RU2802367C1 - Method for selecting moving targets at high pulse repetition rate of a probing linear-frequency-modulated signal with a small duty cycle - Google Patents

Abstract

FIELD: radars.

SUBSTANCE: invention relates can be used in ground (surface) based pulse-Doppler (PD) radar stations (RS) to solve the problem of selecting a group of moving air objects against the background of narrow-band passive interference and, first of all, against the background of intense reflections from underlying earth (water) surface, local objects and slow-moving meteorological formations. In the method for selecting moving targets at a high pulse repetition rate of a probing linear-frequency-modulated signal with a low duty cycle, the selection of the parameters of the probing signal, its formation and emission, the definition and setting of the band-stop filter parameters, the transfer of received signals to an intermediate frequency are performed twice. Noise suppression in the band-stop filter in the areas of zero, low Doppler frequencies and frequencies multiple of the frequency pulse repetition rate and the extraction of the useful signal in the area of high Doppler frequencies are performed in the first probing cycle, and noise suppression in the band-stop filter in the areas of zero, high Doppler frequencies and frequencies multiple of the frequency pulse repetition rate and the selection of a useful signal in the region of low Doppler frequencies are performed in the second probing cycle.

EFFECT: increased noise immunity of the PD RS when probing with quasi-continuous signals (QCS) of low duty cycle with linear frequency modulation (LFM).

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Description

Field of technology to which the invention relates

The invention relates to the field of radar and can be used in pulse-Doppler (ID) ground-based (surface)-based radar stations to solve the problem of selecting a group of moving air objects against the background of narrow-band passive interference and, first of all, against the background of intense reflections from underlying earth (water) surface, local objects and sedentary meteorological formations.

State of the art

Algorithms for the operation of moving target selection devices (MTS) are widely known today and are based on taking into account the frequency differences of signals reflected from moving and stationary objects [1]. Thus, when reflected from moving objects, the signal frequency changes by the value of the Doppler addition (F _d ), which directly depends on the radial speed of the object:

where ν _r is the radial speed of the object;

λ is the wavelength of the probing signal.

The frequency of the signal reflected from a stationary target will not differ from the frequency of the sounding signal.

ID radars using probing quasi-continuous signals (QNS) use the SDC method based on a notch filter (RF), adopted as a prototype [1]. This method ensures the suppression of a signal (interference) reflected from stationary objects and the selection of a useful (having a Doppler frequency additive) signal using RF in one cycle of target sensing, which means the formation and emission of SNS into space, reception of the reflected signal and its processing [1 ]. The essence of this method is as follows (see Fig. 1):

1) Selecting the parameters of the probing signal (block 1.1);

2) Formation of the probing signal (block 1.2) and its radiation (blocks 1.3 and 1.4);

3) Determination and installation of RF parameters (block 1.5);

4) Transfer of received signals to an intermediate frequency (block 1.6);

5) Filter suppression of interference in the region of zero Doppler and multiple pulse repetition frequency (PRF) frequencies and selection of the useful signal in the region of Doppler frequencies.

However, when using probing signals with linear frequency modulation (chirp), the implementation of SDC based on RF has its own peculiarities. Thus, the frequency of the received signal in the process of transfer to an intermediate frequency increases by the amount of the rangefinder frequency (ƒ _long ), which is directly proportional to the distance from the radar to the reflecting object [2]:

where Δƒ is the frequency deviation of the probing SNS;

r _c - range to target;

c is the speed of propagation of radio waves;

τ _s - signal duration.

This feature negatively affects the efficiency of the SDC when operating an ID radar in the near-ground region of space, when reflections from the underlying surface have a rangefinder frequency value (2) comparable to the Doppler addition (1). It was noted in [3] that in this case the noise immunity during primary ranging in ID radars deteriorates and at the same time a solution to this problem was proposed, the feature of which is the use of a probing SNS with a chirp with a slope (P):

where F is the PRF of the probing SNS.

This approach ensures the concentration of signals (interference) reflected from stationary objects in the vicinity of zero and multiple PRF (F _p ) frequencies, regardless of the distance to the reflecting objects, as demonstrated in Fig. 2a and fig. 3 (solid line).

