Commercial civil dual-frequency receivers exist that do not need to
know the Y-code and that provide both L1 and L2 measurements and thus
removal of almost all of the ionospheric effect. Scan the GPS World
website for up-to-date information on GPS and GPS receivers.
-- Richard Langley
On 30-Sep-11, at 1:08 PM, Ian Batten wrote:
On 30 Sep 2011, at 1532, Peter Vince wrote:
If they were using stand-alone caesium clocks, then yes - gravity and
altitude would make big difference. But they locked their clocks
to a
single common-view GPS satellite - surely, then, they were both
ticking at the same rate, and in sync?
If you don't have access to the encrypted L2 frequency, what is the
lower bound on clock precision for two separated stations observing
some common-view satellites? I would have thought that propagation
in the ionosphere would introduce enough uncertainty to make 60ns
precision unlikely. It's difficult for the non-specialist to know
which errors were reduced by the removal of SA and which errors are
inherent to the technology, but one paper I found [1] says that use
of the ionospheric model that is transmitted on L1 isn't anything
like perfect:
Using the broadcast model under normal conditions removes about
half of the error (Fees and Stephens 1987) leaving a residual error
of around 60-90 nanoseconds during the day and 10 to 20 nanoseconds
at night (Knight and Rhoades 1987).
I presume that some of these errors can be corrected if you know
your location accurately, but what is the real state of the art?
ian
[1] Dana, P. H. Global Positioning System (GPS) time dissemination
for real-time applications. Real-Time Systems 12, 9-40 (1997).
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