effective utilization of the geostationary orbit for satellite communication

308 PROCEEDINGS OF THE IEEE, VOL. 65, NO. 3, MARCH 1977

Effective Utilization of the Geostationary Orbit for Satellite Communication

DAVID J. WITHERS

Abstmct-The geostati~ury satellite orbit has a rite capacity for communication satellites operating in the frequency spectrum available, but saturation could be delayed for a long time if various practices and principles of system design and use were agreed internationally and applied. Three broad areas are identified where action is needed, namely system en-g for an interference-limited environment, intersystem coodination of the use of orbit and the spectrum, and the ordering of the use of orbit and spectrum 80 that systems with radkdly different characteristics do not interfere. In each of the thtee areaa, the main factors of concern are identified, the benefits that would be achieved are deduced in broad terms, and the practical and economic feasiiility of actually securing the benefit is reviewed for each factor.

I. INTRODUCTION HE NUMBER of communication satellites in service has increased considerably in the past 2 or 3 years and the upward trend is accelerating [ 11 . Most of these satellites

are geostationary. In some parts of the geostationary orbit it is already necessary to pay careful attention to intersatellite interference problems when choosing the location for a new satellite using the 4- and 6GHz bands. The time is fast approaching when new geostationary satellites using these bands will be constrained as to their location and technical parameters if they are not to cause unacceptable interference to satellites already in service.

Extensive additional bandwidth is already allocated to satellite services in higher frequency bands but it is little used at present. Radio propagation in these higher bands is more dependent upon meteorological conditions than at 4 and 6 CHz and currently, hardware is in a less advanced state of readiness. However, the higher frequency bands also have advantages when compared with 4 and 6 GHz, making them acceptable or even preferable for some applications. These bands wiU come into use on their own merits or as a consequence of congestion in the lower frequency bands and eventually the interference problem will appear there also.

Ultimately, it can be foreseen that there will be no room for further general growth of satellite communications services. When that happens, it will be necessary to decide which satellite services should be given priority. We can perhaps foresee that priority will be given to services for ships, aircraft, and isolated communities on land and to various broadcast, quasi- broadcast, and multi-terminal services for which the satellite medium is particularly well-suited. Heavy-traffic point-to-point services may be progressively squeezed out of the more desirable frequency bands and the geostationary orbit, and perhaps ultimately out of space radio altogether into terrestrial transmission media. However, this problem should surely not become acute for several decades if the space medium is used wisely.

A great deal can be done, by both technical and administrative means, t o optimise the use made of the geostationary sat-

Manuscript received July 12, 1976;revised September 28, 1976.

communications Development Department, Post Office, London ECl , The author is with the Space Communication Systems Division, Tele-

England.

ellite orbit. The limitation of interference between satellite networks is by its nature an international problem and the International Telecommunication Union (ITU) has played an important role, originally at the Extraordinary Administrative Radio Conference - Space Radiocommunication in 1963 and more emphatically at the World Administrative Radio Confer- ence (WARC) in 1971, in dealing with it. Throughout this period CCIR Study Group 4 (FixedSatellite Services) has been working on this problem, and more recently Study Groups 8 (Mobile Services), 10 and 11 (Sound and television broadcasting). A WARC with wide terms of reference is planned for 1979 at which it can be foreseen that the ITU provisions for regulating the use of the geostationary orbit will be revised to incorporate the results of recent experience and studies. The approach of this latter conference, which may become a turning-point in the international management of orbit utilisation, makes the present a good time to review the problems and outline some possible solutions.

There are various ways of stretching the satellite communication medium beyond the capacity of the geostationary orbit used in conventional ways.

1) Satellites in phased inclined-circular mediumaltitude orbits (typically 20 000 km above the earth’s surface)[21, inclined-circular geosychronous orbits [ 31 ,or inclined-elliptical subsynchronous orbits [ 41 could operate without encroaching upon that unique resource, the geostationary orbit. In general, costs and operational complexity would be greater than for geostationary systems, but some of these orbits also have advantages which offset the disadvantages to some degree and for some applications.

2) Intersatellite links, operating at the higher millimeter- wave frequencies or even in the infrared, could be used to enable the microwave traffic links between earth and space to be used to the greatest advantage by concentrating traffic for an earth station, which might otherwise be distributed between several satellites, into a single high-capacity high-efficiency link. Intersatellite links might also relax the geographical constraints on satellite positioning and increase the operating angle of elevation at earth stations, enabling parts of the orbit and spectrum to be used that might otherwise be unsuitable for serving all the earth stations in any one wide-coverage network.

3) Earth-space frequency bands could be used in both directions of transmission, Satellite A receiving from earth in the frequency band that its near neighbor, Satellite B, is transmitting to the earth and vice versa. Given accurate satellite station- keeping, reasonable satellite antenna sidelobe control and well- separated earth stations, the interference between the networks using Satellites A and B need not be significant and the total potential traffic capacity of the orbit could therefore be doubled.

However, the basic problem is to make the best use of geostationary satellites, used conventionally, and it is this problem that the rest of this paper addresses.

For some services it may be possible to draw up a carefully optimized plan for the utilisation of the orbit and specified

WITHERS: UTILIZATION OF GEOSTATIONARY ORBIT 309

frequency bands. This may be possible, for example, for satellite broadcasting [ 51, [ 61. A similar situation may arise in the maritime and aeronautical mobile services via satellite. However, the wide variety of satellite point-to-point services that can be expected to jostle for the use of the spectrum and the speed and unpredictability of advances in technology will prevent efficient large-scale long-term planning of the fixedsatellite frequency bands. This case demands a continuous process of ad hoc adjustment of short-term localized plans.

If the geostationary satellite orbit is to be used effectively for services for “fixed” earth stations, (that is, stations provid- ing permanent communication facilities) systems must be designed and used, not only so that the objectives of individual systems are met but also so that satellite networks using neigh- bouring positions in the orbit live harmoniously together without waste. Such harmony will have to be bought by careful orbit and spectrum planning, good system engineering and good operational control. The price may include some loss of network capacity and flexibility. Nevertheless, in considering what sacrifices may be required of individual systems for the good of all systems, we must not lose sight of the fact that international consent is unlikely to be secured if the cost t o individual systems is too high. It is necessary to seek an acceptable middle way between excessive restriction that would cramp development and self-destructive freedom.

