1、 Rec. ITU-R RS.1166-4 1 RECOMMENDATION ITU-R RS.1166-4 Performance and interference criteria*for active spaceborne sensors (1995-1998-1999-2006-2009) Scope This Recommendation reflects the performance and interference criteria for spaceborne active sensors in the bands allocated to the EESS (active)
2、. The Annex presents the technical bases for development of performance and interference criteria for various types of spaceborne active sensors. The sensor types include altimeters, scatterometers, precipitation radars, synthetic aperture radars and cloud profile radars. The ITU Radiocommunication
3、Assembly, considering a) that spaceborne active microwave remote sensing requires specific frequency ranges depending on the physical phenomena to be observed; b) that certain frequency bands have been allocated for spaceborne active microwave remote sensing; c) that these bands are also allocated t
4、o other radio services; d) that studies have established measurement sensitivity requirements; e) that performance requirements for active sensors can be stated in terms of precision of measurement of physical parameters and availability, measured at the satellite, assuming that degradation from oth
5、er elements in the system will be small; f) that performance objectives for active spaceborne microwave sensors are a prerequisite for the establishment of the associated interference criteria; g) that interference criteria are needed to ensure that systems can be designed to achieve adequate perfor
6、mance in the presence of interference, assess compatibility with systems in other services and, if needed, to assist in developing sharing criteria; h) that Annex 1 presents the technical bases for performance and interference criteria based on representative active sensors, *Interference criteria d
7、o not imply automatically sharing criteria. 2 Rec. ITU-R RS.1166-4 recommends 1 that the performance criteria given in Table 1 should be applied to instruments used in active sensing of the Earths land, oceans and atmosphere: TABLE 1 Performance criteria for remote sensing instruments Frequency band
8、 Scatterometer Altimeter SAR imager Precipitation radar Cloud profile radars 432-438 MHz Minimum reflectivity of 21 dB 1 215-1 300 MHz Minimum reflectivity of 32 dB 3 100-3 300 MHz Sea level precision 3 cmMinimum reflectivity of 26 dB 5 250-5 570 MHz Wind speed 3 m/s Sea level precision 3 cmMinimum
9、reflectivity of 24 dB 8 550-8 650 MHz Wind speed 3 m/s Sea level precision 3 cmMinimum reflectivity of 21 dB 9 300-9 900 MHz(1) Wind speed 3 m/s Sea level precision 3 cmMinimum reflectivity of 18 dB 13.25-13.75 GHz Wind speed 3 m/s Sea level precision 3 cmMinimum rain rates from 0.7-0.75 mm/h 17.2-1
10、7.3 GHz Wind speed 3 m/s Minimum rain rates from 0.7-0.75 mm/h 24.05-24.25 GHz Minimum rain rates from 0.7-0.75 mm/h 35.5-36 GHz Wind speed 3 m/s Sea level precision 3 cmMinimum rain rates from 0.1-0.2 mm/h 17 dBZ 10% 78-79 GHz 27 dBZ 10% 94-94.1 GHz 30 dBZ 10% 133.5-134 GHz 34 dBZ 10% 237.9-238 GHz
11、 44 dBZ 10% dBZ: “Unit” radar reflectivity used in meteorology which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z, referred to a value of 1 mm6/m3. SAR: Synthetic aperture radar. (1)See the relevant decisions of WRC-07. Rec. ITU-R RS.1166-4 3
12、 2 that the interference and data availability criteria given in Table 2 be applied for instruments used for active sensing of the Earths land, oceans and atmosphere. TABLE 2 Interference criteria Data availability criteria (%) Sensor type Performance degradation I/N (dB) Systematic Random Synthetic
13、 aperture radar 10% degradation of standard deviation of pixel power 6 99 95 Altimeter 4% degradation in height noise 3 99 95 Scatterometer 8% degradation in measurement of normalized radar backscatter to deduce wind speeds 5 99 95 Precipitation radar 7% increase in minimum rainfall rate 10 N/A 99.8
14、 Cloud profile radar 10% degradation in minimum cloud reflectivity 10 99 95 For bands with secondary allocation, the interference criteria are provided only to indicate performance degradation with regard to primary services. Annex 1 Performance and interference criteria for spaceborne active sensor
15、s 1 Introduction Performance criteria for active spaceborne sensors are needed in order to develop interference criteria. Interference criteria, in turn, can be used to assess the compatibility of radionavigation and radiolocation systems and active sensors in common frequency bands. This Annex pres
16、ents the technical basis for development of performance and interference criteria for various types of spaceborne active sensors. The sensor types include altimeters, scatterometers, precipitation radars, synthetic aperture radars, and cloud profile radars. Although the criteria are based on current
17、 and planned space science system designs and associated operating requirements, it is anticipated that future space science systems can be designed to accept at least the same levels of interfering signals and associated spatial and temporal conditions. 4 Rec. ITU-R RS.1166-4 2 Altimeters This sect
