University of Leicester Department of Physics and Astronomy

University of LeicesterDepartment of Physics and Astronomy

Lecture NotesCommunication 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 LeicesterDepartment of Physics and AstronomyLecture NotesCommunication and Navigation Satellites

Course: 1604Issue: 1.0Date: April 7, 2012Page: 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 CommunicationLinks 23



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 28

1 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 communicationssatellite. ”Extraterrestial Relays”, Wireless World 51, 305-308, October 1945.

Simple idea; place a satellite in circular orbit above the equator at a radius of ≈ 42000kmwhich gives an orbital period of 1 day, the same as the rotation period of the Earth. Insuch an orbit the satellite remains above the same point on the Earth’s surface. Thesatellite could receive and relay signals.



In principle 3 satellites spaced at 120◦ around the equator could service the whole globeprovided messages could be sent between satellites as well as from ground to satellite.Continuous, reliable communication could be provided between any 2 points on theEarth’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 aroutine 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 communicationsbetween ships and platforms.

The expansion of the market has been remarkable. There is now congestion ingeosynchronous 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 transmissionto 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 stationon eastern seaboard of USA. (Note can’t hop directly to California).

• Transmission over landlines after frequency division and multiplexing to mainexchange in L.A.



• 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 thanthe landline delay. Thus in the above there is a 0.5 second delay between asking a questionand getting an answer. Therefore live conversations don’t use links involving more than1 satellite. UK to Australia uses just 1 satellite over the Indian Ocean. This is not aproblem 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 timetaken for the Earth to complete 360◦ rotation.

Torb = 23hr56mins which is 1 sidereal day.

From Kepler’s 3rd law for elliptical orbits:

Torb = 2π√

a3/GMe where GMe = 4× 105 km3s−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 theinclination of the orbit, the angle between the Earth’s equatorial plane and the orbitplane at the ascending node.

The ground track or sub-satellite path is the locus of points at which the satellite is directlyoverhead during the orbit.



For an inclined geosynchronous orbit the ground track is a figure-of-eight. The centre pointis 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 inclinationand 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 havei < 5◦.

3 Launching Geostationary Satellites

To place a satellite in a GEO requires an acceleration to a velocity of ≈ 3050m/s in a zeroinclination (equatorial) orbit and lifting it ≈ 42000km above the Earth’s surface. Thereare 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 ellipticalorbit 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



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 atthe ascending (or descending) node when the inclined orbit crosses the equatorial plane.The velocity kick required is given by vector addition:

∆v =√

v21 + v2

2 − 2v1v2 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 sometimesperformed during powered flight of the main rocket. It is advantageous to apply ∆vwhen 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 satellitesystem is then deployed. Injection into a GTO (transfer orbit) is then performed using aperigee 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 (moreoften) a specific additional stage using either liquid or solid propellant is added. Somesystems provide both the perigee and apogee ∆vs.



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 inprinciple. They are carried with their axes perpendicular to the longitudinal axis of theshuttle. Can lift 1250kg into GTO.

The inertial upper stage (IUS). Actually contains 2 motors so can perform the AKM burnas well. 2 stage solid propellant system. Can lift 2270kg into GEO. It is carried with axisparallel to shuttle longitudinal axis and is rotated by 60◦ for deployment by springs. TheIUS 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 manoeuvresare required to ensure that the motors are pointing the right way and that the solar panelsand 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 groundstations.

3.5 The Drift Orbit

The final orbit achieved initially is unlikely to be exactly right with e = 0 and i = 0. Thiscauses the satellite to drift about its nominal geostationary position. The satellite mustuse low thrust on-board rockets to perform station keeping manoeuvres. At the same timethese motors are also used to tweek the satellite to the correct longitude position requiredaround the Equator.

The following are performed in the drift orbit:

• Sun aquisition



• 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 satelliteinto the correct geostationary orbit and the system would then remain fixed in attitudeand position for use as a communications link. Life, of course, is not so simple becausethere are several perturbing influences that cause the satellite to drift.

