If you've ever wondered why your GPS receiver sometimes takes longer 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 store it in memory.
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.
If you are interested in the more technical aspects, the GPS almanac also includes GPS clock calibration data, and data to help correct for distortion caused by the ionosphere.
