RU1840929C - Moving target selection device - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: device includes series-connected antenna, antenna switch and receiver, series-connected pulse transmitter, the second output of which is connected to the second input of the antenna switch, a coherent phasing unit, a heterodyne, a phase detector and a Doppler frequency generating unit. A signal carrier frequency divider is connected between the output of the receiver and the signal input of the phase detector.

EFFECT: high probability of detecting moving targets.

1 dwg

Description

The invention relates to the field of radio engineering, in particular to pulsed radar systems (radar) with selectors for moving targets (SDC), and is intended to increase the efficiency of recognition of the difference between stationary and moving objects or between objects moving at different speeds.

, где n=1, 2, 3, …, λ - длина рабочей волны РЛС. Такое перемещение приводит к тому, что принимаемые сигналы имеют фазовый сдвиг между последовательными импульсами, равный 360° или кратный этому значению. При таком фазовом сдвиге напряжение на выходе фазового детектора РЛС не изменяется, поэтому сигнал этой цели подавляется в СДЦ. Следует отметить, что при фазовом сдвиге, близком к 360° (или 0°), СДЦ воспринимает эхо-сигналы как сигналы медленно движущихся целей и компенсирует их. Следовательно, первый недостаток, обусловленный стробоскопическим эффектом, заключается в том, что значительное количество целей, движущихся со скоростями, равными или близкими к "слепым", воспринимаются СДЦ как неподвижные или медленно движущиеся и компенсируются, что исключает возможность их обнаружения.The stroboscopic effect in pulsed radar with SDC is manifested in the ambiguous measurement of the Doppler frequency shift of the echo signals reflected from moving targets. This leads, firstly, to the appearance of so-called “blind” speeds. "Blind" speeds correspond to the movement of the target during the time between the radiation of consecutive probe pulses over a distance $$n\frac{\lambda }{}$$$$\frac{}{2}$$

where n = 1, 2, 3, ..., λ is the radar operating wavelength. This movement leads to the fact that the received signals have a phase shift between successive pulses equal to 360 ° or a multiple of this value. With such a phase shift, the voltage at the output of the radar phase detector does not change, therefore, the signal of this target is suppressed in the SDC. It should be noted that with a phase shift close to 360 ° (or 0 °), the SDS perceives echo signals as signals of slowly moving targets and compensates for them. Therefore, the first drawback caused by the stroboscopic effect is that a significant number of targets moving at speeds equal to or close to “blind” are perceived by the SDC as stationary or slowly moving and compensated, which excludes the possibility of their detection.

The second significant drawback due to the stroboscopic effect is that all Doppler frequencies are brought into the range 0-F _p / 2, where F _p is the repetition frequency of the probe radar pulses. However, it is in this frequency range that the instabilities of the coherent radar regime are manifested. This can be illustrated by such an example. Suppose, from pulse to pulse for any reason, the phase of the probing signals changes relative to the phase of the coherent local oscillator alternately by 0.01 radians (0.57 °). Such instability leads to the fact that in the spectrum of echo signals of all targets appears frequency F _p / 2 with a level of -40 dB relative to their amplitude. This component passes through the SDS without compensation, as optimal, and reduces the coefficient of improvement of the SDS, with the ideality of the remaining parameters, to 40 dB. If the instability of the coherent mode changes more slowly, the new frequency is within the range 0 ÷ F _p / 2, and depending on the value of the lower cutoff frequency of the frequency response of the SDC filter, this component will be allocated or weakened by an amount depending on the slope of the slope of the frequency response of the selector. Thus, the second drawback due to the stroboscopic effect is the need to ensure a high degree of coherence, in connection with which the radar, in fact, turns into a high-precision phase-measuring system, and the ratio of the processed signals reaches 10 ²⁰ . This leads to a significant increase in the complexity and cost of the radar.

The third and perhaps the most unpleasant drawback caused by the stroboscopic effect is the overlap in the frequency domain of the echo signals of targets moving with different radial speeds. In the absence of a stroboscopic effect, for example, in Doppler radars with continuous radiation, the frequency response of each target is proportional to its radial speed. This allows you to choose the frequency of the notch filter SDS so that the motionless signals in slow moving targets are suppressed, and the signals of fast moving targets are allocated. The overlay effect does not allow us to unambiguously establish the threshold (criterion) "moves - does not move", and as a result, the efficiency of the MIS in the radar is low. If the threshold (set by the lower cutoff of the frequency response of the SDC filter) is very low, the effect of suppressing slowly moving passive interference is not provided, if the cutoff frequency is chosen higher, the zones of blind speeds are excessively expanded and the likelihood of skipping high-speed targets increases.