Effective orbit utilization demands three things.

Engineering for an interference-limited environment. The basic characteristics of the equipment on the ground and in space and the techniques used for modulation and multiple access should be chosen to minimise the interference power level received from and injected into other networks. That having been done, it is desirable to increase the tolerability of the interference that does occur. These measures would increase the number of networks that can use the orbit, without increasing the number of networks with which each new network must be coordinated. Effective intersystem coordination. Efficient administrative machinery should be used, constructively, t o coordi- nate the use of the orbit and the spectrum by neighboring satellites. The avoidance of extreme inhomogeneity in orbit and spectrum sharing. The satellites of networks that differ greatly in spectral power density and radio channel bandwidth should be segregated from one another into different frequency bands or, where possible, into different parts of the orbit. When this rough sorting process has been completed, a further improvement in orbit loading might be achieved by the standardisation of key system parameters.

The factors arising from these three requirements will now be examined in more detail.

11. ENGINEERING FOR AN INTERFERENCE-LIMITED ENVIRONMENT

Almost all of the frequency bands used so far for satellite communication are shared with &me&ial he-of-sight radio relay systems. These radio relay systems are widely spread and heed has had to be paid to limiting interference between the two media. This has involved endless planning effort, but operational constraints have not been severe so far. The use of carrier energy dispersal and care in earth station siting has been enough to keep interference in check in most situations. The agreed limits on down-path power flux density from satellites

Fig. 1 . The worst satellite location situation for earth station 2 served

E , G, and H. by satellite F and receiving interference from neighboring satellites D ,

and of eirp for radio relay stations [7] have provided further protection against interference, but these limits have not set bounds on practical commercial systems until quite recently.

However, satellite system characteristics have been determined largely without regard to interference between satellite networks. Satellite antenna radiation patterns have been optimised, within payload mass limits, to provide maximum gain towards the earth stations served. Earth station antenna radiation characteristics have been chosen, within cost limits, to maximize the G / T ratio. The choice of techniques for modulation and multiple access has been a compromise between a low-cost earth/space interface and high per-transponder traffic capacity. No doubt these objectives will continue to occupy a prominent place in system designers’ minds. Nevertheless in the future it will no longer be possible to ignore the need to design also for limitation of interference between satellite networks.

A. Satellite Station-Keeping Solar radiation pressure and irregularities in the earths’ gravi-

tational field, acting upon a satellite which is nominally geostationary, cause the orbit of the satellite t o depart to some extent from the ideal. These orbital errors cause the satellite, as seen from the earth, to move to east or west off its nominal longitudinal station. The nominal spacing between satellites must exceed the minimum necessary spacing by a margin which allows for these stationkeeping errors. Inclination of the orbital plane also causes a daily east-west motion, but this is negligibly small for moderate angles of inclination.

Under favorable circumstances (i.e., with relatively homogeneous orbit utilization), and assuming perfect station-keeping, the minimum necessary east-west spacing between satellites serving the same region would typically be between 2’ and 5’, although spacings as great as IO’ might be necessary if the

station antennas were particularly small. The ITU radio regulations require a satellite to be kept within T 1.0’ of its nominal longitude, but a spacing margin big enough to guard against such large excursions involves a quite significant loss of useful orbit-space.

Fig. 1 shows Satellites D , E , F, G, and H , part of an array of similar satellites all of which illuminate an earth station at 2 served by Satellite F. Each satellite is assumed to move +ao

310 PROCEEDINGS OF THE IEEE, MARCH 1977

TABLE I

Required spacing, given perfect stationkeeping 2.0" 5 .O" 10.0"

Required nominal spacing, assuming * 1 .O" east-west excursion 2.85" 6.25" 11.25' Required nominal spacing, assuming f 0.1" east-west excursion 2.13" 5.11" 10.1"

Increase in orbit capacity given by a reduction of excursion from f 1.0" to i 0.1" 25% 22% 10%

about its nominal orbital position and the angular separation between the nominal locations is 8". (Here and elsewhere in the paper, presentation is simplified by treating angular rela- tionships in orbit as seen from the earth's surface as if they were the same as the corresponding angle seen from the center of the earth. The error involved is fairly small.) In the diagram these five satellites are shown to have drifted into the positions where the interference entering the sidelobes of the earth station antenna at 2 will be worst. Then, making the assumption, discussed below, that the gain of the sidelobe envelope of that earth station can be approximated by

32 - 25 loglo @ dBi

where 4 is the off-boresight angle, the total interference at 2 from satellites D, E , G, and His a function of

Interference from more distant similar satellites, that is further from F than D or H , will be small compared with these four main components. This expression can therefore be used to derive the relationship between the required nominal spacing 8 and the positional tolerance 6. Equating 6 to 0.1, chosen as a good but already attainable operational standard of station- keeping, provides a measure of the waste of orbit allowed by the present international agreement. This calculation has been done for various values of 8 and the results, shown in Table I , indicate that orbit loading efficiency could be increased, typically by over 20 percent, if east-west excursions were limited to f 0.1'. (This example is somewhat simplified because it does not take into account the statistics of satellite positions; nevertheless, it contains the substance of the problem.)

In recent years, several operational satellites in the ITTELSAT and TELESAT systems have been kept within 0.1 of their nominal longitude. This demands careful and relatively frequent adjustment of the orbit period, perhaps as often as once per week for satellites located at longitudes where the east-west component of the earth's gravitational field is strong. The cost of such activity at the control earth station may not be negligible but it is very small compared with the value of the extra orbital space it would release for use. For these spin-stabilized satellites, INTELSAT 111, INTELSAT IV, and ANIK, there is no other significant cost, since little more thruster fuel is required for frequent small adjustments of orbital period than for less frequent large adjustments. For body-stabilized satellites with deployed solar arrays, seasonal correction of orbital ellipticity caused by solar radiation pressure may be necessary if east-west movement from all causes is to be kept within f 0.1' and this will require a small expenditure of fuel.

Clearly, an improvement in required satellite east-west station-keeping to perhaps f0.1" would be an effective, feasible and economical way of making a substantial improvement in orbit utilization efficiency.

