Global Positioning System (GPS) for Submersibles*
Authored by:
Marvin B. May
Rodney A. Martin
Bereket Tanju
Robert A. Lopez, CAPT USN
Representing:
Applied Research Laboratory
Pennsylvania State University
995 Newtown Rd.
Warminster, PA 18974-2933
PMA/PMW 156
SPAWARSYSCOM
4301 Pacific Highway
San Diego, CA 92110-3127
POC: Marvin B. May
Applied Research Laboratory
Pennsylvania State University
995 Newtown Rd.
Warminster, PA 18974-2933
215-682-4003
mbm16@psu.edu
Running Title: "GPS for Submersibles"
Distribution Statement C: Distribution authorized to U.S. Government agencies and their
contractors; Critical Technology, October 2003. Other requests for this document shall be referred
to the Commanding Officer, SPAWARSYSCOM, 4301 Pacific Highway, San Diego, CA 921103127.
Related Presentation given at “Emerging Military Capabilities Enabled by Advances in Navigation Sensors,” NATO
Symposium, 14-16 October 2002. Istambul, Turkey
*
1
ABSTRACT
The operating environment of a Global Positioning System (GPS) is primarily thought of in the context of
air, space, surface and land missions. This paper addresses the role of GPS in submersibles. More
specifically, this paper addresses negative factors within the operational boundaries of the submersible
GPS when compared to air/surface/land GPS, and mitigation actions that can be taken to reduce the
impact of these factors. The distinctions for submersibles fall into the categories of: environmental,
operational, mechanical, navigational and economic factors. Environmental factors include the air-water
interface and the presence of significant ducting effects. Operational factors include sporadic fixing
availability and the requirement for short exposure times. Mechanical considerations include the limited
real estate for the antenna and radio frequency (RF) piping. Navigational aspects involve the availability
of long endurance inertial navigators, atomic clocks, and time averaged mission performance accuracy
criteria. Economic factors principally involve the minimal profit motivation for low volume production
platforms. Potential accommodations to these unique submersible issues are provided in terms of existing
and future system designs.
2
community. This lukewarm reception to the
advent of GPS, combined with the inherent
technical challenges, may have contributed to
the fact that few special accommodations for
the utilization of GPS in submersibles have
been made.
I. BACKGROUND
Ironically, from a historical perspective, the
original impetus for satellite navigation
resulted from the needs of the submarine
community. In the early 1960’s, the Polaris
ballistic missile submarines required a
worldwide, all-weather, high accuracy
navigation positioning aid. The TRANSIT
satellite navigation system was borne out of
this necessity and was installed on Polaris
submarines in 19641.
It was a highly
successful and also a classified program until
1968 when then Vice President Humphrey
announced its availability to the general public.
TRANSIT served as the principal inertial
navigation fixing aid for most submarines until
its decommissioning in 1996. The motivation
for TRANSIT improvement programs began
almost immediately, but did not stem
principally from the submarine community.
TRANSIT’s deficiencies with respect to
proposed new satellite systems such as NOVA,
TIMATION and 621B,2 were from the
perspective of other users, not particularly
from the submarine community. Availability of
fixes on the average of about once every two
hours was just fine for submariners whose
motto it is to “run quiet, run deep”. It is
important to realize that for submariners,
satellite navigation has always been a
navigation aid as opposed to a navigation
source. Furthermore, knowing altitude is of
course a moot point and velocity or attitude
information could be adequately obtained from
the inertial navigation system.3 The horizontal
positioning accuracy of TRANSIT, typically
about 75 meters radial root mean squared (rms)
with perfect velocity aiding, degrading
somewhat due to at sea inertial velocity
Doppler compensation errors, was more than
acceptable for virtually all submarine missions.
Therefore, when the Global Positioning
System (GPS) became a Joint Program Office
for satellite navigation in 1974, there was
marginal interest from the submarine
II. OVERVIEW: UNIQUENESS OF GPS
FOR THE SUBMERSIBLE
In this section we delineate the unique aspects
of submersible satellite navigation operation.
Several of these are obvious but still justify
stating because of their implications. The
following sections expand, where necessary,
on these aspects. We then proceed to discuss
several potential accommodations, including
hardware, software, system or operational
approaches, which may be applicable to
submersible operations.
Attenuation of the GPS Signal
The crux of the submersible uniqueness is the
air-seawater interface that is encountered when
the satellite signal is received by a submerged
antenna4. Whenever an electromagnetic (EM)
signal is incident upon a material with different
dielectric properties, the wave is refracted.
