Moving target imaging synthetic aperture radar

• US 5,122,803 A
• Filed: 11/06/1991
• Issued: 06/16/1992
• Est. Priority Date: 11/06/1991
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Army (Government of the United States of America)
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Alexander, Peter, Stann, Barry L.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US07/788,699
• Time in Patent Office: 223 Days
• Field of Search: 342/25, 342/196
• US Class Current: 342/25.00B
• CPC Class Codes: G01S 13/87 Combinations of radar systems, e.g. primary radar and secondary 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

Abstract

A method and apparatus of imaging moving targets with an aircraft mounted complex radar system has a plurality of independent, but synchronized synthetic aperture radars (SARs) positioned on the aircraft at equal separation distance Δx along the flight velocity vector V_p of the aircraft. Frequency modulated (or otherwise coded) pulses are transmitted therefrom with an interpulse period T_p, where 1/T_p is an integral multiple of V_p /Δx. The pulse repetition frequency, platform velocity, and spacing between adjacent SARs are all chosen to create the effect of a stationary radar momentarily fixed in space. A two dimensional complex IF (intermediate frequency) output signal is recovered by the first SAR on the aircraft. This signal is identical to that recovered by a conventional SAR. The two dimensions are fast time and downtract position. If only one point target is present, the IF output signal is the point target'"'"'s phase history. Typically, many point targets are simultaneously present, and the IF output signal is the sum of point target phase histories. Each additional SAR on the platform recovers a different two dimensional complex IF output signal. These signals are sequentially stacked to form a three dimensional complex data set. The stacking dimension is called subaperture time, and is unique to this invention. A two dimensional cut through the three dimensional complex data set and normal to the downtract position dimension, say at downtrack position x, contains the data which would be collected by a stationary radar at downtrack position x. This radar would transmit a total of M frequency modulated (or otherwise coded) pulses, where M is the number of SARs on the aircraft, at a rate of one pulse every Δx/V_p seconds, as it observes all targets. For each target present, a slow doppler fluctuation, whose frequency is directly proportional to target slow relative velocity, appears along the subaperture time axis. Target slow relative velocity is the target velocity with respect to the fixed radar in space. Subaperture time and slow relative velocity are a Fourier transform pair. Consequently, targets, in terms of phase history

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured, used and licensed by or for the United States Government for Governmental purposes without payment to us of any royalty thereon.

Claims

1. A method of imaging moving targets with an aircraft mounted radar system having a plurality of synthetic aperture radars (SARs) comprising the steps of:

positioning said plurality of SARs at equal separation distance Δ

x along the flight velocity vector V_p of the aircraft,

transmitting a plurality of modulated pulses from said radar system periodically, with period T_p, where 1/T_p is an integral multiple of V_p /Δ

x,

generating a two dimensional complex IF output signal comprising fast time and downtrack position dimensions from return pulses received at a leading first SAR,

generating additional two dimensional complex IF output signals, corresponding to each successive SAR, trailing said first SAR,

sequentially stacking said two dimensional complex IF output signals to form a three dimensional complex data set or IF output space, whereas subaperture time is a third dimension, according substantially to the following criteria, ##EQU32## wherein;

x is aircraft downtrack position (m),

t'"'"' is adjusted fast time (secs),

t_s is subaperture time (secs),

N is number of moving targets present,

f_d.sbsb.i is ith moving target fast doppler frequency,

f_r.sbsb.i is ith moving target range fluctuation frequency,

f_s.sbsb.i is ith moving target slow doppler frequency,

T is transmit signal pulse width (secs),

T_s is subaperture time record length (secs), ##EQU33## is ith moving target fast doppler fluctuation, ##EQU34## is ith moving target range fluctuation, ##EQU35## is ith moving target slow doppler fluctuation, ##EQU36## is ith moving target phase history, whereby subaperture time, as an additional dimension in conjunction with downtrack position and adjusted fast time, provides a means to sort moving target phase histories by slow relative velocity.

2. The method of imaging moving targets as claimed in claim 1 further comprising the step of:

taking a Fourier transform of IF output space with respect to subaperture time thereby sorting moving target phase histories by slow relative velocity.

3. The method of imaging moving targets as claimed in claim 2 further comprising the step of:

processing said slow relative velocity sorted moving target phase histories to produce spatially and velocity resolved images.

