Bandwidth extrapolation of LFM signals for Narrowband Radar Systems

In this paper, we address the problem of improving range resolution in narrowband radar systems. By using linear frequency modulation (LFM) waveform, we propose a method of achieving range resolution improvement through extrapolating the bandwidth of the LFM waveform after it has been radiated by the transmitter and reflected back from targets and clutter to the receiver of a radar system. The proposed method is based on using linear prediction to extend the target frequency response into spectral regions outside of the measured band. There are two key differences between the proposed technique and an existing bandwidth extrapolation technique designed for wideband radar systems. Firstly, it uses correlation rather than stretch processing to obtain the range profile. This has the advantage of allowing the range depth of interest to be as large as the radar unambiguous distance, which is commonly required in narrowband radar systems. Secondly, the bandwidth extrapolation process is carried out after Doppler processing, which allows the proposed technique to work effectively even in the presence of strong stationary or slow moving clutter. We evaluate the performance of the proposed method using real data collected from an over-the-horizon radar system. We have shown that the proposed technique is capable of improving the range resolution in the presence of stationary clutter that is 20 - 50 dB stronger than the targets. In addition to range resolution improvement, we have observed that the proposed bandwidth extrapolation technique also improves the signal to noise ratio of the targets.