University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Dr. R. Willingale
April 6, 2000
Contents
1 Preamble and Books
3
2 Introduction
3
2.1
History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
2.2
The London to L.A. Telephone Call . . . . . . . . . . . . . . . . . . . . . .
4
2.3
Goesynchronous Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.4
Geostationary Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
3 Launching Geostationary Satellites
6
3.1
The ELV Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
3.2
The STS Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3.3
PKM Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
2
3.4
Sequence of Events for GEO Injection . . . . . . . . . . . . . . . . . . . . .
8
3.5
The Drift Orbit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.6
Stability of a Geostationary Orbit - Station Keeping . . . . . . . . . . . . .
9
3.7
Some Properties of GEO . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
4 Communication Satellites - The Spacecraft
10
4.1
AOCS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2
TT&C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3
The Power System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.4
The Payload - Communication Sub-System . . . . . . . . . . . . . . . . . . 13
4.5
Transponders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.6
Spacecraft Antennae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.7
Dish Antennae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 The Design of a Satellite Communications Link
17
5.1
A Typical Link Power Budget . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2
The Signal-to-Noise Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3
Noise in Electronic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.4
The Noise Budget for a Direct TV Broadcast . . . . . . . . . . . . . . . . . 21
5.5
The Earth Station Figure of Merit . . . . . . . . . . . . . . . . . . . . . . . 23
5.6
The Noise Figure for the Receiver . . . . . . . . . . . . . . . . . . . . . . . 23
6 Modulation and Multiplexing Techniques for Satellite Communication
Links
23
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
6.1
Frequency Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . 24
6.2
Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7 International Programmes in Satellite Communications
26
7.1
INTELSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.2
INMARSAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.3
ESA Programme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4
OLYMPUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8 LEO Networks
1
1604
1.0
April 7, 2012
3
28
Preamble and Books
• Communication Satellites - RW - 5 lectures
• Navigation Satellites - BAC - 3 lectures
Library index 621.38. . .
• Satellite Communications, T.Pratt and C.Bostian
• Communication Satellite Systems, J.Martin
2
Introduction
Most authorities credit Arthur C. Clarke with the idea of a synchronous communications
satellite. ”Extraterrestial Relays”, Wireless World 51, 305-308, October 1945.
Simple idea; place a satellite in circular orbit above the equator at a radius of ≈ 42000km
which gives an orbital period of 1 day, the same as the rotation period of the Earth. In
such an orbit the satellite remains above the same point on the Earth’s surface. The
satellite could receive and relay signals.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
4
In principle 3 satellites spaced at 120◦ around the equator could service the whole globe
provided messages could be sent between satellites as well as from ground to satellite.
Continuous, reliable communication could be provided between any 2 points on the
Earth’s surface using such a system.
2.1
History
• 1957 - launch of SPUTNIK 1, Low Earth orbit (LEO), 200 to 600km, period 90mins.
• 1958-64 - early developments mainly related to space race!
• TELSTAR I elliptical orbit 960 to 6080 km, period 2hr 38mins.
• 1965 - INTELSAT I (Early Bird). First geosynchronous satellite that provided a
routine link between USA and Europe for 4 years.
INTELSAT - International Telecommunications Satellite Organization. ≥ 110 countries responsible for providing communication links between its members - hires out a service.
COMSAT is the USA representive.
INMARSAT - International Maritime Satellite Organization. Provides communications
between ships and platforms.
The expansion of the market has been remarkable. There is now congestion in
geosynchronous orbit! Economic pressures have lead to larger individual spacecraft
(size,mass,power,bandwidth) and corresponding reduction in unit costs.
Satellite systems are now an integrated part of international communication networks.
2.2
The London to L.A. Telephone Call
• Direct transmission of analogue speech via wire pair to local exchange.
• Convert to digital form and transfer to time-shared optical fibre link for transmission
to a ground station in UK.
• Modulation on a 6 GHz carrier for transmission to a satellite above the Atlantic.
• Receive at satellite and convert to a 4 GHz carrier for transmission to ground station
on eastern seaboard of USA. (Note can’t hop directly to California).
• Transmission over landlines after frequency division and multiplexing to main
exchange in L.A.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
5
• Conversion to a less intensive multiplexing for distribution to local exchange.
• Convert back to analogue signal and transmit by wire pair to receiver in L.A.
Of course the other half of the conversation has to take the reverse route.
One important aspect of such a system is the time delay introduced by the satellite link.
The up+down delay for a geosynchronous orbit is about 270ms which is much larger than
the landline delay. Thus in the above there is a 0.5 second delay between asking a question
and getting an answer. Therefore live conversations don’t use links involving more than
1 satellite. UK to Australia uses just 1 satellite over the Indian Ocean. This is not a
problem for broadcasts or non-interactive data transmission.
