How do I calculate the Doppler shift of a moving target at a given radar frequency?
Doppler Shift in Radar
The Doppler shift is proportional to the radial velocity component (velocity along the line from radar to target). Targets moving perpendicular to the radar line of sight produce zero Doppler shift. For airborne radar looking down: ground clutter returns have Doppler shifts that depend on the aircraft velocity and look angle: f_clutter = 2 × v_aircraft × cos(θ) / λ. The clutter Doppler spread must be filtered out to detect moving targets.
Frequently Asked Questions
How does Doppler help detect targets?
Moving targets have a non-zero Doppler shift, while stationary clutter has zero Doppler (or a known Doppler from platform motion). MTI (moving target indication) and pulse-Doppler processing separate targets from clutter based on their different Doppler shifts. This is the primary technique for ground-based air surveillance and airborne ground-moving-target indication (GMTI).
What is the blind speed?
Blind speeds occur at v = n × λ × PRF / 2 (integer multiples). At these velocities, the Doppler shift equals a multiple of the PRF, and the target appears at zero Doppler (indistinguishable from clutter). Mitigation: stagger the PRF between pulses so that the blind speeds of different PRFs do not coincide.