Terrain aided navigation using multi-channel monopulse radar imaging

• US 9,110,170 B1
• Filed: 08/29/2014
• Issued: 08/18/2015
• Est. Priority Date: 08/29/2014
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Woollard, Bruce R., Owens, Mark A.
• Primary Examiner(s): Mawari, Redhwan K
• Application Number:
  US14/473,483
• Time in Patent Office: 354 Days
• US Class Current: 1/1
• CPC Class Codes: G01C 21/00 Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 measuring distance traversed on the ground by a vehicle G01C22/00; control of position, course, altitude or attitude of vehicles G05D1/00; traffic control systems for road vehicles involving transmission of navigation instructions to the vehicle G08G1/0968
• CPC Class Codes: G01C 21/005 with correlation of navigation data from several sources, e.g. map or contour matching G01C21/30 takes precedence
• CPC Class Codes: G01S 13/5244 Adaptive clutter cancellation specially adapted for airborne MTI, G01S13/5242
• CPC Class Codes: G01S 13/86 Combinations of radar systems with non-radar systems, e.g. sonar, direction finder
• CPC Class Codes: G01S 13/89 for mapping or imaging
• CPC Class Codes: G01S 13/935 for terrain-avoidance

Abstract

A terrain aided navigation using multi-channel monopulse radar imaging to provide a navigation position update. The monopulse radar transmits a single RF pulse transmission or multiple quick RF pulse train bursts to generate a monopulse radar image that can be correlated with a digital terrain segment to provide navigation updates when requested. The radar has monopulse and off-axis capability that allows for selection of a terrain segment within the radar'"'"'s search area that will provide a good terrain correlation. The radar measurements are made on a range/Doppler cell-by-cell basis that includes angle information. The cells in the range/Doppler map corresponding to the antenna main beam return are converted into a high resolution (x,y,z) image and correlated to the selected terrain segment in the data base reference frame to provide an updated navigation position estimate.

Claims

1. A method for autonomous terrain aided navigation of an airborne vehicle, upon receipt of a request for a terrain navigation update performing the steps of:

determining a radar field-of-regard (FOR) search space about the vehicle'"'"'s flight path from a navigation position estimate and an off-axis capability of the vehicle'"'"'s multi-channel radar transceiver;

searching a terrain elevation database within the search space to select a terrain segment having prominent terrain features that exhibit at least a minimum variation in elevation;

computing an off-axis command angle for an antenna main beam to image the selected terrain segment;

transmitting at least one RF pulse burst in a 2-35 GHz band in the antenna main beam at the off-axis command angle to illuminate terrain encompassing the selected terrain segment;

simultaneously receiving and sampling the RF returns in at least three receive channels to produce a three-dimensional range/Doppler map in which each range/Doppler indexed cell includes a monopulse angle vector (x,y,z) and an intensity Q;

converting the three-dimensional range/Doppler map into a three-dimensional image in (x,y,z); and

correlating the three-dimensional image and the selected terrain segment in a reference frame of the database to provide a navigation position estimate of the airborne vehicle.

2. The method of claim 1, wherein the terrain segment is selected by convolving possible terrain segments with themselves and selecting a terrain segment that satisfies a specified criteria.

3. The method of claim 1, wherein the airborne vehicle has a current navigation position estimate with an error component bounded by an uncertainty region, wherein the terrain illuminated by the antenna main beam is oversized with respect to the selected terrain segment to account for the uncertainty region.

4. The method of claim 1, further comprising varying the width of the antenna main beam to illuminate terrain encompassing the selected terrain segment.

5. The method of claim 1, wherein converting the three-dimensional range/Doppler map into a three-dimensional image in (x,y,z) comprises:

using the range, Doppler and monopulse angle vector of the indexed cells and the off-axis command angle to identify a subset of indexed cells corresponding to the antenna main beam return; and

using the range and monopulse angle vector to map the subset of indexed cells into the three-dimensional image in (x,y,z).

6. The method of claim 5, further comprising:

using the Doppler measurement to refine the range and monopulse angle vector measurements.

7. The method of claim 1, wherein the three-dimensional image and the selected terrain segment are correlated by varying the navigation position estimate to find the best correlation in (x,y,z) space.

8. The method of claim 1, wherein the three-dimensional image and the selected terrain segment are correlated by:

providing an estimate of the vehicle altitude;

computing a slant range vector from the vehicle to a position of the antenna main beam on the ground;

finding the best correlation in (x,y) space that provides the position of the antenna main beam on the ground; and

adding the slant range vector to the position of the antenna main beam on the ground to update the navigation position estimate.

9. The method of claim 1, wherein said at least one RF pulse burst is only transmitted upon receipt of a request for the terrain navigation update.

