What Is a GPS Almanac?

Some GPS devices take time to locate themselves and begin transmitting location information. This is because the device must first acquire information about local satellites in the form of almanac and ephemeris data. While this information may be stored in the device's internal memory, updating almanac and ephemeris is critical for speedy and accurate location data.

The GPS almanac is a data set that every GPS satellite transmits. It includes information about the state of the entire GPS satellite constellation, as well as coarse data on every satellite in obit. The orbital position of each satellite is known as the ephemeris data. While the ephemeris data helps deliver more precise locations to GPS devices, the almanac data helps the device locate a region of eligible satellites. This information is then saved for faster signal location in the future.

The GPS almanac also includes clock calibration data as well as information for correcting distortions caused by changes in the ionosphere. When a GPS receiver has current almanac data in memory, it can acquire satellite signals and quickly determine an initial position.

The total time it takes to gather GPS data is called TTFF, which is an acronym for Time to First Fix. It can take as long as 12 minutes for a GPS receiver to acquire the necessary data to establish the first location fix.

Older GPS receivers without an almanac take significantly longer to boot up because the receiver has to do a lengthy satellite search. However, this process is much faster in newer hardware—even if it lacks an almanac.