Use of an altitude sensor to augment availability of GPS location fixes

• US 6,055,477 A
• Filed: 07/03/1997
• Issued: 04/25/2000
• Est. Priority Date: 03/31/1995
• Status: Expired due to Fees

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: McBurney, Paul, Braisted, Paul
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): PIPALA, EDWARD J
• Application Number:
  US08/887,954
• Time in Patent Office: 1,027 Days
• Field of Search: 701/207, 701/208, 701/213, 701/300
• US Class Current: 701/207
• CPC Class Codes: G01C 21/20 Instruments for performing navigational calculations G01C21/24, G01C21/26 take precedence
• CPC Class Codes: G01C 5/00 Measuring height; Measuring distances transverse to line of sight; Levelling between separated points; Surveyors' levels G01C3/20, G01C3/30 take precedence
• CPC Class Codes: G01S 19/45 by combining measurements of signals from the satellite radio beacon positioning system with a supplementary measurement

Abstract

Methods for GPS-assisted determination of location coordinates of a mobile user or selected position on or adjacent to the Earth'"'"'s surface with improved accuracy. Elevation readings from an altimeter or barometer are integrated with readings of the GPS-determined elevation coordinate for that location, using Kalman filter techniques, minimum least square techniques, or comparison of certain statistically defined parameters associated with the altimeter and GPS variables, such as the standard deviation of the expected errors in these variables. The resulting elevation coordinate may be a statistical blend or filtered blend of the altimeter value and a GPS-determined value for the elevation coordinate; or this resulting elevation coordinate can be chosen to be one or the other of these values, based upon comparison of time varying statistical parameters corresponding to the altimeter and the GPS.

Claims

1. A method for determination of location coordinates of a selected position on or adjacent to the Earth'"'"'s surface with improved accuracy, the method comprising the steps of:

receiving GPS signals from N selected GPS satellites (N≧

3) and determining location coordinates (x_gps, y_gps, z_gps) of a selected location on or adjacent the Earth'"'"'s surface, where the coordinate z_gps (t) is an estimate of a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference location at a time t;

providing an altimeter reading z_alt (t) that is an estimate of the elevation coordinate A(t) of the selected location relative to the selected vertical reference location at a time t;

computing statistically determined estimates σ

_gps (t) and σ

_alt (t) of the standard deviations of the variables z_gps (t) and z_alt (t), respectively, for at least one selected time;

defining an estimate A_est (t) of the elevation coordinate A(t) of the selected position to be equal to z_gps (t) if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected first criterion, defining A_est (t) to be equal to z_alt (t) if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected second criterion, and defining A_est (t) to be equal to a value that lies between the value z_gps (t) and the value z_alt (t) if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy neither the first criterion nor the second criterion; and

calibrating the altimeter reading z_alt against an accurate value for z_alt at least once at a selected time t=t_cal subsequent to a time at which an altimeter reading is initially taken.

2. The method of claim 1, further comprising the step of selecting said first criterion to be σ

_gps (t)<

σ

_alt (t)+Δ

z₁, and selecting said second criterion to be σ

_gps (t)>

σ

_alt (t)+Δ

z₂, where Δ

z₁ and Δ

z₂ are selected real numbers that may be positive, negative or zero, with Δ

z₁ ≦

Δ

z₂.

3. The method of claim 1, further comprising the step of calibrating said altimeter at said selected time t=t_cal by setting said coordinate z_alt (t_cal)=z_gps (t=t_cal).

4. The method of claim 1, further comprising the step of calibrating said altimeter at said selected time t=t_cal by setting said coordinate z_alt (t_cal) equal to a trailhead value for said coordinate z_alt.

5. A method for determination of location coordinates of a selected position on or adjacent to the Earth'"'"'s surface with improved accuracy, the method comprising the steps of:

receiving GPS signals from N selected GPS satellites and determining location coordinates (x_gps, y_gps, z_gps) of a selected location on or adjacent the Earth'"'"'s surface, where the coordinate z_gps =z_gps (t) is an estimate of a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference location at a time t;

providing an altimeter having an altimeter reading z_alt (t) that is an estimate of the elevation coordinate A(t) of the selected location relative to the fixed vertical location at a time t;

computing statistically determined estimates σ

_gps (t) and σ

_alt (t) of the standard deviations of the variables z_gps (t) and z_alt (t), respectively, for at least one selected time t; and

defining the elevation coordinate A(t) of the selected location to be a linear combination of the values z_gps (t) and z_alt (t), defined by

space="preserve" listing-type="equation">z(t)=K.sub.1 z.sub.gps (t)+K.sub.2 z.sub.alt (t),

space="preserve" listing-type="equation">K.sub.1 =c.sub.1 (σ

.sub.gps (t)).sup.P /[c.sub.1 (σ

.sub.gps (t)).sup.P +c.sub.2 (σ

.sub.alt (t)).sup.q ],

space="preserve" listing-type="equation">K.sub.2 =c.sub.2 (σ

.sub.alt (t)).sup.q /[c.sub.1 (σ

.sub.gps (t)).sup.P +c.sub.2 (σ

.sub.alt (t)).sup.q ]=1-K.sub.1,

where c₁, c₂, p and q are selected positive constants.

