Chi-Yung Chang

Application of the multiple PRF technique to resolve Doppler centroid estimation ambiguity for spaceborne SAR

The range correlation technique for resolution of Doppler centroid estimation ambiguity for a spaceborne SAR is reviewed. A new technique using multiple pulse repetition frequencies (PRFs) is presented. An algorithm employing simple integer arithmetic for radar systems, such as moving target indicator radar (MTIR) systems, where the PRFs contain a large common divisor, is formulated. For SAR systems, where other performance factors control selection of the PRFs, an algorithm that uses PRFs of arbitrary numerical values is devised to resolve the ambiguity. The performance of this multiple PRF technique is analyzed using a statistical error model. An example is presented for the Shuttle Imaging Radar-C (SIR-C) C-band SAR. >

Squint Mode Sar Processing Algorithms

The unique characteristics of a spaceborne SAR (synthetic aperture radar) operating in a squint mode include large range walk and large variation in the Doppler centroid as a function of range. A pointing control technique to reduce the Doppler drift and a new processing algorithm to accommodate large range walk are presented. Simulations of the new algorithm for squint angles up to 20 deg and look angles up to 44 deg for the Earth Observing System (Eos) L-band SAR configuration demonstrate that it is capable of maintaining the resolution broadening within 20 percent and the ISLR within a fraction of a decibel of the theoretical value.

VLSI systolic binary tree-searched vector quantizer for image compression

A high-speed image compression VLSI processor based on the systolic architecture of difference-codebook binary tree-searched vector quantization has been developed to meet the increasing demands on large-volume data communication and storage requirements. Simulation results show that this design is applicable to many types of image data and capable of producing good reconstructed data quality at high compression ratios. Various design aspects of the binary tree-searched vector quantizer including the algorithm, architecture, and detailed functional design are thoroughly investigated for VLSI implementation. An 8-level difference-codebook binary tree-searched vector quantizer can be implemented on a custom VLSI chip that includes a systolic array of eight identical processors and a hierarchical memory of eight subcodebook memory banks. The total transistor count is about 300000 and the die size is about 8.67/spl times/7.72 mm/sup 2/ in a 1.0 /spl mu/m CMOS technology. The throughput rate of this high-speed VLSI compression system is approximately 25 Mpixels per second and its equivalent computation power is 600 million instructions per second. >

Doppler Centroid Estimation Ambiguity For Synthetic Aperture Radars

A technique for estimation of the Doppler centroid of an SAR in the presence of large uncertainty in antenna boresight pointing is described. Also investigated is the image degradation resulting from data processing that uses an ambiguous centroid. Two approaches for resolving ambiguities in Doppler centroid estimation (DCE) are presented: the range cross-correlation technique and the multiple-PRF (pulse repetition frequency) technique. Because other design factors control the PRF selection for SAR, a generalized algorithm is derived for PRFs not containing a common divisor. An example using the SIR-C parameters illustrates that this algorithm is capable of resolving the C-band DCE ambiguities for antenna pointing uncertainties of about 2-3 deg.

SAR Processing Based On The Exact Two-dimensional Transfer Function

The two-dimensional transfer functions of several synthetic aperture radar (SAR) focusing algorithms are derived considering the spaceborne SAR environments. The formulation includes the factors of the earth rotation and the antenna squint angles. The resultant transfer functions are explicitly expressed in terms of Doppler centroid frequency and Doppler frequency rate, which can be accurately estimated from the SAR data. Point target simulation results show that the algorithm based on the two-dimensional Fourier transformation outperforms the one-dimensional one for processing data acquired from high squint angles. The two-dimensional Fourier transformation approach appears to be a viable and simple solution for the processor design of future spaceborne SAR systems.

Spatial compression of Seasat SAR imagery

The results of a study of the techniques for spatial compression of synthetic-aperture-radar (SAR) imagery are summarized. Emphasis is on image-data volume reduction for archive and online storage applications while preserving the image resolution and radiometric fidelity. A quantitative analysis of various techniques, including vector quantization (VQ) and adaptive discrete cosine transform (ADCT), is presented. Various factors such as compression ratio, algorithm complexity, and image quality are considered in determining the optimal algorithm. The compression system requirements are established for electronic access of an online archive system based on the results of a survey of the science community. The various algorithms are presented and their results evaluated considering the effects of speckle noise and the wide dynamic range inherent in SAR imagery. >

First SIR-C ScanSAR results

During the two space shuttle missions in 1994, the Space-borne Imaging Radar-C (SIR-C) acquired several experimental datatakes: using the ScanSAR mode. Same of these ScanSAR data have been processed into images of swath wider than 200 km. Example images are shown. These early results demonstrate that ScanSAR operation is not only feasible, but also represents a potentially useful tool for regional and global scale imaging by the future SAR missions.

Attitude Steering For Space Shuttle Based Synthetic Aperture Radars

A technique is presented to steer the antenna beam to the zero-Doppler line for a spacecraft platform which operates in a non-zero roll attitude. The purposes of employing this attitude steering technique are reduction of the Doppler centroid error caused by the variation in target elevation; reduction of the Doppler centroid bound; and reduction of the Doppler drift. It is shown that a fixed attitude sequence can be easily accommodated in the radar command sequence to achieve nearly zero-Doppler steering. Implementation of this technique improves the image quality as well as simplifies the processor design.

Techniques in processing multi‐frequency multi‐polarization spaceborne SAR data

This paper presents the algorithm design of the SIR-C ground data processor, with emphasis on the unique elements involved in the production of registered multi-frequency polari-metric data products. A quick-look processing algorithm used for generation of low-resolution browse image products and estimation of echo signal parameters is also presented. Specifically, we will discuss the following: 1) Azimuth reference function generation to produce registered polarimetric imagery; 2) Geometric rectification to accommodate cross-track and along-track Doppler drifts; 3) Multi-look filtering designed to generate output imagery with a uniform resolution; and 4): Efficient coding to compress the polarimetric image data for distribution.

Data Compression For Eos On-board Sar Processor

A lightweight, low-power, real-time data compressor design for the Earth Observing System (Eos) onboard synthetic aperture radar (SAR) processor is presented. The implementation is based on VLSI design of a pipelined binary tree-searched vector quantizer (VQ), utilizing space-qualifiable 1.25-micron CMOS technology. The implementation exploits VLSI system design principles such as the modularity, regular data flow, simple interconnection, localized communication, simple global control, and parallel/pipelined processing. The overall system requires 30 chips with only one VLSI processing element design. The total weight is about 1.2 lbs, with an estimated power dissipation of approximately 4 watts operating at the maximum input data rate. The projected throughput rate exceeds 5 MHz.