[time-nuts] Using a UBlox NEO-6 GPS module for calibrating a PIC microprocessor based timer.

[David J Taylor]

From: Luke Mester

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Luke,

I meant to add - there could be periods of several seconds or even a minute 
or more where the receiver loses its view of a sufficient number of 
satellites.  You will need to place it in a location with a good view of the 
sky, preferably a 360 degree horizon view.  These devices are very 
sensitive, though, and mine work well enough on the top floor of a house, 
located in a good VHF location with no local blockages such as industrial 
premises or blocks of flats.  One of my GPS receivers is located downstairs, 
but near a window, and you can see that it likely lost GPS lock just after 
02:00 UTC this morning:

  http://www.satsignal.eu/mrtg/raspi2_ntp.html

Temperature variations are apparent throughout the plots here:

  http://www.satsignal.eu/mrtg/performance_ntp.php
  http://www.satsignal.eu/mrtg/performance_disk_temp.php

Heating comes on around 05:30 UTC.

The final accuracy tweak is to compensate for the quantisation of the PPS 
signal caused by the finite clock rate in the GPS receiver - sometimes 
called sawtooth error.  Some GPS receivers will output that error in the 
data stream, and you can then correct for it in your software.  A good 
starting point:

  http://www.leapsecond.com/pages/m12/sawtooth.htm

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk