Artist's rendition of the TecSAR spacecraft in Earth orbit. Photo Credit: IAI

March 20, 2009, (Sawf News) - The RISAT radar imaging satellite that ISRO is set to launch in the first week of April is most likely an off-the-shelf Israeli TecSAR satellite acquired purely for military purposes.

ISRO has been understandably tight lipped about the purpose or details of the satellite.

The RISAT radar imaging satellite that ISRO is set to launch in the first week of April is most likely an off-the-shelf Israeli TecSAR satellite acquired purely for military purposes. Photo Credit: ELTA Systems Ltd.

On January 21, 2008, India launched a TecSAR satellite with a X-Band synthetic aperture radar for Israel atop the PSLV-C10, a core alone version of the PSLV (PSLV-CA).

At 300 kg RISAT weighs exactly the same as TecSAR, probably because it is a TecSAR.

It will be launched along with a 38 kg mini communication satellite, Anusat, built by Anna University, Chennai atop a PSLV-C12, also a core alone version of the launcher.

"RISAT is likely to be launched in the first half of April. We are looking at April five or six," an ISRO official told PTI.

Illustration of the TecSAR spacecraft. Photo Credit: IAI Systems Ltd.

ISRO has been independently developing its own Radar Imaging Satellite but the project is over 2 years behind schedule.

The ISRO built RISAT reportedly weighs 1,780-kg and features a C-band Synthetic Aperture Radar (SAR) operating with a 6 x 2 meter planar active array antenna.

The ISRO RISAT is expected to be ready for launch later this year.

Multi modes of SAR. Photo Credit: ELTA Systems Ltd.

TecSAR

The Mini class, low earth orbit TecSAR, is offered as an off-the-shelf product by Israeli Aerospace Industries (IAI) and ranks among the world's most advanced space systems.

Its Synthetic Aperture Radar (SAR) uses a large dish-like antenna to transmit and receive radar signals that can penetrate darkness and thick clouds providing images with 1m resolution.

The Multi mode SAR is capable of high resolution imaging of Spot (1m), Strip (3m), Mosaic(1.8m) and Wide coverage (8m).

The highly agile bus design in combination with the body-pointing parabolic antenna dish system permits greatly increased viewing capabilities from the spacecraft. The spacecraft/antenna system may be dynamically redirected into 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.

Strip 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.

Illustration of the wide coverage mode. Photo Credit: 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. The swath width can be increased by using more antenna beams. In principle the swath width may get to more than 100 km for some incidence angles. However, this reduces the ground resolution to about 20 m.

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.

Illustration of the mechanical and electrical beam steering in the mosaic mode. Photo Credit: 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.

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