Ship position and velocity using satellite ephemerides and radar range measurement of satellite

• US 9,459,344 B1
• Filed: 01/14/2011
• Issued: 10/04/2016
• Est. Priority Date: 01/14/2011
• Status: Active Grant

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Gray, Jack, Boardman, Jonathan A.
• Primary Examiner(s): Adams, Tashiana
• Assistant Examiner(s): Mull, Fred H
• Application Number:
  US13/007,213
• Time in Patent Office: 2,090 Days
• Field of Search: 342/126, 342/107, 342/109, 342/357.2, 342/357.31
• US Class Current: 1/1
• CPC Class Codes: G01S 13/06 Systems determining position data of a target
• CPC Class Codes: G01S 13/876 Combination of several spaced transponders or reflectors of known location for determining the position of a receiver G01S13/874 takes precedence
• CPC Class Codes: G01S 13/88 Radar or analogous systems specially adapted for specific applications electromagnetic prospecting or detecting of objects, e.g. near-field detection, G01V3/00

Abstract

There may be situations in which a ship at sea is lost and GPS is not available due to jamming, and neither a position fix nor GPS is available. A system and method allow estimation of ship position (SPOS) using only single radar range measurements and satellite ephemeris data. The same radar can determine ship velocity using radar range rate information.

Claims

1. A method for geolocation of a movable platform, said method comprising the steps of:

operating a radar system mounted on said movable platform;

measuring, with said radar system, a range from the movable platform to each of N satellites in Earth orbit, where N is greater than 2, and where N is the total number of range measurements made using the radar system;

providing a memory having ephemerides data for each of said N satellites;

obtaining from said memory on said movable platform, said ephemerides data for said satellites;

establishing an initial estimated position of said movable platform;

iteratively performing in a computer coupled to said memory and to said radar system, and using as input the N range measurements from the movable platform to the corresponding satellite, the steps of;

estimating a range to each of the N satellites based on the estimated position;

comparing the estimated ranges to the N range measurements obtained from the radar system;

setting up range equations which express the geolocation relative to said satellites; and

updating the estimated position of the movable platform by applying a least-squares solution based on the range equations, the N range measurements and the estimated ranges; and

determining the geolocation of the movable platform based on the estimated position in response to a determination that an iteration threshold has been reached.

2. A method according to claim 1, wherein said step of applying a least-squares solution includes the steps of:

receiving the estimated position; and

performing computer steps of;

loading said satellite ephemeris data from said memory on said movable platform,

computing a geometry matrix composed of a set of linearized range equations, and

solving for an incremental position estimate.

3. A method according to claim 1, wherein determining that the iteration threshold has been reached includes one of:

a) comparing the change in the estimated position with a threshold value; and

b) counting the total number of iterations and comparing the number to a threshold.

4. A method according to claim 1, further comprising the steps of:

operating said radar system mounted on said movable platform to determine a range rate of said N satellites in Earth orbit; and

using said computer, and from said range rates to each satellite, the velocity of each of said satellites as established by said ephemerides, and the lines of sight between said moving platform and each of said satellites, determining the velocity of the movable platform, relative to the Earth.

5. A method for geolocation of a movable platform, said method comprising the steps of:

operating a radar system mounted on said movable platform;

measuring, with said radar system, a range from the movable platform to each of N satellites in Earth orbit, where N is equal to 2, and where N is the total number of range measurements made using the radar system, wherein said movable platform is an earth-bound sea-based ship determined to be on an ellipsoid approximating the Earth'"'"'s surface;

providing a memory having ephemerides data for each of said N satellites;

obtaining from said memory on said movable platform, said ephemerides data for said satellites;

establishing an initial estimated position of said movable platform;

iteratively performing in a computer coupled to said memory and to said radar system, and using as input the N range measurements from the movable platform to the corresponding satellite, the steps of;

estimating a range to each of the N satellites based on the estimated position;

comparing the estimated ranges to the N range measurements obtained from the radar system;

setting up range equations which express the geolocation relative to said satellites; and

updating the estimated position of the movable platform by applying a least-squares solution based on the range equations, the N range measurements and the estimated ranges, wherein said step of applying a least-squares solution includes the steps of;

receiving the estimated position; and

performing computer steps of;

loading said satellite ephemeris data from said memory on said movable platform,

computing a geometry matrix composed of a set of linearized range equations,

computing a Jacobian matrix relating ship latitude and longitude to three-dimensional position, and

solving for an incremental position estimate; and

determining the geolocation of the movable platform based on the estimated position in response to a determination that an iteration threshold has been reached.

6. A method according to claim 5, wherein the computer step of forming the initial estimate of the position further comprises determining the initial estimated position of the earth-bound sea-based ship to be [0,0,0]^T in an Earth-Centered, Earth-Fixed (ECEF) coordinate system.

