Ambiguity resolution

Some types of measurements introduce an unavoidable modulo operation in the measurement process. This happens with all radar systems.^[1]

Radar aliasing happens when:

• Pulse repetition frequency (PRF) is too low to sample Doppler frequency directly
• PRF is too high to sample range directly

Pulse Doppler sonar uses similar principles to measure position and velocity involving liquids.

Radar systems operating at a PRF below about 3 kHz pulse rate produce true range, but produce ambiguous target speed. Radar systems operating at a PRF above 30 kHz produce true target speed, but produce ambiguous target range.

Medium PRF systems produce both ambiguous range measurement and ambiguous radial speed measurement using PRF from 3 kHz to 30 kHz.

Ambiguity resolution finds true range and true speed by using ambiguous range and ambiguous speed measurements with multiple PRF.

Doppler systems involve velocity measurements similar to the kind of measurements made using a strobe light.

For example, a strobe light can be used as a tachometer to measure rotational velocity for rotating machinery. Strobe light measurements can be inaccurate because the light may be flashing 2 or 3 times faster than shaft rotation speed. The user can only produce an accurate measurement by increasing the pulse rate starting near zero until pulses are fast enough to make the rotating object appear stationary.

Radar and sonar systems use the same phenomenon to detect target speed.