A novel digital receiver concept for ISRO's future remote sensing radars

Abstract

Technology development related to digital, antenna and RF subsystems for Microwave Radar Sensors like Synthetic Aperture Radar, Scatterometer, Altimeter and Radiometer is one of the major activities under ISRO's microwave remote sensing programme, since 1980s. These technologies are now being gainfully utilized for building ISRO's operational Earth Observation missions involving microwave sensors like Radar Imaging Satellite, RISAT SAR, Oceansat-2 Scatterometer, Megha-Tropiques, MADRAS and Airborne SAR for Disaster Management, DMSAR. Concurrently, advanced technology developments in these fields are underway to meet the major technological challenges of building ISRO's proposed advanced microwave missions like ultra-high resolution SAR's, Synthetic Aperture Radiometer (SARAD), Milli-meter and sub-millimeter wave sounders and SAR Constellations for Disaster management as well as Interferometric, Polarmetric and polarmetric interferometry applications. Also, these hardware are being designed with core radar electronics concept, in which the same RF and digital hardware sub-units / modules will be utilized to build different microwave radar sensors. One of the major and common requirements for all these active and passive microwave sensors is the moderate to highspeed data acquisition and signal processing system. Traditionally, the Data acquisition units for all these radar sensors are implemented as stand-alone units, following the radar receivers. For ISRO's C-band airborne SAR (ASAR) and RISAT high resolution SAR, we have designed and developed High Speed 8-bit ADC based I/Q Digitisers, operating at 30.814 MHz and 250 MHz sampling rates, respectively. With the increasing demand of wide bandwidth and ultra-high resolution in imaging and non-imaging radar systems, the technology trend worldwide is towards a digital receiver, involving bandpass or IF sampling, thus eliminating the need for RF down converters and analog IQ demodulators. In order to evolve a generic configuration for all the microwave sensors, we have initiated design and development of a generic L-band digital receiver, consisting of receiver elements (LNA, digital attenuator and Bandpass filter) followed by Analog-to-Digital Converter. The digitised data can then be output in parallel or serial format. Additionally, a digital signal processor performing tasks like data compression, convolution or correlation and formatting can also be integrated with this generic digital receiver. The front end of the receiver is wide-band, catering to bandwidths of upto 2 GHz while the digitisation rates are also of the order of 1-2 GHz. It is proposed to standardize the design and use this generic receiver for front end data acquisition of all the future microwave sensors. It will meet the digitisation requirements of 500 MHz to 1 GHz for ultra-high resolution (0.25-0.5 meter) SAR as well as direct sampling of the signal around 1.4GHz for L-band Synthetic Aperture Radiometer. After initial prototyping using discrete receiver elements and ultra-high speed 8-bit ADC, it will be taken up as a custom ASIC or multi-chip module consisting of RF MMIC's and a mixed signal ADC ASIC. These designs will be fabricated using InP, GaAs or SiGe process technologies at competent foundries like GATEC, SCL, Infineon/Germany, X-Fab/Germany and Ommic-Philips/France. This novel digital receiver will offer several advantages like flexibility, stability, reduced RF hardware and miniaturisation. This paper describes the ultra-high speed design requirements, configuration details and target specifications and salient features of this generic L-band digital receiver for ISRO's future spaceborne and airborne radar missions. It also addresses the associated signal integrity, EMI/EMC and thermal issues.