1、 Recommendation ITU-R RS.2043-0(02/2014)Characteristics of synthetic aperture radars operating in the Earth exploration-satellite service (active) around 9 600 MHzRS SeriesRemote sensing systemsii Rec. ITU-R RS.2043-0 Foreword The role of the Radiocommunication Sector is to ensure the rational, equi
2、table, efficient and economical use of the radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunicatio
3、n Sector are performed by World and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolutio
4、n ITU-R 1. Forms to be used for the submission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information datab
5、ase can also be found. Series of ITU-R Recommendations (Also available online at http:/www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed se
6、rvice M Mobile, radiodetermination, amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM S
7、pectrum management SNG Satellite news gathering TF Time signals and frequency standards emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2014 ITU 2014 All rights rese
8、rved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R RS.2043 1 RECOMMENDATION ITU-R RS.2043-0 Characteristics of synthetic aperture radars operating in the Earth exploration-satellite service (active) around 9 600 MHz (2014) Scop
9、e This Recommendation provides characteristics for synthetic aperture radars operating in the Earth exploration-satellite service (active) allocated around 9 600 MHz. This information should enable sharing and compatibility studies with other radio services coexisting in the same frequency range or
10、nearby frequency ranges. The use of this frequency range comprises remote sensing satellite systems that are implemented with different radar transmission bandwidths ranging from 100 MHz up to 1 200 MHz. The ITU Radiocommunication Assembly, considering a) that spaceborne active microwave remote sens
11、ing 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 the transmission bandwidth of a radar sensor is directly related to the achievable measurement resolu
12、tion; d) that a growing demand for high-resolution radar information exists as shown in Report ITU-R RS.2274; e) that observations in the 9 GHz frequency range provide data critical to the study of the characteristics of the Earth and its natural phenomena, including data relating to the state of th
13、e environment; f) that only the 9 600 MHz frequency range offers the most advantageous situation of highest possible bandwidth in a frequency band which offers good propagation conditions, recognizing that Recommendation ITU-R RS.1166 provides performance and interference criteria for Earth explorat
14、ion-satellite service (active) sensors including synthetic aperture radars operating around 9 600 MHz, recommends that the characteristics of typical space-borne synthetic aperture radar systems operating in the 9 GHz range, as described in the Annex, should be used for sharing and compatibility stu
15、dies involving the Earth exploration-satellite service (active) around 9 600 MHz. 2 Rec. ITU-R RS.2043-0 Annex Characteristics of synthetic aperture radars operating in the Earth exploration-satellite service (active) around 9 600 MHz 1 Principles of synthetic aperture radars (SAR) A synthetic apert
16、ure radar (SAR) is a coherent spaceborne side-looking radar system which utilizes a satellites flight path to emulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. In principle, a SAR is an active phased array antenna. But instead o
17、f using a large number of parallel antenna elements, SAR uses one antenna element in time-multiplex. The different geometric positions of the antenna elements are the results of the moving platform. The satellite travels forward in the flight direction with a nadir pointing towards the centre of the
18、 Earth. The microwave beam is transmitted obliquely at right angles to the flight direction illuminating a swath. Range refers to the across-track dimension perpendicular to the flight direction, while azimuth refers to the along-track dimension parallel to the flight direction. Swath width refers t
19、o the strip of the Earths surface from which data is collected as a side-looking radar. It is the width of the imaged scene in the range dimension. The longitudinal extent of the swath is defined by the motion of the aircraft with respect to the surface, whereas the swath width is measured perpendic
20、ularly to the longitudinal extent of the swath. Over time, individual transmit/receive cycles (pulse repetition time, (PRT) are completed and the gathered data from each cycle is stored in on-board memory. The signal processing uses the magnitude and phase of the received signals over successive pul
21、ses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined to create a high resolution image of the terrain being overflown. 2 Modes of operation of synthetic aperture radars (SAR) 2.1 Geometry The SARs operating near 9.6 GHz are controlled via a ground
22、command to turn on and off as required to view only specific areas on the Earth. From all SAR modes shown in Fig. 1, the full 1 200 MHz chirp bandwidth is only intended for use when operating in spotlight mode. Other modes may use the frequency band 9 300-9 900 MHz, in accordance with provisions giv
