By Stefan Nitschke
Operating in two modes, imaging via synthetic aperture radar (SAR) and moving target indication (MTI), SAR/MTI sensors fitted to manned/unmanned aircraft are able to obtain high-resolution imagery of static targets and to detect, locate, and identify the deployment and movement of potentially hostile forces. The industry has made encouraging progress in the field of integrating SAR sensors having advanced MTI to help establish an enemy’s order of battle from standoff distances. These are increasingly smaller SAR/MTI radars sized to fit applications ranging from larger medium-altitude, long-endurance (MALE) unmanned aerial vehicle (UAV) down to smaller platforms as best represented by the German Army’s LUNA X-2000 system.
EADS Defence Electronics has developed such a SAR/MTI radar sensor which is named Miniature SAR (MiSAR) to expand the usefulness of small unmanned aircraft, light aircraft, and reconnaissance pods. MiSAR principally takes advantage of EADS’ development of Ka-band technology for missile seekers. The company successfully tested this newly developed miniature SAR/MTI sensor during a flight demonstration in the Baumholder military training area in Rheineland Palatinate in February last year. During these trials, the sensor was carried by a German Army’s LUNA reconnaissance UAV which was developed by the German company EMT Ingenieurgesellschaft GmbH. LUNA stands for Luftgestuetzte Unbemannte Nahaufklaerungs-Ausstattung.
The 4 kg MiSAR sensor which fits inside a 10dm3 payload bay, enables the LUNA UAV to deliver high-resolution real-time images for tactical military missions or time-critical security purposes. When operating in SAR strip mode, MiSAR has a resolution of only 0.5 metre over a 0.5km- to 2km-wide swath at ranges from about 500 metres to almost 1,000 metres. The two principal features of this Ka-band SAR/MTI sensor operating at 35 GHz are its adverse weather capability and the capability to deliver high-resolution imagery about the type, speed and direction of a moving ground target. While when operated during such tasks, MiSAR employs frequency-modulated continuous wave mode of operation, the microwave link for the air vehicle tracking operates at C-band. For any of these purposes, the MiSAR sensor has a gimbal-mounted dual antenna unit with separate transmit and receive antennas allowing both the command uplink and downlink functioning at 5 MHz analogue and 10.7 Mbit/s digital over distances of more than 80 kilometres. But this datalink range can be extended by other airborne or ground relays.
For operation in a dense electromagnetic environment, the system employs frequency hopping techniques. SAR/MTI sensors like the MiSAR sensor were found to be very effective when combined with an EO/IR sensor suite. When functioning in extreme weather conditions, a SAR/MTI radar mission payload operator did not know whether the moving object that he detected was of military significance unless he used the SAR to detect and classify it when it is stationary, or he cross-cued the EO/IR sensor to classify the object. As a consequence, the LUNA UAV carries a nose-mounted colour camera for navigation and pilot control, a downward looking colour video CCD camera, and, for operation at night, a low-power consumption, low- weight IR camera having a 3 to 5 microns band 256×256 platinum silicide (PtSi) detector array. However, the performance characteristics of currently available SAR/MTI sensors are likely to be limited in providing advanced target recognition (ATR) of moving objects. MTI can detect moving targets over a wide search area, but will not be capable to adequately classify them, whereas SAR sensors are able to classify them, but only if they are stationary.
Meanwhile, ten air vehicles of the lightweight, medium-range LUNA surveillance UAV plus two catapult launchers and two vehicle-mounted ground control stations (GCS) were delivered to the German Army since March