Almanac and ephemeris

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In order to determine your position, a GPS receiver must know two things. First, it must receive very accurate timing information from several GPS satellites. Second, it must know precisely where those satellites are.

Timing information is broadcast continuously, so that several measurements can be taken each second. In addition to timing information, each GPS satellite transmits "almanac" and "ephemeris" data.

Almanac data

The almanac tells the GPS receiver roughly where all the GPS satellite are. Each satellite transmits the full almanac every 12.5 minutes or so, so a GPS receiver which knows nothing at all can determine the rough location of the satellites in less than 15 minutes. Because it is so coarse, almanac data is considered valid for several months.

Ephemeris data

Once it's determined their rough location, a GPS receiver can listen for ephemeris data. Each satellite broadcasts this data, which tells the receiver its precise orbit, every 30 seconds. Because it is very precise, ephemeris data is only valid for 30 minutes or so.

Because of this, it's important to turn the FlySight on for 15 minutes at the start of each day. Although you could wait longer, it's good to develop a regular routine so that you won't forget. This ensures that the FlySight always has a valid almanac. In addition, the FlySight should be turned on for a minute or so while waiting for the plane. This ensures that the FlySight has reasonably accurate ephemeris data, which helps it get a fix quickly when you jump.