This is observed by looking at a branch coming
out of the water. The branch almost seems
“broken” at the air-water interface. Figure 1
illustrates a signal incident upon some
dielectric with a relative permittivity different
than that of air. The angle at which the signal
hits the boundary with respect to the normal is
the incident angle (i). Some of the energy is
reflected away at the same angle (r). The
remaining energy is transmitted into the
dielectric at the transmission angle (t). The
relationship between the incident and
transmitted angles is governed by Snell’s law.
 1 sin  i   2 sin  t
(1)
Where 1 is the permittivity of the dielectric
space from which the EM signal is
3
approaching the interface and 2 is the
permittivity of the dielectric upon which the
signal is incident.
Ts 
2 2 cos  i
 2 cos  i  1 cos  t
Tp 
2 2 cos  i
1 cos  i   2 cos  t
(2)


Where Ts is the transmission coefficient for Spolarized waves and Tp is the transmission
coefficient for P-Polarized waves. The symbol
 represents the permeability of the material.
In many cases, this value is the same as that for
air. The properties of seawater are such that it
has a relative permittivity (’sw) of 80.0 and a
conductivity (sw) of about 4.3 Siemens/meter.
The addition of the conductivity yields a
complex permittivity (3) which is defined as
follows:
  j sw

 sw   sw

Fig. 1 – Plane Wave Incident upon a Dielectric
 r  j
In this case, the dielectric is lossless.
Therefore, the sine of the transmitted angle is
real and the wave transmitted into the dielectric
has uniform wavefronts, where the planes of
constant phase and amplitude are parallel.
 sw

(3)
The value,, is the radial frequency of the
signal. Since seawater has non-zero
conductivity, it is considered a lossy dielectric.
By examining the corresponding equations, we
can see that Ts, Tp and t will be complex as
well.
The amount of energy transmitted into the
dielectric from the incident signal is dependent
on three things: the parameters of the
dielectric, the angle of incidence, and the
polarization of the signal. The polarization of
the incoming wave is taken with respect to the
incident plane, which is defined as the plane
that contains both the direction vector of the
incident wave and the normal to the surface of
the interface. S-polarization (S is for senkrecht,
which is German for perpendicular; Transverse
Electric - TE) is when the electric field of the
incoming wave is perpendicular to the incident
plane. P-polarization (P for parallel; Transverse
Magnetic - TM) is the case when the electric
field is contained in the incident plane. The
equations for the transmission coefficients (2)
are given using the intrinsic impedance () of
the two materials.
Another property used to characterize a
material is its wavenumber, k. This is used to
characterize the propagating characteristics of
a given wave in a material and is defined as
follows:
(4)
k   
Since we will be dealing with complex values
for the wavenumbers, we define the variable 
as follows:
jkair   air  j air
jksw   sw   sw  j sw
(5)

 
j0  sw  j sw
Using this new variable, Snell’s law from (1)
may be written as:
4
 air sin  i   sw sin  t

j air
sin  t  air sin  i 
sin  i
 sw
 sw  j sw
From (8), we can see that the transmitted field
is not only attenuated at the surface, but it is
non-uniform once it penetrates the seawater
(6)
We can now get the value of the cosine of the
transmission angle and thus the values of the
transmission coefficients. Figure 2 shows the
decibel value of the transmission coefficients
for an air-seawater interface for varying
incidence angles.
Fig. 3 – Plane Wave Incident upon Seawater
surface. This is illustrated in Figure 3.
The value,, represents the angle between the
planes of constant magnitude (which are
parallel with the interface) and planes of
constant phase. It can be shown that the value
of this angle is defined as:
  sin i 

  tan 1  air
(9)
q


Even for the case in which the planes are
parallel (normal incidence, i = 0), the
magnitude of the wave is attenuated by the
lossy nature of the seawater. At normal
incidence, the attenuation constant is expressed
as:
 sw  p
Fig. 2 – Magnitude of Transmission Coefficients for
S- and P-Polarized Electric Fields at Air/Seawater
Interface
We can see that even the best case (normal
incidence), the signal encounters a 14 decibel
loss from just above the interface.
