Monopulse motion compensation for a synthetic aperture radar

• US 4,134,113 A
• Filed: 04/18/1977
• Issued: 01/09/1979
• Est. Priority Date: 04/18/1977
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Corporation
• Original Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Inventors: Powell, Norman F.
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Goodwin, Lawrence
• Application Number:
  US05/788,208
• Time in Patent Office: 631 Days
• Field of Search: 343/5 CM, 343/16 M, 343/ 5 ST
• US Class Current: 342/25.00C
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/9089 SAR having an irregular aperture
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques

Abstract

A synthetic aperture radar in which radial motion compensation is provided by a monopulse null tracking loop which tracks the null position of a stabilized monopulse antenna, and in which tangential motion compensation is provided by a tangential velocity measurement loop which tracks the cross-over angle of the monopulse antenna pattern. Since the motion compensation for the synthetic aperture radar is made in relation to a stabilized monopulse antenna, the imaging process for the synthetic aperture radar may be controlled in response to the angular scan rate of the antenna.

Claims

1. Apparatus for compensating for the radial motion of a synthetic aperture radar which correlates the echo returns of a microwave signal propagated from a stabilized monopulse antenna to form an image of the reflecting surface, said apparatus comprising:

an offset generator for compensating for the phase shift in the echo returns caused by the doppler frequency imposed on the return signals by the radial motion of said synthetic aperture radar;

a prefilter for averaging the phase of a predetermined number of the echo returns contained within a given frequency bandwidth to provide prefilter beam segments; and

means for tracking the amplitude of the echo return signals in relation to the amplitude of the echo return signals at a predetermined angle of the monopulse antenna to provide a phase correction signal in relation to the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals to said offset generator for the correction of said phase errors.

2. The apparatus of claim 1 further comprising:

a rate stabilized monopulse antenna for providing received signals in a difference channel and also for providing received signals in a sum channel.

3. The apparatus of claim 2 in which the predetermined value of the return signal which is tracked by said tracking means is the difference-over-sum value of the null position of the monopulse antenna.

4. A synthetic aperture radar that focuses the echo signals from a signal reflector to form an image, said synthetic aperture radar comprising:

an offset generator for translating the doppler frequency of the echo signals at the null position of said antenna to a substantially zero doppler frequency in response to a monopulse null reference signal;

a prefilter for limiting the bandwidth of the output of said offset generator to substantially match the azimuth resolution of said synthetic aperture radar;

a monopulse null tracker for providing said monopulse null reference signal to said offset generator in response to the output of said prefilter;

a tangential velocity signal generator for determining the tangential velocity of the synthetic aperture radar in response to the frequency translated output of the offset generator; and

means for focusing the bandwidth limited output of said prefilter in response to the tangential velocity determined by said tangential velocity measurement generator.

5. A synthetic aperture radar system operative with a stabilized monopulse antenna, to provide a focused image from echo signals returned from a signal reflector, said radar system comprising:

an offset generator which compensates for radial motion of the synthetic aperture radar platform by translating the doppler frequency of the echo signals at the null position of said antenna to a substantially zero doppler frequency in response to a monopulse null reference signal;

a prefilter for limiting the bandwidth of the motion compensated output of said offset generator to substantially match the sampling frequency of the echo signals to the azimuth resolution of the synthetic aperture radar;

a monopulse null tracker which tracks the null position of the monopulse antenna to provide the monopulse null reference signal for said offset generator;

a tangential velocity signal generator for determining the tangential velocity of the synthetic aperture radar platform in response to the motion compensated doppler frequency at the cross-over angle of the monopulse antenna; and

means for focusing the bandwidth limited output of said prefilter in response to said tangential velocity signal generator and the angular rate of said monopulse antenna.

6. The apparatus of claim 5 in which said tangential velocity signal generator includes:

a multiplier for multiplying the motion compensated, bandwidth limited output of said prefilter by a cross-over translation signal to reference the prefilter output to the cross-over angle of the monopulse antenna;

a digital filter for determining the frequency of the output of said multiplier;

means for controlling the cross-over translation signal to said multiplier in response to the cross-over angle of said monopulse antenna; and

means for generating the tangential velocity in response to the doppler frequency of said digital filter.

7. The apparatus of claim 6 wherein said control means determines the cross-over angle of the monopulse antenna in response to the remainder of the difference signal of said digital filter from the sum signal of said digital filter.

8. The apparatus of claim 6 wherein said generating means generates the tangential velocity of the synthetic aperture radar platform in accordance with the relation:

##EQU13## where;

Δ

f_d is the doppler frequency of the motion compensated output of said prefilter at the cross-over angle of said monopulse antenna;

λ

is the wavelength of the microwave energy propagated from the monopulse antenna; and

θ

_c is the cross-over angle of the monopulse antenna.

