Index Analysis of Radar Moving Target Processing System

Abstract: Based on the radar's ability to detect and display moving targets, the constraints that affect the improvement of the radar's moving target improvement factor are discussed, and the possibility of implementing a moving target processing system is analyzed. Through computer simulation calculation verification, provides a basis for system design.
Keywords: moving target display, improvement factor, adaptive cancellation

1 Introduction In recent years, enhancing the capture, tracking, display and detection of moving targets has become one of the key technologies for the development of new photoelectric integrated measurement systems. With the improvement of the performance of electronic devices and the advancement of digital signal processing technology, moving target processing technology has made great progress. The main purpose of the development of moving target display technology is to enhance the radar's target detection capability and display moving targets. It can detect the reflected signals of targets with different radial velocity in various clutters. The radar clutter refers to the echoes reflected from the ground, sea surface, air clouds and rain, chaff, birds, insects and aurora, etc., expressed as ground clutter, sea clutter, meteorological clutter, "senny wave", etc. Wait. The radar clutter signal listed above is essentially different from the noise signal of the radar receiver. The clutter signal is correlated between two consecutive transmission pulses, while the noise is uncorrelated. The moving target display technology is to use the characteristics of clutter to filter out the clutter signal in the radar echo, thereby displaying the moving target within the radar's range of action, so that the photoelectric integrated measurement technology is automated, intelligent, high precision, all-weather, real-time The direction of processing.
2 The relationship between the improvement factor I of the moving target processing system and the related system of the whole machine The work quality of the moving target processing system can be measured by the improvement factor I. The improvement factor is limited by the performance of the radar-related parts. Such as the undulating motion inside the clutter, the instability of the radar system components, the quantization noise of the A / D converter, and the inconsistency of the amplitude and phase of the quadrature dual channel. The improvement factor I can always be expressed by the following formula:

From equation (1), it can be seen that the total value of I depends on the smallest of various improvement factors.
2.1 Constraints of quantization noise on the improvement factor If the number of bits of the A / D converter is b and the highest bit is the sign bit, then the limitation of the A / D quantization noise on the improvement factor is:

2.2 Orthogonal dual-channel amplitude and phase inconsistency Constraints on improvement factors Using orthogonal dual-channel processing technology, the impact of the amplitude and phase inconsistencies of the I and Q channels on system performance must be considered. The power ratio of the spectrum component of the mirror image to the ideal spectrum component caused by the amplitude inconsistency Δ of the quadrature signals UI (t) and UQ (t) and the phase quadrature difference Q is:

It can be seen from Table 2 that if the improvement factor of the moving target processing system is desired to be greater than 40 dB, the amplitude inconsistency of the I and Q channels must be less than 1%, and the phase quadrature difference must be less than 1 degree. 2.3 Constraints of instability on improvement factors The limitation of radar instability on improvement factors is mainly manifested in the instability of phase, amplitude and pulse delay. In the radar using a magnetron transmitter, the improvement factor I of the moving target processing system is unstable at about 23dB. In a certain type of radar, a solid-state microwave source is used, which is 2 times higher in frequency and phase stability than the magnetron transmitter. Above the order of magnitude, the phase noise at a frequency offset of 1 kHz is lower than -100 to 110 dBC / Hz, the bottom phase noise is below -120 dBC / Hz, and I is unstable between 50 and 60 dB.
2.4 Constraints of internal motion of clutter on system improvement factors
2.4.1 The spectral characteristics of the clutter are well known. Due to the irregular motion inside the clutter, the echoes reflected by the reflection units have different Doppler frequencies, which leads to the spread of the clutter spectrum. The clutter spectrum is usually Gaussian, and its power spectrum is usually approximated as:

In the formula, W (f): the power spectrum of the clutter, W0 = W (f) | f = 0;
f: operating frequency; λ: radar operating wavelength;
δc: standard deviation of the clutter power spectrum, reflecting the degree of clutter spectrum broadening at different operating wavelengths;
δv: The standard deviation of the clutter motion is related to the degree of the undulating motion inside the clutter and has nothing to do with the operating wavelength; there is δc = 2δv / λ.
2.4.2 Analysis of clutter suppression filter (1) Fixed canceller To achieve good suppression of ground clutter, you can use a canceller with a notch fixed at zero frequency.
The transfer function of a canceller is:
H1 (Z) = (1-Z-1) (5)
The transfer function of the quadratic canceller is:
H2 (Z) = (1-Z-1) 2 (6)

For the discrete clutter with zero mean, the δC limits the improvement factors of the fixed primary canceller and the fixed secondary canceller: when the Fr of the system is 1000 Hz and δC = 13 Hz, there is

When the working Fr is higher, the improvement factor will increase. The above-mentioned primary fixed canceller and secondary fixed canceller have great restrictions on ocean waves and meteorological clutter.

