2.4.1 Introduction
The analysis of the risk of interference into space stations assumes a probability basis, since specific data on locations, etc. of radiorelay stations are not available. The analysis is based on estimating the probability for the GSO case and then using the same model to assess the relative risk for the IO case. For the IO case, 5° inclination has been chosen as representing a reasonable value for the analysis. Lower inclination values yield a proportionally lower estimate of potential interference, while higher values will yield a higher estimate.
2.4.2 The model
Radiorelay stations which can have mainbeams which intersect the GSO are limited to those with a particular radiation azimuth at a specific latitude. There are four such points for each satellite location on the GSO accounting for both North and South latitudes as well as locations East and West of the GSO location.
Making an allowance for the beamwidth of the radiorelay antenna, its elevation angle and accounting for refractive effects, a small strip similar to that of Fig. 2a is established, which contains all the stations which could have an intersection with the GSO or the IO at some point in time. The width of this strip is a function of the values assumed for the model parameters.
The model parameters assumed here are: elevation angles from 1° to 4° for the radiorelay antenna and a range for the radio refractivity from 250 to 400. This latter factor adds 2° to the effective range of visibility at the outer edge of the strip. It is also assumed that the beam centre is separated from the orbit under study by 1.5° to account for the beamwidth with some margin.
For the assumed parameters, the width of the strip for the GSO case is approximately 7°. The number of stations located within this strip is a function of the area of the strip and an assumed density for the terrestrial stations. The area need only be calculated for one quadrant from the equator to 70° latitude and, by symmetry, it applies to all quadrants. Intersection with a specific point on the GSO can take place from all four quadrants.
For the 7° (775 km) strip of one quadrant, the area of the strip is 7 875 000 km^{2}.
The width of the strip is not changed by the inclination at low latitudes, but it increases with latitude depending upon the inclination. For the case of 5° inclination, the area of the strip in one quadrant is 13 230 000 km^{2} and the number of stations would be expected to be 1.68 times that of the GSO case.
This result will vary directly with inclination and it can be taken as representative of the effects of IO.
2.4.5 Quantitative assessment
Estimates of the number of potential intersections can be made by assuming a maximum density of radiorelay stations for all of the land area contained in the strip which has sufficient population to justify the assumption. The maximum density of one station per 2 500 km^{2} allows for a station every 50 km in all directions. This corresponds to the normal single hop distance used by radiorelay designers.
It is further assumed that with considerations of population density and the effects of ocean areas, the area of concern would be of the order of 20% of the total. Random pointing of the radiorelay antenna with a 2° beamwidth is assumed and applying these considerations to the GSO case, the total number of stations with possible intersections would be about 14 while those for the inclined orbit case would be about 24.
2.4.6 Practical considerations
This model contains several assumptions which are very conservative, such as:
– radiorelay elevation angles from 1° to 4°,
– the use of a uniform density of radiorelay stations,
– uniform 2° range of elevation angle, considered to be representative,
– a standard nominal refractivity value of 300.
Adjustments can be made to this model, the net effect of which is to reduce the area of concern to 42% of the original model for the GSO case and to 64% for the inclined orbit case. The number of potential exposures is reduced to about 6 for the GSO case and 15 for the inclined orbit case.
2.4.7 Actual experience
A review of INTELSAT satellites in the GSO experiencing interference from terrestrial radio relay sites has shown that the effects have been minor. In fact only one such case has been recorded over the last ten years.
3 Earth station/terrestrial station interference 3.1 Introduction
The absence or cessation of NorthSouth station keeping of a geostationary satellite will cause it to change its orbital inclination continually. An earth station operating with such a satellite may have to track it with its antenna mainbeam through an apparent diurnal trajectory (a narrow figureofeight). When such an earth station has been coordinated with stations of terrestrial services for “strictly” geostationary operation (satellite movement within prescribed or stated small positional tolerances), the need to follow a satellite having or acquiring significant orbital inclination will cause the earth station antenna mainbeam to assume elevation angles and, associated with these, azimuth angles that are different from (both less and greater than) those for which coordination had been effected. Of particular concern is the case when elevation angles are less than required for geostationary operation because in this case the resulting higher earth station horizon antenna gain can cause potential interference from and to a terrestrial station.
