What are Radar Signals?

Radar uses radio to find targets and determine their spatial position. Therefore, radar is also called "radiolocation". Radar is an electronic device that uses electromagnetic waves to detect a target. Radar emits electromagnetic waves to irradiate the target and receive its echoes, thereby obtaining information such as the distance from the target to the electromagnetic wave emission point, the distance change rate (radial velocity), orientation, and altitude. The typical frequency band occupied by radar signals is from 500 MHz to 18 GHz, and the working frequency of millimeter-wave radars reaches 40 GHz or higher. The radar reconnaissance system cannot know in advance exactly which radars will work, and it is impossible to know that these radars emit The frequency of the signal.

Radar uses radio to find targets and determine their spatial position. Therefore, radar is also called "radiolocation". Radar is an electronic device that uses electromagnetic waves to detect a target. Radar emits electromagnetic waves to irradiate the target and receive its echoes, thereby obtaining information such as the distance from the target to the electromagnetic wave emission point, the distance change rate (radial velocity), orientation, and altitude. The typical frequency band occupied by radar signals is from 500 MHz to 18 GHz. The working frequency of millimeter-wave radars reaches 40 GHz or higher. The radar reconnaissance system cannot know in advance exactly which radars will work, and it is impossible to know that these radars will emit The frequency of the signal.

The basic task of the radar is to find the target and determine its coordinates. Usually the echo signal of the target is always mixed with noise and various interferences, and the noise and various interference signals have random persistence. Under this condition, the problem of finding the target belongs to the signal. Detection category The detection theory of signals is to solve the method of judging whether a signal exists and its best processing method.

The task of the detection system is to perform necessary processing and calculations on the input signal and then to determine whether there is a signal in the input according to the output of the system. It can be described by threshold detection.

During the detection process, errors caused by different threshold values detect noise as the target. Such errors are called false alarms, and the probability of occurrence is called false alarm probability. Conversely, signals are mistakenly regarded as noise, which is called leakage. Detection or missed alarm, corresponding occurrence probability is missed detection probability.

The determination of the threshold is related to the best criterion chosen. The best criteria often used in signal detection include the Bayesian criterion, the minimum error probability criterion, the maximum posterior probability criterion, the minimization criterion, and the Newman-Pearson criterion.

The detection of radar signals is also difficult to determine because the probability of the target's appearance is not known in advancethe loss caused by missed inspections is usually selected by the Newman-Pearson criterion, which means that leakage is maintained while maintaining a certain false alarm probability. The detection probability reaches the minimum or the maximum probability of correct detection is maximized.Under the constraint of this criterion combined with the concept of the probability density function of the signal, we can deduce the following conclusions:

The best detection system for radar signals The best reception system is composed of a likelihood ratio calculator and a threshold decision device. Different criteria are reflected in different threshold values. Among them, the likelihood ratio is defined as the ratio of the probability density function when there is a signal and when there is noise. And the core of the likelihood ratio calculator for the additive stationary Gaussian white noise is the matched filter.

There are two main background random processes of radar detection: The noise received by the antenna and generated by the front end of the receiver is a stable random process with a constant power spectral density, called white noise; The transmitted signal is subject to random fluctuations. The clutter caused by the reflection of objects such as ground objects, clouds and rain, foil strips, etc., is related to fluctuations, and its power spectral density is not constant, which is called non-white noise or color noise. Human interference can be classified into the above two types according to the relative spectral width.

There are three types of detection signals discussed in general detection theory: a completely known certain signal; a certain signal containing unknown parameters; and a random signal. In radar detection, the simplest case is the reflection signal of an ideal point target, and the signal amplitude and phase are unknown. This is the second signal mentioned above, which is called a non-volt signal. For complex targets with fluctuations, the correlation and distribution of random fluctuations must be considered. In the pulse search radar, two cases that are completely uncorrelated during each repeated cycle and completely uncorrelated between the number of antenna scans are focused on, namely, fast fluctuation signals and slow fluctuation signals.

