What Is a GPS Almanac?

GPS Almanac Definition

Caucasian woman using GPS system in car
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If you've ever wondered why your GPS receiver sometimes takes a while to become ready to navigate after it's turned on, it is because it 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 turned off. In these cases, the GPS must update its almanac and ephemeris data and then store it in memory.

Older GPS hardware that doesn't have an almanac, take significantly longer to "boot up" and become usable because it has to do a lengthy satellite search. However, this process is much faster in newer hardware even if they lack an almanac.

The total time it takes to gather this GPS data is called TTFF, which means the Time to First Fix, and is usually around 12 minutes long.

What's Included in the GPS Almanac Data

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.

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

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

You can download almanac data from the ALM, AL3, and TXT file format from the United States Coast Guard's Navigation Center website.

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