TOPS SAR Sub-Azimuth Processing for Azimuth Aliasing
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Solution Overview
Problem
Current methods for processing TOPS SAR raw data into high-precision image data are inefficient and prone to azimuth aliasing, requiring pre- and post-processing steps and interpolation, which increases computational effort and reduces precision.
Innovation Solution
The method employs sub-aperture processing with baseband azimuth scaling, allowing for range compression and target range variation correction within each sub-aperture, using the extended chirp scaling algorithm without interpolation, and applying a derotation function to maintain phase accuracy and reduce sidelobes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional TOPS SAR processing methods are used, then azimuth coverage is achieved, but azimuth aliasing occurs and processing efficiency is reduced due to required pre- and post-processing steps
Solution Approach 1:
The patent divides the azimuth processing into sub-apertures, where each sub-aperture is processed independently with its own azimuth scaling. This segmentation allows the system to avoid azimuth aliasing by limiting the bandwidth within each sub-aperture while maintaining overall azimuth coverage through concatenation of processed sub-apertures, thereby eliminating the need for complex pre- and post-processing steps.
Solution Approach 2:
The patent applies range-dependent azimuth scaling with a scaling factor that varies with range distance. By changing the azimuth scaling parameter as a function of range, the system compensates for the range-azimuth coupling effect in TOPS mode, preventing azimuth aliasing and eliminating the need for additional correction steps that would reduce processing efficiency.
2Ease of manufacture
If interpolation methods are used in azimuth scaling, then processing can be performed, but manufacturing precision and phase accuracy are reduced
Solution Approach 1:
The patent replaces interpolation-based azimuth scaling with a direct computational approach using range-dependent scaling factors applied in the azimuth frequency domain. This substitution eliminates the need for interpolation operations that compromise phase accuracy, while maintaining processing simplicity through efficient FFT-based implementation and direct multiplication with scaling kernels.
3Measurement precision
If azimuth scaling is applied to correct range-azimuth coupling, then processing accuracy improves, but computational effort increases
Solution Approach 1:
The patent segments the azimuth processing into independent sub-apertures, each processed with its own scaling factor. This segmentation reduces the computational complexity of azimuth scaling by limiting the scaling operation to smaller bandwidth segments, thereby reducing overall computational effort while maintaining the accuracy benefits of range-dependent scaling through concatenation of processed segments.
Data Source
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AI summary
Sub-aperture processing is carried out. Within each sub-aperture, range compression and a correction for the target range variation are carried out. Baseband azimuth scaling is used for processing the azimuth signal, wherein a long azimuth reference function and thus a wide azimuth dimension are prevented. The scaling range is not constant and depends on the range, which is not equal to the original range vector. It is calculated such that, in combination with a subsequent derotation step, constant azimuth scanning is achieved for all ranges. The selected derotation function, which is applied in the azimuth time domain, makes it possible for all the targets to be in base band, in this way varying the effective chirp rate. Since the phase is purely quadratic because of the azimuth scaling step, it is thus possible to use an optimal filter which takes account of the effective chirp rate. IFFT results in a focused image and a final phase function in the time domain allows phase maintenance. Application for SAR, SONAR and seismic raw data processing in the TOPS mode, as well as other modes which make use of the antenna polar diagram being scanned in the azimuth and/or elevation direction.