1、 Rec. ITU-R RS.1449 1 RECOMMENDATION ITU-R RS.1449*FEASIBILITY OF SHARING BETWEEN THE FSS (SPACE-TO-EARTH) AND THE EARTH EXPLORATION-SATELLITE (PASSIVE) AND SPACE RESEARCH (PASSIVE) SERVICES IN THE BAND 18.6-18.8 GHz (Question ITU-R 215/7) (2000) Rec. ITU-R RS.1449 The ITU Radiocommunication Assembl
2、y, considering a) that the 18.6-18.8 GHz band is allocated to the FSS (space-to-Earth) on a primary basis; b) that the 18.6-18.8 GHz band is allocated to the Earth exploration-satellite service (EESS) (passive) and space research service (SRS) (passive) on a primary basis in Region 2 and on a second
3、ary basis in Regions 1 and 3; c) that RR No. S5.523 states that administrations are requested to limit as far as practicable the pfd at the Earths surface in the band 18.6-18.8 GHz, in order to reduce the risk of interference to passive sensors in the EESS and space research service; d) that passive
4、 sensing in this band is necessary to obtain critical environmental measurements on a worldwide basis and no other frequency band is available that could replace this band; e) that the band 18.6-18.8 GHz is very important for use by FSS in view of the large number of systems in the planning stage an
5、d of the presence of some systems already in operation; f) that studies have been conducted assessing the aggregate interference into a passive spaceborne sensor from multiple FSS transmitting satellites, all utilizing spot beams and having a common service area, and have found that constraints are
6、needed on both FSS and EESS (passive) systems if sharing is to take place (see Annex 1); g) that studies have shown that FSS satellites in 8, 12 and 24 h highly elliptical orbits (HEO) cause less interference to the sensor than GSO FSS satellites (see Annex 2), recommends 1 that passive sensors oper
7、ating in the EESS and SRS and acquiring data over land masses be designed to collect data only when travelling north in the northern hemisphere and south in the southern hemisphere and utilize an antenna that is inclined about 40 from nadir and scans in azimuth about the velocity vector of the space
8、craft; 2 that passive sensors be designed to operate in an interference environment based on a pfd produced by a FSS at the surface of the Earth limited to 95 dB(W/m2) across the 18.6-18.8 GHz band; this value could be exceeded by up to 3 dB for no more than 5% of the time. ANNEX 1 An evaluation of
9、potential interference into the EESS (passive) in the 18.6-18.8 GHz band from GSO satellites operating in the FSS 1 Introduction Spaceborne microwave science sensors make use of very narrow beam antennas to obtain such information as surface temperature, moisture content, sea state and others. Certa
10、in passive sensors make use of the 18.6-18.8 GHz band. This same band is shared with GSO satellite systems operating in the FSS. There is potential for excessive interference into the passive sensors from these GSO satellites. This would be dominated by energy scattered from terrestrial targets into
11、 _ *Radiocommunication Study Group 7 made editorial amendments to this Recommendation. 2 Rec. ITU-R RS.1449 the receiving antennas of these passive sensors. The level of interference is affected by: the individual GSO pfd; number of simultaneous GSO systems; methods of frequency reuse within the FSS
12、; the reflectivity of the terrain as characterized by the terrain scattering coefficient; and the mode of operation of the spaceborne sensor. In the United States of America, passive sensors have a primary allocation in the 18.6-18.8 GHz band. The United States of America limits the FSS pfd to 101 d
13、B(W/(m2 200 MHz). Internationally, passive sensors have a primary allocation in Region 2 and secondary allocations in Regions 1 and 3. RR Table S21.4 limits FSS pfd to 92 dB(W/(m2 200 MHz) at low elevation angles up to 82 dB(W/(m2 200 MHz) and higher elevation angles. Consequently, there is a potent
14、ial of pfd from the FSS from 101 dB(W/(m2 200 MHz) to 82 dB(W/(m2 200 MHz) globally. Since the spaceborne passive sensors would be exposed only intermittently to scattered energy from FSS coverage areas, it is of interest to determine the rate of occurrence of excessive interference events. Recommen
15、-dation ITU-R RS.1029 states that in shared frequency bands (except in the absorption bands), the interference levels given above (155 dB(W/100 MHz) for 18.6-18.8 GHz) can be exceeded for less than 5% of all measurement cells within a sensors service area in the case where the loss occurs randomly,
16、and for less than 1% of measurement cells in the case where the loss occurs systematically at the same locations. The objective of this work is to identify the areas of excessive interference for the different levels of potential FSS pfd. The criteria from Recommendation ITU-R RS.1029 indicate the a
17、ppropriate metric for interference depends on the particular sensor application and the nature of the interference that occurs. Herein, we report estimates for both conditional events (rate of occurrence given the spaceborne sensor is within a FSS coverage area) and unconditional events (rate of occ