B. Satellite Antenna Radiation Characteristics Subject to the constraints of payload mass, attitude stability

and accuracy of station-keeping, the communications capacity of a satellite will usually be maximised by its designer by making the satellite antenna gain as high as is consistent with en- closing the whole of the service area by the -3 dB or 4 d B contour relative to the peak gain of the main lobe. It may be worthwhile to tailor the cross-section of the beam, departing from a simple circular or elliptical form in order to fit more accurately the projected shape of the service area. The mean gain within the service area may also be increased by modifying the typical sin2X/Xz distribution of power across the main lobe. Limitation of antenna response outside the service area is not a major factor in system design at present, except for satellites using spaced-beam frequency reuse, where the objec- tive is likely to be specifically to achieve an adequate carrier- to-interference ratio in each of the satellite's own service areas, not a general minimization of out+f-beam response.

Effective orbit utilization demands proper attention to satellite antenna response outside the service area and this demand will have to be imposed administratively. Various approaches to the problem are conceivable and the problem is to find one that will be feasible in practice and will substantially increase the practical capacity of the orbit without unduly restricting the development of technology or of new facilities. This will not be an easy combination to achieve and the approach indi- cated below is offered with some diffidence.

TWO typical interference situations arise from satellite antenna characteristics.

1) Two satellites may be well-separated in orbit but their service areas are close together and satellite antenna main- lobe overspill is more than the directivity of the earth- station antennas can control.

2) Two satellites are relatively close together in orbit but their service areas are well-separated and interference arises via the sidelobe response of the satellite antennas.

These two situations call for different treatment. It would be severely restrictive to require the satellite antenna

main-lobe response (case 1) to fall away rapidly in all directions outside the service area. Such a requirement might also limit undesirably the width of the orbital arc within which the satellite could be located and still fulfill its mission (that is, the "service arc"). Furthermore, the maximum gain slope that could reasonably be required in all directions, whether neces- s a r y to control interference or not, would be less than could be achieved in selected directions to control specific instances of foreseeable interference. An alternative would be to seek to achieve the best match between the operational needs of the networks serving a region and the available satellite antenna technology by negotiation between the owners of the systems involved, such negotiations forming part of the coordination process. This latter approach has the disadvantage that it would take no account of the requirements of networks that are not in service or planned at the time of coordination; nevertheless it seems more likely to produce good results that the application of rigid main-lobe gain-slope regulations. The guidance of an internationally approved radiation pattern for the main-lobe

WITHERS: UTILIZATION OF GEOSTATIONARY ORBIT 31 1

Fig. 2. Limitation of satellite antenna sidelobe radiation; suggested contours relative to the service area.

Y 0 4

' I m -10

UDIAL LOUTION

Fig. 3. Limitation of satellite antenna sidelobe radiation; suggested sidelobe gain limits at contours A , , A , , etc.

and the first one or two sidelobes might, nevertheless, exert a useful influence on the standards attained.

It is not likely to be feasible to use a coordination process to control interference via satellite antenna sidelobes (case 2); perhaps the most substantial source of difficulty would be the sheer number of parties that would be involved in such discus- sions when the orbit is approaching saturation. It seems, therefore, that the best solution would be to apply an internationally agreed limit t o satellite antenna gain (or related characteristics such as satellite receiving sensitivity and radiated power spectral density) in directions well removed from the m i c e area. The limit might take the following form.

Let a contour A0 be drawn outside the service area of a beam so that there is an angular clearance of J? between the contour and the nearest point in the service area as seen from the nominal location of the satellite. A family of three contours A to A3 might then be drawn, outside the first contour, each separated from its neighbours by as seen from the nominal satellite location ( F i g . 2). Then the antenna gain might be required to fall below +20 dB relative to isotropic at contour A . , and so linearly to 0 dBi at contour A 3 , the sidelobe peaks remaining below 0 dBi outside A3 in all directions intercepted

by the earth (Fig. 3). The value of L might be related to the size of the service area, allowing a generous margin within which the radiation pattern would be determined by system economics, coordination with networks serving nearby areas, and the provision of an adequate service arc. It is, however, important that the response should cut off rapidly outside contour A'. The limiting gain slope need not be determined by the size of the service area, since large service areas may be covered by overlapping beams generated by multiple feeds serving an antenna of large aperture. The value of 0 should therefore be related to the frequency and the largest satellite antenna aperture that might reasonably be used; possible values are 1.0' at 4 and 6 GHz, 0.4' at 11/12 and 14 GHz and 0.2' at 20 and 30 GHz.

C. Polarization There are prospects of improving the efficiency of orbit-

spectrum utilization by a factor approaching 2 by dual polarization operation. The technical problems of dual polarization are being explored in a number of experimental programmes involving ATS-6, INTELSAT IVA and the ESA Orbital Test Satellite (OTS) and its use is foreshadowed in the RCA and COMSTAR US domestic systems, INTELSAT V and ESA European Communication Satellite (ECS) systems. The benefit is most evidently realizable for networks, such as those men- tioned above, having a traffic capacity requirement that is larger than the bandwidth allocated in the frequency bands used (typically 500 MHz) can readily supply in the single polarization mode.

When the network traffic requirement is less than that which the allocated bandwidth could carry with dual polarization, orbit-spectrum utilization could be improved in the same way, the bandwidth occupied by the system being cut to 100 MHz, 200 MHz, or whatever is sufficient in the dual polarization mode. An apparent alternative, namely for such networks to operate in the single polarization mode but with high purity of polarization, so that interference from another network using a nearby orbital location in the reverse polarization mode could be rejected, seems unlikely to be feasible owing to several difficulties, perhaps the greatest of which is the rapid deterioration in the rejection of cross-polar signals by a typical earth station antenna- as the unwanted satellite moves away from the boresight direction. However, while experimental results obtained so far give confidence that dual polarization will be feasible, it is clear that the increase of cost of some elements in systems will not be insignificant, including:

1) dual-polar feeds; 2) antenna systems of good polarization purity; 3) additional earth station high power amplifiers and low

noise amplifiers where access is needed to both polarization modes;

4) adaptive interference suppressors if needed to deal with rain depolarization.

