
different
Minimizing
reduces the manpower and
but
altitudes
the
equipment
operate the system.
82
affect
need
asset
the
have
overhead
different
to reboost greatly
base
needed
to
3.
Trade-off Analysis
Mission
number
of
determine
need and program funding will determine the
satellites.
coverage
number
The
gaps
and
the
of
satellites
number
of
will
separate
operational areas that can be broadcast to.
The next chapter will briefly examine program costs.
The
last
chapter
will
be
a
summary
recommending future work or studies.
83
and
conclusions,
ESTIMATED PROGRAM COSTS
VII.
Estimating
the cost for the SUBCOM satellite program is
very difficult
early
this
at
stage,
but
ballpark
some
assumptions and educated calculations can give a feel of the
cost.
The source document that will be used is the" Unmanned
Spacecraft Cost Model" [Ref.
current
examining
By
39].
satellite
the
satellites,
subsystems may be very roughly estimated as a percentage
total
satellite
Satellites
mass.
differ,
of
course,
of
and
surely this one will be very different, but comparisons with
previous missions
SUBCOM
will
subsystems
should
broken
be
approximation.
first
a
very
seven
into
general
that will have to be mission "all encompassing".
The subsystem mass
peculiar
provide
down
ratios
were
of
tethered
properties
mass allocation,
satellite
etc...).
as
estimated
a
mass
to
include
the
satellites (i.e., tether
Tabulated below are
those
seven
and for each subsystem the estimated
subsystems,
mass ratio
adjusted
percent
satellite
total
of
mass,
the
in kilograms based on a 10,000kg satellite,
and the lg weight in pounds is given.
Table
will
2
subsystems
based
weighting.
The
1979 dollars,
break
on
first
down
their
the
mass
costs
and
of
each of
associated
the
dollar
column will be nonrecurring costs in
and the second column will be recurring
84
costs
TABLE
1
SUBSYSTEM MASS RATIOS
SYSTEM
1.
2.
3.
4.
5.
6.
7.
MASS RATIO
Structure
Thermal control
Mission communication
Telemetry, tracking
and control equip.
Electrical power sys.
Attitude control sys.
and 3-axis AKM
Computer/Data storage
satellite
per
etc.
.
.
.
LBS
20%
8%
2000kg
100kg
2000kg
800kg
7801b
401b
7801b
3201b
30%
20%
3000kg 11801b
2000kg 7801b
20%
1%
401b
100kg
1%
1979 dollars.
in
to one time start up costs,
satellites
KGS
Nonrecurring costs
independent
of
the
This is the design, development,
built.
Recurring costs are the actual costs to
satellite,
satellites
based on the first unit cost.
will
be
Subsystems
curve.
refer
number
of
testing,
build
each
The costs for later
adjusted for an efficiency in learning
one
two
and
(structure
and
thermal
control) will be combined for analysis.
The total one
approximately
100
time nonrecurring
development costs
are
million 1979 dollars, and the first unit
production costs are approximately 50 million dollars. For a
constellation of 12 satellites
will
times,
not
be
12 times
the
the
total
recurring
first unit cost,
because the learning curve decreases
costs
but about ten
the
production
cost of follow on units. For a 12 satellite system then, the
85
TABLE
2
SUBSYSTEM COST ESTIMATION
SYSTEM
$
NONRECURRING
$ RECURRING
Structure
$10M
and Thermal
3.
Mission
$20M
Commun i cat i on
4.
Telemetry, Tracking $14M
and Control
5.
Electrical
$15M
Power Sys.
Attitude Control
$40M
6.
and AKM $40M
7.
Computer and
$4M
Data Storage
1979
Total
Dollars
$103M
$2M
1&2.
recurring
total
costs
program costs are the
$7M
$10M
$16M
$12M
$51M
be 500 million dollars.
will
sum
$14M
nonrecurring
of
or 600 million dollars in 1979.
costs,
approaching
1990,
and
This is not 1979,
or 800 million 1990 dollars.
of a 12 unit satellite
million
but
a rough inflation adjustment for 11
years compounded annually at 2.6% per year is
33%,
Total
recurring
and
production
convenient
a
The average per unit cost
line
is
over
just
dollars per satellite. A full 36 satellite,
3
65
plane
constellation, would bring per unit costs down to 55 million
dollars
per
copy
in
1990
dollars.
Table
3
summarizes
what has just been discussed.
This analysis did not take into account
factors
some
important
that have significant costs, but are extremely hard
to estimate at this point.
For the nonrecurring
86
costs,
the
TABLE
3
AQUISITION COST SUMMARY
Nonrecurring costs
(rounded,
$100 Million
1979)
Recurring costs
(12 satellites)
Total program cost
(1979 dollars)
Inflation correction
(1.33,
Aerospace
$500 Million
$600 Million
$800 Million
1990)
Ground Support Equipment must be included; ten to
fifteen million dollars is a first guess
nonrecurring
10%
of
total
Recurring costs will have two factors:
costs.
