[time-nuts] How long to monitor GPS? (was: Re: Rapco 1804M - serial problems?)

David J Taylor david-taylor at blueyonder.co.uk
Fri Oct 21 11:54:21 EDT 2011

> I've now written a small plotting program to show the PDOP, satellite 
> signal strength, inactive/coarse/fine lock mode etc.  This ideally needs 
> to run for several days with the box connected to each puck in turn, but 
> preliminary results show that the unit achieved "fine" lock several 
> times with the new antenna, but never with the old.  The reported signal 
> strength and number of active satellites appears more consistent as 
> well. The extra cable length meant that I could bypass a short BNC patch 
> lead I had, so at least reducing the number of connectors.

Running this monitoring for a while is producing unexpected results.  From 
about midday yesterday to midday today, I reverted to the Garmin GPS puck, 
to establish a baseline.  On changing back to the new puck, the signal now 
seems worse!  The pucks are in approximately the same location - about 140 
mm from each other - outside on a SSW-facing window, with rather more than 
135 degrees of clear view.

This raises the question - how long should one monitor GPS satellites 
(e.g. the reported number and strength) to get a good average value when 
comparing pucks?  I vaguely recalled the orbits being 12 hours, so I had 
thought that one day would be enough.  Any comments?

SatSignal software - quality software written to your requirements
Web:  http://www.satsignal.eu
Email:  david-taylor at blueyonder.co.uk 

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