10. A method for autonomous terrain aided navigation of an airborne vehicle,

providing an initial navigation position estimate of the vehicle;

providing inertial measurements of motion of the airborne vehicle;

updating the navigation position estimate in (x,y,z) of the airborne vehicle based on the inertial measurements, said position estimate having an error component bounded by an uncertainty region that increases with time based on accumulated errors of the inertial measurements;

issuing a request for the terrain navigation update when the uncertainty region reaches a certain threshold;

determining a radar field-of-regard (FOR) search space about the vehicle'"'"'s flight path from a navigation position estimate and an off-axis capability of the vehicle'"'"'s multi-channel radar transceiver;

searching a terrain elevation database within the search space to select a terrain segment having prominent terrain features that exhibit at least a minimum variation in elevation;

computing an off-axis command angle for an antenna main beam to image the selected terrain segment;

transmitting at least one RF pulse burst in a 2-35 GHz band in the antenna main beam at the off-axis command angle to illuminate terrain encompassing the selected terrain segment;

simultaneously receiving and sampling the RF returns in at least three receive channels to produce a three-dimensional range/Doppler map in which each range/Doppler indexed cell includes a monopulse angle vector (x,y,z) and an intensity Q;

converting the three-dimensional range/Doppler map into a three-dimensional image in (x,y,z); and

correlating the three-dimensional image and the selected terrain segment in a reference frame of the database to provide a navigation position estimate of the airborne vehicle and to reset its uncertainty region.

11. The method of claim 10, wherein converting the three-dimensional range/Doppler map into a three-dimensional image in (x,y,z) comprises:

using the range, Doppler and monopulse angle vector of the indexed cells and the off-axis command angle to identify a subset of indexed cells corresponding to the antenna main beam return; and

using the range and monopulse angle vector to map the subset of indexed cells into the three-dimensional image in (x,y,z).

12. The method of claim 11, further comprising:

using the Doppler measurement to refine the range and monopulse angle vector measurements.

13. The method of claim 10, wherein the three-dimensional image and the selected terrain segment are correlated by:

providing an estimate of the vehicle altitude;

computing a slant range vector from the vehicle to a position of the antenna main beam on the ground;

finding the best correlation in (x,y) space that provides the position of the antenna main beam on the ground; and

adding the slant range vector to the position of the antenna main beam on the ground to update the navigation position estimate.

14. A terrain aided navigated airborne vehicle, comprising:

a multi-channel radar configured to transmit at least one RF pulse burst in a 2-35 GHz band in an antenna main beam at a commanded angle and to simultaneously receive and sample the RF returns in the multiple channels;

a radar processor configured to process the sampled RF returns to produce a three-dimensional range/Doppler map in which each range/Doppler indexed cell includes a monopulse angle vector (x,y,z) and an intensity Q and to convert the range/Doppler map to an image in (x,y,z);

a terrain elevation database comprising elevation z of terrain at given locations (x,y) in a reference frame, said database formatted independently of a flight path of the airborne vehicle over the terrain;

an inertial measurement unit configured to provide inertial measurements of motion of the airborne vehicle;

a navigation processor configured to update a navigation position estimate in (x,y,z) space of the airborne vehicle based on the inertial measurements, said position estimate having an error component bounded by an uncertainty region that increases with time based on accumulated errors of the inertial measurements, to issue a request for a terrain navigation update when the uncertainty region reaches a certain threshold and to define a radar field-of-regard (FOR) search space about the vehicle'"'"'s flight path from the current navigation position estimate and an off-axis capability of the vehicle'"'"'s multi-channel radar transceiver;

a terrain selector configured to search the terrain elevation database within the search space to select a terrain segment having prominent terrain features that exhibit at least a minimum variation in elevation;

said navigation processor computing the commanded angle for the antenna main beam to image the selected terrain segment and passing the commanded angle to the multi-channel radar; and

a terrain correlator configured to correlate the three-dimensional image and the selected terrain segment in a reference frame of the database to provide a correlation offset to the navigation processor to update the navigation position estimate and reset its uncertainty region.

15. The terrain aided navigated airborne vehicle of claim 14, wherein the radar processor is configured to convert the three-dimensional range/Doppler map into a three-dimensional image in (x,y,z) by using the range, Doppler and monopulse angle vector of the indexed cells and the off-axis command angle to identify a subset of indexed cells corresponding to the antenna main beam return and to use the range and monopulse angle vector to map the subset of indexed cells into the three-dimensional image in (x,y,z).

16. The terrain aided navigated airborne vehicle of claim 15, wherein the radar processor is configured to use the Doppler measurement to refine the range and monopulse angle vector measurements.

17. The terrain aided navigated airborne vehicle of claim 14, wherein the terrain correlator is configured to correlate three-dimensional image and the selected terrain segment are correlated by:

providing an estimate of the vehicle altitude;

computing a slant range vector from the vehicle to a position of the antenna main beam on the ground; and

finding the best correlation in (x,y) space that provides the position of the antenna main beam on the ground,

wherein the navigation processor is configured to add the slant range vector to the position of the antenna main beam on the ground to update the navigation position estimate.

18. The terrain aided navigated airborne vehicle of claim 14, wherein the multi-channel radar comprises an active electronic steered array (AESA).