The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
Once a tracking radar has detected a target, it will place range gates to either side of it. Range gates essentially blank out all signals which originate from ranges outside a narrow window, substantially increasing the signal-to-noise ratio and protecting the radar against unsynchronised jamming pulses. The radar 'concentrates' on a range interval of a few hundred metres which encloses the target's location, and it no longer looks out for other targets. This state is known as 'lock-on'. But range gates can be 'stolen', and it is the objective of the Range Gate Pull-Off (RGPO) technique to break lock and escape from out of the window. RGPO works as follows:
How RGPO works
Upon detection (or assumption) that a tracking radar has locked on, the on-board jammer is switched on and starts to work in a couple of phases:
First, a sample of the illuminating pulse signal is taken and the radar's pulse repetition frequency (PRF) is determined. This sample is amplified and sent back immediately when further pulses are received. At first glance this doesn't seem to be a good idea, as the aircraft actually highlights itself on the radar screen. The jamming power is steadily increased, and phase one continues until the replica is much stronger than the echo from the aircraft's body (the 'skin echo'). Now, the sensitivity of the tracking radar's receiver needs to be reduced in order to avoid overload. This, in turn, has the consequence that the skin echo vanishes below the noise floor.
Now it's time for phase two: another replica is transmitted after each of the 'dummy' skin echoes. The power of the second replica is increased while the dummy is made weaker.
In phase three, the tracker has locked on to the delayed replica, whereas the skin echo has sunk down into the noise. With respect to each of the radar's pulses, the replica is now being delayed by small, but increasing amounts of time. The range gates, of course, follow the dummy target which appears to be receding. This continues until the range gates have been moved away from the target's real position. The result is that the radar is tracking a phantom target and the skin echo is being blanked out by the range gates.
Phase four is a simple one. The jammer is switched off and leaves the radar with just nothing but noise inside the window between its range gates. Break-lock was successfully achieved and the radar needs to switch back into a search or acquisition mode and loses time. The whole cycle will start again if the target is still within range and is reacquired.
In the way described above, RGPO creates only false targets which appear at greater ranges than the real target because the deceptive signal is transmitted after the skin echo. If the victim radar's PRF is constant then the time of incidence of the next radar pulse can be calculated and jamming pulses can be placed such that false targets at closer ranges are produced.
In a taxonomy of jamming techniques, RGPO is sorted into the 'Range Deception' kind because the radar still measures angles correctly and it's the range indicator which shows false readings.
Protection against RGPO
Of course, counter-countermeasures against RGPO are available. Varying the PRF (by switching to another PRF or jittering) renders the jammer incapable of anticipating when the next illuminating pulse is due to arrive. False targets at closer ranges can be sorted out because their range readings vary as the PRF does. False targets at greater ranges can be eliminated by monitoring the echo signal strength (which increases significantly in phase one) and switching over to leading-edge tracking, that is, taking measurements not according to where the centre of the return signal is but rather at the leading edge. Last but not least, pulse-to-pulse frequency-hopping is another way of protection because the jammer needs some time to find the radar signal and tune in to it.
History: Overview | Isle of Wight Radar During WWII
Technology: Basic Principle | Main Components | Signal Processing | Antennae | Side Lobe Suppression | Phased Array Antennae | Antenna Beam Shapes | Monopulse Antennae | Continuous Wave Radar
Theoretical Basics: The Radar Equation | Ambiguous Measurements | Signals and Range Resolution | Ambiguity and PRFs
Civilian Applications: Police Radar | Automotive Radar | Primary and Secondary Radar | Airborne Collision Avoidance | Synthetic Aperture Radar
Military Applications: Overview | Over The Horizon | Low Probability of Intercept | How a Bat's Sensor Works
Electronic Combat: Overview | Electronic Combat in Wildlife | Range Gate Pull-Off | Inverse Gain Jamming | Advanced ECM | How Stealth Works | Stealth Aircraft