Almanach

In GPS terms, the almanac is the predicted health and location of the satellites in their orbital planes. It is modulated on the L1 carrier phase of every satellite and is automatically received whenever you're tracking at least one satellite. It's used by the receivers to locate satellites when powered up. In the event that a receiver doesn't have an almanac, has an extremely old one, or is powered up in a very different location on earth than the last time, it can take a few minutes for the receiver to locate a satellite. However, as soon as at least one is located, the almanac will begin to refresh and the receiver's internal clock will be corrected. It takes about 30 seconds per satellite to download so about 16 minutes or so to totally refresh.

Prior to GPS the term referred to a yearly publication of tables predicting the location of the sun, moon, planets and prominent stars at specific Greenwich Mean Time (GMT).?ÿ The location is given as a predicted angle from the Greenwich Meridian and above or below the equator at a date and GMT.?ÿ Using standard surveying equipment and a clock set to Universal Coordinated Time (GMT) the observers latitude, longitude and astronomic North may be computed.?ÿ Current versions of these publications may be ordered from the U.S. Government Bookstore?ÿAstronomical Almanac For The Year 2021 or UKHO Nautical Publications Star Almanac for Land Surveyors 2020. The observation procedures have been part of basic surveying texts for many years.

Almanac and Ephemeris Data as used by GPS receivers (4 July 1998)

The satellites broadcast two type of data, Almanac and Ephemeris. Almanac data is course orbital parameters for all SVs. Each SV broadcasts Almanac data for ALL SVs. ThisAlmanac data is not very precise and is considered valid for up to 
several months. Ephemeris data by comparison is  very  precise orbital and clock correction for each SV and is necessary  for precise positioning. EACH SV broadcasts ONL its own Ephemeris data. This data is only considered valid for abou 30 minutes. The Ephemeris data is broadcast by each SV every 30 seconds.

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