23、en by RR footnotes Nos. 5.475A, 5.476A, 5.478A and 5.478B. The conventional SAR strip map mode assumes a fixed pointing direction of the radar antenna broadside to the platform track. A strip map is an image formed in width by the swath of the SAR and follows the length contour of the flight line of
24、 the platform itself. In the scanSAR mode, the SAR can illuminate several sub-swaths by scanning its antenna into different positions. Rec. ITU-R RS.2043 3 FIGURE 1 Modes of operations for SAR system in the 9 GHz Earth exploration-satellite service (EESS) allocation RS.2043-01555520Spotlight modeStr
25、ipmap modeScanSAR mode550 km190 km190 km550 km514 km202055Spotlight is the SAR mode for obtaining highest resolution by electronic steering of the radar beam pointing at a target in the beam thus forming a longer synthetic aperture. The spotlight mode is capable of improving the resolution of SAR im
26、aging capability to less than 30 cm. As more pulses are used, the azimuth resolution also improves. Spotlight mode of operation is usually at the expense of spatial coverage, as other areas within a given accessibility swath of the SAR cannot be illuminated while the radar beam is spotlighting over
27、a particular target area. Details on the imaging geometries of this mode are shown in Fig. 2. Data will typically be collected by taking between 49 and 65 sub-swaths of 20 km in range by 0.35 km in azimuth. This data can then be used to create a mosaic of sub-swaths in azimuth to process a 5 km by 5
28、 km image. All SARs are controlled via ground command to turn on and off as required to view specific areas on the Earth. The “on”-command triggers a transmission of radio-frequency pulses (chirps) for a short period of around five seconds or less depending on the intended observation. 4 Rec. ITU-R
29、RS.2043-0 FIGURE 2 EESS SAR imaging geometry for high resolution spotlight mode (wideband with 1 200 MHz chirp bandwidth) RS.2043-0255RfAccessrangeSatellite trackSpot area5 5 km2190km550km20Simplified flat spot areaSwathwidthCross trackor rangeor elevation directionIncident angleGrazing angleOff-nad
30、ir angle orlook angle(antenna beam orientation)Antenna cone 1.13 (or elevation beam width)Antenna cross-cone 0.53 (or azimuth beam width)RnNadirh = 510 kmSymmetrical casefor left side ofsatelliteDRDATVsat = 7630 m/sSAR antennaphase centrePixelresolutionATRSub-satellite ground trackor along trackor a
31、zimuth direction2.2 Timing characteristics for SAR-4 in high resolution mode As noted in 2.1 above, the maximum bandwidth of 1 200 MHz is only used in spotlight mode of the SAR-4 system, when the highest radar picture resolution is required. In this mode, SAR-4 transmits for a short period (typicall
32、y 5 seconds) per event of SAR exposure (“snapshot”). During the five seconds of transmission, the satellite travels actually more than 38 km along the orbit track, thus permanently changing the effective incident angle into the exposed spot area as shown in Fig. 2. In spotlight mode, there can be up
33、 to 20 snapshot events per satellite orbit (95 minutes), with a minimum time of 1 second between consecutive events, corresponding to a distance of 45 km on the ground. A graphical representation of the situation is given in Fig. 3. In a sun-synchronous orbit, the satellite permanently travels along
34、 the day-night terminator. With a typical altitude of 515 km, the periodicity of the sun-synchronous orbit results in a track of the sub-satellite point that repeats every 11 days. As a result of orbit requirements, and depending on geographic latitude, a radar location and adjacent areas are not vi
35、sible more often than twice a day. Adjacent areas are considered to be areas affected by antenna sidelobe illumination. Peak level areas and adjacent areas are illuminated not more than once per orbit period. A SAR-4 system transmits pulses at a fixed duty cycle of 50 s followed by 120 s of silence.
36、 The pulse timing is adapted to a fixed pulse repetition rate. During the 50 s of each transmission pulse the unmodulated (CW) transmit carrier sweeps over the entire bandwidth of 1 200 MHz (“chirp”). The resulting transmission duty cycle, as shown in Table 1, remains fixed under all conditions of p
37、ulse width and pulse repetition rate. Rec. ITU-R RS.2043 5 FIGURE 3 Minimum separation distance between two consecutive targets RS.2043-03Orbit velocity = 7.630 km/scorresponds tovelocity of sub-satellite point = 7.060 km/sMinimum separation distance between 2 consecutive snapshots = 45 kmMinimum ti
38、me = 1 seconde 5 sec5 sec11Other SAR modes are described in Report ITU-R RS.2178. Figure 4 shows the ground tracks of the sub-satellite point for 14 orbital periods of a SAR-4 satellite. During each orbit period the Earth rotates by about 23.7. In case that there is an overhead (90 elevation) path o
39、ver one location, the orbit before and the orbit thereafter will appear at lower maximum elevation angles (close to the horizons) of the station. FIGURE 4 SAR-4 track of sub-satellite points for 14 orbital periods of 1 h 34 m 49 s each (15.19 rev/d) RS.2043-04Figure 5 provides examples for overhead
40、paths and their corresponding illumination condition at three typical latitudes. In each of the pictures in Fig. 5, an area in blue can be seen on both sides of the satellite track. This shows the domain where a SAR instrument would illuminate an area in spotlight mode at one moment in time. 6 Rec.