18、urrence globally). A secondary goal is to evaluate potential interference mitigation techniques and to describe a potential method of mitigating interference by avoiding geometry where the sensor might be pointed directly, or nearly so, into a specular reflection. It has been suggested that addition
19、al mitigation could be achieved by restricting the sensor scan range from 70 to 35. Both methods were evaluated for a specific case of 4, 8, and 16 GSO systems in the FSS simultaneously serving a coverage area described with 24 spot beams. The approach was to make use of Monte-Carlo simulations wher
20、e the interference into a spaceborne sensor is estimated as it orbits the Earth. The motion of the GSO satellites, the Earth, and the spaceborne sensor are all accounted for. Since the scattering of energy from the Earths surface is a random phenomenon (due to independent fading effects and terrain
21、variability), this variability is also included in the simulation. At each simulation time instant, the interference from all 16 satellites was accounted for including the weighting by the directivity and angular offset of the individual spot beams. 2 Interference scenario It is of interest to estim
22、ate a worst-case level of interference into an EESS satellite passive sensor from a maximum constellation of 16 FSS satellite systems that each provides a common coverage area that is described by 22 conterminous beams and two nearby, but geographically separate beams. Each of the systems uses four
23、times frequency segmentation to minimize intra-system interference. The 16 satellite systems are then grouped by systems of four that offset their segmentation choices to further minimize inter-system interference. Further reduction of mutual interference is achieved by a 2 orbital separation betwee
24、n systems. Figure 1 illustrates a microwave passive sensing satellite passing over a FSS coverage area and receiving interference from the ground reflections. The sensor makes use of a highly directive receiving beam that is scanned 70 perpendicular to the direction of motion. The sensing area is ve
25、ry small compared to the FSS spot beams that make up the total FSS coverage area. In the simulations performed for this work, the interference is calculated at 2 increments of scan. As mentioned, each system will segment its spectrum to minimized intra-system interference. Figure 2 illustrates the i
26、solation of four spot beams by such segmentation. The pattern is repeated throughout the coverage area. The next three systems will, it is assumed, stagger the segmentation so the second system, for example, might use F2 in place of F1, F3 in place of F2, etc. Additional systems after the fourth wil
27、l simply repeat the pattern. Isolation of the systems will be Rec. ITU-R RS.1449 3 aided by spatial separation in orbit at 2 increments and use of directive antennas by terrestrial-based users. By this scenario each of the 22 conterminous spot beams will scatter power from four GSOs with spectrum th
28、at overlaps that of the EESS, as well as other GSOs that contribute through side lobes that overlap from adjacent beams. 1449-01FIGURE 116 GSO FSS systems interfering with spaceborne sensorEESSsatelliteScattered FSSenergy16 FSS systemsFIGURE 1/SA.1449.D01 = 3 CM 1449-02F1 F2 F1 F1 F1F2 F2F3F4 F3F3F4
29、F4F4F1 F1F1F2 F2 F2F3F3F4F4FIGURE 2Typical FSS coverage, 22 conterminous beams and two off-shore beamsAlaskaHawaiFIGURE 2/SA.1449.D01 = 3 CM In the event such intra-service interference mitigation techniques are not used, it is possible that all satellites would use identical frequency segmentation.
30、 In that case each spot would scatter energy from 16 satellites. However, since the spots would still have the F1-F4 allocation pattern, only a subset of these spots would scatter energy that overlapped the EESS band. In the scenario shown in Fig. 2 this could range from 5 to 7 spot beams depending
31、on the selections of F1-F4 segments. 4 Rec. ITU-R RS.1449 A set of typical characteristics for the FSS and EESS systems are described in Tables 1 and 2 respectively. The scattering coefficient model described in 3 is used as the basis for estimating the scattered power. TABLE 1 FSS parameters in the
32、 18.6-18.8 GHz band TABLE 2 Spaceborne passive sensor parameters in the 18.6-18.8 GHz band Coverage 22 conterminous, 2 off-shore 5 dB beamwidth 1.0 Maximum antenna gain 46.5 dBi pfd at surface of the Earth 101.0 dB(W/(m2 200 MHz) each polarization Polarization RHC and LHC each beam Bandwidth 200 MHz
33、 Frequency reuse Every fourth beam Antenna pattern Recommendation ITU-R S.672 with LN= 25 dB and LF= 0 dBi Orbital altitude 500 km Orbital inclination 90.0 Boresight elevation angle 45.0 Antenna pattern dBi Off-axis angle, 57.0 0.2 21.0 0.2 90.0 Antenna scan angle 70 Polarization Linear Receiver ref
