Basic knowledge of jammers

The discussion of signal interference was mainly limited to the electronic warfare forum. But with the rise of autonomous driving technology, interference has become a common topic. Now, it is useful to understand some basic knowledge about the test setup required to evaluate anti-jamming techniques and signal propagation principles. Many complex advances in signal scrambling are due to the work of radar. Therefore, it is helpful to start with some basic radar knowledge.

The idea of ​​radar is of course that the target reflects part of the energy of the transmitted signal. Receive and process the returned energy to detect the target and extract its position and relative velocity. If the original antenna beam is narrow enough, the direction of arrival of the return signal can also show the angular position of the target. And if there is relative motion between the target and the radar, the carrier frequency offset of the reflected wave (ie, the Doppler effect) is a measure of the relative radial velocity of the target. Can be used to distinguish moving targets and stationary objects.

A typical radar transmitter emits signals in pulses. And pulses usually have a low duty cycle, where the duty cycle is the ratio of the pulse duration to the pulse period. In addition, the resolution of radar (and lidar) is limited by the pulse width. For example, a 1 microsecond pulse spans a space of 150 m. Therefore, a radar emitting a 1-microsecond pulse cannot distinguish distances less than 150 m.

However, FM modulation is a method used to increase the range resolution of radar signals to less than 1/(pulse width). Chirp is a signal that increases or decreases in frequency over time. There are several types of whistle-linear, non-linear, exponential, etc. Some systems also emit compressed pulses in an attempt to reduce transmitter power and the possibility of interception. Overall, the subject quickly becomes complicated. Therefore, FM radar includes a stage that modulates the transmitted pulse, transmits the pulse, receives the echo, and then correlates the received signal with the transmitted pulse.

After the radar has sent the second pulse, there may be ambiguity when the energy returns from a distant target. This second time is called echo. In order to clearly measure distant objects, the radar must wait for its pulse to return. However, the fastest target that can be clearly measured by radar is determined by the frequency of the transmitted pulse, that is, the repetition frequency of the pulse. In order to measure the speed of fast objects, the PRF must be high, which is a trade-off between radar range and speed detection.

One thing to note is that radar portable jammer usually have a range advantage over target radars. This is because the signals from the jammer to the radar all indicate a path loss of 1/R2, while the radar feedback is 1/R4 loss when it bounces back from the target.

With these points in mind, let us consider the different types of radar jamming methods defined in electronic warfare. The barrage jammer attempts to flood the radar receiver with jamming signals in the receiving band. The noise jammer uses phase or AM noise to modulate the jamming signal. A deceptive jammer uses a repeater or memory to generate a copy of the radar echo and make appropriate changes in time or frequency. Repeated interference will modify and retransmit the received radar signal, so the generated echo will relay inaccurate locations. Similarly, a transponder jammer is a transponder. After being triggered by radar signal reception, it will copy the stored signal copy.

A related jamming mode is established in which the receiver measures the frequency of the threat radar and adjusts the oscillator to resend it. The same is true for minesweeper jammers, which sweep oscillators in the frequency band used by radar receivers. Finally, there are some electronic warfare terms applicable to military exercises. The long-range jammer is usually an aircraft. When it blocks the attacking aircraft and the aircraft’s radar, it will avoid hostile weapons, while the repeat jammer will shoot at a close enough distance. In a hostile way

Anti-jamming techniques usually try to counteract these efforts by taking measures that include changing the radar pulse synchronization pattern. But the synchronization properties can also vary according to the radar mode (search, acquisition, tracking). In addition, adjusting the pulse timing allows the radar to clearly assess the range and speed of the target. In terms of radar development and anti-jamming, the emphasis is on using test instruments to simulate radar or anti-jamming settings before working on hardware or software design.

A common method is to synthesize complex pulse and pulse modulation waveforms on tools such as Matlab, and then pass the information to an arbitrary waveform generator to create the waveform. There are also special software packages for creating waveforms, such as Keysight Technologies’ Signal Studio.

In the test setup, the waveform generator is usually used to add real effects to the signal flow, such as jitter and sweep, pulse repetition frequency and center frequency changes. Pulse creation software can also add effects produced by specific antenna patterns and radiation quality.