As a result of patent research on the sources of patent and scientific and technical information, methods and devices for compensating for the stroboscopic effect in pulsed radars in the CDS were not detected.

At the same time, technical solutions are known aimed at eliminating the phenomenon of "blind" speeds - the first of the considered disadvantages of radar with SDC, due to the influence of the stroboscopic effect. The second and third considered flaws are perceived as objectively inevitable and methods of dealing with them are not known.

Known methods of dealing with "blind" speeds are described, for example, in the book "Selection of moving targets", G.M. Vishin, M., Military Publishing, 1966, p. 213. They come down to the following basic solutions:

- work with radar with a high repetition rate;

- radar operation at a variable repetition rate (with a change in interpulse intervals);

- change in the carrier frequency of the radar transmitter (tuning the frequency of the probing signals);

- simultaneous use of several carrier frequencies of the radar.

The repetition frequency of probe pulses in radars operating with a high repetition rate is selected from the condition of providing an unambiguous measurement of the target speed, due to which the possibility of Doppler filtering is achieved. As noted, this solution has the following disadvantages:

- the range of the target varies ambiguously, in connection with which it is necessary to complicate the equipment of the range finder;

- requires a stronger suppression of passive interference, due to their imposition with ambiguity in range;

- more stringent requirements for stability and spectral purity of transmitted signals, due to the stronger influence of passive interference when superimposed;

- the average transmitter power should be increased in proportion to the decrease in duty cycle.

The operation of the radar at a variable repetition rate is based on the fact that with a decrease in the interpulse intervals, a shift in the values of the "blind" speeds is achieved. This method is not suitable for CDS with a fixed delay for the repetition period. In addition, the stabilization of the transmitter, necessary for the good functioning of the CDS with a constant repetition period, is associated with large financial costs and an increase in the mass of the system. However, the stabilization of the transmitter, which is necessary for the system to function well with a change in the period from pulse to pulse, requires overcoming even greater difficulties (see the Manual on Radar, edited by M. Skolnik, (trans. From English, M., Sov. Radio, 1979, volume 3, p. 319) At the same time, this method in some cases is the only way to weaken the effect of “blind” speeds.

Elimination of "blind" speeds is also fundamentally possible by changing the carrier frequency of the radar transmitter. However, the limits of the frequency tuning of the transmitter necessary for a significant change in the "blind" speeds are so wide that practically such tuning is not feasible.

The method of simultaneous use of several carrier frequencies of the radar can be implemented in the simplest version at two frequencies, selected in such a way that the corresponding "blind" speeds do not match. Both frequency channels are combined in front of the compensating device. Channel aggregation can be achieved by mixing frequency-spaced signals. Further signal processing is performed at the differential frequency. Essentially, these are two radars with common processing, which limits the possibility of implementing this method.

The analysis shows that the considered methods of dealing with “blind” speeds are particular solutions to one drawback and, therefore, are palliative (half measures) because they do not provide a complete, radical solution to the problem.

The closest in technical essence to the described invention is a method of dealing with "blind" speeds by changing the interpulse intervals (with wobble repetition rate) described, for example, in the book "Radar Reference", ed. M. Skolnik, per. with English., M., Sov. Radio, 1979, vol. 3, pp. 319-326 and in US patent No. 3902174, MKI G01S 9/42, publ. 08/26/75, It is known that the "blind" speeds correspond to Doppler frequencies that are multiples of the repetition frequency F _p . If the radar repetition frequency is slightly changed, then a number of Doppler frequencies, previously multiple of F _p , will become non-multiple and, therefore, targets with such frequencies will be allocated by the SDC. However, at the same time, the signals of other targets, previously allocated, will be multiples of F _p and will be compensated. The device for changing the repetition frequency of the probe pulses of the transmitter operates automatically according to a certain program optimized for detecting targets in the expected speed range. The method is carried out by a device consisting of a pulsed radar with an SDS, containing an antenna, an antenna switch, a receiver, a coherent phasing circuit, a coherent local oscillator, a pulse transmitter with a device for changing the repetition frequency of probing pulses, and a phase detector whose output is connected to a moving target selector (see, for example , a book by M.I. Finkelstein, “Fundamentals of Radar,” M., Sov. Skolnik, vol. 3, M., 1979, fig. 51-52, p. 293-330, fig. 4 on page .285).

