Dual cancellation interferometric AMTI radar

• US 5,559,516 A
• Filed: 11/27/1981
• Issued: 09/24/1996
• Est. Priority Date: 11/27/1981
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Guarino, Robert A., Didomizio, John A.
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US06/325,522
• Time in Patent Office: 5,415 Days
• Field of Search: 343/5 CM, 343/5 FT, 343/7.7, 343/368, 342/118, 342/147, 342/156
• US Class Current: 342/118
• CPC Class Codes: G01S 13/5242 with means for platform motion or scan motion compensation, e.g. airborne MTI
• CPC Class Codes: G01S 13/5244 Adaptive clutter cancellation specially adapted for airborne MTI, G01S13/5242
• CPC Class Codes: G01S 13/582 adapted for simultaneous range and velocity measurements
• CPC Class Codes: G01S 13/72 for two-dimensional tracking, e.g. combination of angle and range tracking, track-while-scan radar
• CPC Class Codes: G01S 13/9029 specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques
• CPC Class Codes: G01S 3/46 using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
• CPC Class Codes: G01S 7/2813 Means providing a modification of the radiation pattern for cancelling noise, clutter or interfering signals, e.g. side lobe suppression, side lobe blanking, null-steering arrays specially adapted to secondary radar systems G01S13/762; aerials or aerials systems H01Q21/29, H01Q25/00
• CPC Class Codes: G01S 7/2883 using FFT processing
• CPC Class Codes: G01S 7/2886 using I/Q processing
• CPC Class Codes: G01S 7/4004 of parts of a radar system
• CPC Class Codes: G01S 7/415 Identification of targets based on measurements of movement associated with the target

Abstract

An AMTI radar employs a dual cancellation format to cancel the clutter in the radar returns received by the apertures of an interferometric radar antenna. The three-aperture antenna presents the radar returns to three receivers which demodulate the returns to a complex in-phase signal I and a quadrature signal Q which are sampled at a pulse repetition interval at all ranges of interest. After conventional motion compensation four data sets are derived from the returns of the three apertures, L(t), C(t-τ), C(t) and R(T-τ) which represents samples taken over multiple pulse repetition intervals for each range interval or bin of interest. Fast Fourier transforms change the data sets into the frequency domain and phase compensations for the data sets are calculated in a triple interferometric signal calibration unit. Compensation phase is further adjusted in a clutter phase calculator allowing a much reduced phase quantity to be applied to the delayed data sets as the second part of the dual cancellation signal. Following subtraction of doppler filter outputs of the delayed data sets from the undelayed data sets in the cancellation unit, maximum clutter cancellation is achieved. The resulting clutter-cancelled signals are presented to a detection and validation processor to determine range, doppler, amplitude and angle measurements to the moving targets with a high degree accuracy.

Claims

1. An airborne moving target indicator (AMTI) for effectively detecting and measuring slowly moving targets in main beam clutter from a moving platform, comprising:

antenna means having a transmit aperture for transmitting radar pulses at a pulse repetition frequency (PRF), and having three spaced-apart receive apertures;

a left aperture (12), a center aperture (14), and a right aperture (16) for receiving radar return signals;

receiver means (18, 20,

     22) for providing in-phase (I) and quadrature (Q) component signals associated with each receive aperture in response to the radar return signals;

clock means (51) for providing a timing signal (52) related to each transmitted radar pulse;

delay means (53) for providing a delay signal in response to platform movement, PRF and interaperture spacing;

storage means (70, 72, 74,

     76) for providing delayed and undelayed sets of data in a time domain format in response to the timing signal, the delay signal and the in-phase and quadrature component signals;

FFT means (84, 94, 104,114) for providing delayed and undelayed sets of data in a frequency domain format in response to the delayed and undelayed sets of data in time domain format;

interferometer means (158) connected to the FFT means for providing phase calibration signals (126,

     146) indicative of the measured angle versus doppler characteristic for a left/center interferometer comprising the left and center apertures and a center/right interferometer comprising the center and right apertures in response to the delayed and undelayed sets of data in frequency domain format, and for providing amplitude calibration signals (134,

     154) indicative of the ratio of the average clutter powers for the left/center and center/right interferometers in response to the delayed and undelayed sets of data in frequency domain format;

phase computation means (169) for providing a phase difference signal indicative of the phase difference between the delayed and undelayed delta sets of data in response to the delay signal;

phase calculation means (160) for providing phase correction signals for the left/center and center/right interferometers indicative of a time delay phase correction, the positional phase relationship between adjacent apertures, and a phase due to motion of the targets in response to the coherent addition of the phase difference signal and the phase calibration signals;

cancellation means ( 250 ) for providing clutter-cancelled in-phase and quadrature component signals that have significantly suppressed clutter characteristics in response to the phase correction signals, the amplitude calibration signals and the delayed and undelayed sets of data in frequency domain format; and

detect ion means (350) for providing range (r_t), doppler (f_t), amplitude (a_t), and angle (θ

_t) signals indicative of target motion in response to the clutter cancelled in-phase and quadrature component signals.

