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
'Inverse Gain' is a countermeasure which works against the angle measurement circuits of a certain kind of tracking radar: those that sweep their antenna beam about the area where a target was detected. The sweeping movement can be found in some outdated nodding height-finding radars (which keep moving their antenna up and down), or in conical scan tracking radars. Conical ccan was invented in WWII but the technique is still in use.
A tracking radar must be capable of precise angle measurements, even if the beamwidth cannot be made as small as is desirable. An early idea to accomplish comparably good results is the conical scan (or ConScan) technique: the beam is made to rotate around the axis of the radar dish, by using a feed which is slightly offset from the focal point and rotates or nutates about the axis of symmetry.
Searching for a target is done by moving the whole antenna arrangement. A search window in space is divided into lines which are stacked above each other and are visited sequentially in a line-by-line pattern1. Once a target is found, the antenna dish stops its motion and the beam merely rotates around the last position in space because of the rotating feed.
If the antenna dish is somewhat misaligned with the target direction then the echo will show alternating strength. This is because there are times when the beam is right on the spot, and times when it isn't. On the other side, if the dish is fully-aligned with the target then the echo will remain constant in amplitude. Therefore, the presence of a varying amplitude indicates antenna misalignment. In this case, the time of maximum echo (in relation to some 'index' beam position) indicates the direction in which the dish should be moved in order to get the target into the centre. This technique is able to measure angles down to fractions of a degree even with a beamwidth of 12°, as was the case for the German Würzburg radar of WWII.
Inverse Gain Jamming
A warning receiver on board the aircraft detects the presence of the tracking radar's signal as well as a varying signal strength (because the beam is sometimes on target and sometimes isn't). The jamming method is to fool the radar by transmitting copies of the signal which are transmitted by the jammer. The idea is to use a strong replica when the illuminating signal is weak, and a weak jamming signal when the illumination is strong. The technique works because back at the radar receiver, the jamming signal is added to the real echo and either evens out the phases when weak echoes are received, fooling the radar's signal processor into believing it was right on spot, or over-compensates the weak phases, and has the radar dish move away from the target.
Because of the 'weak when strong, strong when weak' rule this technique is called inverse amplitude or inverse gain jamming.
In a taxonomy of jamming techniques, inverse gain is sorted into the 'angle deception' kind because the radar is still able to measure range correctly and it is the angle indicator which shows the wrong readings.
Of course, countermeasures against inverse gain jamming itself are available. Inverse gain jamming relies on measuring the parameters of the rotating beam. The idea which led to the development of an ECCM was that there is no need to let the radar beam exhibit a rotating movement when transmitting the signal. It is fully sufficient if the scanning motion is performed only on part of the radar's receiver. One way to do this is to use two antenna dishes, one with a fixed beam for transmission, and the other with a rotating beam for reception. This type of scanning is called COSRO (Conical Scan on Receive Only) or LORO (Lobe On Receive Only), which is the more generic term. Radars using LORO deny a jammer the information which is required to fully deceive them.
Another Cycle in ECM History
However, LORO radars aren't safe against jamming. A counter-ECCM (ECCCM, if you so wish) works like this: LORO is effective in denying the jammer any information about the scanning frequency ('How fast does the beam rotate?') and scanning phase ('When is the beam on target?'). But the jamming equipment can 'assume' that there is a LORO radar around, and more or less blindly 'sweep' its inverse gain signal through a range of probable scanning frequencies in a repeating cycle. This technique is called SSW (for 'Swept Square Wave'). SSW doesn't protect the aircraft at 100% probability, but it's better than nothing.
More Cycles in ECM History
Inverse Gain Jamming, LORO and SSW were all invented between the 1950s and 1970s. The counter-counter-counter-countermeasures (EC4M) and the related EC5M didn't take overly long to make their appearance, but you'll already have got the hang of this particular 'game' by now.
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