Radar target recognition in complex electromagnetic environment

1.1 Definition of complex electromagnetic environment and its relationship with informationization conditions The informationization combat background mainly refers to the combat environment under complex electromagnetic interference environment, that is, the so-called complex electromagnetic environment. There is no unified strict definition of complex electromagnetic environment at present, but many definitions of complex electromagnetic environment have appeared in various non-academic publications. The so-called complex electromagnetic environment, in general, is the sum of artificial electromagnetic emissions and various electromagnetic phenomena during a certain combat period. The main factors that constitute the complex electromagnetic environment are the electronic confrontation between the enemy and our side, the high-density, high-intensity, multi-spectrum electromagnetic waves released by various weapons and equipment, the radiation of civilian electromagnetic equipment and the electromagnetic waves generated by nature. Specifically, the so-called complex electromagnetic environment refers to the multi-type, full-spectrum, high-density electromagnetic radiation signals generated under the conditions of fierce confrontation between the warring parties on the informationized battlefield, as well as the mutual influence and interference caused by the large-scale use of electronic equipment by our side, resulting in sudden changes in the time domain, criss-crossing in the air domain, and crowded overlap in the frequency domain, which seriously affects the effectiveness of weapons and equipment, combat command and the invisible battlefield environment of the combat operations of the troops. The complex electromagnetic environment mainly includes the electromagnetic radiation and reconnaissance environment of military equipment, the electromagnetic radiation environment of civilian electronic equipment, and the natural electromagnetic radiation environment.
1.2 Characteristics of complex electromagnetic environment
The complexity of the electromagnetic environment is caused by the development of electronic technology and the continuous application of electronic technology in weapons and equipment. The complexity is mainly reflected in the increase in military and civilian electromagnetic use activities; the dependence of the warring parties on the electromagnetic spectrum has led to an increase in the interference and anti-interference equipment and technical means used by both sides to compete for electromagnetic dominance; the increase in electromagnetic spectrum bands, almost covering the entire electromagnetic spectrum band, etc.

In addition to these man-made electromagnetic activities, there are also natural electromagnetic activities, mainly solar system and interstellar electromagnetic radiation, earth and atmosphere electromagnetic fields, lightning and its electromagnetic pulses, etc. All of these together constitute a complex electromagnetic environment, of which the electromagnetic environment caused by intentional human interference is the main part and the part that has the greatest impact on combat under information conditions. Its main characteristics are:
(1) Extensiveness
In order to weaken the other party's electronic warfare capabilities, reduce or destroy the effectiveness of the other party's electronic equipment, and at the same time ensure the normal performance of their own equipment, the warring parties will take various measures to fight for the dominance of the electromagnetic spectrum in multi-dimensional spaces such as land, sea, air and even space. The targets involve various electronic equipment such as radio communications, radar, guidance, navigation, sonar and telecommunications, broadcasting, and television, covering the entire electromagnetic
spectrum space.
(2) Intensity
In a certain airspace, time domain, and frequency domain, a large number of military and civilian electronic equipment are used simultaneously, the electromagnetic waves are very dense, and the working frequency is very concentrated, resulting in a very complex electromagnetic environment in the combat area.
(3) Dynamicity
In the informationized battlefield, the warring parties will continue to use new radars, radio stations and new communication frequencies to maintain smooth communication and uninterrupted combat command, causing the battlefield electromagnetic spectrum environment
to change with the changing situation of the two sides in the electromagnetic spectrum field.
(4) Confrontation
In future wars, in order to accurately grasp the enemy's combat operations, the warring parties will strengthen reconnaissance and surveillance of electronic equipment, and carry out soft and hard attacks on command, communication, radar and other systems. The struggle between reconnaissance and counter-reconnaissance, interference
and counter-interference, suppression and counter-suppression, destruction and counter-destruction will be very fierce, and the electronic information system will work in a fiercely confrontational electromagnetic environment.
1.3 The impact of complex electromagnetic environment on combat operations
In the informationized battlefield, the electromagnetic environment has penetrated into all aspects of battlefield perception, command and control, combat coordination, etc., and has had a very broad and profound impact on the correctness of judgment and decision-making, the performance of combat effectiveness, and even battlefield construction. In general, these impacts are mainly manifested in the following aspects:
(1) Complex electromagnetic environment increases the difficulty of battlefield perception;
(2) Complex electromagnetic environment restricts the efficiency of command and control;
(3) Complex electromagnetic environment increases the difficulty of combat support;
(4) Complex electromagnetic environment affects the performance of combat effectiveness.
Among them, the most direct and first impact is the restriction of complex electromagnetic environment on our ability to perceive and grasp the battlefield environment. The enemy's electronic interference will make the radar lose its combat effectiveness and make us "blind", unable to command our own targets; unable to distinguish between enemy and our own targets.

