RU2505838C1 - Method for radio location of objects with radar-absorbent coating - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: method involves irradiating an object with microwave transmitter pulses and recording the reflected echo signal with a receiver, wherein irradiation is carried out at two close frequencies and the echo signal is recorded at difference frequency thereof. Generation of the difference frequency occurs in the composite coating material owing to its nonlinearity. Due to that the transmitter operates in the centimetre wavelength range and the receiver operates in the metre wavelength range, there is virtually no absorption of the echo signal by the thin coating layer, wherein the accuracy of determining coordinates of a target does not deteriorate since the azimuth and elevation angle are situated in the direction of the short-wave and narrow-directed antenna of the transmitter.

EFFECT: high sensitivity and range of action.

Description

The invention relates to the field of active radar and relates to the detection and determination of the coordinates of objects covered with radar absorbing material, for example, stealth aircraft.

Typically, the radar of objects is carried out by sending a transmitter to the surrounding space after a certain period of time (repetition period) of probing pulsed electromagnetic signals in the microwave range and registering an echo signal reflected by the object by the receiver. The distance to the object (target) is determined by the time interval between the emitted and received pulses, and its azimuth and elevation angle are determined in the direction of the antenna. If there is no reflected signal, then it is concluded that the sky is clear. However, there may not be a reflected signal even in the presence of objects with a radio-absorbing coating. In this case, the probe signal is absorbed by the coating and is not reflected back, as a result of which the target is not detected.

For radar detection of objects with a radar absorbing coating, the transmitter power and receiver sensitivity are increased [1]. However, the shape of the covered objects is constantly being improved, and the reflection coefficient of coatings is becoming lower and wider in the frequency band (for example, self-consistent coatings [2]). Therefore, this method of radar in the future may be unpromising.

A known method of radar objects with a radar absorbing coating, called the “single wave” method, in which the transmitter and receiver of the locator are located on opposite sides of the detected object, for example, the transmitter is located on a satellite in space, and the receiver is on the surface of the earth [3]. If there is an object between the transmitter and receiver, the received signal will decrease due to scattering or absorption of radio waves by it. It doesn’t matter if it has coverage or not, the signal flow will be interrupted in any case. The operator will conclude that some object appears in the sky between the transmitter and receiver.

The disadvantage of this method is that part of the radar (transmitter or receiver) must be placed on the satellite. The system is also complicated by the need for a large number of receivers (or transmitters) on the surface of the earth to cover the entire area of the most likely airspace violation.

Radar absorbing materials are usually composites containing ferrites, ferroelectrics, metal particles, semiconductors, carbon (graphite, carbon black, fullerenes) and a polymer binder [4]. They are fundamentally nonlinear, since ferrites have magnetic nonlinearity, ferroelectrics - dielectric, and semiconductors and metals in the places of contact of the particles form electron-hole transitions with nonlinear current-voltage characteristics. Therefore, in principle, nonlinear radiolocation of objects with such coatings is possible.

A known system for detecting objects including nonlinear elements [5]. The radar method used in it consists in irradiating the object with a pulse transmitter and registering the echo signal at the frequency of the second and third harmonics of the transmitter signal (prototype).

The disadvantages of this method are the low sensitivity and short detection range, due, firstly, to the inevitable presence in the transmitter's own radiation of the 2nd and 3rd harmonics, which create noise at the input of the receiver, and, secondly, the strong absorption of harmonics in the radar absorbing coverage of the object due to the short wavelength, which leads to low echo power.

The objective of the invention is to increase the sensitivity and, accordingly, the radar range of objects with a radar absorbing coating.

This is achieved by the fact that the object is irradiated at two close frequencies, and the echo signal is received at the difference frequency.

For this, the pulse transmitter of the radar station is performed on two generators (magnetrons) operating from one modulator at close frequencies f ₁ , f ₂ . The radiation from the generators is directed to a common radiating antenna. The receiver has a separate antenna and is tuned to the difference frequency f ₁ -f ₂ . Both antennas are connected mechanically on a common axis. The radiation from the transmitter lies in the centimeter wavelength range, and the receiver in the meter.

When the object is irradiated at frequencies f ₁ and f _{2, the} radiation from the transmitter is absorbed by the coating material. Due to the nonlinearity of this material, Raman frequencies are generated in it, including the difference frequency f ₁ -f ₂ to which the receiver is tuned. The difference frequency signal is practically not absorbed by the coating material due to the fact that its wavelength (meters) is much larger than the layer thickness (millimeters). It is reflected from the metal of the object, radiated into the surrounding space and recorded by the receiver. The distance to the target is determined by the time the signal arrives at the receiver, and the azimuth and elevation angle are determined by the direction of the narrowly directed emitting antenna. In this case, the receiving antenna may be omnidirectional or have a wide radiation pattern.

The increase in the sensitivity and range of the radar in comparison with the prototype is associated with a significantly lower frequency of the reflected signal, which is practically not absorbed by a thin coating layer. In this case, the accuracy of measuring the coordinates of the target does not drop, since the azimuth and elevation angle are located along the short-wavelength radiating antenna.

The method can be used to radar even such objects that are covered with completely non-reflective materials.

INFORMATION SOURCES

1. P.A. Bakulev. Radar systems. M., Radio Engineering, 2003.

2. D.I. Mirovitsky, V.M. Petrov. Self-consistent absorption of electromagnetic waves by a ferro-magnetodielectric layer. Radio Engineering, 1989, No. 3, pp. 60-62.

3. A. Tsurikov. Can I see the invisibility? Engineer, 2008, No. 11, p. 32, 33.

4. V.M. Petrov, V.V. Gagulin. Radar absorbing materials. Izv. RAS. Inorganic Materials, 2001, vol. 37, No. 2, pp. 135-141.

5. Pat. RF №2166769, cl. G01S 13/02. Claim October 9, 2000, publ. 05/10/2001.

Claims (1)

A method for radiolocation of objects with a radio-absorbing coating, including irradiating an object with microwave pulses of a transmitter and registering a reflected echo signal with a receiver, characterized in that the irradiation is carried out by a narrow antenna in the centimeter range at two close frequencies f ₁ and f ₂ , and the reception and registration of the echo signal at their difference frequency f ₁ -f ₂ , where the wavelength is much greater than the thickness of the radar absorbing coating.