Clutter rejection in a passive radar receiver of ofdm signals

• US 20040066331A1
• Filed: 07/30/2003
• Published: 04/08/2004
• Est. Priority Date: 02/07/2001
• Status: Active Grant

• Current Assignee: ONERA
• Original Assignee: ONERA
• Inventors: Poullin, Dominique
• Granted Patent: US 6,924,763 B2
• Time in Patent Office: Days
• US Class Current: 342/378
• CPC Class Codes: G01S 13/003 Bistatic radar systems; Multistatic radar systems
• CPC Class Codes: G01S 13/524 based upon the phase or frequency shift resulting from movement of objects, with reference to the transmitted signals, e.g. coherent MTi
• CPC Class Codes: G01S 7/414 Discriminating targets with respect to background clutter

Abstract

The invention concerns a passive radar receiver for a received orthogonal frequency division multiplex-type signal consisting of symbol frames each emitted on coded orthogonal carriers. After formatting the received signals into digital symbols (S₁ S₁), a filtering circuit (2) eliminates by subtraction or using a covariance matrix, in the symbol signal at least unwanted signals with null Doppler effect so as to apply a filtered signal (X′) including essentially signals backscattered by mobile targets to a Doppler-distance correlator (4).

Claims

1. A radar receiver processing a radio signal received via a propagation channel (EM-RE) and comprising frames of symbols each emitted on coded orthogonal carriers (f₁-f_K), comprising shaping means (1;

1b) for converting the received signal into a digital symbol signal (X, S₁-S_I) and Doppler-distance correlation means (4;

4b) for discriminating mobile targets, characterized in that it comprises filtering means (2;

2b) for eliminating in the symbol signal (X) at least unwanted zero Doppler effect signals (TD) in order to apply a filtered signal (X′

) including essentially signals (SC) backscattered by targets (CB) to the correlation means (4;

4b).

2. A radar receiver according to claim 1, wherein the filtering means (2) comprises means (21) for producing spectral lines (SP₁-SP_K) of the symbol signal (X) corresponding to the orthogonal carriers (f₁-f_K), means (3) for detecting the unwanted zero Doppler effect signals (TD) by estimating coefficients of the transfer function ({overscore (H)}₁-H_K) of the propagation channel respectively in the spectral lines, means (22) for subtracting the spectral lines of the unwanted zero Doppler effect signals (TD) deduced from the coefficients of the estimated transfer function from the spectral lines of the symbol signal (SP₁-SP_K) , and means (23) for synthesizing the spectral lines produced by the subtraction means (22) into the filtered signal (X′

).

3. A radar receiver according to claim 2, wherein the spectral lines of the unwanted signals (TD) are estimated for each symbol and averaged over each frame in the detecting means (3) before being subtracted from the spectral lines (SP₁-SP_K) of the symbol signal.

4. A radar receiver according to claim 2 or 3, comprising means (5) for estimating an emitted signal replica (Re) as a function of the spectral lines of the unwanted zero Doppler effect signals (TD), the estimated replica being correlated to the filtered signal (X′

) in the Doppler-distance correlation means (4).

5. A radar receiver according to claim 1, wherein the shaping means (1b) comprises a plurality of receiver means (11₁, 12₁ to 11_N, 12_N) and shapes a plurality of signals received by the receiver means in the form of digital symbol signals (X₁-X_N) , characterized in that the filtering means (2b) comprises means (21b) for producing groups of spectral lines (SP₁₁-SP_1N to SP_K1-SP_KN) of the symbol signals (X₁-X_N) respectively corresponding to the orthogonal carriers (f₁-f_K), means (24b) for estimating covariance matrices (R₁-R_K) each depending on products of spectral lines (SP_k1to SP_kN) two by two in a group relating to a respective carrier (f_K), means (25b) for deducing inverse matrices (R₁⁻

1 to R_K<sup>−

1</sup>) of the covariance matrices, means (26b) for filtering the groups of spectral lines respectively relating to the carriers (f₁f_K) by multiplying the groups of lines by the respective inverse matrices to produce filtered groups of spectral lines and means (23b) for synthesizing the filtered groups of spectral lines into filtered symbol signals (X₁′

-X_N′

) including essentially signals backscattered by targets (CB) applied to the correlation means (4b).

6. A radar receiver according to claim 5, wherein the spectral line products on which the covariance matrices (R₁-R_K) are dependent depend on spectral lines of symbols and are averaged over each frame.

7. A radar receiver according to claim 5 or 6, comprising means (3, 5) for estimating an emitted signal replica (Re) as a function of the spectral lines (SP₁₁-SP_K1) of one of the symbols signals (X₁) by estimating coefficients of the transfer function of the propagation channel in the unwanted zero Doppler effect signals, the estimated replica being correlated to the filtered symbol signals (X₁′

-X_N′

) in the correlation means (4b).