51.3 Spacecraft technology

51.3.1 Orbits

By far the most useful orbit for communication satellites is the geostationary satellite orbit. This is a direct equatorial orbit about 35800km above the ground, the period of which is the same as the length of the sidereal day, about 23 hours 56 minutes. A satellite in this orbit, moving in the same direction as the Earth's rotation, remains stationary as seen from points on the Earth's surface. A geostationary satellite has line of sight coverage of a great area of the Earth and, as Clarke noted in 1945, three of them suitably located around the Equator could cover almost all the Earth's surface. Figure 51.1 shows, for example, the coverage provided by satellites located at 30°W, 150°W and 90°E longitude.

There are, however, other orbits of interest for satellite communi­cation. The USSR, with a territory which has an exceptionally wide span in longitude at high latitudes, found 12-hour elliptical orbits inclined at about 63° to the equatorial plane to be preferable for its domestic ORBITA network. These satellites are operated for periods of about eight hours when they are close to their apogee, about 40(X)0km high above Siberia, but three satellites are required to provide continuous coverage.

Satellites in geostationary and 12-hour elliptical orbits are at great distances from the surface of the Earth and the transmission loss is very high; at 1.6GHz, for example, the free space loss is about 188dB. This loss has particularly serious consequences for satellite communication with mobile earth stations and in particular for hand portable and vehicle mounted stations, which typically have little antenna gain. Links with satellites in low orbits would have signifi­cantly less loss; for example, for an orbit only 1500km above the ground the loss would be about 28dB less, which would make a great difference to the weight of batteries required in a pocket radio telephone. For reasons such as these, low orbits are being seriously considered for this application.

51.3.2 Launchers and launching

The classic procedure for launching a geostationary satellite falls into three phases. In the first phase (boost phase) a powerful two stage liquid propellant rocket (the booster) places the satellite and the necessary upstage rockets in an orbit about 200km above the ground. This orbit, called the 'parking' orbit, has a period of about 100 minutes, it is direct (that is, the satellite revolves in the direction in which the Earth rotates), it is approximately circular and the plane of the orbit is inclined to the plane of the equator unless it happens that the launching site is located on the equator. The booster will have been jettisoned before this parking orbit is attained and the second stage rocket may be detached in the parking orbit.

In the second phase, as the satellite is passing through the equa­torial plane, another rocket, called the 'perigee motor', is fired to accelerate the satellite out of the parking orbit and into the elliptical 'transfer' orbit. This orbit has a period of about 11 hours, and the same inclination to the equatorial plane as the parking orbit. The perigee motor casing is then discarded. Close to its apogee and about 35800km above the ground, this transfer orbit passes through the equatorial plane.

In the third phase, at a time when the satellite is close to its apogee and about to pass through the equatorial plane, the firing of the final stage rocket, called the 'apogee motor', accelerates the satellite to a velocity of about 3kp/s and removes the inclination of its orbit. This change of velocity and direction causes the orbit to become roughly circular and equatorial and its period becomes roughly equal to one sidereal day; it will, in fact, be approximately geostationary. Low-energy thrusters incorporated in the satellite itself, typically fuelled with hydrazine, are then used to move the satellite to the point in longitude at which it is to operate and to correct any remaining errors in circularity, period or orbital inclination.

The United States Space Transportation System (Space Shuttle) enables the first phase of launching to be carried out by reusable rockets, although this facility has not been available for commercial launching since 1986. Other reusable launch and upper stage sys­tems are under development. However, all other launch vehicles in current use are expendable.

The classic procedure described above can be varied to take advantage of the capabilities of particular launch vehicles. 'Strap-on' solid propellant rockets are usually added to the booster to increase the payload mass which it can launch. The second stage rocket may also perform the functions of the perigee motor, allow­ing the first and second phases of the launch procedure to be fused into one phase. A single liquid propellant rocket may combine the functions of apogee motor and perigee motor. This, in fact, is the procedure that is most commonly used today; it is illustrated in Figures 51.2 and 51.3. Despite such variants, however, the prin­ciples of the classic procedure are generally applicable for all geostationary launchings and may also be used, with appropriate changes, for launching satellites which are not to be geostationary.