However, the noted solution is not always effective. For example, when working on low-flying, unobtrusive objects. Thus, their high-quality detection requires an increase in the energy potential of the radar, which is realized at a fixed average transmitter power, as a rule, by reducing the signal duty cycle. Reducing the duty cycle of the marked SOS with chirp slope (3), both due to an increase in the pulse duration and a decrease in their repetition period, entails an increase in the frequency deviation in each of the pulses - the structure of the amplitude-frequency spectrum of the signal changes. Such changes lead to the appearance of reflections from stationary objects not only in the vicinity of the zero and multiple PRF frequency regions, but also in the regions of low Doppler frequencies as shown in Figs. 2b and 3 (dashed line). Those. elements of passive interference (PP) go beyond the RF suppression zone. Therefore, echoes from low-speed targets will be masked by the PP.

Thus, the operation of an ID radar in the low duty cycle SNS mode with a chirp with parameters (3) will entail a decrease in the quality indicators of the SDC device that implements the method chosen as a prototype. One of these indicators is the coefficient of improvement of the signal-to-interference ratio ( _Kу ):

Where - the ratio of signal and noise at the output and input of the SDC device, respectively. The value of indicator (4) for the prototype will be equal to one, i.e. 0 dB (see Fig. 6).

Disclosure of the invention

The technical problem solved by the developed method is to increase the noise immunity of the ID radar when probing a low-duty signal station with a chirp.

The technical result, which provides a solution to the stated technical problem, is achieved by the fact that in the method of selecting moving targets in the high pulse repetition rate mode of a probing linearly frequency-modulated signal with a low duty cycle, which consists in selecting the parameters of the probing signal, its formation and probing, selection and setting the parameters of the notch filter, transferring received signals to an intermediate frequency, filter suppression of interference in the region of zero Doppler frequencies and multiples of the pulse repetition frequency, and isolating the useful signal in the Doppler frequency region, according to the invention, selecting the parameters of the probing signal, its formation and emission, definition and installation parameters of the notch filter, the transfer of received signals to an intermediate frequency is carried out twice, while suppression of interference in the notch filter in the regions of zero, low Doppler and multiple frequencies of pulse repetition frequency and selection of the useful signal in the region of high Doppler frequencies is carried out in the first probing cycle, and suppression of interference in the notch filter in the areas of zero, high Doppler and multiple pulse repetition frequencies and the selection of the useful signal in the area of low Doppler frequencies - in the second probing cycle.

The physical essence of the proposed method is as follows.

The quality of operation of protection devices against PP is determined by the correct choice of parameters of the frequency response of the SDC device K(ƒ), which is strictly related to the frequency structure of the interference:

where C is an integer;

N(ƒ) is the spectral power density of the interference.

In turn, the parameters of the PP, which is a consequence of the reflection of the probing signal from stationary objects interfering with radar observation, depend on the parameters of the latter. Thus, the connection between the parameters of the probing signal and the characteristics of the SDC devices is obvious, which explains the solution to the problem posed by parametric optimization of the probing low-duty system with a chirp.

At the first stage, according to the method, the optimal parameters of the probing SNS are selected in accordance with (3), its formation and radiation.

The next step is to determine and set the RF parameters according to (5), which is done in accordance with the well-known analytical model of the two-dimensional mismatch function of the considered SNN to calculate the spectral level of the normalized power of filtered interference signals [5, 6]:

where ⏐ρ(τ,0)⏐ is the signal mismatch function;

ƒ - frequency mismatch value;

τ is the magnitude of the time mismatch.

In this case, for periodic signals (N(ƒ)) is calculated within one pulse repetition frequency (F _p ). The calculation of the power of the PP (6) showed that for a low-duty probing SNS with a chirp slope (3), the maximum interference power (N(ƒ)) is concentrated in the region of zero, low Doppler frequencies and frequencies that are multiples of the PRF, and the minimum in the region of high Doppler frequencies frequencies, as shown in Fig. 3 (dashed line) and FIG. 4 (dashed line).

At the next stage, the received signals are transferred to an intermediate frequency.

The next step is to suppress interference in the notch filter in the regions of zero, low Doppler and multiples of the pulse repetition frequency frequencies and highlight the useful signal in the region of high Doppler frequencies.

Since, in accordance with (5), the RF parameters suppress interference in the region of low Doppler frequencies, the useful signal, if present in this range, will also be suppressed. Therefore, it is necessary to perform another cycle of target probing, but with different parameters of the target, so at the next stage of the method a decision is made to change the parameters of the probing signal.

