Almanac and Ephemeris Data as used by GPS receivers (4 July 1998) The satellites broadcast two types of data, Almanac and Ephemeris. Almanac data is course orbital parameters for all SVs. Each SV broadcasts Almanac data for ALL SVs. This Almanac 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 ONLY its own Ephemeris data. This data is only considered valid for about 30 minutes. The Ephemeris data is broadcast by each SV every 30 seconds. When the GPS is initially turned on after being off for more than 30 minutes, it "looks" for SVs based on where it is based on the almanac and current time. With this information, appropriate SVs can be selected for initial search. When the GPS receiver initially locks onto a SV, the Garmin display then shows "hollow" signal strength bars. At this time, the Ephemeris data has yet to be completely collected. Once the ephemeris data is collected from EACH SV in turn, the associated signal strength bar will turn "solid" black and then the data from that SV is considered valid for navigation. If power is cycled on a GPS unit, and when turned back on, the Ephemeris data is less than 30 minutes old, lock-on will be very quick since the GPS does not have to collect new Ephemeris data. This is called a "warm" start. If it is later than 30 minutes, this is considered a "cold" start and all Ephemeris data will have to be recollected. If the GPS has moved more than a few hundred miles or accurate time is lost, the Almanac data will be invalid and if you are far enough off, none of the SVs that the Almanac thinks should be overhead will be there. In such case, the GPS will have to "sky search" or be reinitialized so it can download a new Almanac and start over. (Note: Yes! We know this is somewhat simplified information. Yes, we know that the Ephemeris data may not have to be updated as often as the G-12XL does it to get data to the G-12XL accuracy.) ================================================================ Joe Mehaffey