Angle Deception

The Angle Deception goal is to generate false scan modulations that appear to come from a target at some angle different from the true target angle preferably symmetrically opposite to the real target with respect to the boresight axis.

The Angle Deception Techniques can be grouped into three categories:

• Envelope modulation techniques (which belong to the category of Amplitude Modulations) The objective of Envelope (or Amplitude) Modulation Techniques is to produce angular tracking errors in “passively scanned or lobed” radar, by distorting the information of amplitude necessary to track the target accurately.We can distinguish two main applications of Amplitude Modulation in Angle Deception, depending on the capability of the jammer to receive the modulation of the radar and then to synchronize its transmission to the received signal:
• • Inverse Gain:
    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 with synchronized Amplitude Modulations.
    The jammer is able to receive the Amplitude Modulations from the radar (motion of the detected transmitted beam) and can synchronize its modulation patterns with the motion of the radar beam (Synchronized Inverse Gain to compensate for radar scanning).

• Swept Amplitude Modulation (SAM):
  Swept Amplitude Modulation (SAM)  is applied against radars whose modulation cannot be received by the jammer: they are referred to as Lobe-On-Receive-Only (LORO) or Scan-On-Receive-Only (SORO) systems.
  Against these radars, the AM jammer is forced to operate blindly, without synchronization.
  SAM produces an ON-OFF Amplitude Modulations, whose Repetition Interval is frequency modulated around the scan rate value of the radar.
  This technique assumes the knowledge of the scan rate of the victim radar (Mission Data dependent).

• Mono-pulse techniques:

Mono-pulse is a simultaneous beam lobing technique of angle measurement, that uses the relative amplitudes (and/or            phases) of the target return received simultaneously by a couple of beams.The measurement is unaffected by the time variation of the received target return amplitude.Amplitude Modulation is not effective against these kind of radars because the jamming signal provides an equally acceptable (and very likely stronger) signal for mono-pulse angle measurement.Techniques effective against Mono-pulse radar are Cross-polarization (X-pol) and Cross-eye(X-E) also known as Wave Front Distortion (WFD).

• Frequency offset Techniques
  • Image Frequency Jamming:
    The target of this technique is to produce a wrong angular measurement by radiating a signal at the image frequency of the intermediate frequency stage of the victim radar.
    The jammer exploits the frequency conversion process of the victim radar.
    The image frequency in the victim receiver is at the same distance as the target frequency from the local oscillator, but on the opposite side (Figure 1).

Figure 1: Spectrum of Image Frequency Jamming

When the false signal beats with the local oscillator it generates an intermediate frequency with a relative phase angle between the sum and difference channels opposite of that of the true target.
This type of jamming changes the sign of the angular error correction voltages.
This technique assumes the knowledge of the intermediate frequency of the victim radar (Mission Data dependent).

• • Skirt Jamming
    Intent of this technique is to produce errors in the angular measurement of a radar by forcing it to follow a signal that is on the side of the passband of its intermediate frequency (IF) filter or Doppler filter.
    This type of jammer must have enough power to overcome the rejection of the filter in its dark band (skirt of the frequency response) while at the same time it must be greater than the true target signal that is in the pass band.
    The amplitude and phase characteristics of the filters are generally uncontrollable after the “3 dB” point of the pass band, so that incorrect angular measurements are produced by the tracking of these erroneous signals.
  • Delta frequency Jamming
    The intent is to produce errors in angular tracking in a mono-pulse radar by transmitting two RF signals in the passband of the victim receiver that generate two false IF signals after the beating with the local oscillator.
    The two injected signals have frequencies that are apart by an amount equal to the IF.
    After the beating, intermodulation products are generated at that particular intermediate frequency (Figure 2).

Figure 2: Spectrum of Delta frequency jamming

The amplitude of the resulting signal is proportional to the product of the amplitudes of the disturbing signals.
During the product, the phase information of the difference channel is lost causing an error of the measurement.
This technique assumes the knowledge of the intermediate frequency of the victim radar (Mission Data dependent).