• North-South drift - changing inclination of the orbit. This is the result of thedeparture of the local gravitaional field from a central source field due to the pull ofthe Sun and the Moon. Lunisolar perturbations.

• East-West drift - changing longitude of the orbit. This is due to varations in theEarth’s gravitational field (inhomogeneity and departures from spherical symmetryof 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 performingstation keeping manoeuvres at frequent intervals. 1 manoeuvre per 3 months to 3 yearsdepending on how much drift can be tolerated.

The East-West drift can be controlled by the tennis ball approach. This requires about 1manoeuvre 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?)



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 Azimuthand Elevation.

Azimuth is the angle around the horizon from North measured eastwards. Ranges from0◦ 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 AutumnalEquinoxes 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 totalof 44 nights per year in March-April, September-October.

Most communication satellites carry batteries so that they can continue operating duringeclipse.

More serious is the passage of the satellite in front of the Sun. During these timescommunication is blocked by the very large radio noise output from the Sun. This lastsabout 10 mins and occurs on 5 consecutive days twice a year. The only way to get aroundthis 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



4.1 AOCS

The attitude control must be precise enough such that the narrow beam communicationssub-systems are pointed correctly at the Earth. Requirement can range from 17.5◦ to0.5◦.

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 forspin-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 orthogonalaxes. 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 toprovide the appropriate ∆v to tweek the orbital parameters.

The jets use either a single propellent which is ignited by catalyst or heating, propaneor hydrazine (N2H4) are common, or a bi-propellent propulsion system which requires 2gases 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 Earthstation. The tasks are:

• attitude and orbit control - by command

• monitoring status of all the sub-systems - by telemetry



• 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 ratecommunications link (UHF) - high signal power - low error rate.

For safety might use an omni-directional system to avoid loss of signal if the AOCSmalfunctions. INTELSAT uses horns with full Earth coverage once attitude is established.

The command structure must possess numerous safeguards to prevent accidentalcommanding 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. Cellsalso 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 sothe body must be large. Only 1/3 of the area faces the sun at any time. The surfacetemperature is 20− 30◦C.

On a 3-axis stabilized craft the solar panels are deployed like sails and entire area facesthe Sun at once. However they tend to run rather hot, 50 − 80◦C, and this reduces theefficiency.



Batteries are required to provide power during eclipse. TV broadcast satellites require toomuch power for battery operation. They are usually sited 20◦ West of service longitudeso 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 relayingof voice, video and data streams. All other sub-systems exist solely to support thecommunications sub-system. However the latter represents only a small fraction of thevolume, mass and cost of the complete package.

Modes of radio propogation:

• Line-of-sight systems. e.g. microwave links using dishes and towers. Curvature ofthe Earth limits the distance. A 60m tower gives 60km line of sight. e.g. Gas Boardin Regent Road.

• Surface or ground wave propogation. The radio wave travels along the Earth’ssurface as a result of currents flowing in the ground. This is the dominant mechanismat 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 withheight. The return wave can in turn be reflected back up again. The gap betweenthe ionosphere and the ground acts as a waveguide.

• Tropospheric scattering. Radio waves are scattered from small particles in the loweratmosphere to provide over the horizon communications.

Satellite communication is an extreme example of line-of-sight radio links. One tower isof height 35600km!

Radio waves propogating in free space diminish in power as 1/r2 so after 36000km theyare very weak. Typically the received power is < 10−12W . Compared with a normalground 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 theground, amplify it, change its frequency and retransmit it to the ground.

The change of frequency between Rx and Tx is absoluely essential because otherwise theup-link signal would be completely jammed by the relatively powerful down-link signal.



Satellite communication systems use UHF or SHF(microwaves). This ensures that theypenitrate 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 centralcarrier 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 witha much larger bandwidth of 3500 MHz. The larger the bandwidth the greater the trafficand the greater the commercial return.