Where the network traffic capacity requirement is high enough, these costs would be accepted because dual polarization will enable one satellite and one set of earth terminals to do the work of two; there will therefore be no need for external pressure to urge the adoption of dual polarization in such cases. Where the traffic demand is less, the cost penalty arising from a requirement to use dual polarization would not, however, be negligible. At some time in the future the congestion of the orbit and spectrum may become so severe that general use of


dual polarization will be necessary. In anticipation of this, it would be desirable for new earth terminals t o be designed so that they could radily be converted to dual polarization operation. For the time being, however, it does not seem to be necessary to make dual polarization a universal requirement; there are more cost-effective ways of improving orbit-spectrum utilization in the short term.

D. Earth-Station Antenna Sidelobe Radiation INTELSAT requires the antennas of earth stations with un-

qualified access to the system ("Standard A") t o have a figure of merit G / T not less than 40.7 db/K at 4 CHz. Lower values of G / T , typically between 27 and 32 db/K, are specified for other systems. Such antennas have D/X ratios between 200 and 500 and if realized in ideal form their sidelobes would be negligible a few degrees from boresight. Practical antennas have quite considerable sidelobe skirts, due to their various imperfections, and interference to and from unwanted satellites, coupled through these sidelobes, plays a major part in deter- mining how closely satellites sharing the same frequency band may be located.

The symmetrical Cassegrainian configuration is used almost invariably today for large earth station antennas and F i g . 4 shows the form of the envelope of the sidelobe peaks for a typical antenna. The sidelobe envelope of the great majority of large Cassegrainian antennas would fall within a few decibels of this curve. It is convenient t o fit a simple law to the curve, for the purpose of analysis. A convenient form would be

E = A - BlogIo $dBi

where A and B are numerical coefficients and @ is the angle between the direction of interest and the boresight axis. In 1965, the CCIR adopted E = 32 - 25 log @ dBi as the reference radiation pattern for use in large-antenna interference calcula- tions when specific antenna data are not available [81, this envelope being assumed to include 90 percent of the sidelobe peaks and to apply from 1' off-boresight out to the points where E = - 10 dBi (that is, to 48" off-boresight). Beyond 48' it is assumed that the envelope continues constant at - 10 dBi. That law was based upon the antenna data then available. It st i l l fits the available information on operational antennas reasonably well, and is widely used in generalized studies of interference.

It is instructive to consider briefly what change would be effected in the calculated maximum load of the orbit if these coefficients A and B were altered. Thus, given homogeneous orbit loading with satellites accessed by earth stations to the INTELSAT standard, using large-capacity FDM/FM :mien, the minimum acceptable spacing would be about 2.9 when E = 32 - 25 log 8 . Figs. 5 and 6 show the substantial effects on minimum satellite spacing that would follow if the coefficients were improved relative to the CCIR reference radiation pattern.

However, to see what order of improvement might be obtain- able in practice, it is first necessary to consider why the practical radiation patterns of large Cassegrainian antennas are so much worse than the ideal case [ 9 ] . Out to about 2' off- boresight, high sidelobes are caused mainly by large-scale in- accuracies of the profile of the main reflector. The use of insufficient illumination taper also has a significant effect here. Between 2' and about 10' off-boresight, small-scale profiie errors of the main reflector are the main source, although scat-

40 E I C C I R REFERENCE PATTERN

30

20 10

0

- 10 Y I 1

lo l o o IWO QF-BOIIESIGHT M L E . @

Fig. 4. Sidelobe envelopes for large earth station antennas.

32 27 22 *I

A (ENVELWf LEVEL AT I * OFF-BORESIGHT)

Fig. 5. The effect on minimum required spacing of satellites of homogeneous networks with large earth station antennas if the gain slope of

= A - 25 log @J but its level varies. the sidelobe peak envelope remains constant at the value given by E

I E

25 I I

30 35

B=COEFFICIENT OF (LOG,o a)

Fig. 6. The effect on minimum required spacing of satellites of homogeneous networks with large earth station antennas if the level of the sidelobe peak envelope at lo off-boresight remains at 32 dBi and B , the coefficient of log, o@J, varies.

tering at the subreflector supports and diffraction at the edge of the subreflector are also significant. Between 10" and about 30" off-boresight, the main cause is subreflector spillover. Finally, beyond 30" the sidelobes are fed from many sources but particularly from scattering at the subreflector and its supports and the sidelobes of the feed radiation pattern.

Thus, two important sidelobe-generating mechanisms emerge for large symmetrical Cassegrainian antennas over the angles where the sidelobes are likely to do most harm, namely small- scale main reflector profile errors and over-illumination of the subreflector. It does not seem to be possible, at present, t o provide an exact quantification of the relationship between these causes and their effects, and the following should be considered as approximate estimates only. An improvement in the small-scale profile tolerance from 0.5-mm rms (which is typical of current practice) t o 0.25-mm rms (probably corresponding to the best current commercial practice) could be expected to improve the envelope of the sidelobe peaks be-


tween 2" and 10" off-boresight from a level somewhat below 32-25 log I$ dBi to about 3 dB lower; the cost would not be negligible but it would be small relative to the cost of an earth station. Further refinement of the profile would be beneficial, but it might be necessary to introduce new techniques of fab- rication if the cost is to remain acceptable. An adjustment of subreflector illumination to make sure that the edge-illumination is at least 20 dB below the maximum should bring the sidelobe envelope level between 10" and 30" off-boresight down by several decibels to the CCIR reference level for a tri- fling cost. A further improvement could be obtained by the use of an offset-fed configuration, but the cost of this would be difficult to estimate, pending the evolution of suitable commercially available designs.

In short, the sidelobe performance of large earth station antennas operating at 4 and 6 GHz could be improved by say 3 dB over the important arcs of the polar diagram for a modest cost, and corresponding improvements could, no doubt be obtained with smaller antennas and at higher frequencies. A general improvement of this order could lead to a reduction in minimum satellite spacings of about 25 percent and a corresponding improvement in orbit-spectrum utilisation.