Another phenomenon that occurs once the
signal enters the seawater is that the wave
becomes non-uniform. Since the cosine of the
transmission angle is now complex, we may
express it as:
cos  t  1  sin 2  t
12
2
 

  sw 
1 




   sw 
1 
 1  Np / m



  sw 
 2 

(7)
 s cos   j sin  
It can then be shown that the transmitted
electric field is of the form:

Etranse  zp exp  j  air x sin  i  zq 
p  s sw cos    sw sin  
(10)
The attenuation constants for the L1 and L2
frequencies are 86.877 and 109.024
Nepers/meter, respectively. The amount of
attenuation is governed by the equation:
A  exp   sw z 
(8)
q  s sw sin    sw cos  
z  depth (in meters)
5
(11)
Figure 4 illustrates the decibel loss for both
frequencies compared to depth.
3 inches in size. Figure 5 shows a typical
commercial GPS antenna and a cutaway view
of its contents. The actual antenna portion of
Fig. 4 – Attenuation in Seawater vs. Depth
From this graph, we can see just how rapidly
the signal attenuates as the depth is increased.
The signal level is reduced by over 350
decibels for both GPS bands in just one-half
meter. This graph does not include the
transmission coefficient losses and assumed a
unit strength signal to start. This loss coupled
with the loss encountered at the surface in
addition to the non-uniform wavefronts
encountered in seawater makes it very unlikely
to be able to receive a coherent GPS signal at
any appreciable depth.
Fig. 5 – Typical Commercial GPS Antenna and
Cutaway View
the unit usually consists of a microstrip patch
mounted on a dielectric substrate.
It would be difficult to predict the effect of
random water droplets on the outside of this
casing. A worst-case scenario of the casing
being completely covered with a layer of
seawater will be examined. For this simulation,
a computational EM program, IE3D, was used.
The geometry used was a half-wavelength
patch antenna mounted on a dielectric substrate
with relative permittivity of 2.33. The
frequency used was roughly the L1 band used
in GPS, 1.58 GHz. An air gap was used to
simulate the protective radome covering over
the patch antenna. To account for the seawater
coating, an additional superstrate layer was
added with the electrical properties of seawater
at varying thickness. Figure 6 illustrates this
geometry.
Effect of Residual Seawater
Most GPS antennas are contained within a
protective casing knows as a radome. This
casing does not interfere with the antenna
performance and provides a waterproof shell
around the actual antenna. The actual size of
the antenna is relatively small – anywhere from
one to two inches square.
The radome houses the antenna as well as a
low-noise amplifier (LNA) and is usually 1 to
6
Even for one-eighth of an inch of seawater, the
VSWR of the antenna is drastically changed.
This shift in VSWR would represent a shift in
the antenna’s resonant frequency. If we also
examine the radiation patterns, we can see that
the effect of the seawater layer is felt there as
well. Figure 8 illustrates the phi and theta
components of the radiated far field of the
antenna for the three conditions mentioned
above. Again, we see that with just a small
amount of residual seawater, the radiation
pattern is altered.
Fig. 6 – Sample Geometry Used for Analysis
The simulation was run at frequencies from
one to two gigahertz to demonstrate the
changing characteristics of the antenna. IE3D
provides the radiating characteristics of the
antenna; however, these can be used to
characterize the receive properties of the
antenna as well. Simulations were run for no,
one-eighth inch (3.175 mm), and one-quarter
(6.35 mm) inch seawater layers. The first
characteristic of interest is the voltage standing
wave ratio (VSWR) at the feed element. Figure
7 shows the effect of adding the seawater layer
on the VSWR.
Fig. 8 - Radiation Patterns of L1 GPS Antenna for
Varying Seawater Layer Thickness
The presence of the layer reduces the gain by
approximately 10 decibels and, in the case of
the theta component, also alters the shape of
the radiation pattern.
Fig. 7 – VSWR of L1 GPS Antenna from 1 to 2 GHz
7
One solution to reducing the effects of the
residual seawater layer on the antenna would
be to design the radome covering such that a
very large percentage of the seawater “runs
off” shortly after breaking the surface. This
could be accomplished by using an inverted
cone shape around the unit. Since it is
transparent at GPS frequencies, the shape
should not alter the receive properties of the
antenna. If nearly complete clearing of the
seawater from the radome is not possible,
another possibility would be to design a GPS
antenna array that could actively adapt itself to
“look” between the residual droplets. This
array would excite elements in a way to receive
the signal from areas that do not contain
seawater on the casing. However, even with an
adaptive array, it would not be able to receive
signals if the casing was completely covered.
Electromagnetic
Propagation
Conditions
(HEPC), ducting is almost always present over
water between +/- 30 degrees latitude. Duct
heights over water seldom reach 40 meters, and
more typically are 25 meters or less. Based on
the HEPC data, submersibles, due to their low
antenna height, would be much more affected
by ducting than surface ship, land or air assets.
