RU2392638C1 - Method for high-precision radiolocation measurement of lift angle of low flying target under signal interference conditions - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to methods for radiolocation measurement of the lift angle of a low flying target under signal interference conditions. The method is based on scanning the target with interference lobes by way of wobble of the carrier of the signal under study from one pulse to another. One measures the amplitude modulation factor and the modulation phase sign of the signal received as well as the signal/noise ratio and defines the interference lobe lift angle (by the modulation factor) and the position of the target therein by the modulation phase sign and the signal/noise ratio.

EFFECT: improved precision of measurement.

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Description

Technical field

The invention relates to the field of radar systems for determining the location of a target, and more specifically, to methods for radar measuring the elevation angle of a low-flying target under conditions of interference of signals over a reflective underlying surface (sea).

State of the art

At present, many methods are known that allow measuring the altitude of a low-flying target (NLC), such as off-axis monopulse tracking, the use of symmetric sum-difference and asymmetric radiation patterns, the method of complex angles, the method of nonlinear logical filtering, and the maximum likelihood method. However, an accurate measurement of the altitude or elevation angle (UM) of a low-flying target under the conditions of signal interference by traditional methods is problematic for various reasons, either, as in the case of sum-difference diagram methods, due to the fact that under these conditions the direction-finding characteristic is destroyed, or -for difficult feasibility and high hardware costs necessary for the application of the maximum likelihood method. When the target flies below a certain level, so that the angular separation between the signals reflected from the target and the signals reflected from the water surface is less than 0.8 of the antenna beam width, the usual mono-pulse methods give large tracking errors.

An analogue of the present invention is the method described (Lo T., Litva J. Low-angle tracking using multifrequency sampled aperture radar. - IEEE Trans. On Aerospace and Electronic Systems, 1991, v.27, No. 5, p.797-805) . The method is based on using the SAMPAR system locator and an advanced algorithm for finding the maximum likelihood function, which uses a high-precision signal model. The method provides a multiple increase in the resolution of the signal, which exceeds any method published in the open literature. Like other types of radar systems, the SAMPAR system consists of a transmitting part and a receiving part. The former is used to irradiate the space given by the locator with radio frequency energy, while the latter is used to receive and process the reflected radar signals in order to extract the necessary information. A key characteristic of the target height estimation algorithm is the use of target range information. For this, it is necessary that the radar operates in the mode of determining the range of the target. This algorithm is based on the concept of mirror reflection. The method may not work when the water surface becomes uneven. When the method does not work, then it is necessary to return to the monopulse method. As a result of the experiments, when the system worked on two X-frequency bands simultaneously: one of the frequency channels worked at a fixed frequency of 10.2 GHz, the other quickly switched in the range from 8 to 12 GHz with a step of 30 MHz, the ability to distinguish signals was shown. which are divided by one tenth of the beam width.

The SAMPAR system was chosen as the prototype of the present invention because the solution to the problem of measuring small elevation angles of low-flying targets above the sea in the present invention is based, like in the SAMPAR system, on a fast tuning of the signal carrier frequency.

To solve this problem, the SAMPAR system emits and receives a signal in a very wide band of the carrier frequency tuning: 10 GHz ± 20%. It works with a signal with a unique range, information about which is key to the algorithm for estimating the target height. This signal is characterized by the fact that it has a short duration and a large duty cycle and therefore requires a high pulse power of the transmitter to provide the potential necessary for the detection and tracking of small-sized NLCs. With a duty cycle of 50-100 it should be at least 50-100 kW. Only a transmitter on a traveling wave lamp with a frequency band of less than ± 3% can have such power. Therefore, you must have at least 6 transmitters. The mass of each with a cooling system is several hundred kilograms. It is very cumbersome, expensive and difficult to implement.

SUMMARY OF THE INVENTION

The proposed method is based on scanning the target with interference lobes by wobbling the carrier of the emitted signal from pulse to pulse.

The width of the petal and the angle of its inclination relative to the horizon are proportional to the wavelength λ (Theoretical Foundations of Radar. Edited by V. E. M. Dulevich, Soviet Radio, 1978):

,

,

where h _a is the height of the antenna.

If you change λ in the emitted signal from pulse to pulse, then the interference lobes of the radiation pattern will scan the target along the CM with a frequency equal to half the repetition frequency F _r .

With increasing λ, the lobe expands and rises above the radio horizon, and with decreasing it narrows and falls. In this case, the angle of elevation of the interference lobe can be determined by the modulation coefficient of the amplitude of the reflected signal, and the position of the target in it can be determined by the sign of the modulation phase and the signal-to-noise ratio.

