Multipath effects arise when the signals transmitted from the satellites bounce off another object before getting to our receiver. Satellite geometry can also affect accuracy of GPS positioning. GDOP can magnify or lessen other GPS errors. In simple terms, the wider the angle between satellites you are using for position calculation, the better the measurement. The ultimate accuracy of GPS is determined by the sum of several sources of error.
Differential correction is required to reduce the error cause by atmospheric interference.
If we put a GPS receiver on the ground in a known location, we can use it to figure out exactly what errors the satellite data contain. It acts like a static reference point. It can then transmit an error correction message to any other GPS receivers that are in the local area, and they can use that error message to correct their position solutions. The DoD intends to maintain the GPS system for the foreseeable future, openly accessible, and without direct user charges. Trimble Navigation, Motorola, Magellan, Garmin International, Rockwell International, Magnavox, Corvallis Microtechnology and many others.