Analysis5 indicates that this would potentially
enable relatively low power level beyond line
of sight jammers to incapacitate submersible
GPS receivers. Figure 9 shows, for a duct of 20
meter height, the required jammer power in
dbW to jam a GPS receiver with a nominal J/S
threshold of 30 db as a function of the range to
the jammer. In this figure, the submarine’s
antenna is 3 meters above the water and the
jamming transmitter’s height is the vertical
axis. Note how the required jamming power
stays at low levels for long ranges when the
transmitter is within the duct.
Ducting
A radiowave propagation effect that appears
most applicable to submersibles is ducting.
Ducting is a “waveguide” phenomenon due to
refractive index gradients layered in the
atmosphere and is primarily due to water vapor
content and somewhat less dependent on local
thermal inversions. The result is the
propagation of a “waveguide-like mode”
whose power decreases inversely with range
rather than inversely with the square of range
as in the case of free-space propagation.
Ducting can provide an efficient means of
propagation at L-band, and is generally
observed over large bodies of water and less
commonly over land as local terrain features
tend to disrupt the layered refractive indices
needed to propagate the waveguide mode. If a
jamming transmitter is located in a duct, the
energy becomes constrained to propagate in
and below a trapping layer that is generally
defined by thermal and/or moisture inversion,
thus significantly extending the range of the
jammer beyond what would be calculated from
normal path losses. Based on Historical
Time RMS Navigation Error
The issue addressed in this paragraph is the
steady state time RMS navigation error versus
the instantaneous GPS fix accuracy. The
submariner’s objective for many missions is to
maximize time at depth. In general, this
precludes continuous or frequent utilization of
GPS fixes.
The submarine’s navigation
subsystem performance is often based on the
time rms radial (horizontal) position error of its
inertial navigation system. In general the error
growth of the inertial navigator can be a
complex combination of platform dynamics
and the manner in which the sensor errors
couple into the principal Schuler, diurnal and
linear resonant error modalities. Consider as a
simplification of the submersible’s inertial
navigator’s error performance, the linear radial
error growth with periodic GPS fixes as
depicted in Fig. 10. This simplification would
be relevant for long term error growth where
the principal error driver would be the effective
8
Fig. 9 - Required jamming power for a -100 dBm receiver at 3 meters through a 20 meter duct
between resets. The sensitivity of the Time
RMS error to GPS accuracy can be examined
with R and T as parameters. Clearly for the
product RT small compared to G, then the
Time RMS Error would be equal to G and for
RT very large compared to G, the Time RMS
Error would have a small dependency on G.
An order of magnitude number for RT for a
submarine would be one nautical mile and a
representative number for G would be 0.005
nmi. Consider the case of T=10 days and
R=1.0 nmi/day. Then the time rms error
changes from about 5.77 to 5.82 nmi as the
GPS error changes from 0.005 to 0.05 nmi.
Thus even a 10 fold increase in GPS error has a
relatively small change in the time rms error.
Figure 11 shows the Time Rms Error with GPS
radial error on the abscissa parameterized
versus different combinations of R and T.
Fig. 10 - Steady State Simplified Submarine
Navigator Error Growth
polar gyro drift rate bias. For this error growth
the time rms error would be:
Time RMS Error  G 2  GRT  R 2T 2 / 3 (12)
In equation (12), G would be the nominal GPS
error (assuming the INS reset Kalman Filter
essentially tracks the GPS), R would be the
assumed linear error growth of the inertial
between GPS resets and T is the nominal time
To some extent, the lack of sensitivity to GPS
error explains the original reluctance of
submariners to adopt GPS over TRANSIT
based on GPS’s improved accuracy.
9
Receiver Initialization-Time to First Fix
The covert nature of submersible operations
dictate that antenna exposure time should be
minimized. In this section we discuss some
attributes that a submarine may utilize to
reduce exposure time. For clarification, we
define the following processes associated with
initialization6:
a. The search time for a conventional
receiver is the time required to achieve
carrier and code lock. Normally this is
dominated by achieving carrier and
code lock on the first satellite to be
acquired.
