Global Positioning System (GPS)

Almanac data is data that describes the orbital courses of the satellites. Every satellite will broadcast almanac data for each satellite. Almanac data includes a set of parameters for each GPS satellite that can be used to calculate its approximate location in orbit.

GPS receiver uses almanac data to predict which satellites are nearby when they’re looking for GPS signals. It can then determine which satellites it should track. Using almanac data saves time because the receiver can concentrate on those satellites it can see and forget about those that would over the horizon and out of view.

GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data does not need to be highly accurate to be useful. Therefore it is not precise and valid for many months.