[time-nuts] "geodetic" receivers for timing
doug at niceties.com
Tue Oct 25 13:32:28 EDT 2005
Maybe look back in the archives a few weeks for a message from me which did
mention dual-frequency timing receivers.
There are probably less than a half-dozen models of "geodetic" receivers
used for timing (Ashtech's Z-12T Metronome was a classic). The idea is that
the receiver runs from your external atomic frequency standard, and the raw
GPS observables (code & phase/carrier) can be post-processed along with some
other clock/data to obtain ongoing frequency difference. However, the
receiver needs some way to "select" the zero-crossing of your external
frequency which represents time. So you take your 1PPS and provide it to
the receiver. The receiver has some calibration equation relating that 1PPS
input and frequency input to the raw observable. Using that equation and
other delays such as your antenna/cables, your geodetic receiver can be used
for time (rather than just frequency). The 1PPS is typically only used at
power on and if the receiver ever resets (or if your frequency ever causes
slip at something like half-millisecond). If I had not supplied that 1PPS
to my receiver at power-on, the reference zero-crossing ends up at some
random value typically +/-500 microseconds (because geodetic receivers are
designed to all record raw observables at the appropriate millisecond).
FWIW, such setup typically removes the need for some continuously operating
external Time Interval Counter.
When you see a receiver which has a 1PPS input, you might think of that
receiver as a time "monitor" - it might have some feature or configuration
to also provide time, but that is not the typical/optimal use.
Two questions about GPS timing receivers:
1) Why most timing receivers have a 1pps input?
For what they use that input signal?
2) Are there some connections between timing receivers and geodetic
I'm looking forward an expert on Timing Receivers with whom to exchange
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