On Fri, 17 Aug 2007 14:30:13 +0100, "George Dishman"
wrote:


"Henri Wilson" HW@.... wrote in message
.. .
On Mon, 13 Aug 2007 22:08:18 +0200, "Paul B. Andersen"

George agreed with me.

I said the carrier is used to improve the
resolution of the time difference measurements
which provide the pseudo-ranges. It's like using
the second hand to improve a measurement of time
on a clock beyond that given by the hour and
minute hands. Paul's description is fine, and in
a way your is too, the number of whole cycles of
the carrier is a measure of the time as a fraction
of the chip cycle time.

So to determine the exact time when the signal
received at an instant was sent, the receiver is
counting chips in the PRN-signal.

....or part of a chip, ie., phase difference.

Right, phase difference is a finer-grained measure
of time.

For the C/A channel, this gives a resolution of 0.98 us.

Assuming the orbit is known precisely.

Which it is by continuous monitoring.

Since all frequencies in the signal are derived from
the same frequency standard, it will be a specific number of carrier
cycles in each chip (1540 cycles of the L1 carrier in C/A PRN chip).
So to increase the time resolution, it is possible to count
the carrier cycles within a chip.
Since a carrier cycle is less than 0.1 ns, I think it
is more "counting the cycles" than it is actually
measuring the phase within a cycle.
(But counting cycles is sort of measuring the phase.)

Which all goes to prove that the 'GR correction' is totally unnecessary...

Nonsense. If all the clocks were synchronised but
they ran without the GR correction, the location
of a receiver would be determined just as
accurately but the use of GPS as a time reference
would be hopeless, it would drift by 38us a day.

George, there is already a huge discrepancy between GPS and UTC times. As long
as all the clock differences are known accurately they can easily be software
corrected out. That is what hapens.

George




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