Device and method for calibrating and improving the accuracy of barometric altimeters with GPS-derived altitudes

• US 6,522,298 B1
• Filed: 04/12/2001
• Issued: 02/18/2003
• Est. Priority Date: 04/12/2001
• Status: Expired due to Term

• Current Assignee: Garmin Limited
• Original Assignee: Garmin Limited
• Inventors: Olivier, Tracy, Burgett, Scott
• Primary Examiner(s): Phan, Dao
• Application Number:
  US09/833,318
• Time in Patent Office: 677 Days
• Field of Search: 342/462, 342/357.06, 342/357.12, 701/213, 701/215
• US Class Current: 342/462
• CPC Class Codes: G01C 21/20 Instruments for performing navigational calculations G01C21/24, G01C21/26 take precedence
• CPC Class Codes: G01C 25/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass testing, calibrating or compensating compasses G01C17/38
• CPC Class Codes: G01C 5/06 by using barometric means
• CPC Class Codes: G01S 19/14 specially adapted for specific applications
• CPC Class Codes: G01S 19/42 Determining position

Abstract

A portable, handheld electronic navigation device includes an altimeter and a GPS unit. An internal memory stores cartographic data, for displaying the cartographic data on a display of the navigation device. Accordingly, the device is capable of displaying cartographic data surrounding a location of the unit as determined by GPS and altitude information as determined by the barometric altimeter and GPS. The device provides an enhancement of the calibration and hence the accuracy of barometric altimeter measurements with the aid of derived altitudes from a GPS. The device is able to determine the need for calibration and perform the subsequent computations necessary to facilitate the calibration. Furthermore, the device is able to determine a correction quantity that should be applied to barometric altitude readings, thereby allowing the device to be calibrated while in motion. Both of these features ultimately result in a more accurate determination of altitude. In accordance with an aspect of the invention, the altimeter of the navigation device may be calibrated with altitude information entered by a user, with altitude information obtained from the cartographic, with altitude information derived from GPS or with any combinations thereof.

Claims

1. A navigation device comprising:

a barometric altimeter for providing a barometric based evaluation reading;

a GPS unit for providing a GPS based evaluation reading, wherein a difference between said barometric based elevation reading and said GPS based elevation reading is determined, and wherein said difference is used to calibrate said barometric altimeter;

wherein said difference is an average difference between said barometric based elevation reading and said GPS based elevation reading over a period of time; and

wherein said device has a memory having stored therein data indicative of an accuracy drift associated with said barometric altimeter over a period of time, and wherein said device determines whether for an elapsed time period, said uncertainty of the average difference between said barometric based altimeter and said GPS based altimeter is less, by a predetermined amount, than said accuracy drift.

2. The navigation device as set forth in claim 1, wherein when said uncertainty of the average difference is less by a predetermined amount than said accuracy drift, said device determines whether said elapsed time is greater than a determined value corresponding to a correlation time associated with GPS vertical errors.

3. The navigation device as set forth in claim 2, wherein when said elapsed time is greater than a determined value corresponding to a correlation time associated with GPS vertical errors, then said navigation device calibrates said barometric altimeter using said difference between said barometric based elevation reading and said GPS based elevation reading.

4. The navigation device as set forth in claim 3, wherein said device (a) computes a preliminary calibrated elevation for said altimeter based upon said barometric based elevation reading and said difference between said barometric based elevation reading and said GPS based elevation reading, (b) computes a base pressure value corresponding to said preliminary calibrated elevation and (c) computes a fully-calibrated elevation.

5. The navigation device as set forth in claim 4, wherein said device has a display, and wherein said fully calibrated elevation is displayed on said display.

6. The navigation device as set forth in claim 4, wherein, when said device takes an additional GPS based and barometric based elevation reading, said device uses said fully-calibrated elevation in computing a difference between said additional readings.

7. A method for determination of an uncertainty estimate for an error in barometric altitude (Δ

H_ave) comprising the steps of;

receiving GPS signals from a plurality of GPS satellites and computing a GPS altitude modeled by H_G(t)=H_T(t)+B_G(t)+C_G(t), where H_G(t)is the GPS altitude (in MSL), B_G(t) is a slowly varying bias term due to ionospheric errors, ephemeris errors, satellite clock errors and other factors, and C_G(t) is a zero mean correlated noise term of a much shorter time constant than either B_G(t) or B_B(t) a slowly varying bias like term relating to barometric pressure;

computing a barometric altitude reading modeled by H_B(t)=H_T(t)+B_B(t)+Q_B(t) where H_B(t) is the barometric pressure indicated altitude, H_T(t) is the true altitude in MSL, B_B(t) is a slowly varying bias like term, and Q_B(t) is a zero mean Gaussian noise term of variance σ