18、ion presents information on the performance and interference criteria for spaceborne altimeters in the frequency bands 3.1-3.3 GHz, 5.25-5.57 GHz, 8.55-8.65 GHz, 9.5-9.8 GHz, 13.25-13.75 GHz and 35.5-35.6 GHz. 2.1 Performance criteria Spaceborne altimeters produce, after data processing, measurement
19、 of sea level with a precision of less than 3 cm. The noise level in height measurements from altimeters is around 2-2.4 cm for low sea-states. An increase of 0.1 cm in the height noise due to interference would not materially affect the data and would be acceptable. In other words, a 4% degradation
20、 in height noise would be consistent with mission objectives. A requirement for altimeter missions is acquisition of 90% of all possible data over oceans. The design goal is higher than the minimum requirement and has been established as 95% of all possible data. Observations must be taken as close
21、to the land-sea interface as possible (below 15 km from the land-sea interface, altimeter waveform distortions occur and prevent accurate height estimation). The budget for lost data must accommodate all sources of loss including those due to spacecraft systems, the altimeter instrument, manoeuvres,
22、 etc. The availability requirement for altimetry data is 95%, assuming that the associated individual outages are brief and randomly dispersed over all observation time and areas (i.e. most outages lasting 2 s or less). The impact of interference that is always present at a given geographical locati
23、on is much more serious than that of random interference, because measurements can never be obtained from those geographical areas. In that event the requirement for altimeters is to obtain valid data for 99% of all geographical areas of interest. 2.2 Interference criteria Typical altimeters have li
24、nk budgets that result in S/N of 13 dB (except for 35.5-36 GHz altimeters) in the receiver range resolution bandwidth of 39.9 dB/Hz. The altimeter height noise varies as 1 + 2 / (S/N). For a return signal having a S/N of 13 dB before interference, the addition of interference causes the following in
25、crease in height measurement noise: S /N (dB) Degradation (%) Interference level Non-white interference White interference Non-white interference White interference None 13 13 Baseline Baseline 10 dB below noise 12.6 12.99 1 0.05 3 dB below noise 11.25 12.5 4.5 1 Equal to noise 10 11.5 9 3.8 10 dB a
26、bove noise 2.6 3 91 82 Rec. ITU-R RS.1166-4 5 For 35.5-36 GHz altimeters, atmospheric effects and technological constraints result in a less favourable link budget (S/N close to 10 dB) and so the sensitivity to interference level is higher, the following values have to be taken into account: S /N (d
27、B) Degradation (%) Interference level Non-white interference White interference Non-white interference White interference None 10 10 Baseline Baseline 10 dB below noise 9.6 9.98 1.7 0.08 6 dB below noise 9.0 9.9 4.2 0.5 3 dB below noise 8.2 9.5 8.4 1.2 1.5 dB below noise 7.7 9.1 11.8 3.8 Equal to no
28、ise 7.0 8.5 17 6.9 10 dB above noise 0.4 0 167 150 Degradation of height measurement noise in excess of 4% will not allow mission requirements to be met. To allow for non-Gaussian interference, the threshold for interference is set at 3 dB below the noise floor. As can be seen, the performance degra
29、dation increases sharply for interference levels above the noise floor. The criterion for harmful interference to altimeters is, therefore, an aggregate interfering signal power level of 117 dB(W/320 MHz) at 13-14 GHz and a level of 119 dB(W/450 MHz) at 35.5-36.0 GHz which would cause an unacceptabl
30、e increase in the height measurement noise. In shared frequency bands, availability of altimeter data shall exceed 95% of all locations in the sensor service area in the case where the loss occurs randomly and shall exceed 99% of all locations in the case where the loss occurs systematically at the
31、same locations. 3 Scatterometers This section presents information on performance and interference criteria for spaceborne scatterometers in the frequency bands 5.25-5.57 GHz, 8.55-8.65 GHz, 9.5-9.8 GHz, 13.25-13.75 GHz, 17.2-17.3 GHz and 35.5-36.0 GHz. It provides performance and interference crite
32、ria for active spaceborne scatterometers that can be used to analyse the compatibility of active spaceborne scatterometers and radionavigation and radiolocation systems in these bands. Unwanted radio frequency emissions reaching the scatterometers receiver can corrupt the radars scatterometer measur
33、ement of 0, where 0is the normalized radar backscatter coefficient. The amount of degradation will depend on the statistics of the external interference. 3.1 Performance criteria In scatterometer systems, an estimate of the echo return signal power is made by first measuring the “signal + noise” pow
