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A chirp is a signal in which the frequency increases ('up-chirp') or decreases ('down-chirp') with time. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum. In spread spectrum usage, SAW devices such as RACs are often used to generate and demodulate the chirped signals.

In a linear chirp, the frequency varies linearly with time:
f(t) = f0 + f×k×t
This type is easy to generate.

In a geometric chirp, the period of the signal varies linearly with time. Although harder to generate, this type does not suffer from reduction in correlation gain if Doppler shifted by a moving target.