Tapan Misra

Ground calibration of multifrequency scanning microwave radiometer (MSMR)

The Multifrequency Scanning Microwave Radiometer (MSMR), flown on-board Oceansat-I (IRS-P4), is a four-frequency and dual-polarization sensor for oceanographic applications. An extensive ground calibration experiment was conducted prior to launch to evaluate calibration coefficients. This paper presents an overview of the on-board system, details of ground calibration methodology, and expected calibration performance derived from ground calibration data.

The NASA-ISRO SAR mission - An international space partnership for science and societal benefit

The National Aeronautics and Space Administration (NASA) in the United States and the Indian Space Research Organisation (ISRO) have embarked on the formulation of a proposed Earth-orbiting science and applications mission that would exploit synthetic aperture radar to map Earths surface every 12 days. The missions primary objectives would be to study Earth land and ice deformation, and ecosystems, in areas of common interest to the US and Indian science communities. To meet demanding coverage, sampling, and accuracy requirements, the system would require a swath of over 240 km at fine resolution, using full polarimetry where needed. To address the broad range of disciplines and scientific study areas of the mission, a dual-frequency system was conceived, at L-band (24 cm wavelength) and S-band (10 cm wavelength). To achieve these observational characteristics, a reflector-feed system is considered, whereby the feed aperture elements are individually sampled to allow a scan-on-receive (“SweepSAR”) capability at both L-band and S-band. In the partnership, NASA would provide the instrument structure for both L- and S-band electronics, the L-band electronics, the reflector and associated boom, and an avionics payload to interface with the radar that would include a solid state recorder, high-rate Ka-band telecommunication link, and a GPS receiver. ISRO would provide the spacecraft and launch vehicle, and the S-band radar electronics.

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}

This paper departs from the popular usage of the Backus-Gilbert inversion (BGI) method as a tool for inversion of antenna temperature measurements in microwave radiometry. The BGI method is applied in this paper to enhance the information content of an existing set of oversampled brightness-temperature (TB) data. The purpose is to isolate the inversion process from its resolution enhancement counterpart. The advantage gained is that the resolution enhancement can be performed in a simplified way and in a different level of processing that starts with the scan-mode TB data product and simply requires with it the knowledge of the antenna gain pattern and the sensors scan geometry. The technique is demonstrated with the 19.35-GHz Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) channel, which provides oversampled TB data. The radiometric resemblance of this channel with that of the 37 GHz and geocollocation of their TB footprints facilitate validation of the enhancement of features. The significance of oversampling the low-frequency (LF) radiometer channels is underscored in the process, which gives the authors the confidence to propose oversampling of the LF data for the forthcoming sensor Microwave Analysis and Detection of Rain and Atmospheric Structures (MADRAS) onboard the Megha_Tropiques mission, which is a joint ISRO-CNES collaboration (due for launch in 2009).

Analyses, and Estimation of Biases

SAR Payload of RISAT (Radar Imaging Satellite), the first SAR satellite from ISRO, is currently under development. This payload is based on active antenna technology, and it supports variety of resolution and swath requirements in C-band. Both conventional stripmap and scanSAR modes are supported with dual polarization operation. Additionally a quad polarization stripmap mode is provided for availing additional resource classification. In all these modes resolutions from 3m-50 m can be achieved with swath ranging 30 km -240 km. On experimental basis, a sliding spotlight mode is also available. The payload hardware is organized in such a way that that co and cross polarization images are available for any operating modes. Additionally, a quad polarization mode is also supported. Active array configuration of this payload called for development of many new technologies ranging from MMICS, TR modules, miniaturised power supplies, high speed digitisers, dual polarized printed antenna and distributed control systems. A completely new bus is being designed for aiding the payload operation. The RISAT spacecraft is configured around the payload to minimize the spacecraft weight, suitable for launching by ISROs PSLV launcher. RISAT will be placed on dawn to dusk sunsynchronous polar orbit to ensure maximum solar power availability. All the basic building blocks have already crossed design stage and have undergone rigorous space qualification program. Presently a complete SAR with one tile has been integrated as design verification model and is under rigorous testing. This development ensured demonstration of end to end hardware, on-board control software and beam control behavior.

Brightness Temperature Reconstruction Using BGI

The National Aeronautics and Space Administration (NASA) in the United States and the Indian Space Research Organisation (ISRO) are developing a synthetic aperture radar (SAR) mission to map Earths surface every 12 days, known as the NASA-ISRO SAR (NISAR) Mission. NISAR has two radars sharing a mechanical structure and reflector, one operating at L-band (24 cm wavelength) and the other at S-band (10 cm wavelength). To achieve wide-swath observations at both wavelengths, NISAR is designed as a reflector-feed system where the feed aperture elements are individually sampled to allow a scan-on-receive capability. In the partnership, NASA provides the instrument structure for both L- and S-band electronics, the L-band electronics, the reflector and associated boom, and an avionics payload to interface with the radar including a solid-state data recorder, high-rate Ka-band telecommunication link, and a GPS receiver. ISRO provides the spacecraft and launch vehicle, and the S-band radar electronics, and an additional high-rate Ka-band telecom package. Hardware prototyping has matured designs for engineering models, which are currently under development.