All this is obviously complicated. The topics we will cover are:
• Geosynchronous, Geostationary Orbits and LEO Networks
• Communications Satellites
• Satellite Link Design
• Modulation and Multiplexing Techniques for Satellite Links
2.3
Goesynchronous Orbit
A geosynchronous orbit is one for which the orbital period of the spacecraft is the time
taken for the Earth to complete 360◦ rotation.
Torb = 23hr56mins which is 1 sidereal day.
From Kepler’s 3rd law for elliptical orbits:
q
Torb = 2π a3 /GMe where GMe = 4 × 105 km3 s−2 .
asyn = 42164 km where asyn is the semi-major axis of the orbit.
If we consider just circular orbits then rsyn = asyn and the only free parameter is the
inclination of the orbit, the angle between the Earth’s equatorial plane and the orbit
plane at the ascending node.
The ground track or sub-satellite path is the locus of points at which the satellite is directly
overhead during the orbit.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
6
For an inclined geosynchronous orbit the ground track is a figure-of-eight. The centre point
is the ascending and descending node of the orbit and the peak deviation in latitude is at
±i the inclination and the largest offset in longitude is ±i2 /4 for small i.
2.4
Geostationary Orbit
This is a special case of the geosynchronous orbit with i = 0 and e = 0, zero inclination
and circular.
In such an orbit the satellite remains above the same point on the ground all the time.
The ground track is reduced to a point.
In practice the word geostationary is used for orbits which are nearly circular and have
i < 5◦ .
3
Launching Geostationary Satellites
To place a satellite in a GEO requires an acceleration to a velocity of ≈ 3050m/s in a zero
inclination (equatorial) orbit and lifting it ≈ 42000km above the Earth’s surface. There
are 2 competing technologies for doing this:
• Expendable Launch Vehicles (ELV)
• The Space Transportation System (STS)
3.1
The ELV Approach
The ELV approach (e.g. Delta and Ariane) place the satellite into an inclined elliptical
orbit called a transfer orbit with the apogee at geosynchronous altitude, perigee of
≈ 370km and a period of ≈ 10.5hrs
Two steps are then required to transfer into GEO:
• transfer to an inclined, circular, geosynchronous orbit
• reduce the inclination to ≈ 0, equatorialize the orbit
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
7
rapogee = rsyn
A rocket engine is fired at apogee to apply a delta velocity:
∆v = vsyn − vapogee = 3050 − 1550 = 1500m/s
The apogee kick motor (AKM) is usually an integral part of the satellite system.
The second step is to reduce the inclination. This is done by applying a delta velocity at
the ascending (or descending) node when the inclined orbit crosses the equatorial plane.
The velocity kick required is given by vector addition:
∆v =
q
v12 + v22 − 2v1 v2 cos ∆i
The inclination of the transfer orbit is governed largely by the latitude of the launch site.
The minimum inclination that can be achieved in a due east launch is:
imin = latitude of launch site
For Cape Canaveral latitude = 28.3◦ N which gives:
∆v =
√
2 × 30502 − 2 × 30502 cos 28.3 = 520m/s
For Ariane launches latitude = 5◦ N giving:
∆v = 16m/s a considerable saving.
So called Dog-Leg manoeuvres to change the inclination of an orbit are sometimes
performed during powered flight of the main rocket. It is advantageous to apply ∆v
when v1 and v2 are small so they are best done near apogee.
In practice the circularization and equatorialization can be done by one burn of the AKM.
3.2
The STS Approach
The Space Shuttle lifts the payload into an inclined LEO, parking orbit. The satellite
system is then deployed. Injection into a GTO (transfer orbit) is then performed using a
perigee kick motor. The sequence then follows as described above for the ELV.
The perigee kick motor (PKM) can either be incorporated into the satellite or (more
often) a specific additional stage using either liquid or solid propellant is added. Some
systems provide both the perigee and apogee ∆vs.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
8
The velocity deltas required for transfer from parking to GTO to GEO are:
∆vperigee ≈ 2450m/s
∆vapogee ≈ 1478m/s
3.3
PKM Systems
The PAM D2 system. Solid propellant. The shuttle can carry upto 4 such systems in
principle. They are carried with their axes perpendicular to the longitudinal axis of the
shuttle. Can lift 1250kg into GTO.
The inertial upper stage (IUS). Actually contains 2 motors so can perform the AKM burn
as well. 2 stage solid propellant system. Can lift 2270kg into GEO. It is carried with axis
parallel to shuttle longitudinal axis and is rotated by 60◦ for deployment by springs. The
IUS can also be launched on Titan.
3.4
Sequence of Events for GEO Injection
As well as launch or deployment from the shuttle to get into GTO, attitude manoeuvres
are required to ensure that the motors are pointing the right way and that the solar panels
and communications antennae are in the correct orientation.
The ground track of the satellite during the transfer orbit (10.5hrs) is quite extended.