Improving the sidelobe radiation pattern of the earth station antenna has little effect on boresight gain and so it is not cost- effective in the optimization of equipment design unless some cash value can be attached to increasing the effectiveness of orbit utilization. Thus, improvement is only likely to be achieved by internationally agreed administrative action. Furthermore, benefit in terms of orbit utilization will not be obtained until most earth station antennas in service have better performance. In view of the large number of earth station antennas already in service and the huge investment that they represent, it would be unrealistic to expect a very rapid improvement in orbit- spectrum utilization efficiency to arise from action taken now. Finally, the achievement of an improvement in sidelobe response by administrative action is made more uncertain by the fact that it is far from easy to measure the radiation pattern of a large earth-station antenna or of any antenna, large or small, that has limited steerability.

However, these admitted difficulties are worth overcoming. Just what would be a realistic target to aim for is a matter for further study, but it may not be unreasonable to require all large new antennas taken into service after, perhaps, 1985, to achieve the standard shown on Fig. 3, and for none which fail to achieve it to remain in service after 1995. The E = 32 - 25 log @ law might serve as an interim goal, to be achieved by all new antennas from 1980. The same standards might be found suitable for smaller antennas also.

Finally, it may be noted that the minimum separation between two satellites is often determined, not by widespread interference, but by interference on one or two carriers at a few earth stations only. Adaptive interference cancellation has already been demonstrated to be feasible when a terrestrial station interferes with the reception of a satellite signal [ 101 , and this technique should be readily applicable in the much simpler case of interference between satellite networks, espe- cially when the interference arises in the down-path.

E. Modulation Technique and Spectral Energy Distribution Depending upon the down-path carrier power available and

the modulation, multiplexing, and multiple-access techniques used, the traffic capacity of a satellite per unit of bandwidth will vary considerably, for example from 10 telephone channels

per megahertz when FDM/FM is used in FDMA to 50 channels in TDMA with 4-phase PSK and digital speech interpolation. The choice of modulation techniques also has a considerable effect upon the level of interference at the receiver input that can be borne without the post-demodulator interference exceeding the permissible level. It would not be practicable to review all the significant aspects of this complicated matter in a general paper' nor would it be easy to draw out generalized conclusions as to the techniques which optimise orbit-spectrum utilization. It may not even be helpful to generalise in this way, since the choice may be too strongly influenced by economic factors arising from the use to which the satellite network is to be put and the nature of the terrestrial system with which it interfaces. Nevertheless, it is useful to consider whether there are factors in this area which are conducive to efficient orbit-spectrum utilisation, yet do not enter fundamentally into the economic viability of the system.

The carrier power required for satisfactory reception of an emission 1s a function of the predemodulator bandwidth, but its potential for causing interference is a function of the interference power failing into the bandwidth of the link suffering interference, and in many cases to the spectral distribution of interference power within that bandwidth. Thus, for FDM/ FM wanted emissions in general and for narrow-band wanted emissions regardless of modulation technique, a wide-band unwanted signal may cause interference out of proportion to its mean flux density if the distribution of energy in the interfering spectrum is strongly nonuniform. In the absence of special arrangements, the spectral energy distribution of emissions is usually markedly nonuniform, particularly when traffic loading is light, the energy tending to be concentrated about the carrier frequency for analogue emissions and about a series of discrete spectral lines in digital emissions. Artificial spectral energy dispersal techniques, such as the addition of a low-frequency sawtooth waveform to an FM baseband and the addition of a long-cycle pseudo-random sequence to digital traffic signals, can improve the uniformity of the distribution of the spectral energy of emissions markedly without high cost or significant degradation of the performance of the wanted Signal.

Three conclusions can be drawn from this:

1) effective artifical spectral energy dispersal should be used whenever the energy dispersal provided by the traffic signal is inadequate,

2) a measure of uniformity of predemodulator bandwidth is desirable,

3) particular care must be taken to deal with the susceptibil- ity of FDM/FM systems to interference from unwanted signals with insufficiently dispersed spectra.

Of these, the first has the greatest impact on orbit-spectrum utilisation and least impact on system design. At present, most satellite carriers have some degree of artificial energy dispersal applied to prevent the down-path power flux density at the earth's surface from exceeding limits imposed by international agreement to prevent interference to terrestrial radio services. In the future, it would be desirable for all satellite emissions to have as large a degree of artificial energy dispersal as is consistent with efficient reception as a wanted signal.

this issue; see "A s w e y of interference problems and application to ' A more extensive discussion of this topic is contained elsewhere in

geostationary satellites" by M. Jeruchim.


F. The Interference Degradation Budget At present, designers of analog satellite systems are recom-

mended to allow 10 percent of all channel noise for interference from other satellite networks and a further 10 percent for interference from terrestrial radio systems [ 111. The makeup of the remaining 80 percent will include allocations for such components as uppath and down-path thermal noise, earth station and satellite intermodulation noise, cross-polar and interbeam interference within the same network, multipath and group delay distortion, but the allocation for each component will depend on the circumstances of each network and each carrier. Recommendations for the interference budget for digital systems are st i l l under study; when they emerge they are likely to be similar in broad effect to those already existing for analog systems, although differences in detail are bound to be made necessary by differences in the nature of the interference mechanism.

It has been estimated that the total traffic capacity of a busy arc of the orbit could be increased by 75 percent if the proportion of the degradations budget allocated to interference were increased from 10 to 50 percent, circuit performance being restored to the nominal state by making corresponding reduc- tions in the allocations to other degradations. It will sometimes be feasible to do this, although at the cost of a modest loss of satellite capacity, where the thermal noise allocation is initially large, since post-detector thermal noise can be reduced by increasing carrier power and/or by using modulation techniques or parameters more resistant to interference. However, it may not be feasible to do this in networks where the thermal noise allocation has already been made small in order that the allocation to intermodulation noise or to interference from other frequency reuse modes within the same network may be large. In the present, relatively early stage in the technical and operational development of satellite communication it seems pre- mature to increase the internetwork interference allowance beyond 10 percent. In the future, when the orbit is more crowded, and the whole of the 10 percent allowance has already been taken up by interference it may well be useful to allow some additional networks to open service by increasing the internetwork interference budget to perhaps 20 percent of all degradations. However, it might be necessary to apply this increase selectively, excluding its application to networks where the frequency spectrum is reused within the network.

