Originally Posted by
ATC Watcher
Ian W : Ah the famous debate between GPS and baro altitude! Just like enbedded lateral off-set on FMS , those are wonderful ideas but full of hidden (good) reasons not to do it.
Heavily debated during the ICAO FANS meetings, , if my memory is correct ( long time ago !) main reason against was that WGS84 ( the GPS reference ) is a spheroidal while earth is irregular (a "potatoidal" was the name used) and therefore not suitable in some parts/areas of the world.
Another one was that , at the time , the GPS precision mode switch was still in the hands of the US military , and also in addition, many States would not accept to depend solely on it.
Indeed, but as you say that was a long time ago. Things have moved on significantly since then. The ellipsoid corrections to geoid are taken care of by software corrections for example.
(For the technical: The World Geodetic System 1984 (WGS84) datum surface is defined as an oblate spheroid or ellipsoid, with major transverse radius at the equator. The coordinate origin of WGS-84 is the center of mass of the Earth. This is being replaced in some applications by the Earth Gravitational Model of 1996. The EGM96 geoid varies from the WGS-84 ellipsoid by between +85 and -105 meters. This can be corrected by software with errors reduced to centimeters
see
Evaluation of EGM96 geoid model in the U.S.
The CARIB97 high resolution geoid height model for the Caribbean Sea
Tutorial: The Geoid and Receiver Measurements | Education | UNAVCO
)
We are also now in the position that
ATC surveillance is based on GPS (or rather satellite based positioning - includes GLONAS, Galileo etc.). As it is one of the little recognized aspects of the move to ADS-B and C reports for the basis of aircraft tracking that the Air Navigation Service Providers are now becoming reliant on GPS.