Repeater Jamming

Repeater jammer are divided in two main categories:

• Coherent repeater:
  The device derives ECM signal directly from the received victim radar signal.
  The jammer captures the radar signal, then delays it by means of an analog memory loop, then modulates and amplifies it before retransmitting back to the radar.

  Complex programming capabilities allow multiple false echoes with complex apparent kinematic laws and waveform synthesis.

  The analog memory loop has a limited delay capability (few microseconds).

Figure 1: Analog Coherent Repeater

• Non-coherent repeater:
  The ECM signal is synthesized internally to the system by means of a receiver that analyses the victim radar signal key parameters, and then an internal device (VCO, Voltage Controlled Oscillator) that is continuously tuned to the victim signal through an Automatic Frequency Control (AFC) loop to optimally counter the victim radar generates the ECM signal.
  Noise modulation is normally added to the carrier frequency by the same VCO (wide band input).
  Typically, the AFC cannot tune the VCO frequency with the required accuracy level to the radar RF so to make effective certain Doppler techniques (e.g. coordinated range-velocity gate pull off).
  Waveform synthesis allows for minimum response delay.

Figure 2: Analog Non-Coherent Repeater

Current repeater jammers use a digital RF memory (DRFM) that not only performs as a coherent repeater with no limitations in delay capability but also is as well capable to direct synthesis of jammer signals (non-coherent repeater).
The DRFM is the core of a super-heterodyne selective receiver that constitutes the basic responsive channel architecture in any jammer system in which a narrow band of signal has to be handled at time for jamming purposes.
A DRFM is designed to digitize an incoming RF input signal at a frequency and bandwidth necessary to adequately represent the signal, and then reconstruct that RF signal when required.
The most significant aspect of DRFM is that it is coherent with the source of the received signal, as required from a digital “duplicate” of the received signal.
Moreover, it can also act as a waveform synthesizer (non-coherent replicas).

A DRFM is capable of implementing coherent techniques (coherent replicas of the received signals) and is provided with complex programming capabilities (Technique Generator DSP based) that allow multiple false echoes with complex apparent kinematic laws and waveform synthesis.

A coherent replica consists in using the same signal received from the threat radar as the seed of the jamming signal, in this way the jamming signal will be not distinguishable from the true echo and will be processed with the same processing gain (increasing the J/S ratio).

Figure 3: DRFM principle of operation

Coherent replicas are used in deception techniques to change the range by transmitting pulses with changed delay with respect to the true echo (Range Gate Pull Off/Pull In) or the speed by transmitting signals with changed Doppler shift with respect to the true echo (Velocity Gate Pull Off/Pull In).

The signal stored in the DRFM can be used also as the seed of a noise modulation. In this way, Spot Noise is adapted to the radar instantaneous frequency bandwidth or to the Doppler bandwidth (coherent Spot noise).
The intent of this technique is to reduce the victim radar’s ability to measure and track the target’s Doppler by producing a noise Doppler band centred on the target’s real Doppler (e.g. to mask semi-active seeker speed gate).

The use of the received and stored signal as RF carrier (to produce coherent noise) instead of the use of locally synthesized RF carrier (non-coherent noise) increases the Jamming Noise performance, giving a much better matching with the radar BW (this is crucial in case of coherent radars).

Jamming Noise performance can be defined in terms of global S/N reduction when jamming is added to noise (signal to noise plus jamming ratio).
It depends upon the matching between the jamming signal and the true echo (how much of the jamming signal is able to enter the radar matched BW without being attenuated by the radar processing gain?) and how much the noise characteristics can be assumed similar to thermal (Gaussian) noise.
A Noise Quality Index (NQI) can be defined, which depends by the closeness of the jamming noise to Gaussian characteristics and by the ratio between the jamming BW and the radar matched BW (a good noise quality is obtained with a minimum jamming BW to Radar BW ratio of a factor 3).

Continuous Noise generation in repeater mode requires periodical look-through or alternate RX-TX operation to tune jamming carrier to threat RF in order to receive, store and process it before transmission (coherent jamming).

As said before to make coherent jamming the incoming signal must be received.
If the pulse characteristics do not change with time, we can use a pulse received in the past to produce the present coherent replica.
Instead, if the pulse characteristics continuously change, we are obliged to receive all pulses, but if the Jammer acquires the whole pulse, the transmission is blocked during the acquisition phase and it is impossible to generate False Targets closer than PW.

Figure 4: DRFM functional mode: Slice Repeater

A way to overcome the problem is the Slice repeater (Figure 4).
Slice repeater is a coherent DRFM functional mode capable to reconstructing a coherent jamming replica by using only a little part (slice) of the incoming signal (Figure 5).

This will allow receiving only for a while and transmitting before the end of the incoming pulse: the alternation between RX and TX operation is made within the pulse only once.

 This technique allows coherent acquisition and replay operation producing replicas with fast Phase and frequency coherency in order to ensure an enough good correlation (at the matched filtering) in radar receiver.

Figure 5: Slice repeater operation

The timing illustrated in Figure 5 is theoretical, as it does not take into account the process latency and the delay due to the HW electrical path.
The challenge is to reduce these parameters as much as possible together with obtaining the best phase coherency in the re-stitching of the various signal slices.

Of course, intra-pulse modulation presence puts constraints and limitations to this technique.
If only a fraction of the pulse is acquired, a degradation occurs, because jamming pulse is not