jetmech

ALTIMETER. If you have four satellites or more, ( exact SAT altitude constant ) and suitable configuration, it will give you an altitude above sea level. It can exhibit short term unstability of minor errors, maybe 3 times horizontal errors. Never a need to calibrate it for sea level air pressure. Newer units, 12 Channel, perform position averaging which stabilizes the reading and improves accuracy. Simply a matter of triangulation between the car and the given SAT's positions.

fenstermaker

I have a nav on a Honda...it is close but not really reliable...tells you when you go up a hill but could be 40 to 80 feet off...