Proc. SPIE. 6412, Disaster Forewarning Diagnostic Methods and Management
KEYWORDS: Digital signal processing, Doppler effect, Synthetic aperture radar, Image processing, Image resolution, Data processing, Signal processing, Radar imaging, Commercial off the shelf technology, Floods
Since last few years, ISRO has embarked upon the development of two complex Synthetic Aperture Radar (SAR) missions, viz. Spaceborne Radar Imaging Satellite (RISAT) and Airborne SAR for Disaster Mangement (DMSAR), as a capacity building measure under country's Disaster Management Support (DMS) Program, for estimating the extent of damage over large areas (~75 Km) and also assess the effectiveness of the relief measures undertaken during natural disasters such as cyclones, epidemics, earthquakes, floods and landslides, forest fires, crop diseases etc. Synthetic Aperture Radar (SAR) has an unique role to play in mapping and monitoring of large areas affected by natural disasters especially floods, owing to its unique capability to see through clouds as well as all-weather imaging capability. The generation of SAR images with quick turn around time is very essential to meet the above DMS objectives. Thus the development of SAR Processors, for these two SAR systems poses considerable challenges and design efforts. Considering the growing user demand and inevitable necessity for a full-fledged high throughput processor, to process SAR data and generate image in real or near-real time, the design and development of a generic SAR Processor has been taken up and evolved, which will meet the SAR processing requirements for both Airborne and Spaceborne SAR systems. This hardware SAR processor is being built, to the extent possible, using only Commercial-Off-The-Shelf (COTS) DSP and other hardware plug-in modules on a Compact PCI (cPCI) platform. Thus, the major thrust has been on working out Multi-processor Digital Signal Processor (DSP) architecture and algorithm development and optimization rather than hardware design and fabrication. For DMSAR, this generic SAR Processor operates as a Quick Look SAR Processor (QLP) on-board the aircraft to produce real time full swath DMSAR images and as a ground based Near-Real Time high precision full swath Processor (NRTP). It will generate full-swath (6 to 75 Kms) DMSAR images in 1m / 3m / 5m / 10m / 30m resolution SAR operating modes.
For RISAT mission, this generic Quick Look SAR Processor will be mainly used for browse product generation at NRSA-Shadnagar (SAN) ground receive station. RISAT QLP/NRTP is also proposed to provide an alternative emergency SAR product generation chain. For this, the S/C aux data appended in Onboard SAR Frame Format (x, y, z, x', y', z', roll, pitch, yaw) and predicted orbit from previous days Orbit Determination data will be used. The QLP / NRTP will produce ground range images in real / near real time. For emergency data product generation, additional Off-line tasks like geo-tagging, masking, QC etc needs to be performed on the processed image. The QLP / NRTP would generate geo-tagged images from the annotation data available from the SAR P/L data itself. Since the orbit & attitude information are taken as it is, the location accuracy will be poorer compared to the product generated using ADIF, where smoothened attitude and orbit are made available. Additional tasks like masking, output formatting and Quality checking of the data product will be carried out at Balanagar, NRSA after the image annotated data from QLP / NRTP is sent to Balanagar. The necessary interfaces to the QLP/NRTP for Emergency product generation are also being worked out.
As is widely acknowledged, QLP/NRTP for RISAT and DMSAR is an ambitious effort and the technology of future. It is expected that by the middle of next decade, the next generation SAR missions worldwide will have onboard SAR Processors of varying capabilities and generate SAR Data products and Information products onboard instead of SAR raw data. Thus, it is also envisaged that these activities related to QLP/NRTP implementation for RISAT ground segment and DMSAR will be a significant step which will directly feed into the development of onboard real time processing systems for ISRO's future space borne SAR missions. This paper describes the design requirements, configuration details and salient features, apart from highlighting the utility of these Quick Look SAR processors for RISAT and DMSAR, for generation of emergency products for Disaster management.
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