1.  Field of the Invention
The present invention is directed to enhancements for incorporation into an electronic navigation device.  More particularly, the present invention is directed to an enhancement of the calibration and hence the accuracy of barometric altimeter measurements with the aid of derived altitudes from a global positioning system. 
2.  Description of the Related Art
In general, altitude measurements are made using two methods of measurement.  One method utilizes a barometric altimeter.  Barometric altimeters are devices that sense local atmospheric pressure and use a standard model of the atmosphere to convert this pressure measurement into altitude.  Altitude measurements are referenced to height above mean sea level (MSL). 
It is well known that local atmospheric pressure at a given altitude varies widely due to the effects of weather, solar heating, and other factors.  Thus, in order to provide an accurate altitude, barometric altimeters must be calibrated to correct for these variations.  The Global Positioning System (GPS) is a worldwide navigation system that can determine a user's position in horizontal and vertical dimensions.  However, GPS vertical measurements are currently all referenced to the WGS-84 ellipsoid, a purely mathematical construct that approximates the shape of the earth.  The GPS receiver must use a model that relates the height above the ellipsoid to the height above mean sea level. 
Further, it is well known that the vertical measurement of a GPS system is inherently less accurate than the horizontal measurements.  This is due to the fact that GPS satellites are constrained to be above the horizon for signal reception to occur.  This geometry is less that optimal for measuring the vertical component of a user's location simply because there can not be satellites visible below the user (an optimal configuration would have satellites above and below the user).  All GPS receivers are able to take into account satellite geometry (Dilution of Precision) and estimates of other satellite-related errors (URA) and provide a statistical estimate of the errors in the horizontal and vertical measurements. 
In practice, a barometric altimeter typically provides a more stable measurement of altitude than GPS over short time periods.  However, over long time periods, pressure variations can be of such magnitude that the barometric altimeter measurement of altitude is less accurate than the GPS measurement.  The pressure-indicated altitude of an uncalibrated barometric altimeter is typically in error by many tens of meters due to normal atmospheric pressure fluctuations, weather fronts and other sources.  However, this error is of a bias like nature—it is slowly varying with time—resulting in less accurate barometric altimeter readings over long time periods.  Accordingly, while an altitude determination derived from barometric pressure may be meaningfully accurate in a short time frame, over time, the accuracy of such a determination becomes undesirable.  Conversely, because GPS derived altitude suffers from different complementary errors, over a short time period (typically minutes time frame), GPS altitude measurements are subject to much larger variations than barometric altimeter measurements. 
In an attempt to overcome the foregoing, one proposal combines a GPS unit and a barometric pressure sensor in the same housing.  However, in that proposal the pressure sensor is used to augment GPS derived altitude information. 
In particular, McBurney et al.,  in U.S. Pat.  No.  6,055,477, disclose a method of combination or integration of measurements made using two systems to provide better availability or accuracy in altitude measurements by estimating a barometric bias using the difference in altitude obtained from the two sources.  The McBurney et al.  method however, fails to recognize that utilizing the difference between a GPS derived altitude and a barometric altimeter altitude, as a term in calibrating the barometer altimeter, will common mode out any dynamic changes due to movement of the barometric altimeter and the GPS in tandem.  As a result, the McBurney et al.  approach undesirably requires that the user not change altitude during calibration periods. 
Additionally, in the stated prior approach, an altimeter may only be calibrated using GPS derived altitude information when the user is stationary, often referred to as a “Calibration mode”.  The present invention makes no distinction between “calibration mode” and “navigation mode”, indeed the barometric error is constantly being estimated and used to calibrate the system.  Furthermore, the present invention provides a method to statistically determine the need for calibration which results in both the calibration and error estimation numbers being calculated and utilized without any user intervention, i.e. the user need not place the device in ‘calibration mode’ to obtain the required bias parameter for calibration computations. 
There exists a need for a method to take advantage of the long term stability of the GPS altitude measurement and the short term stability of the barometric altimeter measurement to produce an altitude measurement that is stable and accurate over long and short time periods.  Additionally, the need exists for a method that uses both GPS-derived altitude and barometric altimeter altitude to produce an altitude measurement that is more stable and accurate than either measurement taken alone.  The need also exists for an improved method to calibrate a barometric altimeter and to compute a barometric altitude correction quantity.  Particularly, the need exists for a method to have GPS and altimeter outputs to be self calibrating while the user is on the move.  The present invention fills the foregoing identified needs, and other needs, while overcoming the drawbacks of the prior art.