Launching facilities are available from a number of providers, government and commercial, in USA, USSR, France, Japan and China. The mass which these launch vehicle systems can deliver into, for example, the geostationary satellite orbit ranges from 350kg to 4000kg, and the more powerful systems can also be used to deliver several light weight satellites into the same, or similar, orbits. A comparison between the payload capabilities of major communication satellite launchers is shown in Table 51.5.

Figure 51.1 The concept of a world-wide geostationary satellite system

Exercise 1: Terms to know

Line-of-sight terrain	-(земная) поверхность в пределах прямой видимости
Slot	-временной интервал, щелевая антенна, канал (в системах пакетной коммутации)
Assigned slot	-выделенный (временной) канал
Channel slot	-канальный (временной) интервал; канал
Busy slot	-занятый (временной) интервал
Empty slot (free time slot)	-свободный (незанятый) временной интервал свободный канал
Time slot	-временной интервал
Digit time slot	-временной интервал для передачи символов
Frequency slot	-частотный интервал
Feeder=feedline	-фидер, питающая линия, линия передачи
Feeding	-питание, подача (энергии),загрузка
Backward feeding	-противоточное питание
Forward feeding	-прямоточное питание
Feedforward	-прямая связь
Trunk feeder	-магистральный фидер
Feed horn	-рупорный облучатель
Corrugated feed horn	-гофрированный рупорный облучатель
Fan-beam feed horn	-рупорный облучатель с веерной диаграммой направленности
Frequency(F.)	-частота; повторяемость
Allocated F.	-выделенная (отведенная) частота
Alternate F.	-резервная частота
Assigned F. (authorized F.)	-присвоенная частота (станции)
Base-F.	-опорная частота
Basic-F.	-основная частота
Boundary F.	-граничная (предельная) частота
Broadcast F.	-частота радиовещательного диапазона
Calibrating(calibration)F	-эталонная частота
Burst F.	-частота сигнала цветовой синхронизации
Call-back F.	-частота (контрольных) запросов
Calling F.	-вызывная частота
Carrier F.	-несущая частота; частота несущей
Clock F.	-тактовая частота; частота синхронизации
Crossover F.	-частота разделения (каналов)
Critical(cutoff)F.	-граничная (предельная) частота
Day F.	-дневная частота; частота для дневной радиосвязи
Dedicated F.	-выделенная (отведенная) частота
Doppler(-beat) F.	-доплеровская частота
Allotment	-распределение, выделение

Exercise 2: Answer the following questions:

1. What systems are the bands given in Table51.1 allocated for ?

2. Are there any other bands allocated for the fixed satellite service ?

3. Where are the bands shown in Table51.2 used ?

4. What can you tell about frequency bands allocated for down-links from broadcasting satellites ?

5. How many frequency bands are allocated for feeder links used in broadcast satellite services ?

6. Whom do satellite systems share the frequency spectrum with ?

7. Why have constraints on frequency assignments been agreed ?

8. What were these constraints placed on in the frequency bands at 12GHz,14.5GHz, 18GHz,at 4.5, 7.0, 10.8, 11.3 and 13GHz ?

9. What does a regulation RR2613 require ?

10. Why have upper limits been placed on the spectral power flux density, which satellites regardless of orbit, may set up… ?

11. How can the problem of interference between earth stations operating with satellites and the transmitting and receiving stations of terrestrial radio systems be solved ?

12. What do you know about the most useful orbit for communication satellites?

13. Are there any other orbits of interest for satellite communication ?

14. How many phases does the classic procedure for launching a geostationary satellite fall into ?

15. Is the classic procedure always the same ?

Exercise 3: Give the Russian equivalents:

1. the possibilities of artificial satellites

2. terrestrial systems for long distance communication and broadcasting

3. to come into being возникнуть

4. to use earth stations at fixed locations

5. newly emerging high power broadcasting satellites

6. the users’ premises

7. an orbital slot allotment plan

8. partly or wholly

9. it is already evident

10. to keep the interference down to acceptable levels

11. assignments are registered internationally

12. to present a persistent interference hazard for geostationary satellites

13. frequency coordination

14. to keep within acceptable limits

15. the spectral power flux density

16. frequency assignments

17. the length of the sidereal day

18. the free space loss

19. to have an exceptionally wide span in longitude in high latitudes

20. the booster will have been jettisoned before

21. the become roughly circular and equatorial