34、erence bandwidth 100.0 MHz Interference threshold 155 dB(W/100 MHz) Along scan spatial resolution 2 km Across scan spatial resolution 2 km Circular to linear polarization loss 1.5 dB Basis for scattering estimates Skylab/University of Kansas, backscatter curve fits and facet theory (see 3) Rec. ITU-
35、R RS.1449 5 3 Scattering model A critical parameter affecting the level of interference into a spaceborne sensor is the terrestrial reflectivity as indicated by the scattering coefficient of the terrain being viewed by the sensor. This can result in energy being scattered backwards toward its GSO FS
36、S source and being intercepted by a spaceborne sensor that is moving away from the GSO nadir. Alternatively, the energy can be scattered forward and intercepted by a sensor that is moving toward the GSO nadir. There is much more data on backscatter phenomena than for the forward scatter case. Herein
37、 we attempt to make use of available backscatter data and extrapolations, and to infer from these the forward scatter effects. 3.1 Backscatter coefficients Several methods have been proposed in open literature for modelling the mean and extreme values for backscattering coefficients. Scatterometer d
38、ata from Skylab, as shown in Fig. 3, indicate a dependence on incidence angle that varies rapidly over the range of 0-10. The maximum is at 0 incidence. 1449-030 1020304050201816141210864202468101214FIGURE 3Data from Skylab S-193 scatterometerAngle of incidence (degrees)Regression meanRegression upp
39、er and lower decilesData meanData upper and lower decilesData upper and lower 5% levelsScattering coefficient(dB)Regression lines shown for mean 0 = From the above: 86.0)/( =LandOverAreaCoverageFSSP Scaling for different GSO pfd limits is accomplished by shifting the exceedence curves to the right b
40、y the amount the new pfd exceeds 101 dB(W/(m2 200 MHz). 4.3 Occurrence rates of excessive pixel loss The methods in prior sections were used to generate the results in Table 3. The results reported in Fig. 10 are dependent on the assumption of a maximum deployment of 16 FSS systems per coverage area
41、, all providing overlapping, dual-polarization coverage to 24 spot beams. Additional scenarios of 4 and 8 satellites per FSS coverage area were also examined. If the allowable FSS pfd limit is fixed at 101 dB(W/(m2 200 MHz), the exceedence distributions for systems having 4, 8, and 16 systems per FS
42、S coverage area is illustrated in Fig. 11. The 16 system case has four common interferers per spot beam, the 8 system case two, and the four system case one. For a particular exceedence probability, the interference level is reduced approximately by the ratio of number of interferers per spot. 1449-
43、11190 180 170 160 150102101101FIGURE 11Exceedence distributionsInterference level (dB(W/100 MHz)Sensitivity to system complexitymaximum pfd/system (101 dB(W/(m2 200 MHz) per polarization)Probabilityofexceeding level4 systems8 systems16 systemsFIGURE 11/SA.1449.D01 = 3 CM 12 Rec. ITU-R RS.1449 TABLE
44、3 Coverage loss for various FSS pfd values The 1% systematic interference criteria is exceeded for all pfd values in the 16 satellite case but is not exceeded for pfd limits less than 98 dB(W/(m2 200 MHz) in the 8 satellite case, nor is it exceeded for pfd limits less than 96 dB(W/(m2 200 MHz) in th
45、e 4 satellite case. The 5% random events criteria is met by several cases. 4.4 Sensitivity to sensor scan range It has been proposed that under certain circumstances a degree of interference mitigation can be achieved by reducing sensor scan range. The approach was tested with the scenario described
46、 in 2 and found to realize insignificant benefit. The solid lines in Fig. 12 reiterate previous analyses that included a 70 scan of the sensor. The symbols represent the same case with scan reduced to 35. Further testing of the concept revealed there was some advantage where the scattering coefficie
47、nt was dominantly specular. Otherwise, the benefit was, indeed, insignificant. pfd Data loss over FSS service area (%) Data loss over all land masses (%) (dB(W/(m2 200 MHz) 4 8 16 4 8 16 101 2.4 2.1 100 0.3 7.8 0.3 6.7 99 0.1 1.1 18.4 0.1 1.0 15.8 98 0.3 3.2 32.1 0.3 2.8 27.6 97 0.8 7.9 43.3 0.7 6.8
48、 37.2 96 2.0 16.1 50.1 1.7 13.8 43.0 95 4.4 26.5 54.5 3.7 22.8 46.9 94 8.6 36.3 58.3 7.4 31.2 50.1 93 14.9 43.6 61.6 12.8 37.5 52.9 92 22.9 48.7 64.6 19.7 41.8 55.6 91 31.5 52.6 67.5 27.0 45.2 58.0 90 38.9 56.1 70.3 33.5 48.2 60.4 89 44.7 59.2 73.0 38.4 50.9 62.8 88 49.4 62.2 75.9 42.5 53.5 65.3 87
49、53.3 65.2 78.9 45.8 56.1 67.9 86 56.7 68.1 82.7 48.8 58.5 71.1 85 59.9 71.0 87.3 51.5 61.0 75.1 84 63.1 74.3 92.2 54.2 63.9 79.3 83 66.2 78.1 95.8 56.9 67.2 82.4 82 69.4 82.8 98.0 59.7 71.2 84.2 Rec. ITU-R RS.1449 13 1449-12180 170175 160165 150155101101102103102FIGURE 12Effect of restricted scan range of sensorInterference level (dB(W/100 MHz)Probabilitydensity oflevel140 spanNorthern pass only70 span140 span70 span00.160.140.120.100.080.
