Nilesh M. Desai

Oceansat-II Scatterometer: Sensor Performance Evaluation,

The Oceansat-II Scatterometer has completed two years in orbit. The instrument has been declared operational, and the normalized radar cross section (σ0) and wind products are being made routinely available to the global operational Numerical Weather Prediction community. The σ0 data from the sensor have been rigorously analyzed for the past two years. Efforts have been put to systematically correlate the biases observed in the data to the onboard functionality of the instrument and to precisely quantify these biases. These analyses have helped not only in the refinement of the ground-processing algorithm but also in the evaluation of sensor performance. This paper presents some of the analyses that have been carried out related to instrument noise calibration with reference to deep-space observations, estimation of biases in the signal bandwidth, and estimation of fixed remnant attitude biases. This paper also addresses the means for rectifying these instrument-related biases.

\sigma^{0}

In the last couple of years, an indigenous airborne synthetic aperture radar for disaster management (DMSAR) has been under development at SAC / ISRO, as a capacity building measure for evolving an effective disaster management support (DMS) system in India. Synthetic aperture radar (SAR) has an unique role to play in the 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. DMSAR is presently mounted on-board Beechcraft B-200 aircraft and will he subsequently carried on a jet class aircraft and utilized for estimating the extent of damage over large areas (~50-75 Km) and also assess the effectiveness of the relief measures undertaken during disasters like floods. Considering the unique application requirements of disaster management, one of the major essential and critical requirements is the generation and availability of DMSAR images in real- or near-real-time with very fast turn-around times. DMSAR image generation involves complex signal processing of the acquired raw data, involving data decompression, two-dimensional SAR processing, motion sensing and compensation tasks, and image mosaicing on display as well as storage of processed SAR images on suitable recording media.

Analyses, and Estimation of Biases

Current conveyor (a current mode circuit) is a good choice for low voltage applications as it provides high gain-bandwidth product. Current mode design technique offers voltage independent, high bandwidth in analog circuits with properties of accuracy and versatility in a wide range of applications. In this paper we present a rail-to-rail low voltage (±1.5 V) differential current conveyor (LVDCC) circuit in standard CMOS technology that is suited for low voltage operation. The proposed circuit uses a low-voltage-current-mirror. The output range is -1.3 to +1.4 voltage which is nearly rail-to-rail. This proposed differential current conveyor is suitable for low power CMOS mixed-mode designs.

SAR for disaster management

One of the major and common requirements for all active microware sensors is generation of the transmit modulation signal-like chirp/LFM signal, MSK, etc., which can be generated by analog or digital means. With the increasing demands of side bandwidth, longer duration chirp signals in radar systems, digital signal generation, and processing has emerged as a preferred alternative. Design and development of programmable and generic digital waveform generator (DWG) system based on Xilinx Virtex XCV600 FPGA and high-speed DAC is carried out at the Space Applications Center. ISRO is to generate required transmit chirp signal of high time-bandwidth product (~ 1000) for ISOs microwave radar sensor missions. This gives a detailed description of the design requirements, implementation details, salient performance features, and test results of this programmable and generic digital waveform generator (DWG).

A new low voltage differential current conveyor

Radiation Hardened By Design (RHBD) combinational circuits/primitive gates using 0.18um CMOS Technology is developed for Space application with help of Cogenda TCAD software suite. The proposed combinational cells are investigated for radiation simulation using three dimensional (3D) device structure. Single Event Transient (SET) caused by proton, α particle and heavy ions like C, Ar and Kr is observed on developed Cells and SET pulse width is measured on primitive gates. The proposed C element based radiation hardened Inverter is simulated using α, Ar and proton energetic particle. Proposed NOR and NAND gates are simulated under the radiation of proton, α and Kr and Single Event Transient Pulse Width is measured.

ISRO's programmable digital waveform generator

During 1990s, high speed data handling and control unit (DHCU) was developed for Indian Space Research Organisations (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.

Development of Radiation Hardened by Design(RHBD) primitive gates using 0.18μm CMOS technology

This paper reports the development of a millimeter-wave space-borne atmospheric Temperature Sounding Unit (TSU) in Indian Space Research Organization (ISRO). This is ISRO’s first leap towards millimeter-wave technology. The sensor has several new accomplishments to its credit which include among others, the philosophy of sounding channel selection, the new assortment of temperature sounding channels, simultaneous observation of both polarizations of all channels, compact dual-band scanning Gregorian reflector antenna, indigenously developed black-body target for in-orbit calibration, in-house developed millimeter-wave RF front-end and pre-detection automatic gain control method. The prime feature of this instrument is its unique set of channels which can profile the earth’s atmosphere from surface to 40 km altitude with vertical resolution ranging from less than a km near surface to ±2.5 km at 30km altitude. The channels are predominantly off-resonant frequencies in the 50―60 GHz O2 absorption spectrum which offer near-uniform attenuation and hence more channel-bandwidth and better temperature sensitivity and yet have adequate overlap of their weighting functions to achieve the desired vertical resolution. These channels are different and have fewer bands from what has been flown in all earlier sounding missions worldwide e.g. AMSU-A, SSMIS, ATMS etc. The TSU radiometer has been characterized thoroughly using ingenious methods such as low-power active RF energizing along with frequency sweep. This is a compact, low-mass, low-power instrument and has been configured for the ISRO mini-satellite (IMS-2) bus. The flight model with improved hardware performance is being built and a suitable opportunity of flying it is being explored.

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

This paper presents the design and simulation of a two stage power management circuit implemented in 0:18μm CMOS that operates from very low voltages starting from 460mV and higher up to a maximum of 800mV. The proposed capacitive power management unit consumes very low power of 11μW @ 500mV sufficient to be operated from tiny photovoltaic cells, dimensions of few mm2. In addition to the lower power consumption, the proposed circuit does not need any off chip components; ideal for ultra low power wireless sensor nodes.

A novel atmospheric Temperature Sounding Unit: system design and performance analyses

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 ISROs microwave remote sensing programme, since 1980s. These technologies are now being gainfully utilized for building ISROs 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 ISROs proposed advanced microwave missions like ultra-high resolution SARs, 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 ISROs 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 MMICs 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 ISROs future spaceborne and airborne radar missions. It also addresses the associated signal integrity, EMI/EMC and thermal issues.