Must choose the point at which transfer occurs so that you get good coverage from ground
stations.
3.5
The Drift Orbit
The final orbit achieved initially is unlikely to be exactly right with e = 0 and i = 0. This
causes the satellite to drift about its nominal geostationary position. The satellite must
use low thrust on-board rockets to perform station keeping manoeuvres. At the same time
these motors are also used to tweek the satellite to the correct longitude position required
around the Equator.
The following are performed in the drift orbit:
• Sun aquisition
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
9
• Solar panel deployment
• Earth aquisition
• Station aquisition
3.6
Stability of a Geostationary Orbit - Station Keeping
Ideally the final manoeuvres performed during the drift orbit would tweek the satellite
into the correct geostationary orbit and the system would then remain fixed in attitude
and position for use as a communications link. Life, of course, is not so simple because
there are several perturbing influences that cause the satellite to drift.
• North-South drift - changing inclination of the orbit. This is the result of the
departure of the local gravitaional field from a central source field due to the pull of
the Sun and the Moon. Lunisolar perturbations.
• East-West drift - changing longitude of the orbit. This is due to varations in the
Earth’s gravitational field (inhomogeneity and departures from spherical symmetry
of the mass distribution of the Earth). Note stable point near Sri Lanka (Ceylon)
A.C.Clarke again!
In broad terms satellites are kept within particular inclination windows by performing
station keeping manoeuvres at frequent intervals. 1 manoeuvre per 3 months to 3 years
depending on how much drift can be tolerated.
The East-West drift can be controlled by the tennis ball approach. This requires about 1
manoeuvre per month.
3.7
Some Properties of GEO
rsync = 42164km from the centre of the Earth
re = 6378km at the equator
The cone angle subtended by the Earth at the satellite is sin(θ/2) = 6378/42164,
θ = 17.4◦ .
Some 42.4 percent of the Earth’s surface is visible. (Can you work that out?)
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
10
The propogation delay ranges from 110.3ms to 139ms, one way.
The position of the satellite as seen from the surface of the Earth is given by an Azimuth
and Elevation.
Azimuth is the angle around the horizon from North measured eastwards. Ranges from
0◦ to 360◦ .
Elevation is the angle above the horizon. The satellite is overhead, Elevation = 90◦ .
at one place on the Equator. The elevation drops to zero along a minor circle through
∆longitude ± 81.3◦ .
The equatorial orbit is inclined to the ecliptic by 23.4◦ so during the Vernal and Autumnal
Equinoxes the satellite suffers eclipses. That is it enters the Earth’s shadow.
Can assume that the Sun is distant and point like (actually it subtends 1/2◦ on the sky).
The maximum eclipse duration is 17.4/360 days or 70 mins. Eclipses occur over a total
of 44 nights per year in March-April, September-October.
Most communication satellites carry batteries so that they can continue operating during
eclipse.
More serious is the passage of the satellite in front of the Sun. During these times
communication is blocked by the very large radio noise output from the Sun. This lasts
about 10 mins and occurs on 5 consecutive days twice a year. The only way to get around
this is to use 2 or more duplicate satellites at different longitudes.
4
Communication Satellites - The Spacecraft
The main spacecraft subsystems on a communications satellite are:
• Attitude and Orbit Control System (AOCS)
• Telemetry, Tracking and Command System (TT&C)
• Power System
• Communication Sub-system - the payload
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
4.1
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
11
AOCS
The attitude control must be precise enough such that the narrow beam communications
sub-systems are pointed correctly at the Earth. Requirement can range from 17.5◦ to
0.5◦ .
Rotational forces include:
• Luni-Solar perturbations - micro gravity
• Radiation and Solar wind pressure - momentum transfer
• Magnetic fields - net dipole of satellite tries to align to the local magnetic field
There are 2 approaches to attitude control:
• Spin stabilization - The main body of the spacecraft is spun at 30 to 100 rpm.
The communications sub-system is mounted on a de-spun table. Jets are used for
spin-up around the pitch axis and to control the roll and yaw axes of the satellite.
• Three axis stabilization - 3 momentum wheels are used on 3 mutually orthogonal
axes. Pairs of jets control the rotation about each axis, roll, pitch and yaw.
We have already noted the need for station keeping, orbit control. Again jets are used to
provide the appropriate ∆v to tweek the orbital parameters.
The jets use either a single propellent which is ignited by catalyst or heating, propane
or hydrazine (N2 H4 ) are common, or a bi-propellent propulsion system which requires 2
gases to be injected into the thrust chamber where they spontaneously ignite.
4.2
TT&C
Spacecraft management is conducted via the TT&C system from a dedicated Earth
station. The tasks are:
• attitude and orbit control - by command
• monitoring status of all the sub-systems - by telemetry
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
12
• finding the range, elevation and azimuth - by tracking
• configuring the antenna pointing and communication subsystem - by command
The command and telemetry functions are provided by a narrow bandwidth, low bit rate
communications link (UHF) - high signal power - low error rate.
