Hi Serge
Unfortunately I no longer have the 315 but have gone ( backwards ) to a
300. During the initialisation the receiver is collecting ephemeris data, a
bit like fine tuning itself to the satellite. But it's not the strength of
the signal itself that is important at this stage, it's more about the
timing. It's a few years now since I worked professionally with GPS
systems, but I remember that the data consists of the signals used to
derive position for most of the time, then at set intervals it transmits
data about itself which is used by the receiver to build up a picture of
the satellites available and their position. You could say that this allows
it to create a timetable of where the satellites are, and with it's initial
position roughly known it can then start to check the received data and
narrow down down it's location exactly. GPS is nothing more than an updated
method of triangulation as used by mariners with a telescope, but it needs
the empheris data to deduce this. It can take a lot longer than the
published 2 minutes or so, I've sometimes had to wait three hours or more.
A bit like trying to build up a timetable of buses by only standing at the
side of the road and watching, and l think that's why initialisation times
can also vary so much - you never get the right satellite when your waiting
for it, and then three come at once !
Regards
Phil
