In principle the SAR uses a relatively small 

 transmitting/receiving antenna with a fairly broad beamwidth. The 

 antenna beam is typically directed broadside, or skewed forward to 

 the vehicle's flight path to function as a side-looking radar. Returns 

 from radar pulse transmissions are recorded to preserve both amplitude 

 and RF phase information as the vehicle moves along the flight path. 

 These return signals can then be processed collectively as though they 

 had all been transmitted and had been received from a single very long 

 antenna aperture. The resulting effect is radar performance, with 

 respect to resolution cell size and sea clutter rejection, that is much 

 superior to conventional airborne radars with their attendant antenna 

 size restrictions. 



A disadvantage of SARs is the large amount of signal 

 processing required to produce images from the raw data collected. Both 

 optical and digital processing have been employed for SAR data. Con- 

 ceptually, raw data can be downlinked from a satellite or airborne plat- 

 form for ground processing. Obviously, the higher the number of resolu- 

 tion cells to be processed in a swath image, the greater the processing 

 load. Although the resolution cell size is uniform over the SAR image, 

 certain limitations of Doppler and range ambiguity limit the surveillance 

 range to something less than conventional radars. 



Current technology can support swath widths of the 

 order of 100 km (62 nmi). Such a system is being developed by Jet 

 Propulsion Laboratory (JPL) for the SEASAT satellite program. Fishing 

 vessels of the order of 15 m have been detected during airborne tests 

 of scaled down versions of the satellite hardware. The resolution cell 

 size for the L-band SAR is 25 by 25 m. Performance projections for the 

 satellite sensor are comparable to those exhibited by the sensor used 

 in the captive flight tests. 



42 