Program Management
at
at
approximately
two
hundred
fifteen
million dollars per year to operate and manage the satellite
program;
and
Operations
Launch
and
thirty to thirty-five million dollars per
and
Orbital Support at
year
10 to 12 year operational
additional
costs
perhaps $15M,
dollars.
are
life of each satellite.
one
ground
time
operate
to
support the 12 satellite constellation system
over the
Therefore,
support costs of
and yearly operating costs of $250M,
Table 4 summarizes program aquisition costs.
TABLE
4
1990 PROGRAM COSTS
12
$815,000,000
Satellite aquisition cost
$250,000,000
Yearly operating budget
:
:
87
in 1990
CONCLUSION
VIII.
A.
SUMMAEY
this
In
thesis,
transmitter
under,
near,
or
conceptual
a
satellite
polar
the
frequencies
Downlink
between
be
established
requirement for such a space based
communications
into
link
space,
transmitter may be obtained, and
Transmissions
will
more
be
for an ELF/VLF
with
submarines
fields has been discussed.
ice
will
arguments
Preliminary
design
communicating
for
asset.
a
along
propagating
the
need
moving
By
this
more survivable
much
increased.
is
with
chance of
less
highly
widespread interception, because the beam pattern is
directional
3khz.
mission
redundancy
covert,
and
lkhz
the
field
line.
The
directivity also focuses the signal pattern onto the earth's
increasing the illuminated energy density available
surface,
for
able
receiving
antennas.
penetrate
to
increased
coverage
a
With the increased signal strength
greater
area
this
depth
system
of
water,
northern operating areas, submarine operations are
and
receiving
periods are
and
the
provides in the far
not going to be
enhanced
restrictive or
vulnerable to submarine safety.
The
proposed
satellite
is
a 10,000kg,
dual satellite
pair that is connected by an antenna tether 10km to 20km
length.
in
The satellite will be gravity gradient stabilized in
88
a
vertical
orientation.
will
It
have a high inclination
orbit to bring it over the polar operating areas,
pass
overhead
in
altitude
an
will
and
window of 300km to 1000km.
Through the unique properties of self-powered electrodynamic
and whistler mode propagation,
forces,
able
generate
to
the satellite will be
substantial
a
amount
of
its
own
transmission power, and then be able to couple that radiated
power along the earth's field lines to the earth's
Both
these
of
surface.
special properties depend upon interactions
with the geomagnetic field around the earth in a manner that
no previous system has utilized.
The success of the proposed
system in fact depends entirely on these
very
unique,
and
particular, properties of space environmental physics.
operating
The
communications
area
orbital plane is a shallow
east-west,
arc
swath size for one
approximately
and almost 1500km north-south.
3000km
All system studies
were made using the most conservative analysis,
geometry,
all numbers,
determined,
exceed,
models.
and
a
feel
since
long
assumptions,
With worst case assumptions made for
the
for
program's
success
can
be
it is expected an operating system would
by a wide margin, the limits of the research
model.
With that again pointed out, the hot-spot illuminated window
will
be
well
over 1000km high
and 500km to 750km wide.
secondary widow from earth-ionosphere
will
be
over
1500km
high
communication time will be
5
wave
guide
A
trapping
and over 1000km wide.
Overhead
minutes
satellite
to
89
10
per
with
pass,
12
a
revisit
If a 7.5 minute
minutes.
satellites
operational
area
different
coverage
75%
time
that
passed
minutes
each
For
outside the 1500X3000km swath, a
is
plane
orbital
overhead.
of
for that
Coverage gaps would be just a few
satellite
next
the
until
the same satellite in 90
for
window is used, a constellation
provide
will
operational area.
time
must
be
used,
with
own
its
complement of satellites.
The study baseline power projection is 10,000 to
of transmitter power.
watts
initial expectations,
length
and
technology.
and
highly
is
Deviations
upwards because of the conservative
burst
transmission
power
20,000
The actual power may vary from
levels,
three on an intermittent basis,
are
dependent
are
most likely to be
Increased
assumptions.
by
also
tether
on
a factor of two or
possible
the
in
system design by temporarily sacrificing orbital energy.
efficient
use
and
other internal energy storage),
power
The
interplay of solar power, batteries (or
and
self
generated
tether
(orbital energy as an energy reservoir) allows for an
amazing flexibility in energy management and
an
intriguing
application for current technology.
Program costs are very hard to evaluate at this level of
examination,
used.
This
but standard procedures in cost modeling can be
model
produces
a
12
satellite constellation
aquisition total cost of $815,000,000 in 1990 dollars,
per
satellite
cost
of just over $65M.
90
for a
Ongoing program and
system operations costs
annually.
will
be
on
the
order
of
$250M
The operational life of the satellite is expected
to be 10 years.
WHAT STILL NEEDS TO BE DONE?
B.
There are so
examined
many
areas
closely
more
of
study
that
greatest
time
by
Stanford.
this
Denis
He
is
Probably
the
earth's
field
lines.
getting a lot of study at the present
Donohue,
is
be
performance variance is the power that can
be coupled from the antenna into the
Fortunately
to
in this proposal that it is hard to
begin mentioning the most important concerns.
single
need
studying
who
works
coupling
for
Peter
Banks
at
models and ray tracing
patterns for a number of situations.
