The Use of Frequency-Randomised Waveforms with Intelligent Processing for UAV SAR Imaging

A stepped-frequency continuous wave (SF-CW) synthetic aperture radar (SAR), with frequency-agile waveforms and real-time intelligent signal processing algorithms, is proposed for operation from a lightweight UAV platform. An SF-CW radar offers some distinct advantages over a pulsed radar. It measures the frequency response of the scene across a set of discrete frequencies over the bandwidth of interest, at each element position along the synthetic aperture. This means the individual frequency measurements are low power, but which are then integrated to simulate a much higher-power pulsed system. This is a cost effective way of providing radars with a low probability of interception (LPI), operating across wide frequency bands to obtain high image resolutions. In contrast, pulsed radars require large peak powers. Low power operation, and associated simple SF-CW circuitry, provides significant savings in the mass and size of the SAR sensor. To alleviate any bandwidth restrictions imposed by the spatial sampling requirements along the aperture, an approach is outlined using frequency randomised waveforms which allows the bandwidth to be greatly under-sampled before the appearance of significant sampling artefacts. This also provides benefits to the LPI performance of the radar. For a fixed transmit power, omission of frequencies naturally produces a decrease in target signal. However, an intelligent frequency selection scheme is proposed to alleviate signal drop-off, such that signals can be maintained 3dB above the level that might otherwise be expected.