Over land the echo from the ground is often much stronger than that of an aircraft. The radar therefore looks at what is moving (the Doppler shift) and throws away echoes that stand still.
In practiceThat is how a fighter can see a low-flying target against the ground (look-down). The flip side: anything hovering or flying crosswise falls into the same filter. A helicopter hovering low and almost still behind cover is hard for such a radar to find — although its spinning rotor blades can still give it away.
The biggest problem of a pulse radar over land is clutter: the echo of the earth's surface can be millions of times stronger than that of a low-flying aircraft. The solution lies in the Doppler effect (Doppler effect): clutter stands still, targets move. Two related techniques exploit this:
Moving Target Indicator (MTI)
An MTI radar compares the phase of the echoes of successive pulses. A stationary object produces exactly the same phase every time: subtract two successive echoes and nothing remains. A moving target changes range between pulses, and therefore phase: the difference is not zero and the target remains. In this way stationary clutter is "wiped away".
MTI has one notorious weakness: blind speeds. If a target moves exactly a whole number of half wavelengths between two pulses, the phase is exactly the same again and the target is filtered out as if it were stationary:
The remedy is staggered PRI (PRI modulation): with alternating PRIs the blind speeds shift from pulse pair to pulse pair, so no speed is permanently blind.
Pulse-Doppler radar
A pulse-Doppler radar goes a step further: it does not just filter "moving / not moving", but measures the Doppler frequency of each echo precisely with a filter bank (FFT). The result is a full velocity measurement per target and much deeper clutter suppression. This requires a high or medium PRF (unambiguous Doppler, Pulse Repetition Frequency) and a fully coherent transmitter/receiver chain.
| MTI | Pulse-Doppler | |
|---|---|---|
| PRF | low | medium / high |
| Range measurement | unambiguous | ambiguous (resolved with PRF switching) |
| Velocity measurement | only "moving or not" | precise velocity per target |
| Clutter suppression | good (20–40 dB) | very good (> 60 dB) |
| Typical use | (older) surveillance radars | fighter radars (look-down/shoot-down), modern GBAD |
Pulse-Doppler radars look in the velocity domain. So do the countermeasures: the "notch" manoeuvre (flying tangentially → zero Doppler shift → the target falls into the clutter filter) and velocity gate pull-off jamming (pulling the tracker's velocity gate away from the real target step by step).