1、 Rec. ITU-R RS.1280 1 RECOMMENDATION ITU-R RS.1280*, *SELECTION OF ACTIVE SPACEBORNE SENSOR EMISSION CHARACTERISTICS TO MITIGATE THE POTENTIAL FOR INTERFERENCE TO TERRESTRIAL RADARS OPERATING IN FREQUENCY BANDS 1-10 GHz (Question ITU-R 213/7) (1997) Rec. ITU-R RS.1280 The ITU Radiocommunication Asse
2、mbly, considering, a) that active spaceborne sensors may be operated in common frequency bands with systems in the radiolocation and radionavigation services; b) that active spaceborne sensors may have technical characteristics which would cause unacceptable interference to terrestrial radars operat
3、ing in frequency bands 1-10 GHz; c) that the last 20 years of experience in sharing the bands have shown no record of harmful interference occurrences; d) that some terrestrial radars are designed to provide an amount of processing gain with respect to certain types of pulsed interference; e) that t
4、he exact value of the processing gain of a terrestrial radar with respect to a particular type of pulsed interference may not be known; f) that some of the technical characteristics of spaceborne sensors can be adapted to enhance the compatibility with terrestrial radars, but this flexibility is lim
5、ited by the sensor performance requirements, recommends 1 that the methodology given in the Annex 1 may be used to improve the emission characteristics of active spaceborne sensors in order to enhance the compatibility with terrestrial radars operating in common frequency bands; 2 that when specific
6、 interference or sharing criteria have been established for a frequency band, these should be used rather than the generic criteria in the Annex 1. _ *This Recommendation was developed jointly by Radiocommunication Study Groups 7 and 8, and future revisions should be undertaken jointly. *Radiocommun
7、ication Study Group 7 made editorial amendments to this Recommendation. 2 Rec. ITU-R RS.1280 ANNEX 1 1 Introduction A methodology is presented that allows an estimate to be made as to whether or not the unwanted signal received by a terrestrial radar from an active spaceborne sensor may cause diffic
8、ulties if operated in common frequency bands. The calculations highlight a number of parameters of the sensor that can be chosen such that the sharing situation is improved. The frequency bands below 10 GHz being considered by this methodology are 1 215-1 300 MHz, 3 100-3 300 MHz, 5 250-5 350 MHz, 8
9、 550-8 650 MHz and 9 500-9 800 MHz. These are commonly called L-, S-, C-, and X-band (the latter two) by radar operators. The characteristics of these various bands are such that L- and S-bands are quite often used for search and surveillance radars, while C- and X-bands are often used for tracking
10、radars. 2 Calculation of interference to terrestrial radars The average interfering signal power level, I (dBW), received by a terrestrial radar from spaceborne is calculated from I = 10 log Pt+ 10 log ( PRF ) + Gt+ Gr (32.44 + 20 log ( f R ) + OTR PG (1) where: Pt: peak spaceborne sensor transmitte
11、r power (W) : spaceborne sensor pulse width (s) PRF : spaceborne sensor pulse repetition frequency (Hz) Gt: spaceborne sensor antenna gain towards terrestrial radar (dBi) Gr: terrestrial radar antenna gain towards spaceborne sensor (dBi) f : frequency (MHz) R : slant range between sensor and radar (
12、km) OTR : radar receiver on-tune rejection (dB) PG : processing gain (dB), rejection of unwanted signals due to radar receiver signal processing (assumed to be zero if not known). Equation (1) gives the average interference signal power level. The average interference power level is used when it can
13、 be determined that such use is appropriate: For example, a radar that performs a fast Fourier transform on the received signal will “smear” the dissimilar pulsed signal across a number of bins, resulting in an averaged interfering signal level. The on-tune rejection term is calculated from: OTR = 1
14、0 log (Br /Bt ) for Br Bt(2a) = 0 for Br Bt (2b) where: Br: receiver bandwidth Bt: bandwidth of the transmitted interfering signal. If the peak interfering signal is of interest, then the second term of equation (1) should be left out, and on-tune rejection is calculated from the following: Input pu
15、lse with no frequency modulation: OTR = 20 log ( Br ) for Br 1 (3b) Rec. ITU-R RS.1280 3 Input pulse with frequency modulation: OTRBBBBrcrc=01for2BBrc(4b) where: Br: terrestrial radar IF bandwidth Bc: chirp bandwidth of spaceborne sensor : sensor pulse width. 3 Interference criteria for terrestrial
16、radars General interference criteria for the terrestrial radars have been postulated based upon preliminary work of JRG 7-8R; if specific criteria exist for specific systems or frequency bands, these should be used. 3.1 Surveillance radars It will be assumed that the received signal-to-noise of the
17、surveillance radars may not be degraded by more than 0.5 dB longer than a single scan time, taken to be 10 s. This equates to an interference-to-noise power ratio of 9 dB at the receiver IF stage. The average interfering signal power level is considered to be of interest in the case of the surveilla
18、nce radars. 3.2 Tracking radars Tracking radars often use “range gates” to exclude all returns other than those at specific ranges of interest. An important consideration in determining the susceptibility of a tracking radar to an interfering pulse train is the fraction of interfering pulses that ar
19、e coincident with the range gate. The coincidence of interfering pulses with the range gate will depend upon whether the desired and undesired pulse repetition frequencies are related by integer multiples (Case I) or not (Case II). The fraction of coinciding pulses, fc, is found from ( )fGCF PRF PRF
20、PRFcigg=,for Case I (5a) ( )fPRFcigi=+ for Case II (5b) where: PRFi: interfering pulse frequency PRFg: gate repetition frequency GCF ( PRFi, PRFg) : greatest common factor of PRFiand PRFgi: interfering pulse width g: gate width. Note that when i gand the desired and undesired PRFs are not related by
21、 integer multiples (Case II), fcis approximately the duty cycle of the interfering pulses. This situation is considered to be the typical case, and is used in the following determination of degradation threshold for a tracking radar. 4 Rec. ITU-R RS.1280 To obtain highly accurate position data on ob
22、jects of interest, tracking radars use high gain antennas with well defined, narrow mainbeams. A servo mechanism attempts to keep the boresight of the antenna mainbeam on the target; the servo mechanism is driven by an error signal generated by the angle error between the target and the antenna bore
23、sight. Undesired signals entering the radar can increase this bias error. A degradation threshold for a tracking radar, expressed as an allowed fraction of coincident interfering pulses, fc, as a function of the signal-to-interference ratio at the IF output is given as: faBSIcr=219011(/ )when S/I 1
24、(6a) faBIScr=219011(/ )when S/I 45 Horizontal (azimuth) Gh(h) = 0.0 19.6(h )2Gh(h) = 24.5 0.47 hGh(h) = 30.5 0 12.7 Beam pattern G() = Gv(v) + Gh(h), 11 max Pattern Gain G() (dBi) as a function of off-nadir angle (degrees) Angular range Elevation Gel(el) = 39.7 0.397(el)2Gel(el) = 19.7 Gel(el) = 4.7
25、 Gel(el) = 17.7 0 3.9 Beam pattern G () = Gel(el) + Gaz(az), 8 max 6 Rec. ITU-R RS.1280 TABLE 4 Spaceborne SAR antenna gain pattern at 5 300 MHz TABLE 5 Spaceborne SAR antenna gain pattern at 8 600 and 9 650 MHz 4.1.2 Terrestrial radars A sidelobe level of 5.38 dBi (corresponding to the minimum gain
26、 for a 38 dBi gain antenna pattern) is used in the calculations for the surveillance radars, since mainbeam conjunctions are rare and very brief due to their scanning characteristics. Mainbeam coupling is more likely in the case of the tracking radars, which may point in a specific direction for lon
27、g periods; a high gain (47 dBi) parabolic antenna gain pattern is used for the tracking radars. For both cases, a 1 MHz IF bandwidth, and a 5 dB noise figure are assumed. 4.2 Analysis approach and results A simulation of 1 500 orbits of the spaceborne sensor was performed. The tracking event repeate
28、d throughout the simulation assumed for the tracking radars (C- and X-band) is shown in Fig. 2. (A target with a 5 dB/m2radar cross section is used). The received unwanted signal was calculated throughout the period simulated, and compared to the criteria. The processing gain, as defined in equation
29、 (1), was assumed to be equal to zero. If the criteria were exceeded, the simulation was re-run with an incremental reduction in the received unwanted signal power. This was repeated until the criteria were not exceeded. The results for the sensor in Table 1 are shown in Figs. 3 to 6. Pattern Gain G
30、() (dBi) as a function of off-axis angle (degrees) Angle range Vertical (elevation) Gv(v) = 42.7 0.478(v)2Gv(v) = 40.1 1.0 vGv(v) = 5 0 45 Horizontal (azimuth) Gh(h) = 0.0 442(h)2Gh(h) = 25.0 2.2 hGh(h) = 31 0 2.7 Beam pattern G() = Gv(v) + Gh(h), 5 max Pattern Gain G() (dBi) as a function of off-ax
31、is angle (degrees) Angular range Vertical (elevation) Gv(v ) = 44.0 0.397(v )2Gv(v ) = 24.5 Gv(v ) = 9.5 Gv(v ) = 22.5 v60 Horizontal (azimuth) Gh(h ) = 0 612.2(h )2Gh(h ) = 12 Gh(h ) = 0 27.0 dB (h ) Gh(h ) = 35 h1.3 Beam Pattern G() = Gv(v ) + Gh(h ), 3 min Rec. ITU-R RS.1280 7 1280-020 100 200 30
32、0 400 500 6000100200300400Time (s)Azimuth angle (degrees)Elevation angle (degrees)Range (km)See legendFIGURE 2Tracking event assumed for terrestrial tracking radarFIGURE 1280-02 = 10 CM 1280-0320010000510Decrease in unwanted signal (dB)# timescriterion exceededFIGURE 3L-band radar (1 240 MHz)FIGURE
33、1280-03 = 7 CM 8 Rec. ITU-R RS.1280 1280-044020001020Decrease in unwanted signal (dB)# timescriterion exceededFIGURE 4S-band radar (3 200 MHz)FIGURE 1280-04 = 7 CM 1280-0501020500100Decrease in unwanted signal (dB)# timescriterion exceededFIGURE 5C-band radar (5 300 MHz)FIGURE 1280-05 = 7 CM 1280-06
34、01020050100Decrease in unwanted signal (dB)# timescriterion exceededFIGURE 6X-band radar (8 600 MHz)FIGURE 1280-06 = 7 CM Rec. ITU-R RS.1280 9 5 Discussion of example analysis results It can be seen from the Figs. 2 to 6 that regardless of whether tracking radars or surveillance radars are being con
35、sidered, the sensor of Table 1 can potentially cause a significant number of interference events. However, a decrease in the unwanted signal power level received by the terrestrial radars can improve the situation dramatically. A reduction of around 13 and 20 dB, for the surveillance radars and trac
36、king radars, respectively, can reduce the number of possible interference events to a number small enough that it is likely to be acceptable. 6 Procedure to use methodology The peak or average power of an active spaceborne sensor should be examined during the design stages. If it is not in the range
37、 of 13 to 20 dB below that of the example sensor of Table 1, then unacceptable interference into a terrestrial surveillance or tracking radar, respectively, is possible (see Note 1). Equations (1) to (4) can be examined to determine parameters that can potentially be adjusted during the design of th
38、e spaceborne sensor, in order to improve sharing with terrestrial radars. Transmitter power, antenna gain (particularly sidelobe levels), pulse width and repetition rate, and chirp bandwidth are all likely candidates for adjustment. NOTE 1 When compatibility between a spaceborne sensor and a particu
39、lar terrestrial radar is analysed, the processing gain, if any, of the terrestrial radar should be considered since the analysis assumed that there was none. This assumption is valid for the general case since not all radars have processing gain. If all radars within a band have some processing gain
40、 a single value will need to be determined. For example, consider two radars operating in 9 500-9 800 MHz: a tracking radar with a 1 MHz IF bandwidth (radar 1) an airborne intercept radar with a 5 MHz IF bandwidth (radar 2). If the spaceborne sensor of Table 1 can be operated with a different pulse
41、width and chirp bandwidth such as in Table 6, then a significant reduction in the unwanted signal power level can be achieved. The reduction in unwanted signal power level obtained in this example is of the order of that needed to reach the improved sharing situation desired. TABLE 6 Example of redu
42、ction in received unwanted sensor power, via changes in sensor pulse width and chirp bandwidth 7 Conclusion It has been demonstrated that it should be possible to meet the protection criteria for terrestrial radars through selection of active spaceborne sensor parameters with this goal in mind. Sens
43、or transmitter power, antenna gain pattern, pulse width, pulse repetition frequency, and chirp bandwidth (if frequency modulation is used) are all possible characteristics that can be adjusted to improve compatibility. New parameter values OTR Pavg I (s) Bc(MHz) (dB) (dB) (dB) Radar 1 3 280 19.7 NA(1)19.7 Radar 2 3 280 5.7 10.5 16.2 (1)It has been deemed appropriate to use average interference signal power for the airborne radar, and peak interference signal power for the tracking radar.
