Originally Posted by
Jetstream67
Receivers start with relative time checks for received signals aided by any still current satellite almanac info they may have from a prior session or other available location or time info (e.g. time or Mobile data/ wifi derived location data on a phone). Any receivable satellite will give a quite accurate time signal and the receiver can then start to play with the subsequent signals to try to assemble a plausible set of 3D spheres from the relative delay of each time signal and assumptions about credible altitude /depth.
They start with the assumption that the receiver clock is GPS time even though it isn't. That is the basis of psuedo-ranging.
- The receiver unit begins by runnning the PRN code for a satellite and time shifting it until it achieves a lock-on, obtaining a psuedo-range in the process.
- After lock-on, the satellite clock error and orbital position data are received in the nav message.
- Psuedo-range from 3 satellite positions gives a false position due to receiver clock error, whilst 4 result in a mismatch that enables the unit to figure out the error, fix the position and update the receiver clock.
Each satellite also transmits a complete set of constellation almanac data so if a rough location is not not already solved the alamanc info becomes available after about a minute and tells the receiver where each 'visible' satellite should be in space at that moment.
With this info too the rough spheres solution becomes trivial and the receiver can now move to refining the location /altitude to a precision fix and maintain those updates in use.
I don't know where you are getting this information. It takes 12.5 minutes for the almanac to be updated because it is spread over 25 nav messages. When the unit starts, the existing almanac is good enough and the precise orbital data for the individual satellite is obtained within 30 seconds of lock-on.