The next step is to change and reselect the optimal parameters of the probing SNS. It has been established that the regions of low Doppler frequencies are not occupied by PP when the slope (β ₁ ) of the chirp of the probing SOS with low duty cycle is equal to half the square of the PRF

The regions of zero and multiple PRF frequencies in (7) are also noise-saturated, as illustrated in Fig. 4 (solid line). Those. the spectra of signals (interference) reflected from stationary objects with parameters (3) and (7) are spaced apart and occupy small for (3) and high for (7) Doppler frequency regions (Fig. 4).

The formation and radiation into space of the SNS with parameters (7) is sequentially carried out at the next stage of the invention.

Re-setting the RF parameters according to (5) ensures suppression of the PP and selection of the useful signal in the region of low Doppler frequencies.

At the next stage, the received signals are transferred to an intermediate frequency.

At the final stage of the invention, interference in the Russian Federation is suppressed in the regions of zero, high Doppler and multiple PRF frequencies and the useful signal is isolated in the region of low Doppler frequencies.

Brief description of drawings

The invention is illustrated by the drawings shown in Fig. 1-6 revealing the principles of operation of SDC devices, the essence of the invention and parametric optimization of the SNS and the effect achieved by it, as well as the physics and laws of linear-shear deformation of the SNS mismatch function during chirp with a decrease in its duty cycle.

In fig. Figure 1 shows a block diagram of the SDC device; the following notations are used:

1.1. Block for selecting parameters of the probing signal;

1.2. Probing signal generation unit;

1.3. Antenna switch block;

1.4. Transceiver antenna;

1.5. Block for selecting RF parameters;

1.6. Mixer block;

1.7. PP suppression block.

In fig. Figure 2a shows a cross-section of the mismatch function of the SOS with a small duty cycle chirp. The contours of areas of high correlation of the two-dimensional FN FN with the chirp slope are visible there is a zone free from passive interference in the region of both low and high Doppler frequencies, which makes it possible to effectively use SDC systems. In fig. Figure 2b shows the effect of “spreading” of local maxima of the FN of the SOS with a chirp with a decrease in its duty cycle, which leads to a significant narrowing of the interference-free spectral range due to the appearance of PP at low Doppler frequencies.

In fig. 3 graphs of the function characterizing the frequency-rate selectivity of the ID radar correlation-filter receiver for the SNS with chirp (solid) and in the case of reducing its duty cycle (dotted) are shown in Fig. 3a and 3b, respectively, illustrating the same effect as shown in Fig. . 2.

In fig. Figure 4 shows the result of parametric optimization of the probing signal. Graphs of a function characterizing the frequency-velocity selectivity of the correlation-filter receiver of an ID radar for a low-duty SNA with a chirp for its steepness (dotted) and (solid) indicate the existence of mutually non-overlapping spectral regions with a low level of interference power for each of the analyzed signals.

In fig. Figure 5 shows a block diagram of a device that implements the “Method for selecting moving targets in the high pulse repetition rate mode of a linear frequency-modulated probing signal with low duty cycle.” In fig. 5 the following notations are adopted:

5.1. Block for selecting parameters of the probing signal;

5.2. Probing signal generation unit;

5.3. Antenna switch block;

5.4. Transceiver antenna;

5.5. Block for selecting RF parameters;

5.6. Mixer block;

5.7. GS suppression block;

5.8. Block for making a decision on changing the parameters of the probing signal.

In fig. Figure 6 shows the dependence of the improvement factor of the signal-to-interference ratio on the duty cycle of the probing SOS with chirp for the proposed method.

Carrying out the invention

The invention can be implemented using the block diagram shown in FIG. 5, and consists in sequentially performing the following operations:

- selection of the probing SNS and setting its parameters, including the slope of the chirp in accordance with (3) (Fig. 5 block 5.1);

- formation of a low-duty probing SNS with a chirp (Fig. 5 block 5.2);

- radiation of the formed probing SOS with chirp (Fig. 5 blocks 5.3 and 5.4);

- setting the parameters of the RF frequency response (Fig. 5 block 5.5) in accordance with expression (5);

- reception of reflected signals by the antenna and their further transfer to an intermediate frequency (Fig. 5 block 5.6);

- suppression of interference in the Russian Federation in the areas of zero, low Doppler and multiple PRF frequencies and selection of the useful signal in the area of high Doppler frequencies (Fig. 5 block 5.7);