Fig. 11 - Time RMS Navigation Error
This unique lack of sensitivity to GPS error
enables one to consider strategies which may
be beneficial to operations. One possibility is
the relaxation of the software requirement in
most GPS receivers to have current ephemeris
available before proceeding with a fix. The
Control Segment’s ability to predict and
extrapolate GPS ephemeredes over long
intervals such as a submarine’s mission
duration has improved dramatically. For
submersibles/stealthy participants a cryptonet
could be created whereby extrapolated satellite
ephemerides would be available to a much
better accuracy than the present ephemeris
format. For instance, presently, the actual
ephemerides are uploaded to the satellites
using the Extended Range Orbital Messages
several months in advance. Whatever specific
methodology might be adopted, the Control
Segment’s ability to predict ephemerides
(including clock and atmospheric errors) is
excellent now over several months. We can
probably meet the navigation performance
requirements of some classes of submersibles
by virtue of their having extended ephemerides
pre-mission dockside. This would often
obviate the need for lengthy ephemeris data
collection thereby enabling quick fixing and
therefore more covertness.
b. The exposure time is the time that
the satellites must be in view of the
antenna for a fix to be obtained.
These should be distinguished from time to
first fix for a GPS receiver which would
normally include factors such as acquisition
and declaration of track for multiple satellites,
bit synchronization, message synchronization
/collection, measurement processing and
Kalman filter fix convergence.
In the case of a conventional GPS receiver (as
opposed to a storage type receiver described
below), the exposure time must be equal to or
greater than the search time, while, for the case
of a storage receiver, exposure time represents
the signal collection interval: the search can
continue after the data collection interval by
recirculating the stored signal. For a
conventional receiver, the search process for
the P/Y code acquisition must be done while
the antenna is exposed. Due to the very low
repetition-rate-to-bandwidth ratio of the P/Y
code, direct acquisition times, can be
prohibitive. Each search must be performed for
every
possible
code
phase/frequency
uncertainty bin. Submersibles should have an
10
advantage, due to their low dynamics, over
other platforms with respect to the number of
bins that must be searched. Unfortunately,
present submersible GPS receivers, are often
not programmed to take advantage of this.
Most importantly with respect to Direct Y
acquisition, is the availability of atomic clocks
on some submarines. The principle factor in
the number of bins to be searched is the initial
time uncertainty with a lesser dependency on
the position dependency. For example, a user
clock uncertainty of one millisecond (random,
-one sigma zero mean Gaussian bias) and an
additive user to satellite range uncertainty of
30000 meters (random independent one sigma
Gaussian bias) would result in a 3-sigma (99%)
search region of 10050by2by2by3=120,600
bins. If, for example, a submarine with an
atomic clock and a high quality inertial
navigator had uncertainties of 10 microseconds
and 300 meters respectively, then the number
of discrete code bins to be searched would be
reduced to 101by2by2by3=1212 bins, with, for
a conventional receiver, a proportionate
reduction in search time.
Similar
considerations apply for the search with respect
to frequency whereby the high quality inertial
velocities and superior frequency stability
(partially due to the generally low dynamics of
a submarine) often should reduce the frequency
search space to a few frequency windows,
where a frequency window size would be
nominally 0.442/Tc, and Tc being the
predetection
interval,
nominally
1-20
milliseconds.
Fig. 12 - Typical Submarine Antenna Configuration
GPS
receiver
(AN/WRN-6),
Antenna
Electronics (AE-4), attenuator, cabling/slip
rings/multiplexer, antenna RF pre-amplifier
and antenna. The antenna is typically a small
(under 5”) patch antenna, which is designed to
fit within the space allowed. The standard
FRPA-GP antenna, which is utilized for
surface ship applications, is not appropriate for
submarine installation due to watertight
requirements and size constraints. As a result,
new antenna configurations were developed
and designed for submarine applications. The
antenna fits inside a pressure and waterproof
dome. This dome must shed water quickly and
be transparent to GPS frequencies.
The
connection to the GPS receiver is via a single
RF cable which is multiplexed with up to 4
other signals as well as DC power. A typical
GPS antenna and radome on a submarine
antenna mast is in close proximity to other
antennas that are transmitters. This proximity
requires sufficient shielding and protection
from the high power signals that are present in
the antenna mast. The antenna electronics
(AE-4) usually is collocated with the GPS
antenna. For submarines, the AE-4 could not
be located within the antenna assembly (due to
size, shock, environmental and EMI
requirements constraints) thus requiring the use
of a small pre-amplifier. A pre-amplifier is
used to amplify the entire received RF from the
Antenna Considerations
The potential vulnerabilities of GPS have been
well documented. In general, the first line of
defense against GPS jamming is an improved
antenna. Many Navy surface ships are now
being equipped with Controlled Reception
Pattern Antennas (CRPAs)7. Figure 12 is a
typical configuration for fast attack
submarines, consisting of a standard shipboard
11
“recirculated” in non real-time among the
correlator resources. Instead of having to slew
the correlator resources in resources in real
time to the incoming signal until
synchronization is achieved, the storage
receiver can “recirculate” the wideband signal
in post time without requiring further signal
collection. Thus the storage receiver could
reduce exposure time for those platforms for
which stealthiness is a premium (Fig. 13)
Since, at least theoretically, the signal can be
“recirculated” forever, the storage concept
guarantees an “eventual” acquisition within the
given exposure time. The technique can be
made complementary to massively parallel
correlator techniques.10
antenna to overcome the system losses from
extended cable lengths, multiplexers, slip
rings, splitters and switches. The attenuator is
needed to return the signal strength to a level
that is compatible with the AE-4. The AE-4
converts the total RF into two Intermediate
Frequencies (IF) for input into the AN/WRN-6.
