Inter-pulse radar scalloping involves two types of systems.
Moving Target Indicator
Scalloping occurs when the radial velocity of the reflector induces an integer multiple of a 360 degree phase shift between two or more transmit pulses.
Radar scalloping for MTI radar begins to become a concern when the radial velocity is greater than the following value.
This occurs near multiples of 15 m/s for an L-Band radar with pulse repetition frequency of 1kHz (10m/s to 20m/s, 25m/s to 35m/s, 40m/s to 50m/s, and so on).
Pulse-Doppler Radar
Scalloping for pulse-Doppler radar involves blind velocities created by the clutter rejection filter. A two PRF detection schemes will have detection gaps with a pattern of discrete ranges, each of which has a blind velocity.
Scalloping happens in a two PRF detection scenario when target velocity produces a blind velocity for one PRF while the target is at the blind range of the other PRF.
The blind velocity for a specific pulse repetition frequency (PRF) is an integer multiple of the following, which causes the signal to have zero doppler.
The blind range for a specific PRF is an integer multiple of the distance between transmit pulses, which is when the reflected pulse arrives at the same time as when the transmitter fires.
This leaves a series of detection gaps at discrete velocity and range combinations.
These detection gaps are filled in by using three or more alternating PRF in the detection scheme.
An L-Band transmitter using a 3 kHz and 4 kHz PRF pair has the following characteristics.
- 3 kHz blind ranges are 50 km, 100 km, ...
- 3 kHz blind velocities are 450 km/s, 900 km/s, ...
- 4 kHz blind ranges are 37.5 km, 75 km, ...
- 4 kHz blind velocities are 600 km/s, 1,200 km/s, ...
This system would fail to detect reflections at 50 km and 100 km that are moving 600 km/s or 1,200 km/s.
It would also fail to detect reflections at 37.5 km and 75 km that are moving 450 km/s or 900 km/s.