Passive radar system

Abstract

A passive radar system, which comprises the following elements: a reference antenna ARI having a polarization preferably aligned with one of the polarizations of an illuminator, directed at a source of a reference signal, in particular a telephony base station of 4G- / 5G- or next standard, a reference antenna AR2 having an orthogonal polarization to ARI, preferably aligned with the other of the polarizations of the illuminator, orthogonal to the polarization of ARI, directed at the source of the reference signal, at least one observation antenna AO1 having a polarization preferably aligned with that of ARI, directed at an area of observation in which there are moving objects, preferably at least one observation antenna AO2 having an orthogonal polarization to that of AO1, preferably aligned with AR2 (orthogonal to ARI), directed at the area of observation in which there are moving objects, a radio receiving channel (TO) equipped with filters and amplifiers of signals coming from each of the antennas, a device for digitizing and processing signals (DPS) for the purpose of detecting or imaging the moving objects, wherein the radio receiving channel (TO) is connected to the device for digitizing signals (DPS), wherein the device for digitizing and processing signals consists of a digitizer (DI), a polarimetric adaptive filter (PF A), and a correlator system (UK), connected to each other.

Description

[0001] Passive radar system

[0002] The object of the invention is a passive radar system with an improved object detection capability against the interference from immobile objects. The system uses illuminators operating in dual-polarization mode, such as base stations of cellular networks having 4G-, 5G- standards and potentially next generations.

[0003] Ordinary passive radars operate with illuminators transmitting a signal of a single and known polarization. In such a case, techniques for removing strong echoes having zero Doppler frequency are well known. However, they will network in the case of illuminators operating in dual-polarization mode, because they do not allow for the removal of cross-polarized components.

[0004] The above problems are solved by the passive radar system according to the invention.

[0005] A passive radar system according to the invention is characterized in that it comprises the following elements: a reference antenna ARI having a polarization preferably aligned with one of the polarizations of an illuminator, directed at a source of a reference signal, in particular a telephony base station of 4G / 5G or next generation, a reference antenna AR2 having an orthogonal polarization to that of ARI, preferably aligned with the other of the polarizations of the illuminator, orthogonal to the polarization of ARI, directed at the source of the reference signal, at least one observation antenna AO1 having a polarization preferably aligned with that of ARI, directed at an area of observation on which moving objects are present, preferably at least one observation antenna AO2 having an orthogonal polarization to that of AO1, preferably aligned with AR2 (orthogonal to ARI), directed at the area of observation on which moving objects are present, a radio receiving channel equipped with filters and amplifiers of signals coming from each of the antennas, a device for digitizing and processing of the signals for the purpose of detecting or imaging the moving objects, wherein the radio receiving channel is connected to the device for digitizing signals, wherein the device for digitizing and processing signals consists of a digitizer (DI), a polarimetric adaptive filter, and a correlator system, connected together. The system of the invention can be used in passive radiolocation systems and joint systems of the radar-communication one (ang. joint radar - communication) working with dualpolarization illuminators. Particularly preferred is the use of the invention in case of the need to improve the ability to detect objects, e.g., at a large distance from the transmitter, low transmitted power, detection of objects having low reflectivity (e.g., drones), in the case of occurrence of the strong clutter (echoes from stationary objects).

[0006] An embodiment of the invention is presented in the drawing, where Fig. 1 shows an operation diagram of the passive radar system, Fig. 2 - a construction diagram of the passive radar system, and Fig. 3 - a construction diagram of the device for digitizing and processing signals.

[0007] As shown in Fig. 1, the signal from the illuminator (a telecommunication transmitter) reaches directly to the reference antennas (ARI, AR2). The same signal, after reflections from moving and stationary objects, reaches to the observation antennas (AO1, AO2) of the passive radar system. Subsequently, the signals reach the DPS device via the radio receiving channel TO. Delays and frequency shifts (resulting from the Doppler effect) between the signals from the reference antennas and the observation antennas are compared. As a result, information is obtained about the speed and bistatic distance of the objects that are moving in the area of observation. The echoes observed from stationary objects, such as buildings, usually have much greater power as compared to smaller moving objects (cars, drones). Echoes of moving objects, the detection of which is the objective of the passive radar system, are therefore obscured by the echoes from the immobile objects. Thanks to the knowledge of the direct signal, it is possible to remove or reduce the power of the component of the zero-Doppler-frequency signal (originating from reflections from immobile objects) using digital adaptive filtering (e.g., LMS (Least Mean Squares) filters, RLS (Recursive Least Squares) filters, lattice filters), which is a necessary condition for detecting weak echoes coming from moving objects.

