Information available from radar echo

Unless you get some information about the goal , Otherwise, the detection of the target is of little value . Again , No target detected , Target information is meaningless .

distance

By measuring the time when the radar signal propagating at the speed of light reaches the target and returns , The radar can determine the distance from the target . This may be the most outstanding feature of conventional radar . Measure the distance of the target at a long distance , Other sensors can not reach the measurement accuracy of radar ( Basically , Over a long distance , The accuracy of radar is limited by the data accuracy of the propagation speed that people know ). On a short distance , The ranging accuracy can reach several centimeters . To measure distance , At measuring distance , A certain time mark must be introduced into the transmission waveform . The time scale can be a short pulse ( Amplitude modulation of signal ), It can also be a striking frequency or phase modulation . The accuracy of range measurement depends on the radar signal bandwidth ： The wider the bandwidth , The higher the accuracy . therefore , Bandwidth is the basic measure of distance accuracy .

Radial velocity

The radial velocity of the target can be obtained by isomorphism with the range change rate over a period of time , It can also be obtained by measuring the Doppler frequency domain . Accurate measurement of radial velocity takes time . Therefore, time is the basic parameter to describe the quality of radial velocity measurement . The velocity and direction of motion of a moving target can be obtained from its trajectory , The trajectory can come from the measurement of the target position over a period of time .

Angular direction

A method to determine the target is to determine the antenna angle corresponding to the maximum echo signal amplitude of the scanning antenna . This usually requires an antenna with a narrow beam ( High gain antenna ). An air surveillance radar with rotating antenna beam can judge the angle at that time . The angle of the target in the one-dimensional angular direction can also be determined by two antenna beams , The two beams are slightly staggered in angle , Then compare the echo amplitude received by each beam . If it is necessary to measure azimuth and pitch angle at the same time , Four beams are required . The accuracy of the angle measurement depends on the electrical dimensions of the antenna , That is, the antenna size given in the number of wavelengths .

Size and shape

If the radar has enough resolution in range or angle , It can provide target size measurement in high-resolution dimension . Distance is usually a coordinate value in a dimension with resolution . Resolution in lateral distance ( Given by multiplying the distance by the antenna beam width ) It can be obtained by the antenna with extremely narrow beam width . However, the antenna beam width is limited , So the lateral range resolution obtained by this method is not as good as the range resolution . The high resolution in the lateral distance dimension can be based on SAR or ISAR Obtain by Doppler shift . To pass through SAR or ISAR Obtain lateral range resolution , There must be relative motion between the radar and the target . When there is enough resolution in both distance and lateral distance , It can not only obtain the size of the target in two orthogonal coordinates , Sometimes it can also distinguish the shape of the target .

The importance of radar bandwidth

Bandwidth basically represents information , therefore , It is very important in many radar applications . There are two types of bandwidth encountered in radar ： First, signal bandwidth , It is the bandwidth determined by the signal pulse width or signal internal modulation ; Second, adjustable bandwidth .
Large bandwidth distinguishes the target in the range , For accurately measuring the target distance , And the limited ability to identify different types of targets .

Signal-to-noise ratio

The accuracy of all radar measurements and the reliable detection of targets depend on the ratio of signal to noise .

Doppler shift in radar

The importance of Doppler shift , Soon after the Second World War, people began to pay attention to the pulse radar , It has become an increasingly important factor in many radar applications . If the Doppler effect does not exist , Modern radar would not be so interesting or useful .
SAR and ISAR They can be used to describe the use of Doppler frequency shift . Airborne Doppler navigation radar is also based on Doppler frequency shift . The use of Doppler frequency shift usually has higher requirements for the stability of radar transmitter , It also increases the complexity of signal processing , However, in order to get the great benefits offered by Doppler , These require that people be willing to accept . It needs to be mentioned that , Doppler frequency shift is the key capability for radar to measure velocity , As the traffic police try to use when maintaining the speed limit of vehicles and other speed detection applications .
notes ： The article is excerpted from 《 Radar manual 》 The third edition