Application of Sidelobe Blanking Technology in Radar
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Electronic countermeasures play an increasingly important role in modern warfare. Radars without radar anti-interference technology completely lose their ability to detect and determine enemy targets. From the perspective of reducing antenna sidelobe interference, radar anti-interference technology mainly includes sidelobe cancellation technology and sidelobe blanking technology. The sidelobe canceller has a very good effect in suppressing an interference source with an auxiliary antenna, but it cannot suppress virtual target forwarding interference. Therefore, another electronic counter-interference (ECCM) technology is needed to counter different interferences, that is, radar sidelobe blanking technology.
1. The purpose of radar sidelobe blanking
The ultra-low sidelobe antenna is designed to minimize the probability of the radar being detected in the sidelobe direction. The radar using the ultra-low sidelobe antenna can implement spatial selection and limit the interference to the main lobe range; in other angle ranges, the radar can work normally and measure the angle information of the jammer, and then use multi-station cross-positioning technology to measure the distance data of the jammer.
2. Principle of radar sidelobe blanking
Sidelobe blanking is also a technology to deal with sidelobe interference. It uses an auxiliary antenna with a gain less than the main antenna main lobe gain but greater than the main antenna sidelobe gain (Figure 1). Radar sidelobe blanking (SLB) uses a two-channel system of main channel and auxiliary channel, which is similar to the sidelobe cancellation technology, but the signal processing method is different. The working principle of sidelobe blanking technology is that each channel consists of a transceiver antenna, a receiver, a detector and a comparator. The amplitude of the two main and auxiliary channel echo signals is compared by the principle of subtraction, and then the interference is eliminated by the principle of selection to determine whether to blank the main channel signal. The antenna of the main channel antenna scanning radar continuously scans 360 degrees in azimuth, usually with a high-gain main lobe and many side lobes with decreasing gain. The target echo signal enters from the main channel main lobe. Generally, the maximum gain of the main lobe is more than ten to several ten decibels greater than the maximum gain of the first side lobe. This is mainly to reduce the possibility of the side lobe detecting the target, and also reduce the interference signal arriving through the side lobe. The secondary antenna usually adopts a weakly directional omnidirectional antenna, whose gain is greater than the gain of the side lobe of the main antenna, but less than the gain of the main lobe of the main antenna. If noise and path difference are not considered, the interference signal entering the side lobe of the main antenna can be completely shielded. However, due to the existence of noise and the path difference between the main and secondary antennas, the interference signal often cannot be completely shielded, but the probability of false alarm has been reduced to a great extent. As shown in Figure 1, the signal gain in the main lobe of antenna A is greater than that in antenna B. For any signal in the side lobe of the main antenna, the gain of antenna B is greater than that of antenna A.
Figure 1
Figure 2
In Figure 2, antennas A and B are connected to their own receivers, and the echo signals received by the main and auxiliary channels are sent to the comparator at the same time, and the amplitude levels of the two signals are compared at the output of the receiver. If the video amplitude of the echo signal in receiver A is greater than the signal amplitude in receiver B, the radar will correctly draw the following conclusions: the signal enters the receiver with the antenna aimed at the target, and then the signal enters the signal analysis circuit after being gated: if the video amplitude of the echo signal in receiver A is less than the video amplitude of the echo signal in receiver B, a blanking trigger pulse is generated and added to the blanking pulse generator, and the blanking pulse generator generates a sidelobe blanking pulse with an appropriate width and adds it to the gate. When the blanking pulse appears, it means that the radar is interfered by the sidelobe. At this time, the gate is closed, and the sidelobe interference is blanked. Otherwise, the blanking pulse does not appear, and the gate is always open, and the echo signal received by the main channel is sent for normal detection and display. When this radar anti-interference technology is used, that is, when the amplitude of the weak echo signal received by the radar A antenna, i.e. the main antenna, may be smaller than the amplitude of the interference signal received by the auxiliary antenna, the gate is closed and the radar loses the opportunity to detect and display small targets. The amplitude of the pulse signal of the side lobe is smaller than the amplitude of the signal in the B-channel receiver, so it cannot reach the signal analysis circuit and radar display.
In this case, the only hazard to main beam target detection would be if a pulse generated by the side lobe arrived at the radar at the same time as a real target signal in the main beam, but this is unlikely to happen because the main beam target and the side lobe generator are in two different locations relative to the radar.
3. Sidelobe blanking project implementation
Low sidelobe antenna and sidelobe blanking technology are anti-interference measures of the new radar system. In order to reduce the sidelobes of the transmitting beam, weighting is adopted in the elevation direction, which can more effectively deal with anti-radiation missiles. Coherent Doppler processing and adaptive digital moving target display technology are adopted. Moving target display is used for low beam coverage, Doppler filtering is used for downward viewing, and side lobe zero value is used in high beam position to eliminate ground clutter.