The
following
issues
are
recommended
for
further
research and need to be studied in-depth:
1.
Tether power production as a function of inclination
and dip angle.
2.
Modeling the Earth's field lines as per application
to a spacebased ELF transmitter.
3.
High and Low latitude ELF/VLF ray tracing.
4.
Modeling the upper and lower ionosphere w.r.t. ELF
wave propagation.
5.
Tether survivability and debris hardening.
6.
The use of multiple tethers on the same satellite
pair.
7.
High current hollow cathode assemblies.
91
8.
ELF noise sources and levels.
9.
Sixty Hertz harmonic interference.
10.
Tether cables: materials, insulation, conductors,
current capacity,
heat tolerance, tension stress,
and thermal cycles.
11.
Kilowatt
12.
Short duty cycle, high density, energy storage
systems.
13.
ELF antenna radiation efficiency in a magnetoplasma.
14.
ELF antenna impedance matching in a magnetoplasma.
level,
digitally controlled RF switches.
It is recommended that a definition study
developed
that can
collect data
payload
instruments.
satellite,
fields
of
various
experimental
the
of
conditions.
satellite
satellite
test
The
will
ELF/VLF
onboard
its
local
free
a
plasma,
satellite.
flying
and
Though dual
piggy-back
field
effects
satellite
test
be
also be a tethered
length.
and propagation paths
transmitter
receiving stations around the world.
include
be
primary
The
will
but the cable need only be 1km to 2km in
Normal inclination orbits can be used,
from
satellite
proof of principle operation, and
test
can be examined at
It is also desirous
to
satellite to study the
around
operations
the
tethered
within
close
proximity of each other are complex, the data obtained would
be invaluable in improving the effectiveness of transmitting
antennas
in
ionized
recently announced a
plasmas.
similar
The
study
92
Soviets
for
[Ref.
similar
40] have
reasons.
Their
tethered
satellite experiment,
subsatellite, will be
altitudes between
in
an
with
inclination
500km and 2500km,
transmission bands.
93
its free flyer
of
83
degrees,
and will be using
VLF
APPENDIX: FIGURES
CENTRIFUGAL
ACCELERATION
Figure 2.1
-
Stabilizing forces acting on tethered masses
[Ref.
94
12].
Contactor
-AV A
1*h7A
Tether
Load
s
Contactor
Figure 3.1
-
Potential diagram for tether as a generator
[Ref 20].
95
— AV Rp
Con factor
AV C
C
/
/
/
\
\
\
\
\
\
\
1
i
j
\
\
\
\
j
w
Vsupply
\l
Power ~ESupply 3EI
1
"5"
1
#-»*»»
P r»n*«
IUUIUI
uon
-,
l?xB-i|
Figure 3.2
-
-AV A
Potential Diagram for tether as a thruster
[Ref.
20].
ui en
Figure 3.3
-
Structure of contactor plasma plume regie
[Ref.
97
22].
Figure
3.
Schematic diagram of electrodynamic tether
system [Ref. 22].
1^
O
O
:
-
'
'
a
o
o
1
f
-*-
•-*
OJ
CD.
—
tj
a.
l/l
cj-h a.
a:
DC anode
supply
1
03
Figure 3.5
-
Electrical and mechanical schematic of a hollow
cathode [Ref. 22].
99
Figure 3.6
Cross section of hollow cathode in operation
[Ref.
22].
100
1000
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103
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29].
Figure 4.6
-
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106
Figure 4.7
-
Maximum available power coupling configuration.
107
Figure 4.8
North-south crosscut of radiation pattern,
looking east.
108
Figure
4.
Radiation coupling pattern as viewed from above
the antenna, looking down.
109
Figure 4.10
-
East-v/est crosscut of radiation pattern,
looking south.
110
is*
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Figure 4.11
-
Primary and secondary footprints with the
sweep coverage area.
Ill
1995
PREDICTED DEBRIS
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Figure 5.1
-
Projected space debris flux for 1995
[Ref.
31:
p.
112
359].
Figure 5.2
-
Tether wire mass vs.
[Kef.
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23].
113
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Electron Beam Experiment on a Sounding Rocket From
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D.C.: Naval Research Laboratory, 28 April 1975.
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Koons,
H.C., Dazey, M.H., and Edgar, B.C., "Impedance
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Plasma Simulation Chamber", Radio Science v. 19, no. 1,
January-February 1984.
,
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Electrodynamic Tethered Satellite Systems", Journal of
Spacecraft v. 24, no. 3, May-June 1987.
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,
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Sawaya,
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,
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,
Sonwalkar, V. S.
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Geophysical Research v. 91, no. Al, 1 January 1986.
,
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Short Dipole Immersed in a Cold Magnetoionic Medium", Radio
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T4322
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Thompson
Design of an ELF/VLF
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