For safety might use an omni-directional system to avoid loss of signal if the AOCS
malfunctions. INTELSAT uses horns with full Earth coverage once attitude is established.
The command structure must possess numerous safeguards to prevent accidental
commanding errors. They can be very expensive.
4.3
The Power System
Elements of the power system are:
• Solar panels. Covered with solar cells - current generators.
• Battery system.
• Power conditioning unit. Copes with changing current, dumps excess power as heat,
stores power in batteries and distributes regulated power to other sub-systems.
Solar cells. The solar constant is 1.39kW/m2 but cells are only 10-15% efficient. Cells
also degrade with time. Typically allow for a 15% loss after 5 years.
INTELSAT IV-A (1975) 20m2 of solar cells providing 900W at start of lifetime.
The latest satellites generate 2900W from 30m2 .
Most of this power is used by the transmitters.
On a spinner type the solar panels must be wrapped around the body in a cylinder so
the body must be large. Only 1/3 of the area faces the sun at any time. The surface
temperature is 20 − 30◦ C.
On a 3-axis stabilized craft the solar panels are deployed like sails and entire area faces
the Sun at once. However they tend to run rather hot, 50 − 80◦ C, and this reduces the
efficiency.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
13
Batteries are required to provide power during eclipse. TV broadcast satellites require too
much power for battery operation. They are usually sited 20◦ West of service longitude
so that any eclipse breaks occur at 1:00am local time.
4.4
The Payload - Communication Sub-System
The function of a communications satellite is to provide a platform in GEO for relaying
of voice, video and data streams. All other sub-systems exist solely to support the
communications sub-system. However the latter represents only a small fraction of the
volume, mass and cost of the complete package.
Modes of radio propogation:
• Line-of-sight systems. e.g. microwave links using dishes and towers. Curvature of
the Earth limits the distance. A 60m tower gives 60km line of sight. e.g. Gas Board
in Regent Road.
• Surface or ground wave propogation. The radio wave travels along the Earth’s
surface as a result of currents flowing in the ground. This is the dominant mechanism
at low frequencies. e.g. Radio 4 λ = 1500m ≡ 200kHz.
• Ionospheric propogation. Radio waves can be reflected from the ionosphere.
Example of total internal reflection, the refractive index gradually increases with
height. The return wave can in turn be reflected back up again. The gap between
the ionosphere and the ground acts as a waveguide.
• Tropospheric scattering. Radio waves are scattered from small particles in the lower
atmosphere to provide over the horizon communications.
Satellite communication is an extreme example of line-of-sight radio links. One tower is
of height 35600km!
Radio waves propogating in free space diminish in power as 1/r2 so after 36000km they
are very weak. Typically the received power is < 10−12 W . Compared with a normal
ground system this is a factor of ≈ (45/36000)2 weaker.
The relay function of the communications system is to receive the up-link signal from the
ground, amplify it, change its frequency and retransmit it to the ground.
The change of frequency between Rx and Tx is absoluely essential because otherwise the
up-link signal would be completely jammed by the relatively powerful down-link signal.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
14
Satellite communication systems use UHF or SHF(microwaves). This ensures that they
penitrate the ionosphere and provides a large bandwidth.
The following up/down channels are used:
• 6/4 GHz - prime communications - C band
• 14/11 GHz - new generation since 1979 - K band
• 30/20 GHz - experimental technology - K band
A wide bandwidth means that there is a large spread of frequencies about the central
carrier frequency. This dictates the volume of information that can be transmitted. e.g.
the number of telephone calls or the number of TV channels.
For example, the 6/4 Ghz bands use a 500 MHz bandwidth so:
uplink 5.925 - 6.425 GHz
downlink 3.7 - 4.2 GHz
500 MHz is also used for the 14/11 GHz bands but the 30/20 GHz bands operate with
a much larger bandwidth of 3500 MHz. The larger the bandwidth the greater the traffic
and the greater the commercial return.
4.5
Transponders
Most satellites have many transponders. The bandwidth they handle differs from one
satellite to another but typically it is ≈ 36M Hz. One such transponder can handle one
of the following:
• one colour TV channel + sound
• 1200 voice channels
• a data rate of ≈ 50M bits/sec
To utilize the full 500MHz bandwidth for a 6/4GHz link a satellite might use 12
transponsers at 40MHz steps. Most systems include redundant items in case of failure of
particular channels.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
15
Often 24 transponders are employed using two polarizations to double the bandwidth.
Both single and double conversion systems are used. In a single conversion system a single
mixing with a local signal, 2225MHz say, converts from the up to the down frequency. In
a double conversion system the up-link is mixed down to an IF, 1GHz say, amplified and
then mixed up to the down-link frequency.