111. INTERSYSTEM COORDINATION

A satellite network needs several years from conception to launch, it is always costly and, once the spacecraft design has been determined in detail, many of the important characteristics cannot readily be changed. Interference between networks which might be severe can often be reduced to a tolerable or even negligible level by minor adjustments to characteristics if the need can be identified early enough. What is needed is a system for bringing together the various entities who are using or planning networks that could interfere, so that the adjustments required can be identified in time and agreed. In practice, this process of coordination will have to be repeated oc- casionally throughout the lifetime of a satellite as new satellite projects designed to use the same frequencies and the same broad arc of the orbit come into being.

Machinery for the coordination of geostationary satellites was agreed at an ITU conference in 1971 [ 121. In brief it is as follows. As soon as an organization planning a new satellite

network has determined the technical basis of the system, but not more than five years before the first launch, provisional system characteristics are passed to the ITU and circulated to a l l national frequency authorities and other interested parties. The system characteristics are given in sufficient detail for any other user or intending user of the orbit to calculate whether interference at a significant level might be suffered by his own network. If the risk is significant (the agreed threshhold con- dition being when the maximum interference energy at an earth station receiver due to the unwanted network would be equivalent to 2 percent of the total noise arising within the wanted system), the parties concerned collaborate to refine the calculation of interference probability and to identify any necessary and acceptable changes in the characteristics of one system or the other that would reduce interference to a permissible level. The agreed characteristics are eventually re- corded by the International Frequency Registration Board, an organ of the ITU.

No doubt these administrative provisions will be improved in detail at the 1979 WARC. For example, it may be possible to devise a method, more selective than the 2 percent-of-thermal- noise criterion, that will identify all the networks for which a proposed new network is a potential source of significant interference without needlessly complicating the Coordination process by bringing into it many cases when interference will not be significant. These processes will, however, become extremely complex as the number of systems, operational and planned, grows, and the prospects of success must diminish as complexity increases. Some reduction in the number of parties could be expected from the application of the principles ad- vocated in section I1 above. Nevertheless, if coordination is to yield solutions which satisfy the parties immediately involved while leaving opportunities for the entry into the orbit of yet further satellites, not then announced, the process must surely be simplified by the adoption of additional technical guidelines to facilitate the achievement of solutions and to eliminate op- tions which are not essential and complicate negotiation. These guidelines might include the following.

A. Frequency Band Pairing Most present-day fixed-satellite spacecraft use the 3700-4200

MHz (down-path) and the 5925-6425 MHz (up-path) frequency bands. Several projected satellites couple down-path bands at 11 or 12 GHz, with up-path bands at 14 GHz. Soon, no doubt, satellites transmitting at 20 GHz and receiving at 30 GHz will be in service. However, these band pairings arise from technical, economic, or even historical circumstances; the internationally agreed frequency allocations identify some bands as up- path bands and others as down-path bands but do not specify pairings. If for one particular application it were technically preferable to use 14 GHz as the up-band and 4 GHz as the down-band, there is no international impediment to that pairing, even though such an unusual satellite, in the company of others using conventional pairings, would provide the same communications capacity as one conventional satellite while using as much orbit-spectrum as two and while likewise de- manding as great a coordination effort as for two conventional satellites. In principle the waste of orbit and spectrum that this would involve could be minimised if another satellite which paired the 6GHz band (up-path) and the 11 or 12 GHz (down- path) were colocated with the 14GHz/4GHz satellite, but such arrangements provide no general solution in practice.


There is an urgent need to determine preferred frequency band pairings and to use them in all normal applications. Complications can be foreseen.

1) Some high capacity satellites will be designed to use two or more pairs of bands. Such satellites may need to be cross- strapped; thus, for example, in a satellite that uses the 4- and 6- GHz bands and the 11- and 14GHz bands, it may be desirable for some transponders to pair 14 and 4 GHz or 6 and 11 GHz. This practice may be economically advantageous since it may provide the best match to service requirements, and it will not necessarily use the spectrum inefficiently for a network which occupies the full bandwidth of all four bands.

2) National frequency allocations differ in detail in different parts of the world. Thus, for example, some systems operating at 4- and 6GHz pair 3700-4200 MHz with 5925-6425 MHz, while others pair 3400-3900 MHz with 5725-6225 MHz.

3) There is an imbalance between the up-path and down- path frequency allocations, and this is aggravated by the fact that up-path bands allocated for the fiied-satellite service may be used for the up-path connections to satellites of other services, for example, broadcasting satellites. Thus, in ITU Region 2 (North and South America) it would be natural to use the 14.0-14.5 GHz up-path band for fixed-satellites transmitting in the 10.95-1 1.2 GHz and 1 1.45-1 1.7 GHz bands and also for both fixed-service and broadcasting satellites transmitting in the 11.7-12.2 GHz band.

These various complexities suggest that no simple, inflexible code of band pairing rules is likely to be acceptable. Neverthe- less, it should be feasible -to agree a set of guidelines that would help to promote efficient orbit-spectrum utilisation. B. A Standard Frequency Translation Between Up-Path and Down-Path Frequency Bands

Many satellites use the whole width of the allocated frequency band and the mean frequency translation introduced in the satellite is therefore made equal to the difference between the up-path and down-path frequency allocations. Where the bandwidth occupied is much less than the allocated bandwidth, it should be quite feasible for two or more satellites, displaced in frequency, to use the same orbital position without interference. Obviously this economical arrangement will be arrived at more easily if all these satellites also have the same frequency difference between up-path and down-path frequencies. However, the technical mechanism which imposes standardisation auto- matically on full-bandwidth satellites does not operate for narrow-band satellites and it would be desirable to substitute for it some form of international agreement.

C. An Agreed Scale of Permissible Single-Entry Interference Noise Allocations

By international agreement, intersystem interference may account for up to 10 percent of all the noise degradations that a satellite link suffers. In the process of frequency coordination it is necessary to limit the interference from the particular interfering network under consideration to some part of that 10 percent. The CCIR recommends that the allocation to any one interference source should not exceed 40 percent of the total intersystem interference budget, this being roughly the fraction of the total that would arise from the nearest of a hypothetical homogenous array of equally spaced satellites to either side in orbit of the wanted satellite. This recommendation is insufficient to provide a good basis for coordination. It would be unwise to allocate 40 percent of the total interference budget to a powerful satellite perhaps 20’ away from the wanted sat-

ellite and serving a different region since one or several other satellites may be established later, closer in orbit to the wanted satellite and serving the same region. Clearly the simple “40 percent maximum” figure needs to be elaborated into a scale of allocations, taking account of orbital separation, and perhaps other factors such as the geographical relationship of the service areas so that coordination agreements, once arrived at, can remain in effect for a long time.

