Frequency and phase modulation of chirp signals in a small-size spacecraft-based SAR

Abstract

The article examines the problem of using LFM (linear frequency modulation) signals in SAR (synthetic aperture radar). The focus is made on a small-size spacecraft-based SAR. The major issue of radar imaging in such systems is the ambiguity occurrence in range and azimuth coordinate. The properties of broadband LFM signals are examined. There was analyzed the uncertainty function of a single signal and sequence as well if there is frequency modulation and phase center displacement of frequency modulation. Temporary position of the phase center or frequency modulation are proposed to be changed while preserving LFM for side-lobe suppression. It is demonstrated that displacement linear variation enables to reduce the side-lobe level by 10...15 dB. Phase modulation as per pseudo-random M-sequence is proposed to use to further reduce side-lobe level. Quantitative evaluation of the key parameters of the integrated in-depth radar surveillance was made following the simulation results. The simulation revealed that the side-lobe level can be reduced to a level of minus 37 dB with M-sequence volume of 2047 due to this. The simultaneous use of pseudo-random phase and frequency modulation reduces the side-lobe level to minus 50...55 dB, while the main response duration to a point target is reduced by 30%. Thus, the article demonstrates that frequency modulation enables besides suppressing interference signals, to reduce the side-lobe level and increase range resolution, which significantly enhances capabilities of in-depth SAR surveillance. Comprehensive use of frequency and phase modulation of LFM signals is reasonable to achieve maximum efficiency of the integrated small-size spacecraft-based SAR.