4 ms·
Minor clarification: the almanac data is generally valid enough for a long time. It's essentially a very, very low-precision version of the ephemeris, and is us
by zb 12y ago
Minor clarification: the almanac data is generally valid enough for a long time. It's essentially a very, very low-precision version of the ephemeris, and is used to give the receiver a hint on which satellites are likely to be in view, and therefore worth searching for. Each satellite broadcasts the almanac for the entire constellation, which is part of why it takes 13 minutes.
The ephemeris is the precise orbital data used to calculate position. Each satellite broadcasts only its own ephemeris, which takes up to about 50s. This is where AGPS can help - if you can get that data from some other channel then you don't have to wait.
- toomuchtodo 12y ago> The ephemeris is the precise orbital data used to calculate position. Each satellite broadcasts only its own ephemeris, which takes up to about 50s. This is where AGPS can help - if you can get that data from some other channel then you don't have to wait. Don't celltowers provide AGPS data to connected client devices (i.e. phones)? Those towers need the AGPS data for enhanced 911 cellular service.
- TickleSteve 12y agoThats what the AGPS system is, the Almanac and Ephemeris data is provided over a data channel from a server rather than directly from the satellites. The idea being that receiving the data over a dedicated data channel is much quicker than a low-power broadcast from a sat. This is often confused for a similar mechanism for triangulation of a position using the relative powers of multiple cell towers. This provides a low-resolution position fix suitable for use as the initial seed of the kalman filter (which narrows down the position iteratively).