The area accessible from a spaceborne imaging radar, e.g. a synthetic aperture radar (SAR), generally increases with the elevation of the satellite while the map coverage rate is a more complicated function of platform velocity and beam agility. The coverage of a low Earth orbit (LEO) satellite is basically given by the ground velocity times the relatively narrow swath width. The instantaneously accessible area will be limited to some hundreds of kilometers away from the sub-satellite point. In the other extreme, the sub-satellite point of a SAR in geosynchronous orbit will move relatively slowly, while the area which can be accessed at any given time is very large, reaching thousands of kilometers from the sub-satellite point. To effectively use the accessibility provided by a high vantage point, very large antennas with electronically steered beams are required. Interestingly, medium Earth orbits (MEO) will enable powerful observational systems which provide large instantaneous reach and high mapping rates, while pushing technology less than alternative systems at higher altitudes. Using interferometric SAR techniques which can reveal centimeter-level (potentially sub-centimeter) surface displacements, frequent and targeted observations might be key to developing such elusive applications as earthquake forecasting. This paper discusses the basic characteristics of a SAR observational system as a function of the platform altitude and the technologies being developed to make such systems feasible.
The NASA/JPL Airborne Synthetic Aperture Radar system (AIRSAR) has been in operation since 1988. The original radar configuration consisted of PIL/C-band quadpolarization mode in both 20 MHz and 40 MHz chirp bandwidths. Over the years, we have added the L- and C-band along track interferometry mode (ATI), the on-board processor, the C-band cross-track interferometry mode (XTI) in 199 1 , and the L-band XTI mode in 1995. In addition, we also replaced the GPS receiver as well as the inertial navigation system in 1995 to improve the accuracy of motion compensation and geolocation of the output products. In the 1996 PacRim Campaign, we flew a new digital chirp generator that has significantly better chirp linearity, which should improve the ISLR of the output images. In this paper, we will briefly describe the instrument characteristics, the evolution of the various radar modes, the instrument performance and improvement in the knowledge of the positioning and attitude information of the radar. In addition, we will summarize the progress of the data processing effort especially in the interferometry processing. Finally, we will address the issue of processing and calibrating the cross-track interferometry (XTI) data.
The primary purpose of GeoSAR is to demonstrate the feasibility of interferometric topographic mapping through foliage penetration. GeoSAR should become a commercially viable instrument after the feasibility demonstration. To satisfy both requirements, we have designed a dual frequency (UHF- and X-band) interferometric radar. For foliage penetration, a lower frequency (UHF) radar is used. To obtain better height accuracy for low backscatter areas, we proposed a high frequency (X-band) interferometric system. In this paper, we present a possible GeoSAR system configuration and associated performance estimation.
Conference Committee Involvement (1)
Microwave Remote Sensing of the Atmosphere and Environment V
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