RISAT-2 (Radar Imaging Satellite-2)
RISAT-2 is an X-band SAR (Synthetic Aperture Radar) reconnaissance satellite of ISRO (Indian Space Research Organization), Bangalore, India. The spacecraft was built for ISRO by IAI/MBT (Israel Aerospace Industries Ltd.) based on the TecSAR minisatellite design of IAI (launch Jan. 21, 2008 provided by ISRO) of the Israeli MoD (Ministry of Defense).
RISAT-2 was built as part of an acceleration of Indian reconnaissance satellite procurement after the 2008 Mumbai (Bombay) terror attacks, and due to delays with the indigenously developed RISAT-1 spacecraft, which is due for launch in 2011. Hence, RISAT-2 is India's first satellite with a synthetic aperture radar (SAR), which possesses a 24-hour, all-weather monitoring capability. 1) 2)
Figure 1: Illustration of the RISAT-2 spacecraft (image credit: IAI)
RISAT-2 is a minisatellite featuring a low-mass design (S/C mass of 300 kg, including a payload mass of 100 kg). The spacecraft is 3-axis stabilized, using the OptSat-2000 platform (of TecSAR bus heritage). The design keeps the bus and payload well separated so that changes due to growth potential in either one element will have a minimum effect on the other element. Electrical power of 750 W (EOL) is provided by two solar panels. The AOCS (Attitude and Orbit Control Subsystem) provides a high degree of pointing accuracy. An onboard data storage capacity of 240 Gbit is provided. The spacecraft design life is 5 years (minimum).
The highly agile bus design, in combination with the body-pointing parabolic dish antenna system permits increased viewing capabilities from the spacecraft. The spacecraft/antenna system may be dynamically redirected to any direction of the flight path (i.e., in the cross-track as well as in the along-track direction). Thus, a wide FOR (Field of Regard) within the incidence angle range may be obtained on either side of the ground track for event monitoring coverage.
Figure 2: The RISAT-2 spacecraft at IAI during integration (image credit: ISRO)
- The secondary payload on this flight was ANUSat, a microsatellite of Chennai-based Anna University, India with a mass of ~ 40 kg.
Orbit: Near-circular mid-inclination orbit, altitude = 550 km, inclination = 41º, period of ~90 minutes (the orbit is nearly identical to that of the TecSAR spacecraft).
RF communications: The payload data are downlinked in X-band at a data rate of 620 Mbit/s.
The ISRO/NRSC (National Remote Sensing Center) is the focal point for distribution of remote sensing satellite data products in India and its neighboring countries. NRSC has an Earth station at Shadnagar, about 55 km from Hyderabad, to receive data from almost all contemporary remote sensing satellites. NDC (NRSC Data Center) is a one-stop-shop for range of data products with a wide choice of resolutions, processing levels, product media, out scales, area coverage, revisit, season and spectral bands. Data products can be supplied on a wide variety of media and formats. 5)
Figure 3: Illustration of the RISAT-2 observation coverage
• In 2011, RISAT-2 spacecraft and its payload are operating nominally.
• NRSC has acquired XSAR data in various observation modes (Figure 4). The data are being used for such applications as: flood mapping, oil slick detection, landslide mapping, agriculture, forestry and urban mapping.
Sensor complement: (X-SAR)
XSAR (X-band SAR instrument):
XSAR (of XSAR heritage flown on TecSAR) was designed and developed by Elta Systems Ltd. of Ashdod, Israel, a subsidiary of IAI.
XSAR operates at a center frequency of 9.59 GHz (3.1 cm wavelength) with a revisit period of 3 or 4 days and a repeat cycle of 14 days, the look angle can vary from 20-45º and the instrument is capable of acquiring data from both left and right look directions of the subsatellite track. The RISAT-2 data enhances ISRO's capability for earth observation, especially for management of disasters like floods, cyclones, landslides, etc. in a more effective way. 6)
XSAR uses a large dish-like antenna to transmit and receive radar signals that can penetrate thick clouds providing images up to 1m resolution. The multi-mode XSAR is capable of high resolution imaging of Spotlight (≤ 1 m), Stripmap (3 m), Mosaic (1.8 m) and Wide coverage (8 m) modes.
The highly agile bus design in combination with the body-pointing parabolic antenna dish system permits greatly increased viewing capabilities of the spacecraft. The spacecraft/antenna system may be dynamically redirected in any direction of the flight path (i.e. in the cross-track as well as in the along-track direction). Thus, a wide FOR (Field of Regard) within the incidence-angle range may be obtained on either side of the ground track for event monitoring coverage.
- RSC (Radar Signaling and Control) system
- MTT (Multi-Tube Transmitter)
- Deployable paraboloid mesh antenna with electronic beam steering
- OBR (Onboard Recorder) of 256 Gbit capacity
- DLTU (Data-Link Transmission Unit).
The payload modules are separated from the bus so that only cables and wires connect the two. The payload modules occupy a section close to the antenna (Figures 5 and 8). The OBR and DLTU components are part of the bus modules.
