On 20 Apr, 01:42, "Clive D. W. Feather" cl...@on-the-
train.demon.co.uk wrote:
As I understand it, the signals are not on a single frequency but rather
jump around a set of adjacent frequencies ("spread spectrum"). Each
satellite uses a different pattern of jumps repeating every millisecond,
so you can tell which satellite you're picking up by recognising the
pattern

I think how it works is the receiver has to jump between frequencies
on the same pattern as the satellite it wants to listen to. The
receiver therefore needs to know which pattern each satellite is
using, and it also needs a separate tuner for each satellite. Because
most have far fewer tuners than there are satellites (usually 12, vs
30ish) they need to know in advance which satellites are overhead.

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

U

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