4.5 Transponders

Most satellites have many transponders. The bandwidth they handle differs from onesatellite to another but typically it is ≈ 36MHz. One such transponder can handle oneof the following:

• one colour TV channel + sound

• 1200 voice channels

• a data rate of ≈ 50Mbits/sec

To utilize the full 500MHz bandwidth for a 6/4GHz link a satellite might use 12transponsers at 40MHz steps. Most systems include redundant items in case of failure ofparticular channels.



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 singlemixing with a local signal, 2225MHz say, converts from the up to the down frequency. Ina double conversion system the up-link is mixed down to an IF, 1GHz say, amplified andthen mixed up to the down-link frequency.

Mixing the signals involved multiplying and then filtering:

x1 = a1 cos ω1t and x2 = a2 cos ω2t

x1x2 = (a1a2/2)(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 orTx 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 whereomnidirectional coverage is important since commands must be received when thespacecraft is in an anomalous pointing.

However for the communications payload, where bandwidth and signal strength areimportant, directionality of the antenna is important.

To obtain global coverage requires a beam width of 17◦ but spot beams of ≤ 5◦ may beappropriate 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 sameradiated 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



where λ is the wavelength and η is the aperture efficiency. This will dealt with in the 2ndyear EM-Optics course.

Thus if 2m diameter reflector is used at 4GHz and η = 0.5 the gain over an isotropicantenna is:

G = 0.5× 4π2/0.0752 = 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 anddimensions of the aperture. The width of the beam is usually described using the −3dBpoints 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 pointfrom 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 signalas possible. The collected power is ∝ the area of the antenna so we must make the dishas large as possible.

An antenna has the same beam response on reception as transmission. So a large, highgain 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 highradiated power over a reasonably wide beam. For a narrow beam system such a horn isused to feed a reflector dish which produces a much narrower beam.



Array of horns can be used for different directions, frequencies and polarizations withina single dish reflector.

5 The Design of a Satellite Communications Link

A communications system must be designed to meet certain minimum performancecriteria. For example it must be decided how much Tx power is required and how largethe antennae must be.

One important criterion is that adequate S/N ratio be maintained in the communicationschannel.

A first step is to consider the link power budget.

Consider a transmitter radiating power Pt (Watts) isotropically. At a distance r the fluxdensity will have dropped to:

F (r) = Pt/(4πr2) Wm−2

i.e. the original power is spread over an area of 4πr2.

If the transmitter antenna has a gain Gt then:

F (r) = PtGt/(4πr2)

assuming we are at θ = 0, the centre of the beam. This signal is collected by an antennaof area Ar m2. However reflection losses at the face of the dish and absorption in lossycomponents mean we should use an effective area Ae = ηAr. Therefore the power receivedis:

Pr = FAe = PtGtAe/(4πr2)

Previously we noted that the gain of the antenna is:

Gr = 4πηAr/λ2 = 4πAe/λ

2

Therefore we get the so called Friis transmission equation for the power received:

Pr = PtGtGr(λ/4πr)2

PtGt is the effective isotropic radiated power, EIRP



(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 productbecomes 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) +33.0 dBWEarth antenna (30m) +62.5 dB

EIRP +95.5 dBW

Lp -199.0 dBLa (rain) -3.0 dBSpot beam (2m) +39.0 dB

1.8× 10−7W -67.6 dBW



Down-link 4GHz

Tx power (6.3W) +8.0 dBWSpot beam +35.5 dB

EIRP 43.5 dBW

Lp -195.5 dBLa (rain) -3.0 dBEarth dish (30m) +59.0 dB

2.5× 10−10W -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 systemsare characterised by very large signal losses due to the massive distance between the Txand Rx. However the very small signals can be amplified to compensate for the low signallevel. 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 torandom noise voltages on the output. For example a common type of noise is thermalnoise 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 noisetemperature (ENT) at the input. The system is modelled as an ideal noiseless amplifierwith a source of noise strapped to the input. The noise power at the input is:

Pn = kTrB where



k is Boltazmann’s constant, 1.38× 10−23J/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 thecritical items are at the front-end, the low noise amplifier and mixer. Any noise generatedthere is likely to be large compared with the signal. Typical values are:

mixer/low noise block 700Klow noise solid state amplifier 150Kcooled parametric amplifier 35Kmaser 10K

In the real world there is also a contribution to the noise from the sky, radiation receivedby 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 theantenna temperature.