Any type of signal detection system found in any kind of noise background can be divided into two parts in principle: The received signals that may contain echoes are processed to obtain statistics. This process can be linear or non-linear. Compare the obtained statistic with a threshold level, and make a judgment as to whether there is a target based on whether it is greater or smaller than the threshold level. This threshold can be constant or automatically adjusted with the nature of the signal. There is also a sequence detection method that uses two thresholds, that is, when the statistic is greater than the upper threshold, it is judged as a target; when the statistic is less than the lower threshold, it is judged as no target; and when the statistic is between the two thresholds, it is increased Signal continued observation time to obtain new statistics for judgment ^[1]

In the modern war environment, radar detection and radar countermeasure technology are a process of continuous development, improvement and mutual promotion. With the emergence of new system radar technology, signal sorting technology in radar countermeasures will encounter more and more difficulties. Especially in the complex electromagnetic environment where radar radiation sources are increasing sharply and signals are becoming more and more complex and diverse, the real-time performance of radar signal sorting and the accuracy of signal sorting and identification are the biggest challenges encountered in radar signal sorting. Based on the existing signal sorting technology, in-depth research on high-precision frequency measurement and direction finding technology, new system radar signal sorting technology, parallel algorithm and hardware implementation technology for signal sorting, and modularization of radar signal sorting system , Miniaturization and low power consumption technology, this is the cutting-edge research direction of radar signal sorting in the future radar electronic countermeasures ^[2]

Radar signal introduction

The function of modern radar signal processing lies in its ability to find and send information to targets within a certain range. The so-called radar signal processing is to analyze the data of modulation signals, signal data coding and other communication signals by radar electronic equipment. This improves the reliability of receiving radio signals and the randomness of target recognition, while reducing the possibility of its own radar signal being found or identified on a certain basis. Therefore, modern radar detection signal processing technology must perform a full range of technical processing in signal identification, parameter estimation, and source identification to ensure the necessary technical support for subsequent military operations or production processes.

Radar signal technology

Object recognition and classification technology

In modern radar detection technology, there is a very precise requirement for target recognition. When the target enters the radar detection control area, it needs to be able to find the target in time and lock the geographical coordinates of the target. The determination of the initial coordinate data and the tracking and collection of the motion trajectory parameters provide accurate positioning of the space where the target is violated in the shortest time. A single radar search device is not sufficient for the basic requirements of target locking. Usually, a radar observation network is set up for target search in a full range of three-dimensional space. In the target recognition, the echo string characteristics of the radar output signal to the target can be realized, and the target characteristics and attributes obtained by the high-resolution image forming technology can be used to achieve.

Anti-electronic interference technology

The signal processing technology of modern radar detection systems is facing the interference and threat of many kinds of signals, because the radar signals are transmitted by using radio electromagnetic wave signals and get accurate return signals, so as to determine the invasion target. When electromagnetic signals are transmitted in the air, they will encounter effects similar to "four defenses" and space electromagnetic fields. Such electromagnetic fields have a great interference effect on the transmission of modern radar signals. Technical processing of modern radar signals is a solution to a variety of problems. One of the methods of electronic signal interference. Anti-electronic interference technology is generally passive radar detection technology.

Signal processing technology

The signal processing of modern radar detection systems is no longer limited to the use of a large number of electronic pipe fittings in the past. In the development of modern science and technology, large-scale or ultra-large-scale digital circuit integration technology, processing technology and communication technology detection The device is already in use. Its signal processing capacity will increase, and more complex signals can be processed more accurately. The digital signal processing technology can also effectively separate the target echo from the aliased signal, which greatly reduces the influence of the interference signal. Digital processing techniques include signal generation, signal extraction, and signal transformation.

Signal detection and accumulation

Modern radar processing technology is facing the effects of many interference and signal anti-recognition technologies. In order to be able to accurately identify target signals in a complex and complex space environment, the use of video signal imaging technology to process target signals is an important method, which can accurately The target signal appears intuitively in front of people in the imaging mode. The application of imaging technology can greatly enhance the detection capability of the target signal, and through the large capacity of the modern digital imaging database, the previous target signal data can be used. Storage and accumulation, which can be traced to the generation characteristics of the target signal. Reduce the probability of false alarms.

Pulse compression technology

Pulse compression technology can widen the time and coverage of radar signals in radar signal processing. Using digital signal processing system technology can detect targets at high speed and improve the resolution of target discovery. Compression technology can also modulate unmatched waveforms, and reduce the problem of mutual interference between radar networks through the receiver-side unmatched filtering technology ^[3] .