The amplitude modulation coefficient is proportional to the serial number of the petal: for example, for the 10th petal it will be 10 times greater than for the 1st. Thus, the magnitude of the modulation coefficient carries information about the PA: it is proportional to the measured target PA. The elevation angle of the maximum of the nth interference lobe relative to the direction to the horizon is (Theoretical basis of radar. Edited by V. E. M. Dulevich, Sovetskoe Radio, 1978):

CM _n = nδθ + nm _f δθ-δθ / 2

where n = 1, 2, 3 ... is the serial number of the petal;

δθ = λ (2h) is the width of the lobe;

m _f - carrier wobulation coefficient.

As a result of scanning, the signal reflected from the target located on the “slope” of the lobe will be modulated in amplitude with a modulation coefficient

m _an = nm _a1

where m _a1 is the modulation coefficient of the signal in the first lobe, corresponding to m _f .

From here

By measuring the modulation coefficient of the signal m _an , you can determine the number of the lobe n.

The phase of modulation of the signal amplitude as a result of scanning will change to the opposite depending on which “slope” of the petal is on the target: if the phase is positive, the target is on the upper “slope” of the petal, it will be negative when the target is on the lower “slope” .

Recognizing the position of the target in the lobe increases the accuracy of the interferometric measurement method by 2 times: the maximum error in the estimation of the CM in this case does not exceed δθ / 4 and gives a complete picture of the direction of motion of the target along the CM.

This invention is aimed at obtaining high accuracy in the measurement of CM without the use of additional equipment for this purpose and with a very simple algorithm for processing the received signal. This technical result is achieved by the fact that when the signal amplitude modulation coefficient is used as a criterion for evaluating the target amplitude, the problem of measurement ambiguity does not arise during signal interference; on the contrary, interference is used as a positive factor contributing to an increase in measurement accuracy.

The processing circuit of the signal carrying information about the phase sign of the modulated signal and the modulation coefficient is shown in the drawing.

The amplitude of the modulation signal is

U _m = m _a U _c ,

where m _a is the modulation coefficient of the amplitude of the signals;

U _c is the amplitude of the signal pulse.

Expressing the amplitude U _c at the output of the receiving device through the signal-to-noise (S / N) ratio and taking into account that this ratio in free space is 18 dB, under the conditions of signal interference for the same range it will be 12 dB higher and will be 30 dB, we get

;

,

;

,

where σ _W is the effective noise voltage.

The derivative near the middle of the first interference lobe shows that for m _f = 3% m _a1 = 5%, for which U _m1 / σ _w = 1.6.

The probability that U _{m1 is} not less than 1.6 σ _W is ρ ₁ = 0.89.

For the second lobe, wherein the modulation index is 2 times higher, m _a2 ≈10%, the ratio of U _m2 / σ _w = 3.2 and ₂ ρ probability close to 1.

Thus, the proposed method for recognizing the “slope” of the petal is functional even with a small m _f .

An example of determining the target MIN from the measurement results at δθ = λ / 2h _a = 1 mrad for λ = 3 cm and h _a = 15 m is shown in table 1.

Table 1 No. p / p Measurement result Target elevation one - S / N> 20 dB; Target near the middle of the lower - m _a ≈5%; “Slope” of the 1st petal, - sign U _m "-" UM≈0.25 mrad 2 - S / N> 20 dB; Target near the middle of the upper - m _a ≈5%; “Slope” of the 1st petal, - sign U _m "+" UM≈0.75 mrad 3 - S / N> 20 dB; Target near the middle of the lower - m _a ≈5%; "Slope" of the 10th petal, - sign U _m "-" UM≈9.25 mrad

The proposed method provides an order of magnitude higher accuracy in determining the NLM CMs than the accuracy in measuring CMs in free space at a signal-to-noise ratio of 18 dB.

The drawing shows a diagram of a signal processing that carries information about the phase sign of the modulated signal and the modulation coefficient.

Information confirming the possibility of carrying out the invention

The present invention can be implemented in any radar having electronic tuning of the carrier frequency in the band of ± 1.5%. Additional equipment for the implementation of this invention is not required. The algorithm for processing the signal and determining the target mind based on the measurement results is extremely simple.

EFFECT: ensuring high accuracy of measuring the elevation angle of a low-flying target under conditions of signal interference without hardware costs and simplifying the algorithm for processing the received signal.

Claims (1)

A method of high-precision radar measurement of the elevation angle of a low-flying target using signal interference, characterized in that the target is scanned by interference lobes by wobbling the carrier of the emitted signal from pulse to pulse, the amplitude modulation coefficient and the sign of the modulation phase of the received signal are measured, as well as the signal-to-noise ratio, and determine by the modulation coefficient the elevation angle of the interference lobe, and by the sign of the modulation phase and the signal-to-noise ratio, the position of the target in it, and radiate the exact value of the target elevation angle.