The total input noise is therefore:

Pn = k(Tr + Ta)B = kTsysB

where Tsys is the system temperature.

It is against this total noise in the receiver system that we must measure the signal powerreceived at the antenna.



The received carrier signal-to-noise is:

Pr

Pn

=Pr

kTsysB

In our example of the down-link power budget we had a received power of −97dBW , froma 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× 10−14W

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 communicationslink the carrier S/N ratio (C/N) must remain above a threshold which depends on thetype 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 easilymeasured 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 reducethe 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 (1700kmacross).

We require θ3dB = 2.5◦ therefore D = 0.75m (note the high carrier frequency).



Tx power +23.0 dBWTx antenna gain +36.5 dB

EIRP (1 channel) +59.5 dBW

atmospheric loss (clear) +00.0 dBpath loss -205.0 dBreceiving dish +36.5 dB

carrier (best case) -109.0 dBW

station at edge of zone -3.0 dBlosses in Rx before LNA -1.0 dBpointing error -1.0 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 toimprove the Pr/Pn. Of course this limits the information transfer rate.



5.5 The Earth Station Figure of Merit

The effectiveness of the Earth station for reception depends on the collecting power ofthe dish antenna and the noise seen from the local sky or introduced by the receiver. Wecan rewrite the Friis equation in terms of the C/N ratio and group the terms:

C/N =EIRP × Lp × La

kB× Gr

Tsys

The left hand fraction contains constants of the satellite system while the righthandfraction is a function of the Earth station. The ratio Gr/Tsys is sometimes called thefigure 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 definedby:

NF =(S/N)in

(S/N)out

The noise temperature is more useful in satellite communication systems and it is best toconvert from NF to Tsys using:

Tsys = To(NF − 1) where To is a reference temperature, usually 290K.

6 Modulation and Multiplexing Techniques for

Satellite Communication Links

Satellite communications links are usually wide-bandwidth. They have the capacityfor relaying multiple, independent communications signals. e.g. many telephoneconversations.

The method employed to ensure that the different channels don’t mutually interfere isrefered to as a multiplexing scheme.

The most common forms of multiplexing in current use are:



• frequency division multiplexing - FDM

• time division multiplexing - TDM

In FDM a particular communications channel uses a particular band of frequencies withinthe 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 majorityof video signals.

• TDM is used for the transmission of digital format, digital telephony, numericaldata.

The rigorous explanantion of these techniques is beyond this course. We shall just touchon the general ideas.

6.1 Frequency Division Multiplexing

Consider multiplexing a number of voice channels. The signal from the microphone (inthe 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 usefulinformation is represented by a narrow frequency band. The important range is 300 to3400Hz. 4kHz is required for each voice channel.

To send the signal using a much higher frequency radio carrier we must modulate someproperty of the carrier in sympathy with the signal amplitude.

For a typical FDM system amplitude modulation is used several times on a sinusoidalcarrier and then frequency modulation is used to load the composite message signal ontothe radio link carrier.

Amplitude modulation is effected by multiplication of the voice signal by a carrier. Eachfrequency in the voice signal is shifted in frequency:

a1 cos ωvt× a2 cos ωct = a1a2(cos(ωc + ωv)t + cos(ωc − ωv)t)



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, eachchannel is allocated a frequency band within the carrier. The typical heirarchy used is asfollows:

1 voice channel 4kHzgroup (12 channels) 48kHzsupergroup (5 groups) 240kHzmaster group (5 super) 1.2MHzsuper master group (900 channels) 3.6MHz

The radio carrier is typically 6GHz. This is frequency modulated by the super mastergroup. 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). Thebandwidth of νi is given by:

B = 2νd + 2W

where νd is a constant and W is the bandwidth of the message signal. FM transmissionis not spectrally efficient (it uses a large bandwidth) but it is immune to noise andinterference.

