Orthogonal NLFM Waveforms for SAR Range Ambiguity Suppression
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Solution Overview
Problem
Synthetic aperture radar (SAR) systems face limitations in simultaneously increasing azimuth resolution and range swath width due to the trade-off between azimuth and range ambiguities, which affects imaging quality by allowing range ambiguity energy to be received by higher sidelobes, leading to reduced image quality.
Innovation Solution
The use of orthogonal nonlinear frequency modulation (NLFM) waveforms to improve signal orthogonality, minimize cross-correlation energy, and suppress range ambiguity by constructing range matched filters based on the waveform sequence of transmitted and received signals, thereby enhancing imaging quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pulse repetition frequency (PRF) is increased to improve azimuth resolution, then azimuth resolution is improved, but the selectable swath width becomes narrower
Solution Approach 1:
The patent applies parameter changes by using nonlinear frequency modulation (NLFM) instead of traditional linear frequency modulation (LFM). The NLFM signal uses a nonlinear chirp rate function that varies with time, allowing the signal bandwidth to be efficiently utilized while maintaining low sidelobe levels. This parameter change in the signal modulation approach enables improved azimuth resolution without proportionally increasing PRF, thereby preserving swath width.
2Measurement precision
If pulse repetition frequency (PRF) is increased to improve azimuth sampling rate, then azimuth over-sampling rate is improved, but range ambiguity energy increases as range ambiguity area moves closer to signal area
Solution Approach 1:
The patent changes the signal modulation parameter from linear to nonlinear frequency modulation. The NLFM signal's nonlinear chirp rate function is designed to compress the signal energy in the range dimension while maintaining azimuth resolution. This parameter change creates a mismatch between the range ambiguity signal and the matched filter, causing the ambiguity energy to be suppressed rather than enhanced.
Solution Approach 2:
The patent converts the potentially harmful range ambiguity energy into a beneficial suppression mechanism. By using NLFM modulation with specific nonlinear chirp rate functions, the range ambiguity signals that would normally interfere with the main signal are transformed into signals that are effectively rejected by the matched filter processing. The ambiguity energy is converted from a harmful interference source into a suppressed component that improves overall signal clarity.
3Measurement precision
If traditional LFM signals are used with high PRF, then azimuth resolution is improved, but range ambiguity suppression deteriorates due to poor signal orthogonality
Solution Approach 1:
The patent fundamentally changes the signal modulation parameter from linear to nonlinear frequency modulation. The NLFM signal uses a nonlinear chirp rate function that provides superior time-frequency localization compared to LFM. This parameter change creates better orthogonality between the transmitted signal and range ambiguity signals, enabling effective ambiguity suppression while maintaining high azimuth resolution performance.
Data Source
AI summary
Provided are a method and an apparatus for range ambiguity suppression based on orthogonal nonlinear frequency modulation (NLFM) waveforms. The method includes that: according to transmitted orthogonal NLFM signals, a waveform sequence of the transmitted signals corresponding to an obtained echo signal is determined; a set of range matched filters is constructed according to the waveform sequence and the orthogonal NLFM signals; range compression is performed on the echo signal by using the set of range matched filters to obtain range-compressed data; and synthetic aperture radar (SAR) imaging is performed according to the range-compressed data, to obtain an imaging result, and the imaging result is outputted. A non-transitory computer-readable storage medium is also provided.