GPS is entirely passive to the end user. Your receiver contains 1 to 12 receivers that are tracking the PN chipping sequences of the satellites known to be visible. If you do a "cold start" with no clue about your location or time, you can work through a series of PN choices until you lock one. Once the first one is locked, you can use it to download an almanac and set your time. You look at the location of the satellite that you successfully locked vs the almanac data, then try another PN sequence of a satellite that should be visible. You continue doing that until you have 3 or 4 satellites "locked". At that point, you can calculate a fix.
The PNs of the satellites currently launched are usually in the ROM of the receiver. You can update your GPS firmware with many of the higher quality units. This 'cache' of good PN prospects is only needed as a starting point. The almanac will contain the whole list of current satellites...including ones that are newer than your firmware.
Thanks you for all of your information...and I assure you that it won't be used wrongly upon others...low power and minimal R umbrella (10metersq).
Thanks again,
EBUCK