Hi Pete,

I found that not all GPS receivers with 1PPS signal are created equally. Some only 
output a short 1 microsecond pulse at the top of the second. Since I'm polling the 
1PPS signal at 1 millisecond rate a short pulse will not work. The other thing I 
found with some receivers is the NMEA time message and the 1PPS signal are not locked 
together so it's hard to know what time the pulse happened at. The best receiver for 
timing was the Motorola ONCORE series, but they don't make them anymore.

The Garmin GPS 18 LVC sensor (http://www.seismicnet.com/gps/gps18.html) is nice 
because of it's size, about the size of a hockey puck. The device contains both an 
antenna and receiver and can be bought online for under $80.00.

Regards,
Larry Cochrane
Webtronics

Pete Rowe wrote:
> 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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> 
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