Mixing the signals involved multiplying and then filtering:
x1 = a1 cos ω1 t and x2 = a2 cos ω2 t
x1 x2 = (a1 a2 /2)(cos(ω1 + ω2 )t + cos(ω1 − ω2 )t)
Filtering removes either the sum or difference term.
4.6
Spacecraft Antennae
The function of an antenna is to provide a match between electrical signals in the Rx or
Tx and the electromagnetic waves in free space.
In some applications the antenna should radiate (or receive) isotropically.
e.g.
wire antennae (monopoles and dipoles) are used primarily to provide TT&C where
omnidirectional coverage is important since commands must be received when the
spacecraft is in an anomalous pointing.
However for the communications payload, where bandwidth and signal strength are
important, directionality of the antenna is important.
To obtain global coverage requires a beam width of 17◦ but spot beams of ≤ 5◦ may be
appropriate for a specific application, e.g. East Coast USA ↔ Western Europe.
The advantage of a narrow beam is that the gain of the antenna is increased. The same
radiated power is concentrated into a particular direction (solid angle).
4.7
Dish Antennae
A dish antenna of aperture area Am2 has a gain given by:
G = η4πA/λ2
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
16
where λ is the wavelength and η is the aperture efficiency. This will dealt with in the 2nd
year EM-Optics course.
Thus if 2m diameter reflector is used at 4GHz and η = 0.5 the gain over an isotropic
antenna is:
G = 0.5 × 4π 2 /0.0752 = 3500
Electrical engineers usually quote gain on a logarithmic scale. If the power ratio is P2 /P1
then the gain in decibels (dB) is:
x = 10 log10 (P2 /P1 )
Therefore G = 3500 ≡ 35dB
The beam profile from an antenna is a diffraction pattern created by the shape and
dimensions of the aperture. The width of the beam is usually described using the −3dB
points on the beam. Very roughly this is given by:
θ3dB ≈ 75λ/D degrees
where D is the diameter of the aperture. Note −3dB corresponds to the half power point
from the centre of the beam.
In the above example, 2m reflector, λ = 7.5cm, θ3dB = 3◦
which is 1800km on the ground.
The requirement on a receiving antenna is to collect as much of the (feeble) incident signal
as possible. The collected power is ∝ the area of the antenna so we must make the dish
as large as possible.
An antenna has the same beam response on reception as transmission. So a large, high
gain device will have a small beam and must be pointed in the correct direction.
There are 2 types of high gain antennae:
• horns - wide beams
• reflectors - narrow spot beams
In practice horns are used to match the end of a waveguide to free space giving high
radiated power over a reasonably wide beam. For a narrow beam system such a horn is
used to feed a reflector dish which produces a much narrower beam.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
17
Array of horns can be used for different directions, frequencies and polarizations within
a single dish reflector.
5
The Design of a Satellite Communications Link
A communications system must be designed to meet certain minimum performance
criteria. For example it must be decided how much Tx power is required and how large
the antennae must be.
One important criterion is that adequate S/N ratio be maintained in the communications
channel.
A first step is to consider the link power budget.
Consider a transmitter radiating power Pt (Watts) isotropically. At a distance r the flux
density will have dropped to:
F (r) = Pt /(4πr2 ) W m−2
i.e. the original power is spread over an area of 4πr2 .
If the transmitter antenna has a gain Gt then:
F (r) = Pt Gt /(4πr2 )
assuming we are at θ = 0, the centre of the beam. This signal is collected by an antenna
of area Ar m2 . However reflection losses at the face of the dish and absorption in lossy
components mean we should use an effective area Ae = ηAr . Therefore the power received
is:
Pr = F Ae = Pt Gt Ae /(4πr2 )
Previously we noted that the gain of the antenna is:
Gr = 4πηAr /λ2 = 4πAe /λ2
Therefore we get the so called Friis transmission equation for the power received:
Pr = Pt Gt Gr (λ/4πr)2
Pt Gt is the effective isotropic radiated power, EIRP
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
18
(λ/4πr)2 is the path loss Lp (due to radiation spreading out)
Gr is the gain of the receiving antenna
We should also include other losses such as absorption in the intervening atmosphere La
Pr = EIRP × Gr × Lp × La
In communications systems a logarithmic scale is normally used - dB. Then the product
becomes a sum. So if we express each term in dB using powerdBW wrt 1 Watt.
PrdBW = EIRPdBW + LpdB + GrdB + LadB
The possible sources of La are:
• O2 molecules
• water vapour
• rain
• fog and cloud
• snow and hail
• free electrons
5.1
A Typical Link Power Budget
Consider a 4-6 GHz satellite system with a large Earth station antenna.
Up-link 6GHz
Tx power (2kW)
Earth antenna (30m)
+33.0 dBW
+62.5 dB
EIRP
+95.5 dBW
Lp
La (rain)
Spot beam (2m)
-199.0 dB
-3.0 dB
+39.0 dB
1.8 × 10−7 W
-67.6 dBW
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
19
Down-link 4GHz
Tx power (6.3W)
Spot beam
EIRP
+8.0 dBW
+35.5 dB
43.5
Lp
La (rain)
Earth dish (30m)
2.5 × 10−10 W
dBW
-195.5 dB
-3.0 dB
+59.0 dB
-96.0 dBW
Note that the antennae gain, path and absorption losses depend on the frequency.
In this case the received up-link power is very large - generous.
The received down-link power is limited by the Tx power on-board.
If there are 24 channels only 150W is required to power them all. This is rather modest.
5.2
The Signal-to-Noise Ratio
The above discussion of the link power budget indicates that space communication systems
are characterised by very large signal losses due to the massive distance between the Tx
and Rx. However the very small signals can be amplified to compensate for the low signal
level. What limits the effectiveness of the communications link is the signal-to-noise ratio.
5.3
Noise in Electronic Systems
All electronic systems are subject to random (stochastic) processes which give rise to
random noise voltages on the output. For example a common type of noise is thermal
noise due to the thermal motion of the electrons within the electronic components.
The noise in a system like a receiver is characterised in terms of an equivalent noise
temperature (ENT) at the input. The system is modelled as an ideal noiseless amplifier
with a source of noise strapped to the input. The noise power at the input is:
Pn = kTr B where
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
20
k is Boltazmann’s constant, 1.38 × 10−23 J/K
Tr is the equivalent noise temperature on the input of the receiver
B is the bandwidth of the amplifier (receiver).
So the noise power per unit bandwidth is kTr
The actual source of the ENT can come from various components in the receiver but the
critical items are at the front-end, the low noise amplifier and mixer. Any noise generated
there is likely to be large compared with the signal. Typical values are:
mixer/low noise block
low noise solid state amplifier
cooled parametric amplifier
maser
700K
150K
35K
10K
In the real world there is also a contribution to the noise from the sky, radiation received
by the antenna.
Sources of such noise include:
• the Sun ≈ 100000K source!
• the Earth ≈ 250K source as seen from GEO
• the Moon (reflected solar radiation)
• Galactic noise, more important at low frequencies
• sky and atmospheric noise
• man-made noise - interference local to the antenna
All these add up to a second input noise to the receiver which is characterised by Ta the
antenna temperature.
The total input noise is therefore:
Pn = k(Tr + Ta )B = kTsys B
where Tsys is the system temperature.
It is against this total noise in the receiver system that we must measure the signal power
received at the antenna.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
21
The received carrier signal-to-noise is:
Pr
Pr
=
Pn
kTsys B
In our example of the down-link power budget we had a received power of −97dBW , from
a 6.3W transponder with 36MHz bandwidth.
Say the system temperature is 180K (Tr = 150K and Ta = 30K), then
Pn = 1.38 × 10−23 × 180 × 36 × 106 = 9 × 10−14 W
Pn = −130dBW
So the carrier signal-to-noise ratio is +33dB in good weather.
How large must the signal-to-noise ratio be? In order to maintain a usable communications
link the carrier S/N ratio (C/N) must remain above a threshold which depends on the
type of modulation employed but typically 8 − 15dB is needed.
The effect of reducing the C/N ratio will be to increase the error rate. This is most easily
measured in a digital system as the bit error rate or BER.
On INTELSAT systems a typical specification is:
C/N = 18dB, 11dB threshold + 7dB safety margin for poor conditions.
Therefore the above example is possibly overspecified by about 15dB so we could reduce
the transmitter power or perhaps use a smaller Earth station antenna. 15dB ≡ 30×area ≈
5 × diameter so 30m → 6m.
5.4
The Noise Budget for a Direct TV Broadcast
Say 1 TV channel is transmitted in 27MHz at 200W at 12GHz over a 2.5◦ region (1700km
across).
We require θ3dB = 2.5◦ therefore D = 0.75m (note the high carrier frequency).
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Tx power
Tx antenna gain
+23.0 dBW
+36.5 dB
EIRP (1 channel)
+59.5
atmospheric loss (clear)
path loss
receiving dish
+00.0 dB
-205.0 dB
+36.5 dB
carrier (best case)
-109.0
station at edge of zone
losses in Rx before LNA
pointing error
-3.0
-1.0
-1.0
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
22
dBW
dBW
dB
dB
dB
carrier (likely)
-114.0
dBW
noise from mixer 700K
-126.0
dBW
C/N ratio
+12.0
dB
So the system is OK in good weather but gets flaky when it’s raining.
To improve the system you can:
• point the antenna more accurately −0.5dB if ±0.5◦
• use a low noise block amplifier LNB, gives +1.5dB
• move house to get within 2dB of peak
• buy a bigger antenna and point it accurately
• turn off the TV when it’s raining!