41、ITU-R RS.2043-0 Due to the movement of the satellite itself, the sub-satellite point moves along the sub-satellite track1at 7.06 km/s. The target is only illuminated when it is within this blue area (within the satellite main beam lobe), with a maximum illumination time of 5 to 7 seconds depending o
42、n the actual target location with respect to the satellite track. When both the main beam and the sidelobes are considered, the maximum illumination time would be longer. The consequences in terms of harmful interference will depend on the service and system considered. The information below is base
43、d on the main beam illuminations. 1Track of the sub-satellite points on the Earths surface given by a virtual line between spacecraft and centre of the Earth. Rec. ITU-R RS.2043 7 FIGURE 5 Satellite illumination zone (overhead path of satellite) RS.2043-058 Rec. ITU-R RS.2043-0 Figure 6 and Table 1
44、provide, for a given location on Earth, the potential illumination times and accumulation over 11 days after which the track of the sub-satellite points will exactly repeat. There are up to four potential illuminations per day at high latitudes. As shown in Fig. 6, the number of illuminations varies
45、 per day from zero to four. FIGURE 6 Illumination opportunities over a full 11-day period at high latitudes RS.2043-06Wed 10 Thu 11 Fri 12 Sat 13 Sun 14 Mon 15 Wed 17 Thu 18 Fri 19 Sat 20Tue 16Apr 2013Time (UTCG)Access times - 09 April 2013 15:59:38Rec. ITU-R RS.2043 9 TABLE 1 Accumulated time of po
46、tential illuminations over a full 11-day period at high latitudes Start time (UTCG) Stop time (UTCG) Duration (s) 9 Apr 2013 14:04:58.246 9 Apr 2013 14:05:05.008 6.762 9 Apr 2013 15:38:58.735 9 Apr 2013 15:39:06.027 7.292 10 Apr 2013 04:29:51.820 10 Apr 2013 04:29:58.819 6.999 10 Apr 2013 06:03:50.3
47、10 10 Apr 2013 06:03:57.310 7.000 10 Apr 2013 13:47:56.501 10 Apr 2013 13:48:04.209 7.708 10 Apr 2013 15:21:49.102 10 Apr 2013 15:21:55.247 6.145 11 Apr 2013 05:46:48.240 11 Apr 2013 05:46:54.287 6.047 13 Apr 2013 14:30:27.763 13 Apr 2013 14:30:33.100 5.337 14 Apr 2013 04:55:30.126 14 Apr 2013 04:55
48、:35.471 5.345 14 Apr 2013 14:13:22.852 14 Apr 2013 14:13:29.144 6.291 14 Apr 2013 15:47:27.234 14 Apr 2013 15:47:35.117 7.882 15 Apr 2013 04:38:19.663 15 Apr 2013 04:38:26.098 6.435 15 Apr 2013 06:12:13.167 15 Apr 2013 06:12:20.630 7.464 15 Apr 2013 13:56:19.267 15 Apr 2013 13:56:26.513 7.246 15 Apr
49、 2013 15:30:15.649 15 Apr 2013 15:30:22.348 6.699 16 Apr 2013 04:21:07.202 16 Apr 2013 04:21:14.804 7.602 16 Apr 2013 05:55:10.750 16 Apr 2013 05:55:17.266 6.517 16 Apr 2013 15:13:05.560 16 Apr 2013 15:13:11.149 5.589 17 Apr 2013 05:38:06.586 17 Apr 2013 05:38:12.148 5.562 19 Apr 2013 14:21:42.280 19 Apr 2013 14:21:48.106 5.826 20 Apr 2013 04:46:41.507 20 Apr 2013 04:46:47.403 5.896 20 Apr 2013 06:20:30.148 20 Apr 2013 06:20:38.073 7.924 Global statistics Value Min duration (s) 5.337 Max duration (s) 7.924 Mean duration (s) 6.617 Total duration (s) 145.568