D. Maximization o f Service Arc The most elementary and often the most attractive way of

reducing internetwork interference is to move the satellites further apart. This solution is available only if one satellite or the other may have it nominal location changed without losing its ability to fulfill its mission. This freedom to change location should preferably be retained throughout a satellite’s operating lifetime, but at least the option should be kept open until shortly before launch. The range of orbital location over which a satellite could fulfill its mission is called the “service arc.’’

A satellite serving earth stations spread on a global scale may have a very small service arc; if the satellite moves far to east and west, it will go below the horizon as seen at some of its earth stations. There are, however, many other circumstances that limit the service arc.

Some of these circumstances are basically economic, opera- tive regardless of whether the spacecraft has been built or launched. For example, it may be necessary to locate a satellite west of the orbital position where eclipse occurs when it is midnight in the service area, so as to provide maximum commercially useful communication capacity without full battery cover. Other limitations are technical; for example a satellite with multiple spot beams may have the shape and relative directions of the beams optimised for a narrow orbital arc, and the coverage pattern might become suboptimum or even totally unacceptable if the satellite location were changed significantly; it may be possible to adjust the coverage pattern up to a few months before launch if the coordination process is concluded in time, but once launched the option may be completely lost. A third kind of problem is operational. Once a satellite is in service, operational continuity may be very important. It may be extremely difficult to maintain service while a satellite is being moved from one location to another, particularly if other satellites working in the same frequency bands must be passed on the way round the orbit or if the wanted satellite is used by numerous unattended earth stations; such difficulties could often be overcome if a spare satellite were available in orbit.

These are reasons why a satellite may have a narrow service arc, and there are others. Some of these difficulties can be overcome at moderate expense, but a rigid requirement to overcome them all might be unacceptably expensive to imple- ment. Nevertheless, a code of guidelines could be drawn up for maximising the service arc; if this were endorsed internationally and applied to the extent that circumstances permit the prospects of successful coordination would be significantly improved.

w. REDUCTION OF INHOMOGENEITY IN ORBITSPECTRUM SHARING

The range of application for satellite communication is very wide, and system parameters cover a correspondingly wide range. It follows that there are marked differences in the ability of systems to inflict interference on one another. The only

316

substantial defence against interference between networks serving the same or neighboring territories and using the same frequency bands is the directivity of the earth station antenna. Where the gain slope of the side-lobe envelope is high, good use can be made of a few degrees of satellite separation in orbit. Thus, for example, an antenna conforming to the sidelobe reference pattern E = 32 - 25 log $ dBi has an envelope gain slope of 11 dB per degree at 1' off-boresight. and 3.6 dB per degree at 3' off-boresight; measurements made on good large antennas show that such performance can be achieved, and perhaps somewhat exceeded. However, at large angles off- boresight, the reference pattern gain slope falls to such low values as 1.1 dB per degree at 10' off-boresight, and Cassegrain antennas in use tend to have even worse envelope gain slopes at 10' off-boresight owing to the presence of subreflector spillover radiation. In short, if a network causes another network so much interference that the twosatellites cannot operate without excessive interference with an orbital separation of a few degrees, then the required spacing tends to escalate to 'an unmanageably large figure.

This situation can be illustrated by an admittedly extreme hypothetical example involving two satellite networks. Net- work A provides narrow-band links to sensitive earth terminals with large antepnas, as in the SPADE transponder of the INTELSAT Atlantic Primary satellite. Network B provides wide-band links to low-cost earth terminals, for example a TV distribution system using the maximum permitted down-path flux density at 4GHz and serving earth terminals with 5-m antennas. If all earth station antennas are assumed to meet the CCIR sidelobe reference pattern and both systems have effective carrier energy dispersal, it is estimated that an array of satellites like Network A, all sewing the same region, could be spaced as closely as 1.5' without the intersystem interference exceeding 10 percent of all degradations, while the corresponding f i e for an array of satellites like Network B would be about 9'. However, Network B could cause such bad interference to Network A that satellites would have to be spaced by no less than 33' if they served the same region.

Gross inhomogeneity of systems that can interfere is clearly very undesirable. Inhomogeneity cannot be eliminated altogether because of the varied nature of the opportunities for service that the medium provides. Some relief could be obtained if limits were applied to system parameters which primarily determine liability to cause interference and suscepti- bility to suffer it, but this would probably lead to unsatisfac- tory compromises which would circumscribe application without greatly reducing the inhomogeneity. A better solution, if it proves to be feasible, would be to classify systems according to their tendency to cause or suffer interference, and to segre- gate the satellites of the various classes so that interference between networks with very different characteristics does not occur. The details of such a system would need to be worked out. Classification might be based on a small number of parameters, perhaps as follows:

the off-beam up-path spectral ei.r.p. (dBW/Hz); the down-path spectral p.f.d. (dBW/m2/Hz); the up-path interference power flux level (dBW/m2) received at the wanted satellite within the bandwidth of a wanted signal, which causes a threshold of interference noise to be exceeded at an earth station; the off-beam down-path interference p.f.d. (dBW/m*)

PROCEEDINGS OF THE IEEE, MARCH 1977

received at an earth station within the bandwidth of a wanted signal, which causes a threshold of interference noise to be exceeded.

Segregation could be by orbit-division or spectrum-division. However, having regard to the practical needs of some systems

1) to carry at different frequencies in the same satellite a variety of services that would have widely different ratings according to the parameters listed above;

2) to have access to wide bandwidths for high capacity; 3) to have minimal restrictions on the location of satellites

used for any purpose;

it may be supposed that a combination of both orbit and spectrum division will be necessary. Having carried out a rough network sorting process by classification, it may be feasible to bring about a further reduction of inhomogeneity by the standardization of some characteristics within the classes. This might include standardization of uppath eirp spectral density, and earth station C / T and possibly the identification of preferred RF channelling plans.