- making a decision to change the parameters of the probing signal (Fig. 5 block 5.8);

- selection of the probing SNS and setting its parameters (Fig. 5 block 5.1), including the slope of the chirp in accordance with (7);

- formation of a low-duty probing SNS with a chirp (Fig. 5 block 5.2);

- radiation of the formed probing SOS with chirp (Fig. 5 blocks 5.3 and 5.4);

- setting the parameters of the RF frequency response (Fig. 5 block 5.5) in accordance with expression (5);

- reception of reflected signals by the antenna and their further transfer to an intermediate frequency (Fig. 5 block 5.6);

- suppression of interference in the Russian Federation in the regions of zero, high Doppler and multiple PRF frequencies and selection of the useful signal in the region of low Doppler frequencies (Fig. 5 block 5.7).

The proposed method is technically feasible and provides, inaccessible to analogues and the prototype, the possibility of effective selection of moving targets in an ID radar when probing a low-duty signal station with a chirp. The invention was tested on a PC in the form of a mathematical simulation-stochastic model of the selection process of moving targets in an ID radar [4] and can be applied in an ID radar to solve problems of detecting a group of subtle targets in conditions of passive interference with a locally concentrated energy spectrum, and above all, under the influence of intense masking reflections from the underlying surface, local objects and sedentary meteorological formations.

The declared technical result is confirmed by the results obtained by simulation in the MATLAB application package.

Initial data for modeling:

1. Type of probing signal: SNS with chirp;

2. The slope of the chirp of the probing SNS changes in accordance with (3) and (7);

3. The duty cycle of the probing signal varies from 3 to 15;

4. The input signal and interference powers are equal;

Next, the signal power and noise at the RF output were calculated and the improvement factor for the signal-to-noise ratio was estimated in accordance with (4).

Thus, the developed method for selecting moving targets in the mode of high repetition rate of pulses of a linearly frequency-modulated probing signal with low duty cycle is based on an algorithm for optimal signal filtering under conditions of exposure to non-white noise, but, unlike the prototype, provides for the selection of the useful signal in noise-saturated areas frequencies due to wobbling of the chirp slope of the probing SNS, which makes it possible to improve the signal-to-interference ratio up to 24 dB (Fig. 6).

Bibliography

1. Handbook on radar / Ed. M.I. Skolnik. Per. from English Under the general editorship. B.C. Willows. In 2 books. Book 1. M.: Tekhnosphere, 2015. - 672 p. (pp. 40-41, 186, 207-211).

2. Radar signals and their application [Text] / A.A. Trukhachev. - M.: Voenizdat.2005. - 234 p. (pp. 237-238).

3. A method for unambiguous primary ranging of a group of targets against the background of narrow-band passive interference in the mode of high pulse repetition frequency of the probing signal: patent for invention / S.A. Zaitsev, A.M. Lavrentiev, A.A. Kirichenko. - No. 2756034 dated 09/24/2021.

4. Model of the selection process of moving targets in a pulse-Doppler radar station: certificate of state registration of computer programs / Yu.V. Denishchik, A.M. Lavrentiev, S.A. Zaitsev applicants and copyright holders Yu.V. Denishchik, A.M. Lavrentiev, S.A. Zaitsev. - No. 2021665594; application 09.20.2021; registered 09.29.2021.

Claims (1)

A method for selecting moving targets in the mode of high pulse repetition rate of a probing linearly frequency-modulated signal with a low duty cycle, which consists in selecting the parameters of the probing signal, its formation and probing, selecting and setting the parameters of the notch filter, transferring received signals to an intermediate frequency, filter suppression of interference in the region of zero Doppler and multiple pulse repetition frequencies and isolating the useful signal in the Doppler frequency region, characterized in that the selection of parameters of the probing signal, its formation and emission, determination and installation of notch filter parameters, transfer of received signals to an intermediate frequency are carried out twice, in this case, suppression of interference in the notch filter in the regions of zero, low Doppler and multiples of the pulse repetition frequency frequencies and selection of the useful signal in the region of high Doppler frequencies is carried out in the first probing cycle, and suppression of interference in the notch filter in the regions of zero, high Doppler and multiples of the repetition frequency frequency pulses and selection of the useful signal in the region of low Doppler frequencies - in the second probing cycle.

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