Method and apparatus for improving the accuracy of position estimates in a satellite based navigation system

• US 5,390,124 A
• Filed: 12/01/1992
• Issued: 02/14/1995
• Est. Priority Date: 12/01/1992
• Status: Expired due to Fees

• Current Assignee: Caterpillar Incorporated
• Original Assignee: Caterpillar Incorporated
• Inventors: Kyrtsos, Christos T.
• Primary Examiner(s): Chin, Gary
• Application Number:
  US07/984,313
• Time in Patent Office: 805 Days
• Field of Search: 364/443, 364/449, 364/455, 364/454, 364/459, 73/178 R, 342/357, 342/358, 342/450, 342/451, 342/457, 342/463, 342/465
• US Class Current: 701/215
• CPC Class Codes: G01S 19/41 Differential correction, e.g. DGPS [differential GPS]
• CPC Class Codes: G05D 1/0272 comprising means for registering the travel distance, e.g. revolutions of wheels measuring distance traversed on the ground by vehicles, e.g. using odometers G01C22/00
• CPC Class Codes: G05D 1/0278 using satellite positioning signals, e.g. GPS
• CPC Class Codes: G05D 1/0282 generated in a local control room

Abstract

The accuracy of a vehicle position estimate generated using a satellite-based navigation system is improved by accounting for non-linear errors in the vehicle position computations. The standard navigation equation is modified to include the error coefficients α, β, γ and δ. α is used to model errors in the x dimension. β is used to model errors in the y dimension. γ is used to model errors in the z dimension. δ is used to model errors in the pseudoranges. The error coefficients may be computed using an open-ended GPS system or a differential GPS system. The error coefficients may be computed in real time or may be computed once and used for a period thereafter. Once computed, the error coefficients are factored into the computation of a vehicle position estimate for increased precision.

Claims

1. A method for determining the position of a first receiver at or near the surface of the Earth using a second receiver having a known position and using a satellite-based navigation system having a constellation of navigation satellites, the method comprising the steps of:

(a) receiving, from the constellation of navigation satellites, a first plurality of navigation signals at the first receiver;

(b) receiving, from the constellation of navigation satellites, a second plurality of navigation signals at the second receiver;

(c) computing a first error coefficient representing an error in an x direction from said second plurality of navigation signals and the known position of the second receiver, said first error coefficient representing an average error in the x direction for the constellation of navigation satellites;

(d) computing a second error coefficient representing an error in a y direction for the position of the second receiver from said second plurality of navigation signals and the known position of the second receiver, said second error coefficient representing an average error in the y direction for the constallation of navigation satellites;

(e) computing a third error coefficient representing an error in a z direction for the position of the second receiver from said second plurality of navigation signals and the known position of the second receiver, said third error coefficient representing an average error in the z direction for the constellation of navigation satellites;

(f) computing a fourth error coefficient representing a mean pseudorange error from said second plurality of navigation signals and the known position of the second receiver; and

(g) determining a precise position of the first receiver using said first plurality of navigation signals and said error coefficients.

2. The method of claim 1, wherein said steps (c), (d), (e) and (f) of computing are performed at the second receiver and said step (g) of computing is performed at the first receiver, said method further comprising the step between steps (f) and (g) of:

(h) transmitting said error coefficients from the second receiver to the first receiver.

3. The method of claim 2, wherein the constellation of navigation satellites includes four satellites.

4. The method of claim 1, wherein said steps (c), (d), (e), (f) and (g) of computing are performed at the first receiver, said method further comprising the steps between steps (b) and (c) of:

(h) computing, at the second receiver, a satellite position and a pseudorange for each satellite from said second plurality of navigation signals; and

(i) transmitting said satellite positions and said pseudoranges to the first receiver.

5. The method of claim 4, wherein the constellation of navigation satellites includes four satellites.

6. A method for computing the position of a receiver at or near the surface of the Earth using a satellite-based navigation system having a constellation of navigation satellites, the method comprising the steps of:

(a) selecting a first plurality of satellites from the constellation;

(b) computing, for each satellite in said first plurality of satellites, a first satellite position and a first pseudorange;

(c) computing a position estimate for the receiver from said first satellite positions and said first pseudoranges;

(d) selecting a second plurality of satellites from the constellation;

(e) computing, for each satellite in said second plurality of satellites, a second satellite position and a second pseudorange;

(f) computing, from said second pseudoranges, second satellite positions and said position estimate, a first error coefficient characterizing position error trends in an x direction of said position estimate, a second error coefficient characterizing position error trends in a y direction of said position estimate, a third error coefficient characterizing position error trends in a z direction of said position estimate, and a fourth error coefficient characterizing error trends in said second pseudoranges; and

(g) using said error coefficients to refine said position estimate for the receiver to produce a refined position estimate.