The improvement factor of the moving target system is related to the radar repetition frequency. The higher the radar repetition frequency, the better the improvement factor and the better the cancellation effect. For ground clutter, the lower the wind speed, the better the improvement factor. For the wave echo, the lower the wind speed, the better the improvement factor. But when the wind speed is particularly high (gusty wind), the improvement factor will be better than when the wind speed is small. The theoretical analysis has been confirmed by engineering experiment results.
It can be seen that the use of fixed cancellers cannot suppress sea clutter and cloud and rain clutter. Therefore, we adopt an adaptive canceller to suppress sea clutter and cloud and rain clutter.
(2) Adaptive canceller In order to suppress the clutter with variable motion speed such as cloud, rain, snow, etc., the notch of the clutter canceller must be locked in the center of the average Doppler of the clutter in real time. If the average Doppler frequency of the clutter is fdc, the radial velocity of the clutter motion is VC, and the radar operating wavelength is λ (5 cm), then the relationship between the three of them is: fdc = 2VC / λ, if the radial direction of the clutter motion The speed change range is: VC = 0 ~ 60km / h = 0 ~ 16.67m / s, the clutter average Doppler frequency fdc = 0 ~ ± 666.67Hz. Due to the repetition frequency of the radar: fr = 1000 Hz, the variable range of the notch of the adaptive canceller is 0 to 1000 Hz.
The transfer function of the adaptive primary canceller is:

If the input signal is:

W1 = cos (θ1) + jsin (θ1) is called the weight of the filter. The adaptive primary cancellation filter achieves optimal filtering by adjusting its weights.
The transfer function of the adaptive secondary canceller is:

If the input signal of the canceller is:
This is called the weight of the filter. By changing the weights W1 and W2, we can control the position of the filter notch. The secondary adaptive canceller has a wider notch than the primary adaptive canceller, so the cancellation performance for clutter with a wider spectrum is better.
The key problem of the adaptive canceller is how to obtain the weight of the filter corresponding to the average Doppler frequency center fdc of the clutter in real time, so as to lock the notch of the filter to the average Doppler frequency center fdc, and With the change of the average Doppler frequency center fdc of the clutter, the new corresponding filter weights WI and WQ are obtained in real time, so that the notch of the filter is always locked to its center following the change of fdc to achieve the most The best clutter suppression effect, and the corresponding weight to minimize the clutter residual is the best weight.
In adaptive primary and secondary cancellation, we assume that the clutter moves radially at a speed of 10m / s, fdc = 2Vrc / λ, so the Doppler frequency of the motion clutter is fdc = 400Hz. The simulation results are shown in Table 4 Show. The above has analyzed the main factors that affect the improvement of the system improvement factor. It can be seen that the moving target radar improvement factor is subject to many factors, but for a specific radar, only one or two factors are the main contradiction of the problem, and the other factors are all secondary contradictions.
Table 5 shows the comparison of the cancellation improvement effects under the three levels of sea conditions and the four combinations.
3 Implementation scheme of moving target processing system (1) The orthogonal dual-channel complex signal processing mode is adopted. Before entering the moving target processing system, the digital quadrature complex signal has passed the adaptive sidelobe cancellation and digital pulse compression system to form a pulse width Constant digital quadrature complex signal. In order to ensure the performance of the moving target processing system, the amplitude consistency and phase orthogonality of the input I and Q branch signals must meet certain requirements.
(2) Adopt modern digital signal processor ADSP21062 as the core to realize adaptive filtering of ground clutter, sea clutter and meteorological clutter. Multi-mode moving target processing is completed, which has the characteristics of small equipment volume, low cost, high reliability, high calculation density, and strong real-time performance.
(3) CFAR processing technology is a mature technology and will not be repeated here.
4 Conclusion In summary, the following conclusions can be drawn:
(1) The digits of the A / D converter should be greater than 10 digits.
(2) The amplitude and phase inconsistency and phase inconsistency of the quadrature dual channel should be limited to within 1% and 1 degree, respectively.
(3) The radar should select the microwave frequency source with high stability.
(4) Choose different cancellation methods under different circumstances. A secondary fixed canceller can be used when observing strong echoes of ground clutter; a primary one can be used when sea clutter and meteorological clutter are strong Fixed canceler plus secondary adaptive cancellation method, when the cancellation effect is not ideal, you can also automatically adjust the weights of the adaptive canceller to get the best effect.
This scheme can meet the requirements of the improvement factor of ground clutter greater than 40dB, the improvement factor of sea clutter greater than 35dB, and the improvement factor of meteorological clutter greater than 20dB. Improve the accuracy of radar measurement and display of moving targets.
1 M. Edited by I Skornik, translated by Xie Zhuo. Radar Manual. Beijing: National Defense Industry Press, 1978
2 Chen Jianchun, Geng Fulu. Adaptive motion clutter suppression technology. Xi'an: Journal of Xidian University, 1999, 26 (2): 174 ～ 177

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