9. The apparatus of claim 5 wherein said focusing means focuses the bandwidth limited output of the prefilter in accordance with the relation:

##EQU14## in which;

V_t is the tangential velocity of the synthetic aperture radar platform;

λ

is the wavelength of the microwave energy which is propagated from the monpulse antenna;

r is the range of the signal reflector from the monopulse antenna; and

θ

_c is the angular rate of the monopulse antenna.

10. A synthetic aperture radar that is responsive to sum and difference signals of a monopulse antenna to resolve an image of the antenna pattern reflector, said synthetic aperture radar comprising:

an offset generator for translating the doppler frequencies of the sum and difference signals corresponding to a selected angle of the antenna pattern to substantially zero doppler frequency in response to a monopulse null reference signal;

a prefilter that is responsive to the translated sum and difference signals offset generator, said prefilter limiting the bandwidth of the offset generator output to substantially match the azimuth resolution of said synthetic aperture radar; and

a monopulse null tracker that is responsive to the sum and difference signals of the prefilter, said monopulse null tracker generating the monopulse null reference signal for the offset generator with respect to the quotient of the difference signal corresponding to a selected angle of the monopulse antenna pattern divided by the sum signal corresponding to substantially the same angle.

11. Apparatus for focusing the image of a synthetic aperture radar that is responsive to sum and difference signals of a monopulse antenna to resolve an image of the antenna pattern reflector, said apparatus comprising:

means for compensating for the radial motion of the monopulse antenna with respect to the reflector;

means for prefiltering the radial motion compensated output of said compensating means to provide prefilter beam segments; and

means for focusing the prefilter beam segments of said prefiltering means in response to the tangential velocity and angular scan rate of the monopulse antenna, the wavelength of the antenna pattern, and the range to the reflector.

12. Apparatus for determining the tangential velocity of a stabilized monopulse antenna that provides sum and difference signals to a synthetic aperature radar to resolve an image of the antenna pattern reflector, said apparatus comprising:

means for compensating for the radial motion of the monopulse antenna to provide motion compensated sum and difference signals that correspond to a selected angle of the monopulse antenna pattern;

means for translating the doppler frequency of the motion compensated sum and difference signals to the cross-over angle of the monopulse antenna pattern, said translating means being responsive to the sum and difference signals of the compensating means to provide sum and difference signals corresponding to the cross-over angle of the antenna pattern; and

means for determining the tangential velocity of the monopulse antenna with respect to the doppler frequency of the sum and difference signals corresponding to the cross-over angle, said determining means being responsive to the cross-over angle sum and difference signals of said translating means to provide a tangential velocity signal.

13. The apparatus of claim 12 wherein said velocity determining means multiplies the doppler frequency of the cross-over sum and difference signals by a factor of λ

/θ

_c where;

λ

is the wavelength of microwave energy propagated from the monopulse antenna; and

θ

_c is the cross-over angle of the monopulse antenna.

14. A method for compensating for the radial motion of a synthetic aperture radar which correlates the echo returns of a microwave signal propagated from a stabilized monopulse antenna to form an image of the reflecting surface, said method comprising:

averaging the phase of a predetermined number of the echo returns contained within a given frequency bandwidth to provide prefilter beam segments;

tracking a predetermined amplitude of the echo return signals to determine the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals; and

compensating for the phase shift in the echo returns caused by the doppler frequency imposed on the return signals by the radial motion of said synthetic aperture radar in response to the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals.

15. A method for controlling the resolution of a synthetic aperture radar which correlates the echo returns of a microwave signal propagated from a stabilized monopulse antenna to form an image of the reflecting surface, said method comprising:

averaging the phase of a predetermined number of the echo returns contained within a given time-bandwidth product to provide prefilter beam segments;

tracking a predetermined amplitude of the echo return signals to determine the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo signal;

compensating for the phase shift in the echo returns cased by the doppler frequency imposed on the return signals by the radial motion of said synthetic aperture radar in response to the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals; and

controlling the resolution of the synthetic aperture radar by controlling the time-bandwidth products from which the prefilter beam segments are provided.

16. A method for focusing the image of a synthetic aperture radar which correlates the echo returns of a microwave signal propagated from a stabilized monopulse antenna to form an image of the reflecting surface, said method comprising:

averaging the phase of a predetermined number of the echo returns contained within a given frequency bandwidth to provide prefilter beam segments;

tracking a predetermined amplitude of the echo return signals to determine the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals;

compensating for the phase shift in the echo returns caused by the doppler frequency imposed on the return signals by the radial motion of said synthetic aperture radar in response to the angle between the direction of the prefilter beam segments and the direction of the predetermined value of the echo return signals; and

generating the tangential velocity of said synthetic aperture radar in response to the doppler frequency of the prefilter beam segments at a predetermined angle of the antenna pattern of the monopulse antenna.