There are typically two or more antenna masts
that contain a GPS antenna and pre-amplifier.
A selector switch is included in the installation
to choose the antenna that will be used for GPS
operations.
The present configuration points to the
difficulties that may be encountered in
accommodating an antenna/antenna electronics
with special AJ properties such as a CRPA.
These include small real estate for multiple
elements, inability to handle multiple RF
piping connections from CRPA antenna
elements
to
antenna
electronics,
cosite/interference effects and multiplexing
considerations. The Office of Naval Research
Code 31 is supporting two submarine antenna
developments which will be addressing these
issues.
III. NON-CONVENTIONAL
APPROACHES
In this section, we identify four nonconventional approaches to obtaining GPS on a
submersible.
Fig. 13 - Storage GPS Receiver
Figure 14 exemplifies the theoretical advantage
of COGNaC with respect to exposure time to
achieve a Direct Y acquisition for a nominal
initial clock uncertainty of 1 second. For this
large time uncertainty, only the storage
receiver enables viable exposure times,
although at the expense of post exposure data
processing time. For smaller initial time
uncertainties, the storage receiver has less of a
relative exposure time advantage over
conventional receivers, but would still have
Covert GPS Navigation Capability
The first approach is a COGNaC,8,9 (Covert
GPS Navigation Capability), or storage type of
receiver. In conventional GPS receivers, the
acquisition process must occur in real time,
implying that the signal must always be
available in real time. This furthermore means
that the antenna must be exposed during the
lengthy code acquisition process. The storage
receiver provides an alternate strategy. By
storing the broadband signal, the signal can be
12
Fig 14 - Theoretical Direct Y Search Time to First Satellite Fix
some AJ advantages by virtue of its ability to
reprocess data.
The antenna was originally intended to have a
GPS antenna capability. The Applied Physics
Laboratory (APL) has also been developing a
Trailing Wire GPS Antenna. Although these
will potentially enable GPS at depth, they have
the disadvantage of being indelibly tied to the
trailing wire and also have severe real estate
issues. APL has conducted several at-sea tests.
Currently they are examining the results and
developing a plan for future development.
Buoyant Cable Array Antenna or Trailing
Wire GPS Antenna
DARPA has been engaged in developing a
buoyant cable antenna principally for high data
rate two way submarine communications at
depth (Fig. 15).
Recoverable Tethered
(RTOF) System
Optical
Fibre
The United Kingdom’s Defense Evaluation
and Research Agency (DERA) is developing
an interesting variation on the trailing wire
antenna designated the Recoverable Tethered
Optical Fibre (RTOF).11 In this mechanization,
a winch lets out the antenna assembly at a
speed proportional to the submarine’s speed
(Figure 16). This minimizes the relative
velocity of the antenna with respect to the
Fig 15 - Buoyant Cable Array Antenna
13
water, thereby reducing the wake induced by
the antenna.
The buoy would know its GPS position at the
surface and the difference between the
submarine’s
inertial navigator position
changes since launch and the buoy’s inertial
navigator position change since launch would
enable precise positioning of the submersible.
This proposed task would investigate buoy
deployment
techniques,
buoy
design
particularly with respect to its GPS receiver,
GPS antenna and inertial navigation
complement. Small avionics class inertials
such as the GGP will be considered for
incorporation in the buoy. The potential for
propulsion within the buoy to return it to the
submersible may be considered for ultimate
accuracy since this would allow a smoothing
estimation of the buoy’s inertial errors.
