Range Gate Pull-Off (RangeDeception)
The most used deception technique is Range Gate Pull Off (RGPO), based on the manipulation of the time of arrival of the target echo at the Radar receiver.
RGPO is a self-protection jamming technique used against automatic range tracking radars.
It captures the radar range gate, walks it off in range, then turns the jammer OFF leaving the range gate with no signal.
RGPO consists in the generation of a false echo at a slightly longer distance than the platform’s echo and moving far from the radar (reproducing the threat pulses moved across the time scale by an appropriate delay law). This technique is used in order to seduce the range tracker of the Radar. This process is usually repetitive.
The objective of RGPO technique is to break-lock the range tracker, forcing the radar into the acquisition phase, or failing that, to create large measurement errors, for degrading the enemy fire control.
The capability to capture and walking off the tracking gate depends on the relative strength of false echo and target echo at the input of the time discriminator (split gate or early-late gate).
The split gate is positioned in order to capture equal shares of the target pulse energy with the early and late gates so that the difference of the two gates output is zero, and the gate is centred on the target.
Figure 1: Split gate
At the beginning of the RGPO action, the jammer should fall within the victim radar’s range gate with an amplitude similar to that of the target echo.
In the region in which the jammer pulse overlaps the target return pulse, the two pulses add as phasors.
RGPO is sensitive to the relative phase between the jamming signal and the true echo that can affect the effectiveness of pull-off technique.
For this reason RGPO must be a coherent technique (repeater operation), so that the relative phase is consistent pulse-to-pulse.
When the jammer DRFM detects the pulse of the threat to be countered (jammer BW tuned on the threat frequency by designation of the RWR), the jammer transmits a believable (coherent) return echo immediately.
With each subsequent pulse, the intensity of the false echo is increased slightly, to a point far beyond the intensity of the original echo (seduction phase or dwell time).
Then the radar decreases its gain and the contribution of the actual radar return becomes negligible and is no longer distinguishable from background noise (capture of the radar AGC).
Then the jammer begins modifying the transmission time of the false echo by greater intervals, causing also a similar motion of the centre of gravity of the range gate, thus simulating the target getting further away (walk-off or sweep time).
The motion of the centre of gravity during the pull-off manoeuvre causes the move of the gate, and the target pulse remains behind.
If the pull-off is executed slowly enough, the tracking servo will follow the motion of the range gate.
Once the false echo is sufficiently “ranged” away from the actual echo, the jammer signal is progressively attenuated (hold time) and finally is turned off and the radar is left without a target and will need to restart its acquisition process again before it can re-start tracking the actual skin echo.
Figure 2: RGPO principle of operation