3.2 Geometric considerations 3.2.1 Analytic expressions
The elevation angle (_{s}) and azimuth (_{s}) of the mainbeam of an earth station towards a space station in geostationary inclined orbit at the point of maximum excursion are given by the following expressions:
_{s} arc sin ((K A – 1.0) / B) (10)
_{s} 90.0 arc cos (K cos i sin / B cos _{s}) (11)
A cos i cos cos sin i sin (12)
B (1.0 K ^{2} – 2K A)^{0.5 }(13)
Note that arc cos (x) 180.0 arc cos (x)
where:
K : geostationary orbit radius/Earth radius, assumed to be 6.62
i : orbit inclination (ve ascending node East of Greenwich)
: earth station latitude (ve North)
: difference in longitude between the space station and the earth station.
3.2.2 Loss of discrimination
As the satellite goes into an inclined orbit, the elevation angle and azimuth vary with time. This may result in variations in gain toward the horizon, as discussed in the following sections.
Both the elevation angle reduction and the associated azimuth shift are not only functions of the orbital inclination of the satellite being tracked, but also of the latitude and relative longitude (longitude difference to the subsatellite nodal point) of the earth station in question, as shown in Figs. 9a, 9b, 9c and 9d for the two orbital inclinations 5° and 10°, respectively.
In Figs. 9a, 9b, 9c and 9d, the outer circumference describes those locations on the surface of the Earth (in terms of latitude and longitude relative to the subsatellite nodal point) at which the earth station antenna mainbeam elevation angle to the inclinedorbit satellite is never less than 3°. The inner, almondlike area contains that part of the Earth’s surface at which the elevation angle is never less than 48° and, thus, not subject to variations of horizon antenna gain (assuming that beyond an offbeam axis angle of 48° there is no appreciable change in antenna gain).
FIGURE 9a...[D15] 10.5 CM
FIGURE 9b...[D16] 12 CM
FIGURE 9c...[D17] 11 CM
FIGURE 9d...[D18] 11.5 CM
In the upper diagram of each figure, the broken lines show the magnitude of the earth station horizon antenna gain increase (dB), based on an antenna pattern of the form A – 25 log dB. In the lower diagram the broken lines show, for the corresponding earth station locations, the shift in earth station antenna mainbeam azimuth from that associated with strictly geostationary operation to that at which the satellite is seen under the lowest elevation angle in its inclined orbit. The azimuth shift is always towards the equator. Earth stations on the satellite node’s longitude have the greatest horizon antenna gain increase, and the smallest azimuth shift; earth stations near the equator have the smallest horizon antenna gain increase and the largest azimuth shift. The greater the inclination, the greater are both the horizon antenna gain increase and the azimuth shift.
Figure 9d shows, as an additional set of curves, the lateral shift of a “common volume” at 4 km altitude. This is the highest altitude from which rain scatter interference can be expected.
3.3 Effect on earth station coordination area
Due to the variation in azimuth and elevation angle, earth stations which were previously coordinated with terrestrial systems on the basis that they would operate with a satellite on the GSO may be affected by the use of inclined orbits. Additional terrestrial stations may also be affected. The new coordination contours would then be a function of the GSO location(s) and the arc for which they were calculated. There will be a wide variety of situations which will affect many earth stations in the world.
A set of boundary conditions has been examined which may help in assessing the potential problem of re coordinating earth stations where it proves to be necessary.
Earth stations which can operate with inclined orbit satellites with 5° inclination are limited to those with elevation angles of 5° to 10° at the nominal GSO location depending on the earth station latitude. The largest dimension of the coordination contour is in this case based on an antenna gain in the horizontal plane of 7 to 14.5 dB. At a receiving station this gain sets the interference sensitivity, while at a transmitting station, it sets the e.i.r.p. density in the horizontal plane.
For an earth station at the equator which has a low elevation angle, the antenna movement in azimuth is approximately twice the inclination angle. However, in this case the elevation angle changes very little.
For an earth station operating at 5° nominal elevation angle, the increase for the rangeinfluenced azimuthal directions of 50° for the case of 5° inclination is from 0 to 4.9 dB. The impact on the coordination area for an earth station that had been coordinated at 5° at 6 GHz using the maximum allowable e.i.r.p. density of 40 dB(W/4 kHz) is a broadening of the contour around the mainbeam region with no change on the nominal azimuth.
For earth stations at high latitude, the azimuth changes very little while the elevation range will be approximately equal to twice the inclination. Inclined orbit operation would therefore be limited to earth stations with a nominal GSO elevation of 10°. This could mean going as low as 5° during part of the tracking time and would yield a gain increase of 7.5 dB on that azimuth resulting in an increase mainly along the mainbeam azimuth.
There are locations where both the azimuth and elevation angles change by about the same amount, but this change is less than either of the above cases. Here the gain changes will be less than the extreme cases and the coordination area in all cases changes most in the region of the mainbeam of the earth station. Figure 10 provides an overview of such a case.