The coherent phasing circuit is constructed in the same way as the receiving path, i.e. includes a clock mixer, to which the signal of the first local oscillator is applied, the frequency of which in the general case should be very high and differ from the frequency of the probing signal by the value of the intermediate frequency of the receiver.

The disadvantage of this solution is the need to ensure high short-term stability of the first local oscillator and the associated complication of the radar.

In addition to the difficulties noted above associated with a radar transmitter operating with a variable repetition rate, it is necessary to point out another drawback of this solution. A change in the repetition rate leads, in fact, to an averaging of the speed characteristics of the SDS, which reduces the probability of detecting moving targets in comparison with their optimal selection, since dips appear in the pulse train.

The aim of the present invention is to increase the protection of the radar from passive interference and at the same time simplify its hardware implementation.

Another objective of the present invention is to provide a device that implements the proposed method.

Another objective of the present invention is to eliminate the ambiguity of measuring target speed in pulsed radars, as well as to create a device that can normalize the speed characteristic of the radar, make it independent of the operating frequency range, and on this basis provide the possibility of unification of SDCs.

The goals are achieved by the fact that the traditionally performed operations in pulsed radar with SDC, consisting in the formation and emission of sounding pulses, receiving, amplifying and filtering by Doppler frequency offset of the received echo signals, are supplemented by the compression of the dynamic range of the Doppler frequency offset of the echo signals performed before filtering by Doppler frequency offset.

The essence of the invention lies in the fact that the range of Doppler frequencies, depending on the relative (radial) speeds of the targets and on the wavelength of the radar, is compressed by a special device so that the sub-range of Doppler speeds of moving targets of interest is in the passband of the SDC filter, so these signals are not compensated. At the same time, the echoes of passive interference are below the passband of the SDC and are compensated. Elimination of the overlay effect allows you to realize the maximum improvement coefficient provided by the radar with SDC. Moreover, it is possible to form a special form of the frequency response of the SDC filter in the extraction area, for example, increasing, which will further increase the likelihood of detecting high-speed targets with an overall increase in radar protection from passive interference. Since in each particular case the compression coefficient can be chosen differently, based on the working wavelength of the radar, the dynamic range of the Doppler frequency shift at the input of the SLC turns out to be the same and independent of the working frequency of the probe pulses, which will make it possible to unify the SEC.

The invented method differs from the known method of dealing with “blind” speeds in that it does not change the radiation parameters of the probing signals (repetition frequency, radar operating frequency), which helps to reduce the requirements for this equipment and, as a result, to simplify it significantly. The effect is achieved by compressing the dynamic frequency range of the echo signals (before filtering by the moving target selector), performed at the low power level characteristic of the receiving path.

The proposed method is carried out by a pulsed radar with an SDC containing a pulsed transmitter connected to a coherent local oscillator through a coherent phasing circuit and with an antenna switch, the antenna and the radio frequency unit of the receiver are connected to its two outputs, as well as a phase detector, the reference input of which is connected to the output of the coherent local oscillator, and the output is connected to the CDS, into which an additional frequency divider is inserted, connected between the output of the radio-frequency unit of the receiver and the signal input of the phase detector.

The difference between the device that allows the new method and the known one is that the echo signals from the output of the radio frequency block of the receiver, usually directly fed to the signal input of the phase detector, pass through the frequency divider and only then it is fed to the phase detector.

The invention and the achievement of a positive effect can be explained as follows. The output signal of the phase detector is proportional to the cosine of the phase difference of the echo signal and the reference voltage of the coherent local oscillator, i.e. in general, using a phase detector, the frequency difference of the processed signals is determined. The operation of dividing the frequency of the echo signal leads to a compression of its spectrum, as a result of which the values of the Doppler frequencies at the output of the phase detector will be less by the division factor of the divider. It should be emphasized that at the same time the frequency of the components in the spectrum of all echo signals will decrease by the same amount due to the instability of the coherent radar mode.

