Inverse Gain Technique is a form of angle deception used against Conical Scan Radars and is aiming to mislead the tracker from the actual target angle by means of a false-target modulation in which transmitted peaks fall in the valleys of the real target modulation (Figure 1).
Conical scan is a two-dimensional sequential lobing operation in which a pencil beam with circular symmetry executes a scan about the tracker boresight axis: deceptive angle countermeasures against these radars need to synchronize their modulation patterns with the motion of the radar beam.
Inverse Gain implements the so-called On-Off Keying (OOK) technique: the jammer measures the envelope of the signal transmitted by the radar and synchronizes the transmission of the jamming in counter phase with respect to the received signal: the jammer transmission is maximum during the portion in which the envelope is minimum and vice versa.
Figure 1: Example of Inverse Gain
In presence of this form angle deception the automatic tracker shifts its boresight axis to a point on the line joining the real target and false target, closer to the stronger of the two.
In case of jammers designed to operate at a fixed (saturation output level) Inverse Gain deception technique can be implemented by transmitting constant amplitude jamming burst (Figure 2) timed to coincide with the valleys of the real target modulation.
Figure 2: Inverse gain with saturated Jammer