RISAT: first planned SAR mission of ISRO

Since the 2007 National Academy of Science “Decadal Survey” report “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond” [1], the National Aeronautics and Space Administration (NASA) has been studying concepts for a Synthetic Aperture Radar (SAR) mission to determine Earth change in three disciplines - ecosystems, solid earth, and cryospheric sciences. One of the most promising and original concepts involves an innovative international partnership between NASA and the Indian Space Research Organization (ISRO). Previous NASA concepts had focused on exploiting an L-band array-fed reflector SAR configuration that enabled > 200 km swath at full SAR resolution and full polarimetry simultaneously in order to meet requirements in all three disciplines [2]. The feed where the electronics are housed in this design is relatively compact compared to a planar phased array antenna with similar azimuth resolution capability. This compactness allows for straightforward addition of feed array elements at other frequencies. As the partnership concept with ISRO developed, it became clear that flying dual L- and S-band SAR capabilities, with L-band electronics supplied by NASA and S-band electronics by ISRO, would satisfy science and application requirements of the US and India. A dual-frequency fully polarimetric SAR with the potential for global coverage every 12 days would offer unprecedented capability that researchers could exploit in new and exciting ways.

An update on the NASA-ISRO dual-frequency DBF SAR (NISAR) mission

Radar Imaging Satellite (RISAT-1) was launched on 26th April, 2012 and is available to international SAR community and researchers as a source of multi resolution/multi-swath/multi-polarisation SAR data. The first satellite, RISAT-1, carries a multi-mode Synthetic Aperture Radar (SAR) in C-band as the sole payload. RISAT-SAR payload supports variety of imaging modes e.g., stripmap, scanSAR and spotlight. All these imaging modes can be operated in Hybrid polarimetry mode also, making it the first of its kind. The payload is based on active antenna array technology. The active antenna has nominal aperture of 6 m×2m, consisting of 12 tiles which are grouped into 3 panels. The RISAT-1 spacecraft has been built around the SAR payload in order to optimise the spacecraft weight and structure. The satellite is placed in a sun-synchronous orbit with 6 am-6 pm equatorial crossing, to maximize solar power availability. Initial performance assessment indicates very satisfactory performance of the SAR system of RISAT-1 in terms of image quality, calibration, geometric fidelity and consistent product performance and active antenna pattern synthesis. Already from September 2012, data products are available operationally from National Remote Sensing Centre, Hyderabad.

A dual-frequency spaceborne SAR mission concept

This paper presents a new assortment of temperature-sounding channels for a proposed low-Earth-orbit polar Sun-synchronous satelliteborne millimeter-wave atmospheric sounder of the Indian Space Research Organization (ISRO). The newness owes its origin to the exploration of the millimeter-wave O2 absorption spectrum in quest of optimal off-resonance frequencies that impose fewer restrictions on channel bandwidth and temperature sensitivity and yet can sound up to 40 km in the atmosphere with a 4-km vertical resolution. This is ISROs first leap toward millimeter-wave technology. The overall receiver-noise figure for the channels in the 5-mm band (50-60 GHz) has been pessimistically estimated at 5 dB which will severely degrade the system temperature sensitivity. Therefore, channel bandwidth is at premium. The purpose of this design is to limit the number of passbands and simplify the design of frequency-selective filters by choosing center frequencies that have sufficient interleaving and also provide a scope of allotting a reasonable bandwidth. The set of temperature-sounding channels in the current design have 15 channels in 17 passbands in contrast with 15 channels in 29 passbands of the operational Advanced Microwave Sounding Unit (AMSU)-A.

RISAT-1: Configuration and performance evaluation

The conventional matched filter approach uses the replica of thc transmitted, frequency modulated signal to generate a reference function for the range compression. Phase and amplitude errors occuring in the total transceiver chain before digitalization will be contained in the reference function, which is the complex conjugate of the time reversed replica. The impulse response function in this case has the best achievable signal to noise ratio but high sidelobes or paired echoes will appear for severe phase and amplitude errors. We present here a new method for the generation of the reference function from the replica, which provides both, matched filtering and deconvolution of the amplitude and phase errors. The final impulse response function will be like the ideal one irrespective of the amount of errors present. Several simulation results are shown which are significant for the improvement of the image quality and for relaxation in the amount of allowed amplitude and phase errors in the S A R design for range processing. The proposed method can also be extended to azimuth processing of signals received a t high squint angles.