Figure 3
Figure 3 is a block diagram of the engineering implementation of sidelobe blanking in the signal processing system. The beamforming plug-in in the figure mainly completes the digital beamforming function of 30 receiving channels, forming 10 beams covering an elevation range of 35 degrees. Due to the inconsistency of the channels, the performance of beamforming is affected. In severe cases, the beam cannot be formed, so it is necessary to perform receiving and transmitting corrections of the radar system. This work is also completed in the beamforming plug-in. Before the multi-channel receiver performs beam synthesis, it is necessary to calibrate the amplitude and phase of each channel data. The calibration calculation work is completed by the general DSP device. During the calibration, the general DSP device records the amplitude and phase of each receiver output, and after a certain calculation, it forms the coefficients required for the correction of each receiving channel. The DSP synthesizes the coefficients such as beam pointing and weighting coefficient to form the beamforming coefficient for real-time processing of digital beamforming. The auxiliary channels include channels for amplitude and phase monitoring, sidelobe blanking, etc. The general DSP mainly performs channel correction processing, and solves the correction coefficient and adaptive cancellation weight coefficient. The basic processing mathematical model of DBF is as follows:
（1）
In the formula, X is the echo signal of the multi-channel array element channel; C is the channel correction coefficient; W is the weighting coefficient, which is generally symmetrical and real; S is the beam pointing coefficient; B is the data of each beam after DBF synthesis. Finally, the sidelobe blanking channel and the main channel are synthesized together and sent to the subsequent pulse compression processing unit.
4. Compatibility issues between target display radar and SLB
In the moving target indication (MTI) radar, assuming that the clutter suppression filter adopts secondary cancellation (i.e., three-pulse cancellation), the output signal of the MTI clutter suppression filter can be written as U=A-2B+C; where C is the echo signal amplitude of a certain distance unit in the current radar transmission pulse cycle, B is the echo signal amplitude of the same distance unit in the previous radar transmission pulse cycle, and A is the echo signal amplitude of the same distance unit in the previous two radar transmission pulse cycles. Obviously, as long as the echo signals of the same distance unit in these three cycles arrive and have the same amplitude (i.e., fixed clutter interference signals), the output is zero after the MTI clutter suppression filter, that is, the fixed clutter interference is suppressed. If a sidelobe blanking pulse appears in this distance unit in a certain cycle during these three cycles, the radar main receiving channel is closed, and a cycle of echo signals is lost. At this time, not only can the fixed clutter interference not be suppressed, but there will also be output, that is, a false target is generated. It is difficult for ordinary pulse radar to detect and track low-altitude and ultra-low-altitude intrusion targets, which is a major issue in the "four major confrontations" of radar. For this reason, it is necessary to develop a fully coherent radar. For a fully coherent radar, its transmission signal and local oscillator signal are both generated by the same frequency synthesizer, and the signals maintain a strict phase relationship. Only in this way can full coherence be guaranteed. Sidelobe blanking is an effective technology to resist interference from side lobes, while MTI technology is an effective technology for fully coherent MTI radar to suppress fixed clutter interference. Therefore, if you want to use them at the same time, you must solve the compatibility problem of the two. When designing the compatibility of sidelobe blanking and MTI, two problems must be solved: First, when a sidelobe blanking pulse is generated, a blanking pulse of n+1 (n is the number of cancellations) cycles should be continuously generated to lock the output of n+1 cycle echoes. Second, when a blanking pulse appears, the corresponding distance unit is continuously locked for n+1 cycles, and other distance units are not affected.
5. Conclusion
The sidelobe blanking system is effective in preventing interference signals from entering from the side lobes of the radar antenna, and if the gain of the side antenna is properly selected, the ability of the main lobe to detect targets will not be reduced, but it cannot blank the interference signals entering from the main lobe. In the presence of noise and path difference, only part of the interference signal can be blanked, and the improvement effect can be reflected by the improvement factor. The signal-to-noise ratio and fixed phase shift have an impact on the improvement factor. Sidelobe blanking technology cannot deal with continuous wave or noise interference, so sidelobe cancellation technology is needed. The reason why radar has strong anti-interference and anti-clutter capabilities is that its antenna has a very low sidelobe level and is equipped with additional channels for sidelobe blanking or sidelobe cancellation, as well as tracking of interference sources, which can realize adaptive zeroing of the antenna pattern. Since the phased array antenna is composed of independent radiating units or subarrays, it can obtain the best adaptive antenna pattern in an electronic countermeasure environment. The digital beamforming receiver of the phased array radar is a device that uses digital technology to achieve instantaneous multi-beam and real-time adaptive processing. While forming instantaneous multi-beams, it can adaptively zero the interference source and obtain ultra-high resolution and ultra-low sidelobe performance, so it can effectively deal with advanced integrated electronic jamming. In addition, the waveform and lock time of the phased array radar can be adjusted according to the clutter environment requirements. Therefore, the phased array is undoubtedly an extremely excellent radar countermeasure.
Author: Hu Kexin and Hu Aiming, General Department of the 38th Research Institute of China Electronics Technology Group Corporation
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