Radar Doppler Processing
Understanding Radar Doppler Processing
Doppler processing is what separates modern radar from simple range detectors. By measuring the frequency shift of returned pulses, radar can determine target speed, reject stationary ground clutter, and distinguish between multiple targets at the same range.
Doppler Processing Steps
- Transmit a coherent burst of N pulses.
- For each range bin, collect the complex returns from all N pulses.
- Apply a window function (Hamming, Chebyshev) to reduce spectral sidelobes.
- Perform N-point FFT across the slow-time (pulse-to-pulse) dimension.
- Each FFT bin represents a Doppler frequency (velocity) channel.
Range-Doppler Map
- X-axis: Range bins (from pulse compression or range gating).
- Y-axis: Doppler bins (from FFT across pulses).
- Each cell contains the signal power for that range and velocity.
- Clutter appears at zero Doppler; moving targets at non-zero Doppler.
Frequently Asked Questions
What is Doppler processing?
Doppler processing uses FFT across multiple radar pulses to separate targets by velocity. It enables moving target detection, clutter rejection, and velocity measurement. Each FFT bin corresponds to a different target velocity.
How many pulses are needed?
N pulses give N Doppler bins. Velocity resolution = PRF/N in Hz, or lambda*PRF/(2*N) in m/s. 64 pulses at PRF=10 kHz: velocity resolution = 156 Hz = 2.3 m/s at X-band. More pulses = finer velocity resolution.
What is clutter rejection?
Ground clutter returns have near-zero Doppler (stationary). Moving targets have non-zero Doppler. MTI (Moving Target Indication) filters or notch filters reject the zero-Doppler bin, suppressing clutter by 20-40+ dB.