Re: Date/Time accuracy (was Re: First TOM1 image under Rob's Software)

[James Hannon <jmhannon@fmtcs.com>]

I am not sure about the complete GPS units. I have a couple of Motorola GPS engines (guts of a handheld GPS) that I will use for timing a seismic station. They have the 1PPS and NMEA outputs. This is what I have http://www.seismicnet.com/gps/index.html.  Less expensive than a complete GPS.

Jim Hannon

---------- Original Message ----------------------------------
From: Chris Albertson <chrisalbertson90278@yahoo.com>
Date:  Wed, 16 Feb 2005 13:53:48 -0800 (PST)

>
>
>You are right.  These source are better used as frequency 
>standrads.
>
>Do any of the low cost comsummer GPSes offer the
>pulse per second output?  The two units that I own only send out
>NMEA data sreams which are really just 1200 baud serial
>ASCII text with <cr><lf> end of line convention.
>
>
>
>--- James  Hannon <jmhannon@fmtcs.com> wrote:
>
>> Many GPS receivers have a 1 pulse per second output that is aligned
>> with time to a few nanoseconds. by using this and the once per second
>> NMEA data you can get very accurate time. 
>> 
>> All of the WWV clocks that I know of use the 60KHz WWV signal which
>> is mostly ground wave so it does not have as much variation in time
>> as the HF WWV signals.
>> 
>> Jim Hannon
>> 
>> 
>> ---------- Original Message ----------------------------------
>> From: Chris Albertson <chrisalbertson90278@yahoo.com>
>> Date:  Wed, 16 Feb 2005 13:26:39 -0800 (PST)
>> 
>> >
>> >I think NTP is about 100 times more acurate then needed for TASS
>> >but it's free and easy to set up.
>> >At the level of resolution that us humans can notice "anything
>> >works".  Noticing to less than 1/10th second is hard.
>> >
>> >But if we are talking microseconds then even GPS has problems
>> >because of the unpredictable propogation speed through the
>> ionosphere.
>> >The millitary grade GPS signal uses two frequencies so that the
>> >differences in propogation speeds (which are frequency dependent)
>> >tells the reciever enough about the real-time ionosphere that the
>> >reciever can correct out the effects.  Effects are on the order
>> >of a few meters over "C".
>> >
>> >The bigger prolem with GPS, at the consummer level, is the update
>> >rate of the LCD display and delay through the NMEA data port. 
>> >
>> >WWV radio is in the HF frequency range and unless you live very
>> close
>> >to the transmitter comes to you after bounceing offone of the
>> >ionoshpere layers.  The layers are not at constanthight so the path
>> >lenght varies durring the day.   
>> >> 
>> >
>> >
>> >=====
>> >Chris Albertson
>> >  Home:   310-376-1029  chrisalbertson90278@yahoo.com
>> >  Cell:   310-990-7550
>> >  Office: 310-336-5189  Christopher.J.Albertson@aero.org
>> >  KG6OMK
>> >
>> >
>> 
>> 
>> --
>> Jim Hannon
>> http://www.fmtcs.com/web/jmhannon/
>> 42,11.90N,91,39.26W
>> WB0TXL
>> --
>> 
>
>
>=====
>Chris Albertson
>  Home:   310-376-1029  chrisalbertson90278@yahoo.com
>  Cell:   310-990-7550
>  Office: 310-336-5189  Christopher.J.Albertson@aero.org
>  KG6OMK
>
>
>		
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--
Jim Hannon
http://www.fmtcs.com/web/jmhannon/
42,11.90N,91,39.26W
WB0TXL
--