It seems complicated

It seems complicated to use 24 satellites orbiting at 12,600 - 22,000 miles, rising and setting each 12 hours, to find our geographic position. In fact, the solution is simple. Trilateration (the measurement of distance and location) is used to pinpoint our location. Imagine you are on a camping trip armed with compass and map. You want to find your location, so you take four bearings to map landmarks. The cross-bearing is your location. GPS works in a similar way. For GPS trilateration, we need two essential information pieces about each satellite: (1) Orbit position, and (2) Distance from satellite to our position. An almanac transmitted to a GPS receiver during regular operations contains the orbit position. Distance is calculated by multiplying the speed of light (186,000 miles per second) by the lapsed time required for a GPS signal to arrive from a satellite. Accurate satellite and receiver clocks provide timing. Once we have orbit and distance information on four satellites, we have a trilaterated position on the face of the earth. Three satellite solutions are satisfactory on the sea or in the air but four satellites are essential on land. GPS accuracy varies by receiver quality and correction method (described later). Survey grade receivers can provide five millimeter accuracy. The normal range is 5mm to 2cm. Resource grade receivers (used in GIS applications) provide submeter accuracy with a range of 10cm to 5m. A linear relationship exists between accuracy and cost. GPS receiver cost increases as accuracy improves.

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It seems complicated

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