Hi Larry
 I have several types of GPS boards, all of which have 1 pps outputs.
What is special about the Garmin and Oncore?
Pete

--- On Fri, 7/4/08, Larry Cochrane  wrote:

> From: Larry Cochrane 
> Subject: Re: Timing Accuracy Within WinSDR
> To: psn-l@..............
> Date: Friday, July 4, 2008, 4:02 PM
> Chris and Tom,
> 
> A/D data is time stamped at the digitizer board 
> (http://www.seismicnet.com/serialatod.html), when a GPS
> receiver is connected to it. 
> WinSDR does not use the computer's time when GPS time
> is used as a reference. If 
> everything is working correctly the accuracy should be
> under +-4 milliseconds, will 
> within one sample time at 100 samples per second. The GPS
> receiver connected to the 
> board must have a 1 pulse per second (PPS) signal, that is
> why the board only 
> supports the Garmin GPS 16/18 sensor and the now obsolete
> Motorola ONCORE receiver.
> 
> The firmware on the board maintains an accumulator,
> incremented by a 1 millisecond 
> interrupt, that contains the time of day (UTC time) down to
> the millisecond. This 1 
> ms interrupt also monitors the 1 PPS GPS signal. At the top
> of the second the 
> accumulator time is saved and sent to the host computer
> (running WinSDR) along with 
> the ADC data. It's up to the host computer to adjust
> the ADC time accumulator, by 
> sending commands to the board, to stay within a few
> milliseconds of UTC time. The 
> time of day from the GPS NMEA messages is also parsed and
> sent to the host computer 
> so it knows what second the pulse happened at.
> 
> Tom,
> 
> You should not be trying to use earthquakes to check your
> station timing! There are 
> just too many variables. The accuracy of the travel time
> tables being one of them. If 
> you suspect that your timing is not correct you should
> inject a signal at a known 
> time into the A/D board and viewing the results in
> WinQuake. One way of doing this is 
> to disturb the sensor at the top of the minute by listening
> to one of the WWV 
> stations and jumping next to the sensor right at the top of
> the minute mark. Another 
> way would be to connect a 1.5 volt battery to the input of
> the A/D channel, again 
> right at the top of the minute mark. With a little practice
> you should be able able 
> to get you test pulse to within +-100 ms of UTC time. While
> a test like this will not 
> give you millisecond accuracy it should be good enough to
> make sure you are within +- 
> 1/2 second of UTC time.
> 
> Regards,
> Larry Cochrane
> Redwood City, PSN
> 
> ChrisAtUpw@....... wrote:
> >        I am not sure how WinSDR actually does the
> timing, even when corrected 
> > by GPS. The display program will presumably use the
> software clock on your 
> > computer. Have you asked Larry? The software clocks
> fitted to PC computers are 
> > often crap. They can loose or gain 10s of minutes per
> day. 
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