GPS and WGS84

28.10.2004

Submission:

We are committed to provide GPS non-precession approaches to the most widely used local airports in Costa Rica. However, the country has not adjusted its cartography to the WGS84 system. Can the approaches be legally developed prior to the implementation of the WGS84 system, by satellite determination of the co-ordinates (geocentric) and intervening obstacles, with an acceptable level of accuracy? Are known survey points on the ground required? Is there policy on the subject? Do you know of other states that have done it, without having to wait ages before the WGS84 is adopted?

Manuel E. Peralta

Answer or Commentary:

(I.W.

This is a difficult question. The obvious answer, at first glance is "yes" you need a WGS84 survey to fly GPS approaches. However the subject is slightly more involved. The onboard GPS system uses the WGS84 ellipsoid to calculate bearings and distances etc. so the co-ordinates entered into the GPS system must be in WGS84. It is possible to convert, using software, from one datum (i.e. your local datum) to WGS84 if the conversion factors are known. I would not recommend this practise, as quite often runway co-ordinates are slightly incorrect. One can guarantee that an ILS approach will put an aircraft down the runway immaterial as to the runway co-ordinates listed in the AIP. While a GPS approach will put an aircraft exactly at an entered latitude/longitude. That means that if the latitude/longitude values are even slightly incorrect the GPS system may direct you exactly into the ATC control tower, for example. That is one of the reasons that ICAO stipulate that a WGS84 survey of the runways be carried out.)