# A low-power VHF transceiver for airborne SAR with enhanced buried object detection using chirped signal processing

## TL;DR

This paper introduces a low-power radar system for detecting underground tunnels using advanced signal processing techniques.

## Contribution

The novel contribution is the design of an optimized PWL-NLFM chirp waveform using PSO for improved SAR imaging of buried structures.

## Key findings

- The optimized PWL-NLFM chirp reduces PSLR to -33.0 dB and improves ISLR to -21.8 dB.
- The waveform enhances SAR focusing, producing a cleaner mainlobe and better target contrast.
- The system is integrated into a low-power SDR-based transceiver suitable for long-duration airborne missions.

## Abstract

A low-power airborne synthetic aperture radar (SAR) transceiver is presented for high-resolution detection of shallow buried structures, particularly underground tunnels. The system operates in the VHF band to exploit its strong ground-penetration capability, where the limited available bandwidth necessitates advanced waveform shaping to achieve sufficient imaging resolution. To address this challenge, an optimized piecewise-linear nonlinear frequency modulation (PWL-NLFM) chirp is designed using particle swarm optimization (PSO), jointly minimizing sidelobe levels while preserving the required pulse-compression ratio. The tunable parameter \documentclass[12pt]{minimal}
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\begin{document}$$Q$$\end{document} controls the number of PWL segments, enabling a flexible trade-off between sidelobe suppression and pulse compression ratio according to mission requirements. Quantitative evaluation demonstrates that the proposed waveform significantly outperforms standard LFM and quadratic NLFM pulses, reducing the peak sidelobe level ratio (PSLR) to -33.0 dB and improving the integrated sidelobe ratio (ISLR) to -21.8 dB. A full two-dimensional (range–azimuth) point-target simulation further confirms that these improvements translate into superior SAR focusing, producing a cleaner and more isolated mainlobe with only slight broadening. This enhanced 2-D response increases the contrast between weak subsurface targets and background clutter, directly improving the detectability of tunnel features. The optimized PWL-NLFM waveform is integrated into a low-power SDR-based SAR transceiver, demonstrating its suitability for long-duration airborne sensing missions requiring deep penetration and high-contrast imaging.