[ntp:questions] Re: Bought a GPS receiver--What next?

Richard B. Gilbert rgilbert88 at comcast.net
Tue Nov 2 05:28:04 UTC 2004

David Schwartz wrote:

>"Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message 
>news:hISdnS39t4qMdxvcRVn-pQ at comcast.com...
>>We had a long discussion on the topic here a few weeks ago.  The long
>>and the short of it is that Motorola should have written a much better
>>manual.  The chapter on how it all works leaves far too much room to
>>read between the lines.  The leading edge of the PPS signal marks the
>>exact "top of the second".  The receiver adjusts its measurement epoch
>>to keep it within one millisecond of the top of the second..  There are
>>facilities built into the Motorola command set to allow adjusting the
>>"top of the second" for cable delays and other systematic errors.
>    One millisecond is pretty poor PPS accuracy for a GPS receiver intended 
>for timing applications. One microsecond is typical.
>    DS
Evidently I don't write any better than Motorola did!!  The leading edge 
of the PPS is exact!  In the Oncore M12 it is specified to be within 50 
nanoseconds of the UTC second!

The measurement epoch is the instant for which the Oncore grabs the data 
to calculate the time.  The time that is reported via the serial port is 
the time as measured at the previous measurement epoch plus one 
second.   This allows the Oncore one second to do the necessary 
calculations.  The seconds value reported contains a fractional part 
such that the reported time is within one millisecond of the top of the 
UTC second.  It is analogous to a radio station announcer saying "at the 
tone the time will be. . .".   Read the Motorola "Oncore User's Guide", 
particularly Chapter 3 and Chapter 6.   Read carefully!  As I said, 
there is far too much room to read between the lines.

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