Fig 16 - Recoverable Tethered Optical Fibre (RTOF)
GPS Acoustic Ranging Net (GARN)
Submersible Underwater
Receiver (SUGAR)
GPS
Aided
This technique entails the deployment of a net
of GPS surface buoys above the area that the
submersibles would transit (Fig. 18). The GPS
The Submersible GPS Antenna Receiver
(SUGAR)12 (Fig. 17) involves sending out one
buoy and using an avionics-class inertial
navigator within the buoy to constantly keep
track of the vector separation between the
submarine and the buoy.
Fig. 18 - Expendable Buoys with GPS
buoys are, of course, self navigating and
effectively establish an upside down long
baseline transponder net. By acoustically
ranging between the net of GPS buoys and the
host platform, a continuous precise underwater
position reference is established. The technique
could be employed either with an upward or
downward flow of acoustic data. With the
Fig. 17 - Submersible GPS Antenna Receiver (SUGAR)
14
upwards acoustic flow of data, the tracking
principle is based on measuring, on the set of
buoys, the time of arrival (proportional to
range) of an acoustic pulse sent by the mobile
platform. Knowing the sound velocity,
distances from the buoys to the mobile
platform can be calculated. With an adequate
number (typically 2 to 4, depending on
geometry and whether the mobile platform’s
depth could be telemetered up) of surface
buoys, the location of the mobile can be
determined by some centralized processor
which communicates with all the buoys. This
concept is analogous to the GPS Intelligent
Buoy which is marketed and patented (US
Patent: N: 5.579.285) by a French company13.
With a downward flow of acoustic data, the
mobile platform receives coded acoustic
signals from each of the buoys. The equipment
aboard the submersible has an architecture
similar to a GPS receiver but implemented in
the acoustic domain. Knowing the speed of
sound
and
measuring
the
acoustic
pseudoranges to multiple buoys enables the
submersible to calculate its own position. This
technique is discussed in US-AF Youngberg of
the US-Air Force (US Patent N:5.119.341).
V. CONCLUSIONS
The fundamental unique aspects of utilizing
GPS for submersibles have been highlighted.
They include: a) the air-water interface b) the
availability of high accuracy clocks and
inertials c) low dynamics d) unique antenna
constraints and e) a fundamental mission
conflict with exposure. These considerations
have been examined within the context of the
overall lack of large numbers of platforms to
sufficiently influence manufacturers to address
the unique requirements. Although present
submersible GPS equipment and techniques
are virtually identical to ships, we describe
several techniques that have potential
submersible applicability. Ultimately, in the
absence of a port to port inertial navigator,
geophysical
navigation14
or
acoustic
15
transponder
techniques can be used to
complement GPS.
VI. References
1. May, M., Institute of Navigation Newsletter,
Spring 1999.
2. Parkinson, B.W., et al, "A History of
Satellite Navigation," Journal of the Institute of
Navigation, Volume 42, Number 1.
IV. ECONOMIC CONSIDERATIONS
A major factor that has impeded the
incorporation of new specialized GPS
equipment on submarines is the simple fact
that there are not enough submarines to justify
the development cost. Currently, there are less
than 60 fast attack submarines and 14 ballistic
missile submarines in the United States Navy
fleet. The economic reality is there is marginal
incentive for R&D on a product that will only
be used on less than 100 platforms. The
burgeoning applications for Unmanned
Underwater Vehicles may provide further
profit potential.
3. May, M.B., "Inertial Navigation and GPS,"
GPS World, September 1993.
4. Collin, R.E., "Antennas and Radiowave
Propagation," McGraw-Hill, Inc., New York,
1985.
5. Fast, S.A., Young, G.S., Bode, J.N., and
Pelman,
K.E.,
"A
Three-Dimensional
Matching Method for Tropospheric Features,"
Radio Science, Vol. 35, No. 5, pp. 1065-1073,
September-October 2000.
6. May, M.B., Brown, A., Tanju, B.,
"Applications of Digital Storage Receivers for
15
Enhanced Signal Processing," ION GPS '99,
September 1999.
7. Falchetti, C., Abriel, J. “GAS 1 Evaluation
for Navy Ships: DT-B1 Test Report”, April
2002.
8. Thompson, T., Doherty, M, “Data Logging
GPS Receiver”, GPS ION, September 1994.
9. Tanju, B.,
“Covert GPS Navigation
Capability”, Naval Air Development Center
presentation, 19 October 1993.
10. Aein, J. M., “Clock, Correlator, and Inertial
Measurement Unit Technology for SpreadSpectrum
Communications”,
MR-747DARPA, 1997.
11. Lambert, J., CAPT USN, "Recoverable
Tethered Optical Fibre System," update to
DSTL Winfrith, 28 Nov 2001.
