SAR Image Phase Error Compensation via Terrain Elevation Strips
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Solution Overview
Problem
Synthetic Aperture Radar (SAR) image quality is compromised due to deviations from ideal flight paths and variations in terrain elevation, leading to phase errors and image blurring, as existing motion compensation mechanisms assume a rectilinear path and constant ground elevation.
Innovation Solution
A radar system that uses a central reference point, terrain elevation data, and Taylor series expansion to compute phase error corrections, applying these corrections through a navigation system and computer processing to compensate for flight path deviations and elevation variations, thereby improving SAR image clarity by remapping and merging strips with overlapping corrections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If motion compensation is applied assuming a rectilinear path and constant ground elevation, then the SAR imaging process can be simplified, but phase errors are introduced when the actual path deviates from the ideal path or terrain elevation varies
Solution Approach 1:
The patent divides the SAR image into multiple strips (e.g., first strip, second strip) that can be processed separately. Each strip corresponds to a specific portion of the terrain and can have its phase errors compensated independently using local flight path deviation data and terrain elevation information, thereby reducing the overall complexity while maintaining precision.
Solution Approach 2:
The patent applies different phase error compensation methods to different regions (strips) of the SAR image based on their specific characteristics. Each strip uses locally relevant flight path deviation data and terrain elevation information to compute appropriate phase corrections, allowing high precision phase accuracy in each region without requiring the entire image to be processed with a uniform complex method.
2Productivity
If the SAR system processes the entire area as a single image, then processing is simpler, but flight path deviations and elevation variations cause phase errors that blur the entire image
Solution Approach 1:
The patent segments the SAR image into multiple strips that can be processed independently. This allows the system to maintain high processing efficiency by handling smaller data portions separately while improving image quality through targeted phase error compensation in each strip, avoiding the blurring effect that would occur if the entire image were processed as a single unit with uniform assumptions.
Solution Approach 2:
The patent dynamically adjusts the processing approach for each strip based on local flight path deviations and terrain characteristics. The phase error compensation is computed adaptively for each strip using its specific elevation data and flight path information, allowing the system to maintain high reliability image quality while preserving overall processing efficiency through parallel processing of multiple strips.
3Measurement precision
If phase error compensation is computed for the entire SAR array, then comprehensive correction is achieved, but computational complexity and processing time increase significantly
Solution Approach 1:
The patent divides the SAR array into multiple strips that can be processed in parallel. Each strip computes phase error compensation independently using its local flight path deviation data and terrain elevation information, significantly reducing the computational time required compared to processing the entire array as a single unit while maintaining comprehensive correction accuracy across all strips.
Solution Approach 2:
The patent applies phase error compensation to each strip using the necessary local data (flight path deviations and terrain elevation) without requiring computation across the entire SAR array. This partial action approach computes only the required corrections for each region, reducing processing time while maintaining sufficient accuracy for high-quality SAR imaging.
4Device complexity
If the system assumes constant ground elevation, then the motion compensation model is simpler, but phase inaccuracies are introduced when terrain elevation varies
Solution Approach 1:
The patent incorporates terrain elevation information specific to each strip into the phase error compensation process. By using locally relevant elevation data from terrain databases, the system can accurately compensate for phase errors caused by elevation variations without requiring a completely complex global terrain model, thereby maintaining image clarity while managing model complexity.
Solution Approach 2:
The patent uses pre-existing terrain elevation data (such as digital elevation models) to compute phase error corrections before processing the SAR image. This preliminary action allows the system to account for elevation variations in advance, improving SAR image clarity without requiring complex real-time terrain analysis, thus balancing model complexity with imaging precision.
Data Source
AI summary
A radar acquires a formed SAR image of radar scatterers in an area around a central reference point (CRP). Target(s) are within the area illuminated by the radar. The area covers terrain having a plurality of elevations. The radar is on a moving platform, where the moving platform is moving along an actual path. The actual path is displaced from an ideal SAR image acquisition path. The radar has a computer that divides the digital returns descriptive of the formed SAR image into multiple blocks, such as a first strip and an adjacent strip. The first strip is conveniently chosen, likely to generally align with a part of the area, at a first elevation. An adjacent strip covers a second part of the area at a second elevation. The first strip is overlapping the adjacent strip over an overlap portion. The first and second elevation are extracted from a terrain elevation database (DTED). Horizontal displacement of returns (range deviation) is computed for each strip using the elevation information from the terrain elevation database. Taylor series coefficients are computed for the horizontal displacement due to terrain elevation using the ideal path, the actual path and central reference point. Actual flight path deviation is available at each pulse position while azimuth frequency is given in azimuth angle off mid angle point. Remapping between indices in two arrays is also computed. Phase error compensation and compensation in azimuth (spacial frequency) is computed using the Taylor series coefficients, a Fast Fourier Transform and an inverse Fast Fourier Transform for each strip. Phase error compensation is applied to the digital returns from each strip to obtain the SAR image. The SAR image is further improved by having the first strip corrected data and the second strip corrected data merged over the overlap portion to generate a relatively seamless SAR image.