System for resolving velocity ambiguity in pulse-doppler radar

• US 3,935,572 A
• Filed: 11/23/1973
• Issued: 01/27/1976
• Est. Priority Date: 11/23/1973
• Status: Expired due to Term

• Current Assignee: Hughes Aircraft Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Landau, Mark I., Broniwitz, Laurence E., Pearson, John B. III
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US05/418,692
• Time in Patent Office: 795 Days
• Field of Search: 343/5 DP, 343/9
• US Class Current: 342/80
• CPC Class Codes: G01S 13/4472 with means specially adapted to airborne monopulse systems clutter elimination using Doppler effect: G01S13/449
• CPC Class Codes: H01Q 1/421 Means for correcting aberrations introduced by a radome

Abstract

A pulse-doppler radar tracking system is disclosed employing four filter channels for tracking range, velocity, azimuth and elevation. Each channel is mechanized in a Kalman filter form by a stored program in a digital computer. The range channel estimates target range, R_TPR, range rate, V_TPV and acceleration a_TPV from one of many received signal frequency spectra at multiples of the pulse repetition frequency (PRF). Once error in the range rate estimate, V_TPR, is within a velocity corresponding to λ PRF/4,

the velocity channel is reinitialized in its estimate of target velocity, V_TPV, with a corrected velocity computed from the less accurate but unambiguous estimate of velocity, V_TPR, and the ambiguous estimate of velocity, V_TPV.

Claims

1. In a radar system for tracking a target aircraft, said system having antenna for cyclically sensing reflected electromagnetic energy from said target at a particular pulse repetition frequency, PRF, and wavelength, λ

, and producing signals representing discriminants Δ

R_m and Δ

V_m of range and radial velocity, respectively, of said target relative to said tracking system, where said velocity discriminants are derived from doppler shift measurements, a separate filter for tracking target range and velocity, each filter responding to respective discriminants Δ

R_m and Δ

V_m to generate predictions of the next measurements from current estimates of filter states, where the states are vectors X_R (i) and X_V (i), and the subscript i indicates a value of time i, a method for resolving ambiguity of said vector X_V (i) after a period of initial tracking sufficient for said range tracking filter to unambiguously achieve an accuracy in current estimates of states in said range filter for the difference between the first derivative of the range value in X_R (i) effectively estimated as one of said range filter states, and the true doppler velocity to be a velocity less than λ

PRF/4, said method being performed by computer means for computing a corrected target velocity V_TCORR from doppler shift information in accordance with the equation ##EQU43## by the steps of determining f_MLC as an offset in a doppler shift spectra reference introduced to avoid main lobe clutter in the center of a doppler shift spectrum where the true target return is likely to be, f_DAP as the ambiguous doppler shift measurement derived from an ambiguous velocity estimate X_V (i), and f_DP as actual doppler shift estimate derived as the estimate of the first derivative of the unambiguous velocity in X_R (i), truncating the bracketed quantity as indicated by the asterisk by dropping the fractional part thereof of the bracketed quantity after the quotient of (f_MLC +f_DAP -f_DP) is divided by PRF and has been rounded by adding 1/2 to the quotient and subtracting (f_DAP +f_MCL) from the bracketed quantity and multiplying the difference by λ

/2 PRF.

2. A method as defined by claim 1 wherein said states are generated using for each state an equation of the general form where X_i+l is a state vector at a second iteration time.

3. A method as defined in claim 2 wherin said states of said range filter are

4. A method as defined in claim 3 including the step of using corrected target velocity V_TCORR to reinitialize said velocity filter during the next filter cycle by substituting V_TCORR for V_TPV.

5. A method as defined in claim 1 including the step of empirically determining the time when said ambiguity is to be resolved, at a fixed time after tracking of said target is initiated.

6. A method as defined by claim 1 wherein said states are generated using for each state an equation of the general form where X_i+l is a state vector at a second iteration time

7. In a radar system for tracking a target aircraft, said system having antenna for cyclically sensing reflected electromagnetic energy from said target at a particular pulse repetition frequency, PRF, and wavelength, λ

, and producing signals representing discriminants Δ

R_m and Δ

V_m of range and radial velocity, respectively, of said target relative to said tracking system, where said velocity discriminants are derived from doppler shift measurements, a separate filter for tracking target range and velocity, each filter responding to respective discriminants Δ

R_m and Δ

V_m to generate predictions of the next measurements from current estimates of filter states, where the states are vectors X_R (i) and X_V (i), and the subscript i indicates a value at time i, apparatus for resolving ambiguity of said vector X_V (i) after a period of initial tracking sufficient for said range tracking filter to unambiguously achieve an accuracy in current estimates of states in said range filter for the difference between the first derivative of the range value in X_R (i) effectively estimated as one of said range filter states, and the true doppler velocity to be a velocity less than λ

PRF/4, said apparatus being comprised of computer means for computing a corrected target velocity V_TCORR from doppler shift information in accordance with the equation ##EQU44## where f_MLC is an offset in a doppler shift spectra reference introduced to avoid main lobe clutter in the center of a doppler shift spectrum where the true target return is likely to be, f_DAP is the ambiguous doppler shift measurement derived from an ambiguous velocity estimate X_V (i), f_DP is actual doppler shift estimate derived as the estimate of the first derivative of the unambiguous velocity in X_R (i), and the asterisk indicates the bracketed quantity is truncated by dropping the fractional part thereof after the quotient of (f_MLC +f_DAP -f_DP) is divided by PRF and has been rounded by adding 1/2 to the quotient.

8. In a radar system for tracking a target aircraft, said system having antenna means for cyclically sensing reflected electromagnetic energy from said target at a particular pulse repetition frequency, PRF, and wavelength, λ

, a separate filter for tracking target range and velocity, each filter generating predictions of the next measurements from current estimates of filter states, apparatus comprising computer means for computing a corrected target velocity V_TCORR from doppler shift information in accordance with the equation ##EQU45## where f_MLC is an offset in a doppler shift spectra reference introduced to avoid main lobe clutter in the doppler shift spectrum where the true target return is likely to be, f_DAP is the ambiguous doppler shift measurement derived from an ambiguous velocity, and f_DP is a doppler shift estimate range, and the asterisk indicates the bracketed quantity is truncated by dropping the fractional part thereof after the quotient of (f_MLC +f_DAP -f_DP) is divided by PRF and has been rounded by adding 1/2 to the quotient.

9. In a pulse doppler radar system operable to transmit a signal at a selected pulse PRF and a signal λ

for tracking a target, a system for resolving velocity ambiguity of an ambiguous target spectral line in a plurality of PRF regions comprising;

first means for tracking said ambiguous velocity spectral line;

second means for estimating velocity as a function of range to an accuracy of at least λ

PRF/4, and

third means coupled to said first and second means for selecting to track the spectral line in the PRF region defined by the estimated velocity provided by said second means at an accuracy of at least λ

PRF/4.