MTI: analogue delay lines
MTI subtracts the received signal from a delayed copy of the previous pulse. Stationary clutter cancels perfectly; moving targets leave a residual because their phase changed between pulses. Early MTI used quartz delay lines and later digital memory. Simple, cheap, effective — but blind to targets whose radial velocity creates exactly one wavelength of phase shift between pulses (the 'blind speed' problem).
Pulse-Doppler: Fourier in every cell
Instead of subtraction, pulse-Doppler takes a sequence of pulses (a coherent processing interval, typically 10–100 ms) and runs an FFT on each range bin. Clutter sits at zero Doppler (or a known wind offset). Aircraft appear at their specific Doppler frequency. There are no blind speeds — every velocity produces a distinct spectral line — and the radar can simultaneously track hundreds of targets at different velocities.
Range-ambiguous and PRF trade-offs
Pulse-Doppler needs high Pulse Repetition Frequency (PRF) for unambiguous velocity measurement. But high PRF means short range — the next pulse is transmitted before the farthest echo returns. Medium and low PRF modes trade velocity ambiguity for range clarity. Modern military radars switch PRF patterns pulse-to-pulse to resolve both.
Where each lives today
MTI survives in simple airport surveillance and marine radars where cost matters and blind speeds are rare. Pulse-Doppler dominates military airborne interceptors, weather radar and advanced ground systems. The distinction is fading as digital processing makes every radar a hybrid. But the physics — clutter suppression through motion detection — remains the same.