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

DETAILED DESCRIPTION These needs are met by the invention, which combines a GPS with an altimeter or other location-indicating instrument and provides a procedure for combination or integration of measurements made using these two systems to provide better availability or accuracy of two-dimensional and/or three-dimensional location coordinates than with the GPS-determined location coordinates alone or the other instrument's location coordinate(s) alone.
The invention combines measurements of altitude or elevation with GPS-based determination of location to provide accurate location coordinates for a GPS receiver, when GPS signals received from a satellite are noisy or purposely compromised (e.
g.
, because of signal adjustment by Selective Availability, denoted "SA," imposed by the U.
S.
government), or when the GPS satellite signals received are insufficient in number to allow determination of the desired number of location coordinates.
Imposition of SA signal adjustment typically degrades GPS accuracy from the range 0.
1-10 meters to 50-200 meters.
An altimeter, barometer or other altitude sensor can provide altitude or elevation information that is accurate to within 10-20 meters, depending upon the time elapsed since the last calibration, the quality of the last calibration and the local rate of change of barometric pressure.
When only two location coordinates can be determined from the available GPS information, an altimeter will provide a more accurate determination of altitude or elevation and can provide an estimate of horizontal coordinates with improved accuracy for this location, if a local topographic map is also used.
Even where SA signal adjustment is not present, the relative accuracy of a pure GPS-based location determination may be insufficient, and an altimeter or other altitude sensor can often provide more accurate information on a single location coordinate.
This is especially true where GPS signals from at most three GPS satellites are available so that unambiguous three-dimensional location coordinates cannot be determined