34、er (i.e. the echo return plus the system noise contribution), and then subtracting the “noise-only” power (an estimate of the system noise alone, or “noise floor”). The system noise includes thermal emissions from the Earth, as well as those introduced by the antenna, waveguides, and the receiver no
35、ise figure. To optimize system performance, the “signal + noise” and the “noise-only” measurements are made over different bandwidths and/or at different times. This strategy 6 Rec. ITU-R RS.1166-4 relies on the fact that the nominal system noise is inherently white during the measurement sequence (
36、stationary, and with a flat spectral power distribution). If external interference is present, the new composite background noise is the sum of the interference and the nominal system noise. Depending on the strength, modulation, antenna gain pattern, and geometry of the interfering source, the comp
37、osite noise may not be white over the measurement sequence. The “noise-only” measurement will then not correspond to the noise of the “signal + noise” measurement and errors in the estimation of 0will result. The estimated 0error that results from a given “noise-only” measurement error can be quanti
38、fied with the following equation: 0Error (dB) = 10 log 1 + ( 1) / SNR 0(1) where: SNR 0(dB) = 10 log (S/N) = signal-to-noise ratio of the 0estimation process with: S: echo return power spectral density N: nominal noise floor power spectral density (approximately 200 dB(W/Hz) at the scatterometer rec
39、eiver input for both “fan beam” and “spot beam” antennas) and (dB) = 10 log (N + (Is + n/ Bs + n) / N + (In/ Bn) (2) with: Is + n : average power from interfering source in Bs + nduring the “signal + noise” measurement period Bs + n : “signal + noise” measurement bandwidth In : average power from in
40、terfering source in Bnduring the “noise-only” measurement period Bn : “noise-only” measurement bandwidth. The impact of external interference is most severe for winds with low speed. The lowest wind speed to be measured by spaceborne scatterometers is 3 m/s. Results of computer simulations conducted
41、 for non-stationary interference to the NSCAT scatterometer have shown that a maximum value of (see equation (2) that will allow performance requirements to be met for 3 m/s wind speeds is 0.7 dB. Scatterometers in the future may employ spot beam antennas rather than fan beam antennas as are used fo
42、r NSCAT. The main differences, besides the antenna pattern, between the two types of scatterometers are the transmitted e.i.r.p. and receive antenna gain. Results of computer simulations conducted for non-stationary interference have shown that a maximum value of = 6 dB (see equation (2) can be tole
43、rated with the “spot beam” antenna and still meet the performance requirements for 3 m/s wind speeds. The allowable loss of scatterometer data due to interference from radio frequency stations randomly distributed across the oceans is 5% of the data taken over the global oceans. The allowable loss f
44、or systematic interference is 1%. Systematic interference is defined as the loss of coverage at the same points on the oceans for most passes over those points. These maximum allowable losses have been derived from the NSCAT science requirement for measuring 90% of global vector winds over the ocean
45、s and taking into consideration other randomly distributed data losses introduced mainly in areas with intense rainfall. Rec. ITU-R RS.1166-4 7 3.2 Interference criteria Figure 1a is a plot of equation (2) for a scatterometer with a receiver noise floor of N = 200 dB(W/Hz). It shows as a function of
46、 the power spectral density of the interfering signal Is + n /Bs + n). Note that different results for will be obtained depending on how the interference is changing over time or over bandwidth. Figure 1a contains a family of plots for several values of the parameter 10 log (Is + n /Bs + n)/(In /Bn)
47、. The time separation of the “signal + noise” measurement period from the centre of the “noise-only” measuring period is approximately 0.23 s. During this time the angle from the spacecraft scatterometer to a specific point on the ground will change by approximately 0.1. Due to the narrow beamwidth
48、of the fan beam antenna (0.42, 3 dB beamwidth), changes of several dB in received interference levels should be expected as the scatterometer side lobes move through a transmitter beam. Engineering judgement has led to a value of 6 dB as the assumed maximum expected change in 10 log (Is + n /Bs + n)
49、/(In /Bn) during the measurement period. From Fig. 1a, it is therefore concluded that the maximum interference power spectral density that any one of the six fan beam antennas of the NSCAT scatterometer can sustain without degraded measurement accuracy is 207 dB(W/Hz) or 174 dBW over any 2 kHz bandwidth within the 1 MHz bandwidth of the processing channel. For white-noise like interference, the maximum acceptable interference spectral power density would be approximately 194 dB(W/Hz) at