[0008] In the proposed system, the use of receiving reference antennas (ARI and AR2) having two polarizations allows collecting complete information about the signal transmitted from the dual-polarization illuminator. Thanks to this, improved filtering methods can be used to remove cross-polarized components. In the proposed passive radar system, the removal of these components improves the ability to detect objects against the interfering background originating from stationary objects.

[0009] The passive radar system according to the invention in the form of a dual-polarization passive radar allows improving the ability of the radar to detect moving objects against the interfering background originating from stationary objects in the case where the illuminator operates in dual-polarization mode. An exemplary operating scenario of the inventive system is shown in Fig. 1. A base station of a mobile telephony network is located on the roof of a building and transmits a signal simultaneously in two polarizations (marked with the dashed and continuous lines). In the passive radar receiver system, a pair of reference antennas (ARI, AR2) having mutually orthogonal polarizations directed at the base station have been used. A second pair of observation antennas (AO1, AO2), having mutually orthogonal polarizations, are directed at the area of observation. In the area of observation there is a moving object with a low reflection coefficient (symbolically marked as a drone) as well as an apartment building - an immobile object with a high reflection coefficient, causing strong reflections masking the echo from the much smaller object. The observation antennas receive the signal reflected from both objects, wherein the signal reflected from the building typically have much greater power. Additionally, the signal may change polarization upon being reflected from the objects. The use of a pair of reference antennas having orthogonal polarizations provides full information about the signal, whereby it is possible to effectively remove the echoes from immobile objects received from the observation direction.

[0010] The diagram of the entire system of the invention is presented in Fig. 2. First, the signals reach a four-element system of receiving antennas (UA). Then they pass through the receiving channel (TO), which includes filters and amplifiers. From the receiving channel, the signals reach the device for digitizing and processing signals (DPS), the component blocks of which are shown in Fig. 3. First, the analog signal is converted into digital form in the digitizer (DI). In the polarimetric adaptive filter (PF A) block, the direct signal and the echoes of permanent objects are removed. It would not be possible to achieve this with classical lattice filters due to the existence of non-zero correlation between the polarimetric components of the broadcasted signal. For the complete removal, according to the invention, a modified adaptive filter was used, preferably in a lattice structure with two reference signals (from reference antennas ARI, AR2) and at least two measurement input signals (from observation antennas AO1, AO2). At its output, processed signals AO Ip and AO2p are obtained with components ARI and AR2 (direct signal) and their delayed copies (echoes from immobile objects) removed. The filtering process takes into account the coupling between channels ARI and AR2 and changes in signal polarization upon reflections from permanent echoes, what makes it effective. Signals AOlp and AO2p processed in this way are fed into a polarimetric correlator system (UK), generating a space of delays and Doppler shifts, for the purpose of detecting objects and determining their polarimetric properties, and presenting the results on the radar indicator (WR).

[0011] The filtering method described herein enables a significant reduction of the strength of the direct signals and signals reflected from immobile objects in the filtered signals, and consequently achieving high sensitivity of the passive radar. Such a radar system has an improved ability to detect objects.

Claims

Claim1. A passive radar system, characterized in that it comprises the following elements: a reference antenna ARI having a polarization preferably aligned with one of the polarizations of an illuminator, directed at a source of a reference signal, in particular a telephony base station of 4G- / 5G- or next standard, a reference antenna AR2 having an orthogonal polarization to ARI, preferably aligned with the other of the polarizations of the illuminator, orthogonal to the ARI, directed at the source of the reference signal, at least one observation antenna AO1 having a polarization preferably aligned with ARI, directed at an area of observation in which the moving objects are present, preferably at least one observation antenna AO2 having an orthogonal polarization to that of AO1, preferably aligned with AR2 (orthogonal to ARI), directed at the area of observation in which the moving objects are present, a radio receiving channel (TO) equipped with filters and amplifiers of signals coming from each of the antennas, a device for digitizing and processing signals (DPS) for detecting or imaging the moving objects, wherein the radio receiving channel (TO) is connected to the device for digitizing signals (DPS), wherein the device for digitizing and processing signals consists of a digitizer (DI), a polarimetric adaptive filter (PF A), and a correlator system (UK), connected with each other.

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