_Q²;

determining the difference between barometric and GPS derived altitude Δ

H(t) as a function of time;

continuously estimating the average altitude difference Δ

H_ave over multiple samples by recursively averaging Δ

H(t) over many samples according to the equation $\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>H_{\mathrm{ave}}<br><br>\left(k\right)=\frac{k-1}{k}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>H_{\mathrm{ave}}<br><br>\left(k-1\right)+\frac{1}{k}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>H<br><br>\left(t_n\right);<br><br>$

 

estimating the uncertainty of the GPS vertical Channel σ

_V,ave(k) by recursively computing over the estimation interval, the uncertainty of the GPS Channel as a function of time according to the equation ${\mathrm{\sigma <br><br>}}_{V,\mathrm{ave}}<br><br>\left(k\right)=\frac{k-1}{k}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\sigma <br><br>}}_{V,\mathrm{ave}}<br><br>\left(k-1\right)+\frac{1}{k}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\sigma <br><br>}}_{V,\mathrm{GPS}}<br><br>\left(t_n\right);<br><br>\mathrm{and}$

computing the root sum square of uncertainty reduction in σ

_Q and the uncertainty reduction to σ

_V,GPS to yield the uncertainty of the average estimate according to the equation ${\mathrm{\sigma <br><br>}}_{\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>H_{\mathrm{ave}}}<br><br>\left(k\right)={\left(\frac{{\mathrm{\sigma <br><br>}}_Q^2}{k}+\frac{{\mathrm{\sigma <br><br>}}_{V,\mathrm{GPS}}^2}{1+\frac{k*\mathrm{\Delta <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t}{{\mathrm{\tau <br><br>}}_C}}\right)}^{1/2}.$

8. A method for the calibration of a Barometric altimeter comprising the steps of:

evaluating calibration decision criteria for the need to calibrate the barometric altimeter based on the equation σ

<sub>Δ

Have</sub>(k)<

β

*σ

_Baro(t_n−

t_cal), where β

is any non-negative constant and σ

_Baro(t_n−

t_cal) is an estimate of the uncertainty to the barometric altimeter, as a function of the elapsed time since the last calibration and σ

<sub>Δ

H,ave</sub> is the uncertainty of the average difference between barometric altitude and GPS derived altitude;

constraining the averaging period to be some multiple α

of the correlation time of the signal noise term C_G(t) in the following manner k*Δ

t>

α

*τ

_C;

computing a base pressure P_B,cal and a local pressure altitude H_B wherein, $P_{B,\mathrm{cal}}=\frac{P_L}{{\left[\frac{H_{B,\mathrm{cal}}*L}{T_O}+1\right]}^{\frac{-g}{\mathrm{RL}}}};<br><br>$

defining the barometric uncertainty error as the uncertainty estimate for the average difference in altitude values between the barometric altimeter and GPS i.e. σ

_Baro=σ

<sub>Δ

H,ave</sub>; and

calibrating the barometric altimeter measurement H_B using an atmospheric model relating base pressure P_B,cal to altitude in the equation $H_B=\frac{T_O}{L}<br><br>\text{\hspace{1em}<br><br>}\left[{\left(\frac{P_L}{P_{B,\mathrm{cal}}}\right)}^{\frac{-\mathrm{RL}}{g}}-1\right].$

9. A method of calibrating a barometric altimeter, said method comprising the steps of:

obtaining a barometric altitude reading H_B(t) as a function of time using a barometric model;

calculating a GPS signal based altitude H_G(t) as a function of time using a GPS model;

computing a differential altitude Δ

H(t), between the GPS based altitude and the barometric altimeter based altitude, as a function of time; and

calibrating the barometric altimeter by adding or subtracting, from the barometric altimeter reading, the differential altitude.

10. The method as recited in claim 9 wherein, a decision to calibrate is first determined based on a decision statistic comprising the steps of:

computing an uncertainty factor σ

_Baro as a function of elapsed time since the last calibration;

computing a product of the uncertainty factor and a non-negative constant β

; and

comparing said product with the uncertainty associated with an averaging period for the altitude differential.

11. The method as recited in claim 9, further comprising the steps of:

computing a base pressure P_B,cal at the time of calibration;

computing a barometric altitude H_B using said base pressure;

re-initializing recursive estimations; and

continuously estimating an average altitude differential and recursively estimating the uncertainty of the average altitude differential.

12. A method of calibrating a barometric altimeter, said method comprising the steps of:

obtaining a barometric altitude reading H_B(t) as a function of time using a barometric model;

obtaining a barometric altitude reading H_B(t) as a function of time;

calculating a correction term U(t_k) using a state feedback filter; and

calibrating a barometric altimeter by adding said correction term to the barometric altitude reading.

13. A method as recited in claim 12, further comprising the steps of:

computing a calibrated barometric altimeter reading H_B,cal;

computing a base pressure P_B,cal at the time of calibration using said calibrated barometric altitude; and

computing a barometric altitude using said base pressure in a Standard Atmosphere Model.