3.3.2 Change in coordination distances
The effects on the coordination distances are due to elevation angle changes. For nominal elevation angles in the range of 15° to 20°, the gain change would have a maximum value of about 4.4 dB. For elevation angles greater than 20°, a 5° elevation angle change from nominal will result in a maximum increase in the gain in the horizontal plane of 3 dB. For nominal elevation angles larger than about 53°, there will be no gain change.
FIGURE 10...[D19] 16.5 CM
Results of a study on change in maximum coordination distance Mode 1 (great circle propagation) as a function of earth station latitude and longitudinal differences, when compared with a typical 6 GHz earth station coordination contour are presented in Fig. 11. A transmitter power of 20 dBW and 2 MHz energy dispersal have been assumed. The increase of the maximum coordination distance is of the order of 1020% for nominal antenna elevation angles between 10° and 20°, and a few per cent for the higher elevation angles. The change in per cent is independent of the propagation zone considered (Zones A, B and C).
The changes for propagation Mode 2 (scattering due to hydrometeors) are small and in most of the cases, less than 6%.
A stochastic study assessing the input of inclined orbit operations into terrestrial stations has been carried out in which earth stations at latitudes 41° N were used and constrained so that their elevation angles were the local maximum towards the inclined orbit. The results for 15° orbit inclination were a 4.5 dB increase for 23% of the terrestrial receivers for at least 10% of the time (2.4 hours daily) while for 5° inclination 38% of the receivers would receive 1.5 dB increase
in interference for 10% of the time. The expected interference increase will be less for higher elevation angles or for azimuths of the terrestrial receiver that tend to point away from the geostationarysatellite orbit rather than the uniform distribution that was assumed. However, for higher latitudes with smaller elevation angles, the expected interference will be greater.
FIGURE 11...[D14] 19 CM
3.5 Summary
Countries close to the Equator will generally not require recoordination of their earth stations when these operate with an inclinedorbit satellite. Even when their relative longitudes do not fall within the almondshaped zone above 48° elevation angle, there will be only little earth station antenna horizon gain increase which can usually be neglected.
Countries at higher latitudes are increasingly more affected and may, in some cases, have difficulties coordinating and especially recoordinating their earth stations to operate with larger orbit inclinations of their satellite(s). However, in all cases a tradeoff is possible between accepting increased difficulties in earth station coordination and more extensive inclinedorbit operation.
This Annex has examined the sharing situation between the fixed and the FSS when satellites go into inclined orbit. The impact caused to terrestrial networks results from both the space and earth stations. Similarly, satellite networks will also be affected by interference caused to space and earth stations.
The sharing situation when satellites go into slightly inclined orbits is complex. For a short period, it involves greater exposure of fixed service receivers to direct satellite interference and vice versa. The number of terrestrial stations exposed to interference increases with the amount of inclination.
The effect of such exposure upon the system unavailability as well as total interference received has been studied in this Annex. A model has been developed which indicates that under the assumptions of that model an order of magnitude increase in unavailability could be expected. These assumptions include: a system with adaptive power control, all satellites in 10° inclined orbit using 124 dB(W/m^{2}) power fluxdensity and satellites spaced 3° apart. However, it is noted that with the trend toward the use of small earth station antennas, it is doubtful that large inclination angles will be used by satellite operators. Other models, based on an assessment of the distribution of beams from an actual radiorelay network, indicate that the total endtoend interference may in some cases be reduced, depending upon the interference exposure factor.
In a further model, calculation of the aggregate interference over a hypothetical reference circuit shows that the interference does not increase but is redistributed over the length of the network.
Further studies are needed on the models to be used for interference calculations. Additional information is required on the distribution of terrestrial beams around the orbit.
Studies are also necessary which would aid in the development of techniques for both services to ameliorate the interference situation, particularly for low inclination angles. These include such techniques as the methods of using automatic power control, interference cancellers, using wideband pfd limits, satellite antenna pointing restrictions, limits to inclination, coordination procedures, site shielding and others.
With regard to the effect upon the coordination area between earth stations and space stations, the impact varies with elevation angle, azimuth and longitude of the earth station. The resulting increases in coordination distances vary with the degree of inclination. When the satellite inclination is 5° and the nominal elevation angle of the earth station is between 10° and 20°, the increase of the maximum coordination distance, compared with the case where no inclined orbit operation is performed, is of the order of 1020% and for higher elevation angles the increase is a few per cent.