In some cases, it is also advisable to perform a coherent phasing circuit using frequency dividers. This will eliminate the difficulties of implementing the first high-frequency local oscillator radar. Such a solution significantly changes the structure of the radar, the receiver becomes a new type - not superheterodyne and not direct amplification. It may turn out, however, that for specific purposes, for example, to ensure tuning of the radar frequency, the first local oscillator will still need to be used, but in this case its frequency should not be very high, which will reduce the requirement for its stability.

The drawing shows a block diagram of a device for implementing the method. The device comprises a pulse transmitter 1, an antenna switch 2, an antenna 3, a coherent phasing circuit 4, a radio frequency receiver unit 5, a coherent local oscillator 6, a frequency divider 7, a phase detector 8, a moving target selector 9. The output signal of the SDC 9 is supplied to the radar terminal device. The given block diagram rather conventionally denotes a radar with a CDS, however, this was done in order to emphasize the essence of the invention and to avoid unnecessary detail. For example, it is understood that the RF unit of the receiver 5 is a known device and may include a receiver protection device, input circuits, a high-frequency amplifier, a mixer, intermediate-frequency amplifiers, etc. As a pulse transmitter 1, a powerful oscillator can be used as well and a multiplier chain. It also means that the coherent phasing circuit 4 can be quite complex and contain the necessary number of different devices - a probe power of the probe signal, a mixer, an amplifier, gating elements, etc.

The device operates as follows. The pulse transmitter 1 generates a sequence of probe signals that are emitted by the antenna 3 through the antenna switch 2. The echo signals received by the antenna 3 through the antenna switch 2 are sent to the radio frequency unit of the receiver 5, where they are preselected in frequency, amplified, transferred to a sufficiently high intermediate frequency and through a frequency divider 7 are fed to the signal input of the phase detector 8. The coherent phasing circuit 4 ensures the coherence of the radar, by phasing the coherent first local oscillator 6 or probing signal. The signal of the coherent local oscillator 6 is supplied to the phase detector 8 as a reference signal. The output signal of the phase detector 8 through SDC 9 is fed to the terminal device of the radar.

The implementation of the claimed invention is a technical challenge at the level of engineering design.

In this case, the frequency dividers can be constructed according to the scheme of a frequency mixer (converter), covered by feedback. It is a mixer, the output of which includes a selective element tuned to a frequency n times less than the input. An output signal is applied to the reference input of the mixer through a frequency multiplier (n-1) times.

In the simplest case (for n = 2), you can do without a multiplier in the feedback circuit, since in this case its multiplication factor is 1. The required divisor division coefficient depends on the radar frequency range, the speed range of the detected targets and the repetition frequency of the probe pulses. In some cases, a general division factor close to 100 may be required. Such a frequency divider is advisable to perform in the form of a chain of dividers, but this will not lead to a significant complication of the equipment. Even in the case of using dividers with n = 2, all of them will need 6-7 cascades. Amplifiers and decoupling elements can be installed between the cascades.

The main advantage of the claimed invention is to increase the protection of the radar from passive interference while simplifying its hardware implementation.

Using the proposed method of compensation (neutralization) of the stroboscopic effect in pulsed radars with SDS will allow:

- increase the radar's protection from passive interference, which will increase the likelihood of detecting moving targets, especially small ones, against the background of intense interfering reflections;

- significantly simplify the radar transmitting equipment by reducing coherence requirements;

- eliminate the ambiguity of measuring the speed of the target.

Compression of the speed characteristic, elimination of its dependence on the operating frequency of the radar, creates the basis for the unification of SDS.

Claims (1)

A moving target selection device comprising an antenna, an antenna switch and a receiver, a pulse transmitter in series, the second output of which is connected to the second input of the antenna switch, a coherent phasing unit, a local oscillator, a phase detector and a Doppler frequency filtering unit, characterized in that, with the purpose of increasing the probability of detecting moving targets by eliminating the influence of the stroboscopic effect at a low pulse repetition rate of the transmitter, int at the output of the receiver and the signal input of the phase detector, a divider of the carrier frequency of the signal is introduced.

Images