At the other end of the link (i) the message signal is extracted from the carrier bydemodulation of the FM and (ii) the voice signals are extracted by demodulation ofthe AM.

SSB-SC signals are demodulated by multiplying by a replica carrier.

a1a2 cos(ωc + ωv)t× ar cos(ωct + φ) = cos(ωvt− φ) + cos(2ωct + ωvt + φ)

φ is the phase of the replica wrt to the carrier. SSB-SC demodulation is insensitive tothis phase.

6.2 Time Division Multiplexing

Each communications channel makes use of the entire bandwidth of carrier for a briefperiod of time.



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 CodeModulation, PCM.

The input signal is sampled at regular time intervals and the sampled voltage is given abinary code in the range 0 → 2N − 1 where N is the number of bits/sample. Thus theanalog signal is converted to a series of pulses. To sample a 4kHz bandwidth a samplerate of 8kHz is required (this is called the Nyquist rate). If 8 bits are used per sample theoutput digital sequence runs at 64kbits/second.

The next step is to combine many channels using a Time Division Multiplexer. A typicalPCM hierarchy is:

level channels rate Mbits/sec1 30 22 120 83 480 344 1920 140

Note that at the higher levels extra bits are added for housekeeping and error checkingetc..

Finally the very high bit rate digital signal is modulated onto a carrier signal using oneof 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 usedto pick out the individual channels. Finally the a digital to analogue converter is used toreconstruct the analogue speech signal.

7 International Programmes in Satellite

Communications

Several international organizations provide coordination of satellite procurement andoperation. The 2 largest are INTELSAT and INMARSAT.



7.1 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 Astation must have a G/T value of 40.7dBK−1 which operating at 4/6GHz implies a large30m diameter dish.

INTELSAT provides the satellites and the specifications. It is upto the users in theindividual countries to provide the ground equipment.

The ownership is proportional to the use of the system. Capital investment is providedby the member countries who receive a guaranteed 14% return from income raised bycharging 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 orderedand 13 are now in orbit. 2 were destroyed on launch - Atlas/Ariane. The satellites are3 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 useit.

INTELSAT VI 1986. Designed for high volume. Large spinners. Total of 5 satellitesoperating over the Atlantic.

INTELSAT VII to be launched 1992. FORD AEROSPACE won contract in 1988. Initially5 satellites.

7.2 INMARSAT

The International Maritime Satellite Organization formed in 1979. Has 2000 Earthstations on ships, 30 coastal stations and 6 spacecraft. The headquarters is in London.

7.3 ESA Programme

Telecommunications research plus some user services.



OTS - Orbital Test Satellite

ECS1-4 provides TV and telephone services to Europe through EUTELSAT. EuropeanOrganization for Telecommunications Satellites formed in 1977, given permanent status in1985 by intergovernmental agreement between 26 member countries. It is responsible forthe design, contruction, launch, operation and maintenance of European regional satellitesystems. The first ECS satellite was built by ESA in 1983. Launches on Ariane:

F1 June 1983F2 August 1984F3 September 1985 failedF4 September 1987F5 July 1988

7.4 OLYMPUS

According to the glossy brochure:

The OLYMPUS class of communications satellites is the most powerful currently underconstruction in the Western World.

L-SAT was launched by Ariane July 1989. Provides direct broadcasting plus experimentalvideo-teleconference facilities and propogation research. Trying out the new services at11/14GHz and 20/30Ghz. Problems - 1 of the solar panels failed so working on reducedpower.

8 LEO Networks

A new segment of satellite communications is under development. An example is theGobalstar system.

The idea is to combine the technology of cellular telephones with a LEO network ofsatellites 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.



• 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). < 1Watts transmitted power.

• Each satellite stays in range for 10 to 12 minutes. A ”soft handoff” transfers callsto the following satellite.

• Each subscriber can see at least 2 satellites simultaneously.

The band allocation is:

satellite to user L or Suser to satellite Lsatellite to gateway Cgateway to satellite C

The following bar chart gives the present expected time scale:

Documents

University of Leicester Department of Physics and Astronomy