7. The method of claim 6, wherein said first plurality of satellites comprises first, second, third and fourth satellites and said second plurality of satellites comprises third, fourth, fifth and sixth satellites.

8. The method of claim 6, further comprising the steps of:

(i) computing a difference between said position estimate and said refined position estimate;

(j) comparing said difference to a predetermined threshold; and

(k) if said difference is greater than said predetermined threshold, substituting said refined position estimate for said position estimate and repeating steps (d)-(k).

9. A method for computing a position estimate for a receiver located at or near the surface of the Earth using a satellite-based navigation system having a constellation of navigation satellites, the method comprising the steps of:

(a) receiving, from each of at least four satellites, a first navigation signal at the receiver;

(b) computing, from said first navigation signals, a first pseudorange and a first satellite position for each of said at least four satellites;

(c) moving the receiver from a first position to a second position, wherein said second position is related to said first position by a velocity of the vehicle and a travel time between said first and second positions;

(d) receiving, from each of said at least four satellites, a second navigation signal at the receiver located at said second position;

(e) computing, from said second navigation signals, a second pseudorange and a second satellite position for each of said at least four satellites;

(f) selecting one of said first and second positions for computation of a receiver position estimate;

(g) computing a first error coefficient representing an error in an x direction for said receiver position estimate from said first and second pseudoranges, said first and second satellite positions, said vehicle velocity and said travel time;

(h) computing a second error coefficient representing an error in a y direction for said receiver position estimate from said first and second pseudoranges, said first and second satellite positions, said vehicle velocity and said travel time;

(i) computing a third error coefficient representing an error in a z direction for said receiver position estimate from said first and second pseudoranges, said first and second satellite positions, said vehicle velocity and said travel time; and

(j) computing a precise receiver position estimate for said selected position of the receiver using said first pseudoranges, said first satellite positions and said error coefficients.

10. The method of claim 9, further comprising the step before step (j) of:

(k) computing a fourth error coefficient representing a mean error in said first and second pseudoranges from said first and second pseudoranges, said first and second satellite positions, said vehicle velocity and said travel time.

11. An apparatus for determining the position of a vehicle at or near the surface of the Earth using signals from a satellite-based navigation system having a constellation of navigation satellites, comprising:

(a) a first receiver mounted on said vehicle to receive a first plurality of navigation signals from the constellation of navigation satellites;

(b) a second receiver located at a known position to receive a second plurality of navigation signals from the constellation of navigation satellites;

(c) first means, electrically connected to said second receiver, for computing, based on said second plurality of navigation signals and said known position, a first error coefficient representing an average error in an x direction for the constellation of navigation satellites, a second error coefficient representing an average error in a y direction for the constellation of navigation satellites, a third error coefficient representing an average error in a z direction for the constellation of navigation satellites, and a fourth error coefficient representing a mean pseudorange error for the constellation of navigation satellites; and

(d) second means, electrically connected to said first receiver, for receiving said error coefficients from said first means and for computing an accurate vehicle position based on said first plurality of navigation signals and said error coefficients.

12. An apparatus for determining a position of a vehicle at or near the surface of the Earth using navigation signals from a satellite-based navigation system having a constellation of navigation satellites, comprising:

(a) a receiver mounted on the vehicle to receive, from each of at least four satellites, a first navigation signal at a first position of said vehicle and a second navigation signal at a second position of said vehicle;

(b) means for determining, from said first navigation signals, a first pseudorange and a first satellite position for each of said at least four satellites and

for determining, from said second navigation signals, a second pseudorange and a second satellite position for each of said at least four satellites;

(c) means for determining a velocity and a travel time of said vehicle as it is moved from said first position to said second position; and

(d) means for computing, from said first and second pseudoranges, said first and second satellite positions, said vehicle velocity and said travel time, a vehicle position estimate,

a first error coefficient representing an error in an x direction of said vehicle position estimate, a second error coefficient representing an error in a y direction of said vehicle position estimate, a third error coefficient representing an error in a z direction of said vehicle position estimate and a fourth error coefficient representing a mean error in said first and second pseudoranges, wherein said error coefficients account for errors to enhance the accuracy of said vehicle position estimate.