Note that if a satellite has 12 channels 2.4KW of power must be radiated.
Note also that for deep-space (planetary missions) the bandwidth must be restricted to
improve the Pr /Pn . Of course this limits the information transfer rate.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
5.5
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
23
The Earth Station Figure of Merit
The effectiveness of the Earth station for reception depends on the collecting power of
the dish antenna and the noise seen from the local sky or introduced by the receiver. We
can rewrite the Friis equation in terms of the C/N ratio and group the terms:
C/N =
EIRP × Lp × La
Gr
×
kB
Tsys
The left hand fraction contains constants of the satellite system while the righthand
fraction is a function of the Earth station. The ratio Gr /Tsys is sometimes called the
figure of merit of the Earth station and is usually quoted in dB K −1 .
5.6
The Noise Figure for the Receiver
The noise performance of an amplifier or reciever is usually quoted as a noise figure defined
by:
NF =
(S/N )in
(S/N )out
The noise temperature is more useful in satellite communication systems and it is best to
convert from NF to Tsys using:
Tsys = To (N F − 1) where To is a reference temperature, usually 290K.
6
Modulation and Multiplexing
Satellite Communication Links
Techniques
for
Satellite communications links are usually wide-bandwidth. They have the capacity
for relaying multiple, independent communications signals. e.g. many telephone
conversations.
The method employed to ensure that the different channels don’t mutually interfere is
refered to as a multiplexing scheme.
The most common forms of multiplexing in current use are:
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
24
• frequency division multiplexing - FDM
• time division multiplexing - TDM
In FDM a particular communications channel uses a particular band of frequencies within
the transponder bandwidth.
In TDM the transmissions for individual channels are separated in time.
Although many variants are possible it is generally the case that:
• FDM is used for the transmission of analog signals, telephone calls and the majority
of video signals.
• TDM is used for the transmission of digital format, digital telephony, numerical
data.
The rigorous explanantion of these techniques is beyond this course. We shall just touch
on the general ideas.
6.1
Frequency Division Multiplexing
Consider multiplexing a number of voice channels. The signal from the microphone (in
the telephone handset) contains a range of frequencies from 10 to 20kHz.
The time varying signal which represents the voice can be Fourier analysed and the useful
information is represented by a narrow frequency band. The important range is 300 to
3400Hz. 4kHz is required for each voice channel.
To send the signal using a much higher frequency radio carrier we must modulate some
property of the carrier in sympathy with the signal amplitude.
For a typical FDM system amplitude modulation is used several times on a sinusoidal
carrier and then frequency modulation is used to load the composite message signal onto
the radio link carrier.
Amplitude modulation is effected by multiplication of the voice signal by a carrier. Each
frequency in the voice signal is shifted in frequency:
a1 cos ωv t × a2 cos ωc t = a1 a2 (cos(ωc + ωv )t + cos(ωc − ωv )t)
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
25
The second term is removed to produce a Single Side Band - Suppressed Carrier SSB-SC.
Each voice channel is modulated by a different carrier frequency ωc , ωc + 4kHz, ωc +
8kHz, . . . and they are added together to form a composite signal. Hence FDM, each
channel is allocated a frequency band within the carrier. The typical heirarchy used is as
follows:
1 voice channel
group (12 channels)
supergroup (5 groups)
master group (5 super)
super master group (900 channels)
4kHz
48kHz
240kHz
1.2MHz
3.6MHz
The radio carrier is typically 6GHz. This is frequency modulated by the super master
group. The FM signal is not easy to analyse but the basic idea is:
νi = νc + km(t)
where k is constant and m(t) is the message signal (the grouped carrier above). The
bandwidth of νi is given by:
B = 2νd + 2W
where νd is a constant and W is the bandwidth of the message signal. FM transmission
is not spectrally efficient (it uses a large bandwidth) but it is immune to noise and
interference.
At the other end of the link (i) the message signal is extracted from the carrier by
demodulation of the FM and (ii) the voice signals are extracted by demodulation of
the AM.
SSB-SC signals are demodulated by multiplying by a replica carrier.
a1 a2 cos(ωc + ωv )t × ar cos(ωc t + φ) = cos(ωv t − φ) + cos(2ωc t + ωv t + φ)
φ is the phase of the replica wrt to the carrier. SSB-SC demodulation is insensitive to
this phase.
6.2
Time Division Multiplexing
Each communications channel makes use of the entire bandwidth of carrier for a brief
period of time.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
26
Consider the same example of many simultaneous telephone calls multiplexed using TDM.
The first step is to convert the analog voice signal into a digital format using Pulse Code
Modulation, PCM.