The MTT is composed of 10 RF TWT amplifiers referred to as CTWTA (Channeled Traveling Wave Tube Amplifier). Only 8 CTWTAs (out of 10) are being used during an imaging phase; the other two are kept as cold redundancy. The MTT arrangement supports graceful degradation even if only seven of the ten CTWTAs remain in working order. The reflected signal passes through the feeder to a Front End (FE) that contains circulator and LNA (Low Noise Amplifier). A fast electronic switch at the receiver input selects the power coming from one out of the eight FEs. This power is amplified and sampled by a fast A/D (Analog/Digital) converter. The sampled data passes through a 6 to 3 bit per sample BFPQ (Block Floating Point Quantization) compression algorithm. The source data is then recorded onto the OBR (Onboard Recorder).
Figure 5: Exploded view of the TecSAR components (image credit: ELTA Systems Ltd.)
Figure 6: Illustration of the multi-beam feed (image credit: ELTA Systems Ltd.)
The radiating element of XSAR is the parabolic antenna, a deployable umbrella reflector, with a rigid CFRP (Carbon Fiber Reinforced Plastic) central dish and a set of knitted mesh gores stretched by skeleton ribs. The ribs lie on the surface of a parent paraboloid where each gore surface has a paraboloid cylinder contour. The configuration of the antenna is shown in Figure 5. A high paraboloid reflector surface accuracy is achieved by ribs position adjustment after mesh gores mounting. The antenna measurement technique was verified using the method of stereo photogrammetry. The entire reflector mesh has a mass of < 0.5 kg.
In the XSAR system design, a derivative of the BFPQ algorithm is used, namely MBFPQ (Modified Block Floating Point Quantization). The MBFPQ algorithm exploits the fast CPU computing power of the payload (80486DX2 CPU) thus avoiding the need to assume a Gaussian distribution of the data. 9)
All electronics and RF devices (MTT and RSC) of the XSAR payload are part of the hexagonal container as shown in Figure 8.
The electronic beam steering capability is achieved by using an antenna feed array in the focal plane (the power generated in the MTT is being directed toward different feeders in the antenna feed array). Each feeder has a different position relative to the antenna focal point: hence, a multibeam pattern is created (Figure 6). The antenna includes a TTD (True Time Delay) function for the electronic steering with a very wideband beam. As a consequence of the wideband antenna and the TTD feature, the system's range resolution is not limited as in the case of an active phased array antenna.
Figure 7: Antenna test and integration block diagram (image credit: ELTA Systems Ltd.)
All electronic circuits and RF modules are of the payload are installed into a hexagonal container, including the MTT and the RSC (Figure 8). The OBR and DLTU are mounted in the bus section of the spacecraft.
Figure 8: Overview of the payload hexagonal container (image credit: ELTA systems Ltd.)
Modes of operation:
The multi-mode payload employs electronic beam steering, which can be operated in various observation modes including various polarization combinations (optional): HH, HV, VH, VV.
• Stripmap mode: the synthetic apertures are targeted on wide geographical swaths. The spacecraft performs synchronous imaging and does not change its orientation during observations except for some small maneuver due to the need to keep the imaging strip parallel to the ground track. Squinted strip imaging is possible.
Figure 9: Schematic view of the stripmap mode (image credit: NRSC, ELTA systems Ltd.)
• Wide coverage ScanSAR. The coverage of large strips is achieved by electronic beam steering. Three beams are used in the nominal wide coverage mode which create three footprints (subswaths) in the target area. The ground resolution in this mode is decreasing since the integration time is split up among the subswaths.
Figure 10: Schematic view of the wide coverage ScanSAR mode (image credit: NRSC, ELTA systems Ltd.)
• Spotlight mode: this focuses on a specific, pre-assigned target. In spotlight, the spacecraft performs mechanical steering to halt the antenna footprint in a specific target area. The longer integration time over the spot target area yields an improved azimuth resolution. The range resolution is achieved in adjusting the bandwidth to the incidence angle. The TecSAR ability for spotlight imaging in squint allows for multi-look imaging without any loss in resolution. To obtain a multi-look image of a given target area, a number of spotlight images are being observed, each at a different squint angle.
Figure 11: Schematic configuration of the Spotlight mode (image credit: NRSC, ELTA systems Ltd.)
• Mosaic mode: the radar imager slews its focus on a number of spots in the same general target area. The mosaic mode enables to extend the limited coverage of the spot mode by using the electronic steering capability of XSAR. In mosaic mode the radar beam scans in the range direction while the mechanical maneuvering advances the strip line in the azimuth direction. Hence, this mode may also be interpreted as the spot version of ScanSAR.
Figure 12: Schematic view of the mosaic mode (image credit: NRSC, ELTA systems Ltd.)
Table 1: Spatial resolution of TecSAR observation modes
7) Y. Sharay, U. Naftaly, “ TecSAR: design considerations and programme status,” IEE Proceedings- Radar, Sonar and Navigation, Vol. 153, Issue 2, April 13, 2006, pp. 117-121, ISSN: 1350-2395
8) R. Levy-Nathansohn, U. Naftaly, “Overview of the TecSAR Satellite Modes of Operation,” Proceedings of EUSAR 2006, Dresden, Germany, May 16-18, 2006
9) Ury Naftaly, “The Modified BFPQ algorithm,” Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.