EGNOS ‘Open Service’ available

EGNOS is the European Geostationary Navigation Overlay Service, which looks to be the European equivalent of WAAS, the Wide-Area Augmentation Service. Ground stations and geostationary satellites give corrections to GPS satellite position, timing and atmospheric time delay, which improves the positioning … if your receiver is compatible.

Satellite positioning and timing errors correspond pretty directly to positioning errors, so constant updating reduces errors. The atmospheric delay is a variable; if you have two frequencies you can figure out the ionosphere delay (it’s dispersive, so having two frequencies give two delays) but commercial GPS units only currently use 1 frequency; this will change with GPS III.