SAR Autofocus Navigation Error Correction

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Solution Overview

Problem

Existing synthetic aperture radar (SAR) systems face challenges in generating high-quality images due to discontinuities at the boundaries between image blocks, which affect image resolution and subsequent processing tasks like automated target recognition and change detection, especially when using small antenna platforms.

Innovation Solution

The method involves generating a navigation error profile estimate using phase error profiles from an autofocus algorithm, approximated as a vector of low-order polynomials in time, and optimizing polynomial coefficient values to minimize phase errors across selected image blocks, thereby improving the navigation profile and reducing image discontinuities.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If autofocus algorithm is applied to large image blocks, then processing efficiency is improved, but discontinuities appear at the boundaries between image blocks

Engineering Contradiction:

Improveprocessing efficiencyVSAvoidimage continuity at block boundaries

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent divides the SAR image into multiple smaller image blocks and applies autofocus algorithm to each block independently. This segmentation approach allows parallel processing of multiple blocks, improving overall processing efficiency while maintaining image quality within each block.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent merges the autofocus results from multiple image blocks by applying a navigation error correction that is consistent across all blocks. This merging process eliminates discontinuities at block boundaries by ensuring that the phase error corrections are coherent across the entire image.

Inventive Principle:

Principle #5Merging (Combining)

2Manufacturing precision

If small image blocks are used for autofocus, then image continuity is improved, but processing time increases and some blocks may contain low quality targets

Engineering Contradiction:

Improveimage continuity at block boundariesVSAvoidprocessing efficiency

Core Design Contradiction:

Manufacturing precisionVSProductivity

Solution Approach 1:

The patent segments the image into small blocks to maintain continuity, but processes them in parallel to avoid excessive processing time. The small block size ensures that phase error variations within each block are minimal, improving image continuity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent uses the autofocus results from each image block to generate its own navigation error correction, making each block self-sufficient. This self-service approach allows independent processing of each block while maintaining overall image consistency through the shared navigation correction.

Inventive Principle:

Principle #25Self-service

3Ease of operation

If GPS/INS navigation data is used, then navigation information is available, but accuracy is insufficient for sharp SAR images

Engineering Contradiction:

Improvenavigation data availabilityVSAvoidnavigation accuracy

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent uses the autofocus algorithm to measure the actual phase errors in the SAR image, which provide feedback on the navigation errors. This feedback is then used to correct the navigation profile, improving the accuracy of the navigation data for subsequent image formation.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent replaces reliance on mechanical GPS/INS navigation systems with a software-based navigation correction derived from autofocus measurements. This substitution allows the system to achieve higher navigation accuracy by using the actual image focus quality as the reference standard.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS9239383B2Wide beam SAR focusing method using navigation solution derived from autofocus data

Publication Date: 2016.01.19 RAYTHEON CO

AI summary

An algorithm for deriving improved navigation data from the autofocus results obtained from selected image blocks in a wide-beam synthetic aperture radar (SAR) image. In one embodiment the navigation error may be approximated with a vector of low-order polynomials, and a set of polynomial coefficients found which results in a good phase error match with the autofocus results. In another embodiment, a least squares solution may be found for the system of equations relating the phase errors at a point in time for selected image blocks to the navigation error vector at that point in time. An approach using low sample rate backprojection (150) initially to select suitable image blocks, and full sample rate backprojection (156) for the selected image blocks, followed by full sample rate backprojection (164) for the image, using the improved navigation solution, may be used to reduce the computational load of employing the algorithm.