High speed data acquisition systems for ISRO's airborne and spaceborne radars

During 1990's, high speed data handling and control unit (DHCU) was developed for Indian Space Research Organisation's (ISRO) C-band airborne synthetic aperture radar (ASAR) at Space Applications Centre (SAC), ISRO, India. It has been extensively utilised to acquire high bandwidth radar signal during ASAR flights aboard Beechcraft-200 aircraft, conducted regularly since 1997. DHCU supports ASAR data acquisition, formatting and storage. It carries out high speed 6-bit I/Q digitisation (30.814 MHz sampling) of the complex baseband signal (25 MHz bandwidth) received from the ASAR receiver. The digitized data are multiplexed with other auxiliary data before being stored on a redundant array of independent disks (RAID) based recorder through high speed ECL parallel interface. Subsequently, since 2000 A.D., ISRO has embarked upon a very ambitious spaceborne SAR mission, called radar imaging satellite (RISAT). The onboard digital subsystem for this high resolution spaceborne SAR has to cater to data acquisition, control and timing, data compression, buffer storage and formatting requirements. The ultra-high speed data acquisition units of this spaceborne radar consist of dual chains of ultra-high speed 8-bit ECL A-to-D converters based I/Q digitisers. These units perform digitisation of the received complex radar echo signals and calibration signals at more than 200 MHz sampling rate and demultiplexing of ultra-high speed digitised signal into multiple channels. It also implements radar data compression, variable data rate formatting and high speed LVDS interface with satellite. In view of the high bandwidth signals and high sampling rates (/spl sim/ 200 MHz), signal integrity, EMI/EMC and thermal issues assume great importance and pose a great challenge in the PCB and package layout, design and fabrication. This paper describes the high speed design requirements and configuration details for the ultra-high speed data acquisition units of spaceborne SAR as well as for high speed acquisition unit of airborne SAR. It also addresses the signal integrity, EMI/EMC and thermal related issues for these systems.