2.2 Low sidelobe and ultra-low sidelobe antenna technology
When the beam is narrow enough and the sidelobe is low enough, the radar will only receive the target echo signal, and suppress various interferences in the space around the target, which can improve the signal-to-interference ratio of the radar reception. In order to make the radar work effectively in a serious ground interference and electronic interference environment, it is necessary to use low sidelobe antennas as much as possible. Low sidelobe and ultra-low sidelobe antennas can also effectively prevent the radar from being attacked by anti-radiation missiles.
2.3 Sidelobe blanking and sidelobe cancellation technology
The existence of radar antenna sidelobes enables the enemy to implement sidelobe interference, and strong interference can even form an omnidirectional interference fan. In order to eliminate the interference entering from the sidelobes, sidelobe blanking and adaptive sidelobe cancellation technology can be used.
2.4 Antenna adaptive anti-interference technology
Antenna adaptive anti-interference technology is to automatically control the antenna beam shape according to the specific environment of the signal and interference, so that the maximum direction of the main lobe of the beam always points to the target and the zero value direction points to the interference source, so as to receive the most echo energy and the least interference energy, so as to maximize the signal-to-interference ratio.
2.5 Power countermeasure technology
In electronic defense, power countermeasure is an important measure to resist active interference, especially main lobe interference. By increasing the radar's transmit power, extending the beam dwell time on the target or increasing the antenna gain, the echo signal power can be increased and the received signal-to-interference ratio can be improved, which is conducive to detecting and tracking the target. The basic methods of power countermeasure include: increasing the peak power of a single tube; adopting a pulse compression system; power synthesis; beam synthesis; increasing the pulse repetition frequency.
2.6 Frequency selection technology
Frequency selection is to use the difference in frequency domain characteristics between radar signals and interference signals to filter out interference. When the radar changes the operating frequency quickly and jumps out of the interference frequency range, it can avoid interference. Common frequency selection methods include: selecting a frequency close to the enemy radar carrier frequency; opening up a new frequency band; fast frequency modulation; frequency agility; frequency diversity.
2.7 Constant false alarm processing technology
Constant false alarm processing refers to a technical measure to keep the radar false alarm probability constant when the noise and external interference intensity change. In the automatic detection system, the constant false alarm rate processing technology can prevent the computer from being saturated due to too strong interference; in the manual detection radar, the constant false alarm rate processing technology can achieve clear display screen and easy observation under strong clutter interference. At present, the commonly used constant false alarm rate processing methods can be divided into two categories: slow threshold constant false alarm rate processing and fast threshold constant false alarm rate processing.
Although radar anti-interference technology has been rapidly developed, the increasingly complex electromagnetic environment interference technology and the anti-interference ability of radar have put forward higher requirements, and it is urgent to study new technologies and new systems to enhance the survivability and combat capability of radar.
3 New technologies and new systems in development are used to improve the ability of radar target recognition
In recent years, China has increased its investment in the scientific research of electronic anti-interference technology, which has made some breakthroughs in China's new radar technology. Some new technologies and new systems have emerged one after another. Some of these technical systems have been applied, which has greatly improved the information level of our military equipment and its survivability in complex electromagnetic environments. The main ones are:
3.1 Passive radar detection technology
Passive radar is also called passive radar. Passive radar itself does not radiate signals, but uses electromagnetic waves radiated by radar, communication, navigation, interference and other equipment on enemy targets to identify, locate and track targets. Since passive radar does not radiate signals, it can be well concealed, has a strong ability to resist anti-radiation missiles, and has a good ability to detect stealth targets.
3.2 Synthetic aperture radar technology
Synthetic aperture radar is a radar that uses the principle of coherent imaging to image the target to detect and identify the target. Synthetic aperture radar can image the target and use the shape of the target to directly identify the target, which improves the ability to resist deception and interference.
3.3 Radar networking technology
Because different radars have different working systems, frequencies, polarizations, signal parameters, etc., and occupy a large airspace, it is impossible for them to be seriously interfered by the enemy at the same time. By rationally forming these radars into a radar network, and rationally matching high and low airspace, long, medium and short ranges, we can use the data provided by radars in the network that are not interfered with or only slightly interfered with to discover and track targets, so as to achieve identification, tracking and attack of enemy targets under strong interference. Obviously, the radar network must be equipped with reliable communication equipment, accurate coordinate conversion system and efficient command and control system.
3.4 Dual (multi) base radar technology
Dual (multi) base radar adopts separate receiving and transmitting bases, and uses the side scattering of the transmitted signal received by the receiving base to determine the position of the target. Since the receiving base does not radiate electromagnetic waves and is in a hidden position, it is usually not subject to strong electromagnetic interference. At the same time, it can also effectively resist the attack of anti-radiation missiles, and can also identify invisible targets.
4 Conclusion
In summary, complex electromagnetic environment has a great impact on radar target recognition. There is still a big gap between China and developed countries in the research on complex electromagnetic environment. Developed countries started early in the research on combat, training, equipment, technology and talents in complex electromagnetic environment, and have now achieved a large number of results, most of which have been applied to actual equipment. In this regard, our country is already lagging behind. Only by mobilizing all positive factors to catch up and continuously narrow the gap can we be invincible in local wars under future information conditions.