More Background On SUPL, A-GPS, the Almanac and Ephemeris Data

After my previous posts on how to trace and analyze A-GPS SUPL requests (see here and here) I thought I'd also write a quick post with some references to more details on the parameters that are contained in an A-GPS SUPL message. When discussing GPS, two terms are regularly mentioned, The 'Almanac' and the 'Ephemeris data'. Here's a link to some background information on those terms and here's my abbreviated version:

Almanac: This information is broadcast by each GPS satellite and contains rough orbital parameters of each satellite. This information helps a GPS receiver to find other satellites during its startup procedure once it has decoded this information from a downlink signal. Note that this information is NOT contained in a SUPL response as it only gives long term rough orbital parameters that can only be used for satellite search but not for navigation.

Ephemeris: These are the precise orbital parameters of a satellite which is only valid for a short amount of time. While each satellite broadcasts the Almanac of all satellites, a satellite only broadcasts its own Ephemeris data. SUPL responses contain the Epehemeris data of all satellites the SUPL server thinks might be visible at the rough location a mobile device is currently located, perhaps, I'm not sure, in order of certainty, as the satellite IDs in the list were not ordered. I've also run a SUPL request with a cell-id that the SUPL server did not know and as a result the server returned a very long list of Ephemeris data with ordered satellite ID numbers, probably of all the satellites it knew.

And finally, here's a link to additional background information on the individual Epehemeris parameters.