GPS does not get pressure readings from the aircraft static system. Totally independant from the pitot/static system. GPS gets its altitude information from the galaxy of satellites.
Wrong. You have obviously never operated (or maintained) an airborne, approved GPS based navigation system in IFR conditions. Interface to an altitude source is a certification requirement for IFR GPS installations.
The source can be baro-corrected altitude or pressure altitude, but the latter requires baro-correction information to be input on cue prior to GPS approaches.
Excerpt from FAA Advisory Circular AC
-138 (AIRWORTHINESS APPROVAL OF
GLOBAL POSITIONING SYSTEM (GPS) NAVIGATION EQUIPMENT FOR
IFR SUPPLEMENTAL NAVIGATION SYSTEM):
Pressure/Barometric Altitude Inputs: An appropriate input of pressure and/or barometric altitude must be provided to the GPS equipment.
A minimum of three satellites must be in view to determine a two dimensional position, if altitude is known, and four are necessary to establish an accurate three dimensional position.
20-138 has recently been superceded by AC
20-138A (AIRWORTHINESS APPROVAL OF
GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) EQUIPMENT) which now states:
If the GNSS equipment requires barometric corrected (or pressure) altitude data for certain operations (identified in the GNSS installation instructions), the installation should provide an automatic altitude input from the air data system to the GNSS such that additional pilot actions are not required.
This leeway is afforded to accomodate any new technology (like WAAS
) that allows for greater accuracy indepent of altitude sensors. However, for now there aren't many WAAS
approaches around, so all installations, including those with WAAS
capability still have altitude interfaces.
If you fly all the way around the world at 42000' you only add 45.6 nautical miles to the journey at sea level.
I get 25 miles....
[Edited 2004-07-25 15:02:36]