18.70 GNSS Global Navigation Satellite System

18.70.2 Explain the limitations of using GPS/GNSS to supplement normal visual navigation.

GPS works by measuring the amount of time taken for a signal from a satellite to reach the unit in the aircraft.

When it can lock onto 3 or more satellites the unit can calculate the aircrafts position down to less than 10 meters of accuracy

When the unit can lock onto 3 satellites, a 2 dimensional position fix is calculable.

When the unit can lock onto 4 or more a 3D position fix is calculable.

A 3D fix gives position and also height above the earth’s surface.

Under Visual Flight Rules the GPS should only be used as a supplement to visual navigation.

VFR is based on map navigation.

The pilot should always be aware of the aircrafts location in respect to the map.

This becomes especially important when navigating airspace, as airspace changes aren't always reflected in the maps stored in the GPS unit.