6. A method for determination of location coordinates of a selected position on or adjacent to the Earth'"'"'s surface with improved accuracy, the method comprising the steps of:

receiving GPS signals from N selected GPS satellites and determining location coordinates (x_gps, y_gps, z_gps) of a selected location on or adjacent the Earth'"'"'s surface, where the coordinate z_gps =z_gps (t) represents a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference elevation at a time t;

providing an altimeter having an altimeter reading z_alt (t) that is an estimate of the elevation coordinate A(t) of the selected location at a time t;

computing statistically determined standard deviations σ

_gps (t) and σ

_alt (t) of the variables z_gps (t) and z_alt (t), respectively, for at least one selected time t;

estimating the elevation coordinate A(t) of the selected location by an estimated value A_est given by

space="preserve" listing-type="equation">A.sub.est (t)=w(t)z.sub.gps (t)+(1-w(t))z.sub.gps (t),

where w(t) is a weight function that tends monotonically toward the value 1 over a selected time interval, if the standard deviations σ

_gps (t) and σ

_alt (t) associated with the variables z_gps (t) and z_alt (t) satisfy a selected first criterion, and that tends monotonically toward the value 0 if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected second criterion; and

calibrating the altimeter reading z_alt against an accurate value for z_alt at least once at a selected time t=t_cal subsequent to a time at which an altimeter reading is initially taken.

7. The method of claim 6, further comprising the step of selecting said first criterion to be σ

_gps (t)<

σ

_alt (t)+Δ

z₁, and selecting said second criterion to be σ

_gps (t)>

σ

_alt (t)+Δ

z₂, where Δ

z₁ and Δ

z₂ are selected real numbers that may be positive, negative or zero, with Δ

z₁ ≦

Δ

z₂.

8. The method of claim 6, further comprising the step of calibrating said altimeter at said selected time t=t_cal by setting said coordinate z_alt (t_cal)=z_gps (t=t_cal).

9. The method of claim 6, further comprising the step of calibrating said altimeter at said selected time t=t_cal by setting said coordinate z_alt (t_cal) equal to a trailhead value for said coordinate z_alt.

10. The method for determination of location coordinates of a selected position on or adjacent to the Earth'"'"'s surface with improved accuracy, the method comprising the steps of:

receiving GPS signals from N selected GPS satellites and determining location coordinates (x_gps, y_gps, z_gps) of a selected location on or adjacent the Earth'"'"'s surface, where the coordinate z_gps =z_gps (t) is an estimate of a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference location at a time t;

providing an altimeter reading z_alt (t) that is an estimate of the elevation coordinate A(t) of the selected location relative to the fixed vertical location at a time t;

computing a statistically determined estimate σ

_gps (t) of the standard deviation of the variable z_gps (t) for at least one selected time by the following steps;

computing a vertical dilution of precision VDOP(t) and the satellite pseudorange ρ

_i (t) for each satellite number i in the satellite configuration used to determine the GPS location coordinates (x_gps, y_gps, z_gps);

forming a product max_i {VDOP(t)·

ρ

_i (t)}=ε

_V (t) representing a statistical error variable for the pseudorange variables; and

defining the standard deviation σ

_gps (t) to be the standard deviation σ

.sub.ε

V (t) for the error variable ε

_V (t);

computing a statistically determined estimate σ

_alt (t) of the standard deviations of the variable z_alt (t) for the selected time; and

interpreting an estimate A_est (t) of the elevation coordinate A(t) of the selected position to be equal to z_gps (t) if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected first criterion, and defining A_est (t) to be equal to z_alt (t) if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected second criterion.

11. The method of claim 10, further comprising the step of choosing said selected first criterion to be σ

_alt (t)<

ε

_V (t)+Δ

z₁ and choosing said selected second criterion to be σ

_alt (t)>

ε

_V (t)+Δ

z₂, where Δ

z₁ and Δ

z₂ are selected real numbers that can be positive, negative or zero, with Δ

z₁ ≦

Δ

z₂.

12. The method of claim 10, further comprising the step of choosing said selected first criterion to be σ

_alt (t)<

ε

_V (t)+Δ

z₁ and choosing said selected second criterion to be σ

_alt (t)>

ε

_V (t)+Δ

z₂, where Δ

z₁ and Δ

z₂ are selected real numbers that can be positive, negative or zero, with Δ

z₁ ≦

Δ

z₂.

13. A method for determination of location coordinates of a selected position on or adjacent to the Earth'"'"'s surface with improved accuracy, the method comprising the steps of:

receiving GPS signals from N selected GPS satellites and determining location coordinates (x_gps, y_gps, z_gps) of a selected location on or adjacent the Earth'"'"'s surface, where the coordinate z_gps =z_gps (t) represents a true elevation or vertical coordinate A(t) of the selected location relative to a selected vertical reference elevation at a time t;

providing an altimeter having an altimeter reading z_alt (t) that is an estimate of the elevation coordinate A(t) of the selected location at a time t;

computing a statistically determined estimate σ

_gps (t) of the standard deviation of the variable z_gps (t) for at least one selected time by the following steps;

computing a vertical dilution of precision VDOP(t) and the satellite pseudorange ρ

_i (t) for each satellite number i in the satellite configuration used to determine the GPS location coordinates (x_gps, y_gps, z_gps);

forming a product max_i {VDOP(t)·

ρ

_i (t)}=ε

_V (t) representing a statistical error variable for the pseudorange variables; and

defining the standard deviation σ

_gps (t) to be the standard deviation σ

.sub.ε

V (t) for the error variable ε

_V (t);

computing a statistically determined estimate σ

_alt (t) of the standard deviations of the variable z_alt (t) for the selected time t; and

estimating the elevation coordinate A(t) of the selected location by an estimated value A_est given by

space="preserve" listing-type="equation">A.sub.est (t)=w(t)z.sub.gps (t)+(1-w(t))z.sub.gps (t),

where w(t) is a weight function that tends monotonically toward the value 1 over a selected time interval, if the standard deviations σ

_gps (t) and σ

_alt (t) associated with the variables z_gps (t) and z_alt (t) satisfy a selected first criterion, and that tends monotonically toward the value 0 if the standard deviations σ

_gps (t) and σ

_alt (t) satisfy a selected second criterion.