What Is a GPS Almanac?

If you've ever wondered why your GPS receiver sometimes takes a while to be ready to navigate after it's turned on, it's because the receiver must acquire some basic information in addition to capturing GPS satellite signals.

You may encounter a slow start if your GPS has been unused for days or weeks or has been transported a significant distance while it was turned off. In these cases, the GPS must update its almanac and ephemeris data and then store it in memory.

The total time it takes to gather this 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" and become usable 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.

The GPS almanac is a set of data that every GPS satellite transmits, and it includes information about the state (health) of the entire GPS satellite constellation and coarse data on every satellite's orbit. This information is known as the ephemeris data.

When a GPS receiver has current almanac data in memory, it can acquire satellite signals and determine initial position quickly.

The GPS almanac includes GPS clock calibration data and data to correct for distortion caused by the ionosphere.