The input signal is sampled at regular time intervals and the sampled voltage is given a
binary code in the range 0 → 2N − 1 where N is the number of bits/sample. Thus the
analog signal is converted to a series of pulses. To sample a 4kHz bandwidth a sample
rate of 8kHz is required (this is called the Nyquist rate). If 8 bits are used per sample the
output digital sequence runs at 64kbits/second.
The next step is to combine many channels using a Time Division Multiplexer. A typical
PCM hierarchy is:
level channels rate Mbits/sec
1
30
2
2
120
8
3
480
34
4
1920
140
Note that at the higher levels extra bits are added for housekeeping and error checking
etc..
Finally the very high bit rate digital signal is modulated onto a carrier signal using one
of a variety of methods:
• Amplitude Shift Keying - ASK
• Phase Shift Keying - PSK
• Frequency Shift Keying - FSK
At the other end a demodulater is required to unload the carrier and a decoder is used
to pick out the individual channels. Finally the a digital to analogue converter is used to
reconstruct the analogue speech signal.
7
International
Programmes
Communications
in
Satellite
Several international organizations provide coordination of satellite procurement and
operation. The 2 largest are INTELSAT and INMARSAT.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
7.1
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
27
INTELSAT
Set up it 1964 by 11 participating countries. Now there are ≈ 120 member countries International Telecommunications Satellite consortium.
The current INTELSAT network involves ≈ 12 satellites plus over 600 ground stations.
There are classes of ground station (A,B,C,D,E) which are closely defined. A standard A
station must have a G/T value of 40.7dBK −1 which operating at 4/6GHz implies a large
30m diameter dish.
INTELSAT provides the satellites and the specifications. It is upto the users in the
individual countries to provide the ground equipment.
The ownership is proportional to the use of the system. Capital investment is provided
by the member countries who receive a guaranteed 14% return from income raised by
charging fees to telecommunications companies that use the system.
INTELSAT V was built by FORD AEROSPACE (with Marconi as a sub-contractor).
First launched in 1980. Original contract was for 7 satellites. Eventually 15 were ordered
and 13 are now in orbit. 2 were destroyed on launch - Atlas/Ariane. The satellites are
3 axis stabilised. A total of 440 transponders launched. Only 1 has stopped working.
Design lifetime of 2 years for these parts. Said to have cost INTELSAT $650 million.
$650 million is made by INTELSAT each year by charging the ’phone companies that use
it.
INTELSAT VI 1986. Designed for high volume. Large spinners. Total of 5 satellites
operating over the Atlantic.
INTELSAT VII to be launched 1992. FORD AEROSPACE won contract in 1988. Initially
5 satellites.
7.2
INMARSAT
The International Maritime Satellite Organization formed in 1979. Has 2000 Earth
stations on ships, 30 coastal stations and 6 spacecraft. The headquarters is in London.
7.3
ESA Programme
Telecommunications research plus some user services.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
28
OTS - Orbital Test Satellite
ECS1-4 provides TV and telephone services to Europe through EUTELSAT. European
Organization for Telecommunications Satellites formed in 1977, given permanent status in
1985 by intergovernmental agreement between 26 member countries. It is responsible for
the design, contruction, launch, operation and maintenance of European regional satellite
systems. The first ECS satellite was built by ESA in 1983. Launches on Ariane:
F1
F2
F3
F4
F5
7.4
June 1983
August 1984
September 1985 failed
September 1987
July 1988
OLYMPUS
According to the glossy brochure:
The OLYMPUS class of communications satellites is the most powerful currently under
construction in the Western World.
L-SAT was launched by Ariane July 1989. Provides direct broadcasting plus experimental
video-teleconference facilities and propogation research. Trying out the new services at
11/14GHz and 20/30Ghz. Problems - 1 of the solar panels failed so working on reduced
power.
8
LEO Networks
A new segment of satellite communications is under development. An example is the
Gobalstar system.
The idea is to combine the technology of cellular telephones with a LEO network of
satellites to provide a flexible point-to-point global communications system.
Features of the GLOBALSTAR system are:
• Uses 48 LEO satellites in circular orbits of an altitude 1389 km.
• 8 orbital planes at an inclination of 52 degrees.
University of Leicester
Department of Physics and Astronomy
Lecture Notes
Communication and Navigation Satellites
Course:
Issue:
Date:
Page:
1604
1.0
April 7, 2012
29
• Each plane contains 6 satellites.
• Each satellite can handle > 2800 duplex (2-way) voice or data channels.
• > 104, 000 simultaneous users world wide.
• The user set is low power because LEO (> 900 less than required for GEO). < 1
Watts transmitted power.
• Each satellite stays in range for 10 to 12 minutes. A ”soft handoff” transfers calls
to the following satellite.
• Each subscriber can see at least 2 satellites simultaneously.
The band allocation is:
satellite to user
L or S
user to satellite
L
satellite to gateway C
gateway to satellite C
The following bar chart gives the present expected time scale: