1、 Rec. ITU-R RS.1347 1 RECOMMENDATION ITU-R RS.1347*FEASIBILITY OF SHARING BETWEEN RADIONAVIGATION-SATELLITE SERVICE RECEIVERS AND THE EARTH EXPLORATION-SATELLITE (ACTIVE) AND SPACE RESEARCH (ACTIVE) SERVICES IN THE 1 215-1 260 MHz BAND (Question ITU-R 218/7) (1998) Rec. ITU-R RS.1347 The ITU Radioco
2、mmunication Assembly, considering a) that the radionavigation-satellite (space-to-Earth) service is allocated on a primary basis in the 1 215-1 260 MHz frequency band; b) that active spaceborne sensors operating in the Earth exploration-satellite and space research services are allocated on a second
3、ary basis, according to Footnote S5.333, in the 1 215-1 300 MHz frequency band; c) that sharing studies have shown compatibility between the radionavigation-satellite service receivers, including L5 receivers, and active spaceborne sensors in the acquisition and tracking phases (refer to annex); d)
4、that compatibility tests have demonstrated compatibility between the radionavigation-satellite service GPS receivers in the tracking phase and synthetic aperture radars (refer to annex), recommends 1 that, in view of considering c) and d), sharing be considered feasible between the radionavigation-s
5、atellite service and spaceborne synthetic aperture radars in the 1 215-1 260 MHz frequency band. ANNEX Potential interference from spaceborne active sensors into radionavigation-satellite service receivers in the 1 215-1 260 MHz band 1 Introduction The 1 215-1 260 MHz frequency band is allocated to
6、the radionavigation-satellite service (RNSS) and is used by both the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS-M). The 1 215-1 300 MHz band is used by spaceborne active microwave sensors under the provisions of Footnote S5.333 of the Radio Regulations. The o
7、nly active sensor requiring use of this band is the synthetic aperture radar (SAR). This annex presents the compatibility analyses of typical spaceborne synthetic aperture radars (SARs) into GPS and GLONASS-M receivers for both acquisition and tracking phases and presents the compatibility test resu
8、lts between the SAR and GPS for the tracking phase. In addition, the GPS L5 is under consideration for this band. 2 Technical characteristics of a spaceborne SAR The technical characteristics for two standard synthetic aperture radars which use the 1 215-1 300 MHz band are given in Table 1. The para
9、meters of these systems offer a range of possible characteristics to use as representative for an operational SAR. The characteristics chosen in this analysis are those which would result in the worst case interference to a radionavigation-satellite service receiver. _ *Radiocommunication Study Grou
10、p 7 made editorial amendments to this Recommendation. 2 Rec. ITU-R RS.1347 3 Characteristics and protection criteria of GPS and GLONASS-M systems Recommendation ITU-R M.1088 gives the characteristics and system description for the Global Positioning System (GPS) to be used in assessing sharing betwe
11、en other services and a GPS receiver. Recommenda-tion ITU-R M.1317 gives the characteristics and system description for the Global Navigation Satellite System (GLONASS-M) to be used in assessing sharing between a GLONASS-M receiver and other services. The GPS L5 is under consideration for this band.
12、 The characteristics of GPS L5 have been presented as being similar to those of the GPS Coarse Acquisition (C/A) code described in Recommendation ITU-R M.1088. Operation of GPS L5 is expected to behave as the GPS C/A code in the presence of interference signals. TABLE 1 Technical characteristics of
13、spaceborne synthetic aperture radars in the 1 215-1 300 MHz band GPS and GLONASS-M receivers are susceptible to both pulsed and continuous interference for both acquisition and tracking phases. In the case of potential interference from a SAR, the interference falls into the category of pulsed inter
14、ference. Pulsed interference can affect an RNSS receiver in two ways: by causing preamplifier saturation or preamplifier burnout. The principle interference effect is that the pulsed interference causes limiting in the receiver. This occurs when a signal level is received that is strong enough to ca
15、use the high-level limiter diode, located in the RF front-end of the receiver, to saturate in order to prevent burnout of the following receiver stages. When this limiting occurs, the relatively low desired signal would be blocked during the transmission pulse period and any recovery time that is ne
16、cessary for the RNSS receiver. If this period of lost signal is short relative to the GPS information bit length, there should be no appreciable impact on the performance of the receiver. The other possible interference effect occurs when either the peak or average RF power level is high enough to c
17、ause the diode to fail. If this occurs, preamplifier burnout is possible and damage to the receiver may occur. The relevant technical characteristics for the two RNSS systems are summarized in Table 2. The saturation power level (preamplifier limiting level) and the power level required for preampli
18、fier burnout for each of the systems is also given in Table 2. Also in the acquisition phase, the spaceborne SAR at orbit altitudes illuminates a given location on Earth in the mainbeam for only 1-2 seconds, which is typically less time than required for acquisition. Any pulsed signal power level re
19、ceived that is below the preamplifier limiting levels of the RNSS receivers is assumed to have a negligible effect on the performance of the receiver since the SAR transmitted pulse period is relatively short compared to the RNSS information bit length and the SAR transmitter duty cycle is very low.
20、 Standard SAR 1 Standard SAR 2 Peak radiated power (W) 3 200 1 200 Pulse modulation Linear FM Chirp Linear FM Chirp Pulse bandwidth (MHz) 40.0 15.0 Pulse duration (s) 33.8 35.0 Pulse repetition rate (pps) 1 736.0 1 607.0 Duty cycle (%) 5.9 5.6 Maximum antenna gain (dBi) 36.4 33.0 Antenna orientation
21、 (degrees from nadir) 20.0 35.0 Antenna polarization Linear vertical/horizontal Linear horizontal Orbital altitude (km) 400 568 Rec. ITU-R RS.1347 3 TABLE 2 Characteristics and protection criteria for navigation user equipment of the GPS and GLONASS-M systems 4 Compatibility analyses 4.1 Compatibili
22、ty analysis with consideration of SNR degradation The first step in analysing the interference potential from a spaceborne SAR to a GPS or GLONASS-M receiver is to determine if the peak signal power from the SAR is great enough to cause the high-level clipper diode to fail and possibly cause preampl
23、ifier burnout and damage the receiver. The maximum interfering signal power levels received from a spaceborne SAR occur when an RNSS receiver is located in the mainbeam of the SAR antenna. The peak interfering signal power levels from a SAR into a GPS or a GLONASS-M receiver are calculated in Table
24、3. The calculations assume co-frequency operation. These levels for maximum peak power at the receiver input of an RNSS receiver are well below the levels that would cause the high-level clipper diode to fail. Thus, the emissions from a spaceborne SAR will not cause high-level clipper diode burnout
25、or damage to either a GPS or GLONASS-M receiver. The interfering signal level at the GPS receiver input at which the diode will saturate and cause a temporary loss of signal is -70 dBW. For the acquisition phase, this level is 6 dB lower, or -76 dBW. Even in the worst case configuration, this interf
26、ering signal level will not be reached. The interfering signal level at the GLONASS-M receiver input at which the diode will saturate and cause a temporary loss of signal is -80 dBW. In the worst case configuration depicted in Table 3, this interfering signal level may be exceeded by 1.5 dB due to t
27、he transmissions of a SAR. Taking into account the fan-beam shape of the SAR antenna, the resultant necessary off-axis angle to produce 1.5 dB of discrimination is 0.28. A simulation of 15 000 orbits was run (400 km altitude, 57 inclination) to determine how often a stationary GLONASS-M receiver wou
28、ld be within 0.56 of the SAR antenna mainbeam (0.28 on either side). The results showed that this would occur for less than 0.0019% of the time. This means that an interfering signal level greater than the diode saturation level would be received for less than 2 seconds per day assuming this worst c
29、ase interference situation. The compatibility analysis for the GLONASS-M showed that the service should be compatible with multiple spaceborne SAR interference sources, up to four SARs illuminating the GLONASS receivers in the mainbeam of the SAR at the same time, assuming the cumulative duty factor
30、 is 20% for four SARs. Given the small number of spaceborne SARs likely to be in orbit and in operation at the same time, and also given the likely diversity in orbital altitudes, orbital velocities, and orbital periods of the SARs from different administrations, the probability is extremely low tha
31、t four or even two spaceborne SARs will illuminate a GLONASS-M receiver at the same time. Further, two or more SARs would never simultaneously observe the same scene because the resulting mutual interference would preclude acquisition of usable data. GPS GLONASS-M Carrier frequencies (MHz) 1 227.6 1
32、 237-1 261 RF 3 dB filter bandwidth (MHz) 17.0 20.0 Polarization RHC RHC Maximum antenna gain (dBi) 0.0 0.0 Preamplifier burnout level (average) (dBW) 0.0 -1.0 Preamplifier burnout level (peak) (dBW) 10.0 0.0 Preamplifier limiting level (dBW) -70.0 -80.0 4 Rec. ITU-R RS.1347 TABLE 3 Maximum interfer
33、ing signal power levels from SAR into GPS and GLONASS-M receivers Even in the case where the signal level causes the front-end to saturate and a temporary loss of the RNSS signal occurs, there will be no appreciable impact on the performance of the receiver if the SAR transmitted pulse period plus t
34、he RNSS receiver recovery time are relatively short compared to the RNSS information bit length. If a recovery time of 1 s is assumed for the receiver, the transmission from one SAR would remove about 6% of the RNSS signal during the time that the receiver is saturated. (SAR pulse width + RNSS recov