V. CONCLUSION

The geostationary orbit is capable of supporting vast traffic flows or a wide variety of services with flexibility. However, it is not likely to achieve both at the same time, and the effective achievement of either depends upon designing and deploy- ing each network having regard to the presence of other networks. The long term problem, therefore, is to find ways of securing action by all users of the orbit that will lead towards effective orbit utilisation. This will cost the users money and constrain their freedom. If such policies are to succeed in the real world, the costs and constraints must not be too great and must be seen to be justifiable in the not-too-distant future.

In this paper a number of measure have been identified that should be acceptable to the users and should be evidently con- structive. They are:

better satellite station-keeping; limitation of satellite antenna sidelobe radiation; improvement of earth-station antenna sidelobe patterns; carrier energy dispersal; standardized frequency band pairing; standardization of frequency translation in satellites; the adoption of a graduated scale of single-entry interference allowances; the adoption of guidelines for the maximization of service arc ; classification and segregation of networks according to their interference potential.

No one of these measures, except the last, can be expected to provide a large individual contribution to more effective orbit-spectrum utilisation, but collectively they could increase it several-fold. Hopefully it will be possible to reach international agreement on them within the next few years.

ACKNOWLEDGMENT

Acknowledgement is made to the Senior Director of Devel- opment of the British Post Office for permission to make use of the information contained in this paper. The advice of many colleagues is also gratefully acknowledged.

UFTHE EKE, VOL. 65, NO. 3, MARCH 1977 317

REFERENCES

[ 11 W. L. Pritchard, “Satellite communication-An overview of the problems and programs,” this issue, pp. 294-307.

[2] D. I. Dalgleish and A. K.-Jefferis, “Some orbits for communications-satellite systems affording multiple access,” hoc Inst. Elec.

[ 3) H. E. Rowe and A. A. Penzias, “Efficient spacing of synchronous Eng.,vol. 112, p. 21, Jan. 1965.

communication-satellites,” Bell Syst. Tech. J. , vol. 47, no. 10, pp. 2379-2433, Dec. 1968.

[4] I. Petrov, “INTERSPUTNIK International space communication system and organisation,” Telecommun. J. , vol. 29, p. 679, Nov.

151 A. K. Jefferis, D. G. Pope, and P. C. Gilbert, “Satellite television distribution; service from geostationary satellites to community antennas in multiple-coverage areas,” Proc. Znst. Elec. Eng., vol 116,110. 9,p. 1501,Sept . 1969.

1972.

[6] H. Mertens, “Satellite broadcasting; design and planning of 12

[ 7 1 ITU Radio Regulations Article 7 (revised 1971). [ 81 CCIR Recommendation 465-1. Reference earth station radiation

pattern for use in coordination and interference assessment in the frequency range from 2 to about 10 GHz. Documents of the XIIIth Plenary Assembly, Geneva, Switzerland. vol. IV, p. 155, 1974.

191 J. Dijk and E. J. Maanders, “Some aspects of near and far angle sidelobes in double-reflector antennas,” presented at AGARD

[ 101 N. White, D. Brandwood, and G. Raymond, “Some application of Conf. Rome, Italy, May 1973.

interference cancellation to an earth station,” in IEE Conf. Publ. 126, Satellite Communication Systems Technology, p. 233.

Il l ] CCIR Recommendations 356-3, 466-1, and 483. Documents of the XIIIth Plenary Assembly, Geneva, Switzerland, vol. IX, p. 360, and vol. IV, pp. 185 and 186, 1974.

GHz systems,”EBU Rep. Tech 3220 E.

[ 12) ITU Radio Regulations Article 9A (revised 1971).

A Survey of Interference Problems and Applications to Geostationary Satellite Networks

MICHEL C. JERUCHIM

Abstmct-The fundamental limitation in the utilization of the geo- the determination of network performance in an interference *tiom orbit is *e m u w interference genmted b y satellite net- environment. In general terms it is of interest to evaluate an w o r k A review of the theory of operation of communication systems expression of the form m an interference environment is provided, with particular attention to those systems of importance in space communications, namely multi- channel FM telephony, frequency-modulated television, and coherent Q = . f ( s w , D w ; s ~ , D ~ > (1) PSK digital Examples are then given to show the limiting effects of interference on orbit utilization. where

I. INTRODUCTION OR SOME TIME it has been recognized that the geostationary orbit is a natural resource that must be managed efficiently. Much has been written [ 11-[ 161 about the

technical, operational, economic, and regulatory aspects of orbit utilization. The paper by Withers [ 161 provides an up- to-date overview of the problem. While many factors are relevant to orbit utilization, in a sense the most fundamental is interference, since it is the unavoidable presence of interference that creates a problem in the first place.

The inevitability of interference is as physically fundamental as that of noise within a system. In the case of interference, however, the ultimate limitation arises because it is not possible to constrain an antenna beam to a prescribed volume. Hence, geostationary satellite networks operate in an environment in which interference can be considered to be as much a part of the design constraints as noise, or other parameter limitations. Thus, the basic problem to be considered here is

Q wanted signal quality, e.g., signal-to-noise (S/N) ratio and error probability;

S the set of parameters specifying the modulation characteristics, e.g., signal type, modulation index, and baseband bandwidth; subscripts w and I refer to wanted and interfering signals, respectively;

D the set of network link parameters, e.g., e.i.r.p., frequency, and antenna size; subscripts w and I refer to wanted and interfering networks, respectively.

The symbolic equation (1) will be defined in more specific terms subsequently. There are two aspects in the evaluation of (1). The first is the calculation of the interference levels them- selves. This requires a relatively straightforward calculation involving geometrical parameters, power levels and antenna gains. The second aspect, which is the main concern of this paper, is the determination of signal quality Q , given specific interference levels.

The evaluation of interference effects depends upon the nature of both the wanted signal and the interference, including

Manuscript received May 17, 1976; revised September 14, 1976. the modulating signals, the modulation methods, the frequency

The author is with the ~~~~~d Electric Company, Space Division, assignments, the definition of Q , and the operations (filtering, Valley Forge Space Technology Center, Philadelphia, PA 19101. amplification, etc.) that these signals may be subjected to.