Radar Basics

Stepped Chirp Waveform

The stepped chirp waveform is a novel concept for increasing the range resolution of an existing pulse compression radar. This technique is ideally suited to obtaining high range resolution in a radar system that has a limited instantaneous bandwidth but a large tunable bandwidth. This situation is exactly the case when a number of radar sets of the same type have to share a frequency band with each other so that mutual interferences are minimized.

In pulse compression radar the transmitted bandwidth is a measure of the range resolution. Range resolution in radar is inversely proportional to the transmitted signal bandwidth. The stepped chirp waveform splits the full bandwidth linear frequency modulated (LFM) chirp into a sequence of narrow-band sub-chirps, which may overlap in frequency.

However, if the frequency band of the transmitted signal is skipped within sub-pulses in the tunable bandwidth and the received echo signals are properly combined to the carrier frequencies, the composite signal has effectively increased bandwidth and hence improvement in range resolution can be achieved. As the receiver is then only tuned to the narrow-band sub-chirp it offers high rejection to other radars transmitting at other sub-bands.

The radar transmits and receives on one frequency at a time. It then adjusts both transmitter and receiver to operate at the next frequency and transmits the next sub-chirp – and so on. The sub-chirps needn't be transmitted in order (i.e. Costas Code waveform) but this adds to the complexity of processing. However, in this case, each sub-chirp could be transmitted opportunistically when its sub-band is free.

Practically an existing pulse radar such as the ASR-E could use the stepped chirp waveform and can thus take the advantages of the improved range resolution only by software changing. In practice, however, this would be disadvantageous by the time budget of the radar. A disadvantage of the stepped chirp waveform technique is that all frequencies must be received before anyone range bin can be imaged.