4. The method of imaging moving targets as claimed in claim 1 wherein said plurality of SARs are independent and synchronized.

5. The method of imaging moving targets as claimed in claim 1 wherein said plurality of modulated pulses are transmitted simultaneously with said period T_p.

6. The method of imaging moving targets as claimed in claim 1 wherein said plurality of SARs are independent and synchronized, and said plurality of modulated pulses are transmitted simultaneously and periodically with said period T_p.

7. The method of imaging moving targets as claimed in claim 1 wherein said plurality of SARs are synchronized, and said plurality of modulated pulses, which are transmitted simultaneously and periodically with said period T_p, are in addition bi-phase coded in such a way that the bi-phase code in the downtrack position dimension on an unlike return is not correlated with the bi-phase code in the downtrack position dimension on a like return, and further includes the steps of IF processor bi-phase decoding and downtrack position averaging in order to simulate that a return pulse will produce an IF pulse only at the output of the SAR from which it was transmitted.

8. The method of imaging moving targets as claimed in claim 1 wherein transmitting a plurality of modulated pulses with said period T_p occurs at a single transmit antenna, and further includes the steps of receiving return pulses at a plurality of receive antennas, converting said return pulses into IF output pulses, and applying phase corrections to said IF output pulses in order that they substantially approximate IF output pulses from a plurality of independent SARs.

9. A method of imaging moving targets with an aircraft mounted radar system having a plurality of synthetic aperture radars (SARs) comprising the steps of:

positioning said plurality of SARs at equal separation distance Δ

x along the flight velocity vector V_p of the aircraft,

transmitting a plurality of modulated pulses from said radar system periodically, with period T_p, where 1/T_p is an integral multiple of V_p /Δ

x,

generating a two dimensional complex IF output signal comprising fast time and downtrack position dimensions from return pulses received at a leading first SAR,

generating additional two dimensional complex IF output signals, corresponding to each successive SAR, trailing said first SAR,

sequentially stacking said two dimensional complex IF output signals to form a three dimensional complex data set or IF output space, whereas subaperture time is a third dimension, according substantially to the following criteria, ##EQU37## wherein;

x is aircraft downtrack position (m),

t'"'"' is adjusted fast time (secs),

t_s is subaperture time (secs),

N is number of moving targets present,

f_d.sbsb.i is ith moving target fast doppler frequency,

f_r.sbsb.i is ith moving target range fluctuation frequency,

f_s.sbsb.i is ith moving target slow doppler frequency,

T is transmit signal pulse width (secs),

T_s is subaperture time record length (secs), ##EQU38## is ith moving target fast doppler fluctuation, ##EQU39## is ith moving target range fluctuation, ##EQU40## is ith moving target slow doppler fluctuation, ##EQU41## is ith moving target phase history, taking a Fourier transform of IF output space with respect to subaperture time thereby sorting moving target phase histories by slow relative velocity,

processing said slow relative velocity sorted moving target phase histories using matched filtering techniques adjusted for both target crosstrack and downtrack velocity components to produce spatially and velocity resolved images.

10. The method of imaging moving targets as claimed in claim 9 wherein said plurality of SARs are independent and synchronized.

11. The method of imaging moving targets as claimed in claim 9 wherein said plurality of modulated pulses are transmitted simultaneously with said period T_p.

12. The method of imaging moving targets as claimed in claim 9 wherein said plurality of SARs are independent and synchronized, and said plurality of modulated pulses are transmitted simultaneously and periodically with said period T_p.

13. The method of imaging moving targets as claimed in claim 9 wherein said plurality of SARs are synchronized, and said plurality of modulated pulses, which are transmitted simultaneously and periodically with said period T_p, are in addition bi-phase coded in such a way that the bi-phase code in the downtrack position dimension on an unlike return is not correlated with the bi-phase code in the downtrack position dimension on a like return, and further includes the steps of IF processor bi-phase decoding and downtrack position averaging in order to simulate that a return pulse will produce an IF pulse only at the output of the SAR from which it was transmitted.

14. The method of imaging moving targets as claimed in claim 9 wherein transmitting a plurality of modulated pulses with said period T_p occurs at a single transmit antenna, and further includes the steps of receiving return pulses at a plurality of receive antennas, converting said return pulses into IF output pulses, and applying phase corrections to said IF output pulses in order that they substantially approximate IF output pulses from a plurality of independent SARs.