2. An airborne moving target indicator (AMTI) for effectively detecting and measuring slowly moving targets in main beam clutter from a moving platform, comprising:

antenna means having a transmit aperture for transmitting radar pulses at a pulse repetition frequency (PRF), and having three spaced-apart receive apertures;

a left aperture (12), a center aperture (14), and a right aperture (16) for receiving radar return signals;

receiver means (18, 20,

     22) for providing in-phase (I) and quadrature (Q) component signals associated with each receive aperture in response to the radar return signals;

clock means (51) for providing a timing signal (52) related to each transmitted radar pulse;

delay means (53) for providing a delay signal in response to platform movement, PRF and interaperture spacing;

storage means including a first array storage (70) for providing an undelayed set of data (80,

     82) in time domain format in response to the timing signal and the in-phase and quadrature component signals associated with the left aperture, a second array storage (72) for providing a delayed set of data (90,

     92) in time domain format in response to the timing signals, the delay signal and the in-phase and quadrature component signals associated with the center aperture, a third array storage (74) for providing an undelayed set of data in time domain format in response to the timing signal and the in-phase and quadrature component signals associated with the center aperture, and a fourth array storage (76) for providing a delayed set of data in time domain format in response to the timing signal, delay signal and the in-phase and quadrature component signals associated with the right aperture;

FFT means including a first FFT processor (84) for providing an undelayed set of data in frequency domain format associated with the left aperture in response to the undelayed set of data in time domain format associated with the left aperture, a second FFT processor (94) for providing a delayed set of data in frequency domain format associated with the center aperture in response to the delayed set of data set in time domain format associated with the center aperture, a third FFT processor (104) for providing an undelayed set of data in frequency domain format associated with the center aperture in response to the undelayed set of data in time domain format associated with the center aperture, and a fourth FFT processor (114) for providing a delayed set of data in frequency domain format associated with the right aperture in response to the delayed set of data in time domain format associated with the right aperture;

interferometer means (158) for providing a left/center phase calibration signal (126) indicative of the measured angle versus doppler characteristic for a left/center interferometer comprising the left and center apertures in response to the undelayed sets of data in frequency domain format associated with the left and center apertures, for providing a left/center amplitude calibration signal (134) indicative of the amplitude difference in clutter spectrum between the left and center apertures in response to the undelayed sets of data in frequency domain format associated with the left and center apertures, for providing a center/right phase calibration signal (146) indicative of the measured angle versus doppler characteristic for a center/right interferometer comprising the center and right apertures in response to the delayed sets of data in frequency domain format associated with the center and right apertures, for providing a center/right amplitude calibration signal indicative of the amplitude difference in clutter spectrum between the center and right apertures in response to the delayed data sets in frequency domain format associated with the center and right apertures;

phase computation means (169) for providing a phase difference signal indictive of the phase difference between the delayed and undelayed sets of data in response to the delay signal;

phase calculation means (160) for providing a left/center phase correct ion signal (176) for the left/center interferometer indicative of a time delay phase correction, the positional phase relationship between the left and center apertures, and a phase due to motion of the targets in response to the phase addition of the phase difference signal and the left/center phase calibration signal, and for providing a center/right phase correction signal (178) for the center/right interferometer indicative of a time delay phase correction, the positional phase relationship between the center and right apertures, and the phase due to motion of the targets in response to the phase addition of the phase difference signal and the center/right phase calibration signal;

cancellation means for providing left/center clutter cancelled in-phase and quadrature component signals (214,

     216) that have significantly suppressed clutter characteristics in response to the left/center phase correction signal, the left/center amplitude calibration signal, the undelayed set of data in frequency domain format associated with the left aperture, and the delayed set of data in frequency domain format associated with the center aperture, and for providing center/right clutter cancelled in-phase and quadrature component signals (234,

     236) that have significantly suppressed clutter characteristics in response to the center/right phase correction signal, the center/right amplitude calibration signal, the undelayed set of data in frequency domain format associated with the center aperture and the delayed set of data in frequency domain format associated with the right aperture;

detection means (350) connected to the cancellation/enhancement means, for providing range (r_t), doppler (f_t), amplitude (a_t), and angle (θ

_t) signals indicative of target motion in response to the clutter cancelled left/center and center/right in-phase and quadrature signals.

3. An AMTI according to claim 2 wherein said interferometer means (158) includes:

means for providing the left/center phase calibration signal as a function of a linear regression on the undelayed sets of data in frequency domain format associated with the left and center apertures, and means for providing the center/right phase calibration signal as a function of a linear regression on the delayed sets of data in frequency domain format associated with the center and right apertures.

4. An AMTI according to claim 2 wherein said cancellation means (250) includes:

a first ROM (180) for providing signals (184,

     186) indicative of the sine and cosine of the left/center phase correction signal;

first scaling means (192) for providing first correction factor signals (200,

     212) indicative of the product of the left/center amplitude calibration signal and the signals indicative of the sine and cosine of the left/center phase correction signals;

first complex multiplication means (204) for providing compensated center signals (206,

     208) indictive of the delayed set of data associated with the center aperture scaled by the left/center amplitude calibration signal and rotated by the left/center phase correction signal;

first coherent adder means (210,

     212) for providing the left/center clutter cancelled in-phase and quadrature component signals in response to the subtraction of the compensated center signals from the undelayed set of data in frequency domain format associated with the left apertures;

a second ROM (182) for providing signals (188,

     190) indicative of the sine and cosine of the center/right phase correct ion signal;

second scaling means (194) for providing second correction factor signals (220,

     222) indicative of the product of the center/right amplitude calibration signal and the signals indicative of the sine and cosine of the left/center phase correction signals;

second complex multiplication means (224) for providing compensated right signals (226,

     228) indicative of the delayed set of data associated with the right aperture scaled by the center/right amplitude calibration signal and rotated by the center/right phase correction signal; and

second coherent adder means (230,

     232) for providing the center/right clutter cancelled in-phase and quadrature component signals in response to the subtraction of the compensated right signal from the undelayed set of data in frequency domain format associated with the center aperture.