12. ARL/PSU NRDC/SPAWAR System
Center Proposal to ONR BAA “Submersible
Underwater GPS Receiver”, March 2000.
13.
Advanced
Concept
and
System
Architecture, http://www.underwater-gps.com
14. Ringlein, M., May, M. B., Barnett, N.,
“Next Generation Strategic Submarine
Navigator”, AIAA Aerospace Transaction,
November 2000.
15. May, M.B., "Long Endurance GPS Inertial
Transponder (LEGIT)," Patent Application,
July 2003
16
Fig. 17 - Submersible GPS Antenna Receiver
(SUGAR)
VII. LIST OF FIGURE CAPTIONS
Fig. 1 - Plane Wave Incident upon a Dielectric
Fig. 18 - Expendable Buoys with GPS
Fig. 2 - Magnitude
of
Transmission
Coefficients for S- and P- Polarized Electric
Fields at Air/Seawater Interface
Fig. 3 - Plane Wave Incident upon Seawater
Fig. 4 - Attenuation in Seawater vs. Depth
Fig. 5 - Typical Commercial GPS Antenna and
a Cutaway View
Fig. 6 - Sample Geometry Used for Analysis
Fig. 7 - VSWR of L1 GPS Antenna from 1 to 2
GHz
Fig. 8 - Radiation Patterns of L1 GPS Antenna
for Varying Seawater Layer Thickness
Fig. 9 - Required jamming power for a -100
dBm receiver at 3 meters through a 20 meter
duct
Fig. 10 - Steady State Simplified Submarine
Navigator Error Growth
Fig. 11 - Time RMS Navigation Error
Fig. 12 - Typical
Configuration
Submarine
Antenna
Fig. 13 - Storage GPS Receiver
Fig. 14 - Theoretical Direct Y Search Time to
First Satellite Fix
Fig. 15 - Buoyant Cable Array Antenna
Fig. 16 - Recoverable Tethered Optical Fibre
(RTOF)
17
VIII. BIOGRAPHIES
Bereket Tanju
Marvin B. May
Bereket Tanju serves as the Assistant Program
Manager for GPS Modernization for
the Navigation Systems Program Office
(PMW/A-156) at Space and Naval Warfare
System Command (SPAWAR) in San Diego
CA. He is responsible for the execution of the
U.S. Navy Navigation Warfare and
GPS Modernization program.
Among his
specialties is covert GPS navigation.
Marvin B. May is the Chief Navigation
Technologist at ARL Penn State’s Navigation
Research and Development Center in
Warminster, PA. He also manages their
Navigation Education Program. He has a
BSEE from City College of NY and a Masters
Degree from New York University, doctoral
courses at Polytechnic Institute and is a
Professional Engineer. He is an adjunct
professor at several universities and teaches
Master’s Degree navigation courses for the
Penn State Great Valley Graduate Center. He
is a recognized navigation specialist with
expertise in GPS, inertial and geophysical
navigation. During his Navy career he has
worked at the Navy’s Navigation Laboratory of
the Naval Command, Control and Ocean
Surveillance Center (NCCOSC), and his
experience includes eight years as chief analyst
for GPS responsible for satellite navigation
systems analysis, laboratory testing and
integration issues. May has served as Chairman
of the Greater Philadelphia Chapter, the
National Marine Navigation representative,
and is Historian of the Institute of Navigation.
Robert A. Lopez, CAPT USN
Captain Robert A. Lopez serves as program
manager for the Navigation Systems Program
Office (PMW/A-156) at Space and Naval
Warfare System Command (SPAWAR) in San
Diego CA. His responsibilities include the
planning,
coordination, management
and
technical
direction
for
the
design,
development, and Fleet introduction of a wide
range of hardware and software products in
support of improvements to GPS navigation
applications for US Navy aircraft, ships,
submarines, land vehicles and handheld GPS
systems. His previous assignments include
serving as deputy program director for
SPAWAR’s Intelligence, Surveillance, and
Reconnaissance Directorate (PD- 18) and as
deputy program manager for Mobile
Surveillance Systems (PMW- 182).
Rodney A. Martin
Rodney A. Martin received his B.S. and M.S.
degree in electrical engineering from the
Pennsylvania State University, University
Park, in 1999 and 2001, respectively. Since
1998, he has been with the Applied Research
Laboratory at Pennsylvania State University,
involved in the development of a Java-based
suite of antenna models, conformal antenna
analysis software, urban RF propagation
analysis, and fractal antenna engineering. His
research interests include conformal antennas,
numerical methods, and computational
electromagnetics.
18