35、ery time) * SAR pulse repetition frequency = 6.04% When the RNSS receiver is saturated ( 70 dBW for GPS), the signal is essentially lost during the width of the interfering pulse plus any front end recovery time. The GPS navigational data rate is50 bits/s and is mod-2 added to the P-code or C/A code
36、 PRN sequence of 10.23 MHz or1.023 MHz, respectively, before phase modulation onto the 1 227.6 MHz tone. Thus the GPS navigational data bit period is 20 ms long, and during this bit period there are over 204 K P-code chip periods, and from 25 to 43 SAR pulses transmitted. The fraction of signal powe
37、r lost during the bit period is then equal to the ratio of the combined width of the interfering pulse plus any GPS front end recovery time for both acquisition and tracking phases over the interpulse period. The SNR degradation SNR in dB is the ratio of the SNR with interference present to the SNR
38、with no interference and is given by the following: SNR 10*log 1PW RTIPP=+ (1) where PW is the SAR pulse width, RT is the GPS recovery time, IPP is the interpulse period of the SAR, and the argument of the log function is greater than zero. Assuming that the GPS recovery time is from 1 to 30 s, the
39、SNR degradation for the range of SAR 1 pulsewidths and PRFs yields values from -0.1 to -0.6 dB. The SIR-C phased array antenna contains distributed high power amplifiers (HPAs) for amplification and transmission of the 1.25 GHz signal. The HPA dc power is gated on only during the transmit event, and
40、 is gated off during the interpulse period. The HPAs are class C devices in that they are powered to transmit only during the presence of the input pulse. Since these devices are powered off during the interpulse period, there is no interpulse noise present from the spacecraft. Interference to GPS I
41、nterference to GLONASS-M SAR 1 SAR 2 SAR 1 SAR 2 Centre frequency (MHz) 1 227.6 1 227.6 1 250.0 1 250.0 Peak radiated power (dBW) 35.1 30.8 35.1 30.8 Transmitter antenna gain (dB) 36.4 33.0 36.4 33.0 Distance (km) 427.5 709.3 427.5 709.3 Space loss (dB) 146.8 151.2 147.0 151.4 Receiver antenna gain
42、(dB) 0.0 0.0 0.0 0.0 Polarization mismatch loss (dB) 3.0 3.0 3.0 3.0 Maximum received interference power (peak) (dBW) -78.3 -90.4 -78.5 -90.6 Rec. ITU-R RS.1347 5 4.2 Compatibility analysis with consideration to GPS receiver tracking loop gain 4.2.1 Introductory assumptions We present here below a c
43、ompatibility study between spaceborne synthetic aperture radars (SARs) and GPS services in L2 band, i.e. around 1 227.6 MHz with consideration to the GPS receiver tracking loop gain. Since no precise information is available at this time about the L2 P code acquisition phase neither for receivers ab
44、le to track Y/P codes nor for codeless or semicodeless receivers, the analysis below will be limited to interference into GPS L2 receivers in tracking mode. 4.2.2 Information contained in the reference documents The document ITU-R 7-8R/14 presents a typical spaceborne radar to be used for future Ear
45、th observation missions. The centre frequency of the SAR may be somewhat different from that of the GPS receiver, but we consider in the analysis a worst case where the SAR centre frequency would be in the middle of the GPS receiver band. The peak power is 3 200 Watts, the bandwidth 10, 20 or 30 MHz
46、, with a linear chirp modulation, the pulse repetition rate is 1 395 or 1 736 pulses per second. The maximum pulse duration is 33.8 s. The antenna gain is 36.4 dBi; the antenna may scan in elevation from 20 to 55 degrees from nadir. The orbit altitude is 400 km. The Recommendation ITU-R M.1088 prese
47、nts the GPS receivers; the minimal power received at 1 227.6 MHz is -136 dBm; the RF filter 3 dB bandwidth is 17 MHz; an acceptable performance of the receiver can be maintained with spurious incoming signals up to 41 dB above the signal level for the tracking phase, i.e. -95 dBm. The front end ampl
48、ifier input power for saturation is -40 dBm. 4.2.3 Assumptions concerning the GPS receiver operation The GPS signals when entering the receiver are well below the noise floor of the equipment; hence the signal which is sampled and coded by the receiver is essentially noise; for an optimum coding, th
49、e noise has to be maintained by an automatic gain control loop (AGC) at the analog to digital coder input (ADC) at a constant level which is defined by = A/3, where is the noise standard deviation and A the saturation level of the ADC. The AGC loop time constant is assumed to be large compared to input pulse repetition period (716 s maximum). The noise input level may be estimated to -97 dBm, with a receiver 4 dB overall noise figure and with an equivalent input bandwidth of 20 MHz. Hence the equivalent saturation threshold of the ADC corresponds to -