7. A method for geolocation of a ship platform, where the ship platform carries a radar system, a memory, and a computer, said method comprising the steps of:

using said radar system, measuring a range from the ship platform to each of N satellites in Earth orbit, where N is greater than two and where N is the total number of range measurements made using the radar;

storing on said memory, ephemerides data for each satellite of said N satellites;

loading into said computer from said memory coupled to said computer, the static ephemerides data for said satellites;

loading into said computer, an initial estimate as to the position of said ship platform, to define an estimated position;

iteratively performing the following steps using as input, the N range measurements;

using said computer, estimating ranges to each satellite based on the estimated position of said platform, to thereby generate calculated ranges;

comparing said N range measurements to said estimated ranges;

calculating a geometry matrix;

using said geometry matrix, making an estimate of the incremental position of said platform; and

updating the previous estimated position with the current estimate of the incremental position, in response to a determination that an iteration threshold has not been reached.

8. A method according to claim 7, further comprising the steps of:

using said radar system, measuring range rate to each satellite of said N Earth satellites; and

computing at least one of (a) latitude and longitude rate and (b) three-dimensional velocity of said ship platform relative to the Earth.

9. A method according to claim 7, wherein the initial estimate as to the position of said platform loaded into said computer is [0,0,0]^T in an Earth-Centered, Earth-Fixed (ECEF) coordinate system.

10. A system for geolocation of a movable platform, said system comprising:

a radar system mounted on said movable platform configured to measure a range from the movable platform to each of N satellites in Earth orbit, where N is greater than two, and where N is the total number of range measurements made using the radar system;

a memory storing ephemerides data for each satellite of said N satellites;

a source of an initial estimated position of said movable platform; and

a processor coupled to said radar system, to said initial source of the estimated position, and to said memory containing said ephemerides, said processor configured to iteratively perform, using as input, the measured ranges made using the radar, the steps of;

calculating a set of estimated ranges from the estimated position to each satellite of the N satellites, based on the ephemerides data for each satellite of the N satellites;

comparing the input set of measured ranges to the set of estimated ranges;

calculating range equations which express the geolocation of the movable platform based on the comparison; and

applying a least-squares solution based on the set of range equations to update the estimated position.

11. The system of claim 10, wherein said movable platform is an earth-bound sea-based ship, wherein the processor is further configured to receive the initial estimate of the position of said movable platform for determining geolocation of said ship, and wherein said initial estimate received is [0,0,0]^T in an Earth-Centered, Earth-Fixed (ECEF) coordinate system.

12. A system for determining the velocity of a movable platform, said system comprising:

a radar system mounted to said movable platform configured to measure a range from the movable platform to each of N satellites in Earth orbit, wherein N is greater than two and where N is the total number of range measurements and range rate measurements made by the radar system;

a memory storing ephemerides data for each satellite in the N satellites;

a source of an initial estimated position of said movable platform; and

a processor coupled to said radar system, to said source of the initial estimated position, and to said memory containing said ephemerides data of each satellite of said plurality of satellites, said processor configured to iteratively perform, using as input, the N range measurements, the steps of;

calculating range equations which express the range from estimated position of said platform relative to said satellites to determine calculated ranges,

comparing said calculated ranges to said N measured ranges, and

updating the estimated position by applying a least-squares solution based on the range equations, the estimated ranges and the N measured ranges, and

from said range rate, establish the velocity of said satellites by said ephemerides, and the lines of sight between said moving platform and said satellites, determining the velocity of said movable platform relative to the Earth.

13. The system of claim 12, wherein said movable platform is an earth-bound sea-based ship, wherein the processor is further configured to receive the initial estimate of the position of said movable platform for determining geolocation of said ship, and wherein said initial estimate received is [0,0,0]^T in an Earth-Centered, Earth-Fixed (ECEF) coordinate system.

14. A system for geolocation of a ship platform, said system comprising:

a radar system mounted on said ship platform for measuring a range from the ship platform to each of N satellites in Earth orbit, wherein N is greater than 2 and where N is the total number of range measurements made using the radar system;

a memory on said ship platform, having stored thereon, ephemerides data for each satellite of N satellites;

a source of an initial estimated position of said ship platform;

a computer coupled to said radar, to said memory storing said satellite ephemerides, and to said source of the initial estimated position, and configured to iteratively, using as input, the N range measurements, perform the steps of;

(a) calculating ranges to each satellite from said estimated position of said ship platform, to thereby generate estimated ranges,

(b) comparing said measured ranges to said estimated ranges,

(c) calculating a geometry matrix,

(d) using said geometry matrix, making a new estimate of position of said ship platform with the current estimate representing an incremental position of said ship platform.

15. A system according to claim 14, wherein said radar system is further configured to determine the range rate of each satellite of said N satellites in Earth orbit;

and

said computer is further configured to, from said range rates of each satellite in the N satellites, the velocities of each satellite of said N satellites as established by said ephemerides, and the lines of sight between said ship and each satellite of said N satellites, determine the velocity of said movable platform relative to the Earth.

16. The system of claim 14, wherein said movable platform is an earth-bound sea-based ship, and wherein said initial estimate of the position of said movable platform is [0,0,0]^T in an Earth-Centered, Earth-Fixed (ECEF) coordinate system.