SAR/GPS inertial method of range measurement

• US 5,432,520 A
• Filed: 10/18/1993
• Issued: 07/11/1995
• Est. Priority Date: 10/18/1993
• Status: Expired due to Term

• Current Assignee: HE Holdings, Inc. (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Schneider, Arthur J., Olerich, Richard J.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): PHAN, DAO LINDA
• Application Number:
  US08/137,523
• Time in Patent Office: 631 Days
• Field of Search: 342/25, 342/357, 364/458, 364/444, 364/453
• US Class Current: 342/357.27
• CPC Class Codes: F41G 7/007 Preparatory measures taken before the launching of the guided missiles
• CPC Class Codes: F41G 7/2246 Active homing systems, i.e. comprising both a transmitter and a receiver
• CPC Class Codes: F41G 7/2286 using radio waves
• CPC Class Codes: F41G 7/346 using global navigation satellite systems, e.g. GPS, GALILEO, GLONASS
• CPC Class Codes: F41G 7/36 using inertial references
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 19/18 Military applications
• CPC Class Codes: G01S 19/49 whereby the further system is an inertial position system, e.g. loosely-coupled
• CPC Class Codes: G01S 19/51 Relative positioning
• CPC Class Codes: G01S 7/4004 of parts of a radar system

Abstract

A method that provides precise target location measurement using a synthetic aperture radar (SAR) system jointly with a global positioning inertial navigation system (GPS/INS) located on a moving aircraft. The SAR system provides a precise measurement of the round trip elapsed radar wave propagation time from the aircraft location to a selected pixel in a ground map that includes a portion of the target. The velocity of radar wave propagation is measured at the same time that the range time lapse to the designated SAR map pixel is measured. When combined with position and velocity information derived from the GPS/INS inertial navigation system, the radar wave propagation time is used to calculate the position of the target pixel in GPS/INS coordinates, thereby improving measurement accuracy. The method comprises flying an aircraft containing GPS/INS and SAR systems along a predetermined flight path. The relative position and velocity of the aircraft along the flight path is accurately measured using the the global positioning system. A first SAR map is generated. A target pixel in the first SAR map that corresponds to the target is then designated. A minimum of two additional SAR maps are generated and the target pixel in each of the additional SAR maps is designated. The position of the target pixel relative to GPS/INS coordinates and the velocity of radar wave propagation to the target using aircraft position data derived from the global positioning system are then simultaneously measured. Finally, a more accurate position for the target pixel is calculated using the computed value for the radar wave propagation velocity.

Claims

1. A method of more accurately determining range to a target from a moving aircraft containing a global positioning system (GPS/INS) inertial navigation system and a synthetic aperture radar (SAR) system, said method comprising the steps of:

flying the aircraft along a predetermined flight path;

accurately measure the relative position and velocity of the aircraft along the flight path using the the global positioning system with an inertial navigation system;

generating a first synthetic aperture radar (SAR) map;

designating a target pixel in the first SAR map that corresponds to a target;

generating a minimum of one additional SAR map and designating the target pixel in each of the additional SAR map;

simultaneously calculating the position of the target pixel relative to the position of the aircraft in GPS/INS coordinates and measuring the velocity of radar wave propagation to the target using aircraft position data derived from the global positioning system; and

computing a more accurate position for the target pixel using the computed value for the radar wave propagation velocity.

2. The method of claim 1 wherein the step of generating a minimum of two SAR maps comprises correlating the relative positions of the target pixels in each of the SAR maps.

3. The method of claim 1 wherein the step of calculating an accurate value for the radar wave propagation velocity at two successive positions uses the equations ##EQU3## R₁ =CT₁ /2 and R₂ =CT₂ /2,

where V₁ is the velocity of the aircraft at a first measurement position, V₂ is the velocity of the aircraft at a second measurement position, C is the velocity of radar wave propagation, f_o is the radar frequency, df is the change in frequency received by the SAR system from the target, L₁₂ is the distance moved by the aircraft between the first and second measurement positions, T₁ and T₂ are radar wave round trip travel times between the aircraft and the target at the first and second measurement positions.

4. A method of more accurately determining range to a target from a moving aircraft, said method comprising the steps of:

disposing a global positioning system (GPS/INS) inertial navigation system and a synthetic aperture radar (SAR) system on an aircraft;

flying the aircraft along a predetermined flight path;

accurately measuring the relative position and velocity of the aircraft along the flight path using the the global positioning system;

generating a first synthetic aperture radar (SAR) map;

designating a target pixel in the first SAR map that corresponds to a target;

generating a minimum of one additional SAR map and designating the target pixel in each of the additional SAR map;

simultaneously calculating the position of the target pixel relative to GPS/INS coordinates and measuring the velocity of radar wave propagation to the target using aircraft position data derived from the global positioning system; and

computing a more accurate position for the target pixel using the computed value for the radar wave propagation velocity.

5. The method of claim 4 wherein the step of generating a minimum of one additional SAR maps comprises correlating the relative positions of the target pixels in each of the SAR maps.

6. The method of claim 4 wherein the step of calculating an accurate value for the radar wave propagation velocity at two successive positions uses the equations ##EQU4## R₁ =CT₁ /2 and R₂ =CT₂ /2,

where V₁ is the velocity of the aircraft at the first measurement position, V₂ is the velocity of the aircraft at the second measurement position, C is the velocity of radar wave propagation, f_o is the radar frequency, df is the change in frequency received by the SAR system from the target, L₁₂ is the distance moved by the aircraft between the first and second measurement positions, T₁ and T₂ are radar wave round trip travel times between the aircraft and the target at the first and second measurement positions.