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

[David J Taylor]

>
I didn't have any source of accurate time available.  After searching the
internet and finding this mailing list I decided to try a GPS module. I
bought a $20 module from DX.com. It has a built in antenna, voltage
regulator, serial interface and most important, a 1 PPS output.The GPS is a
UBlox NEO-6M. After reading the specs on this module I see that they claim
a 99% accuracy of <60ns for the time pulse signal. What does this mean?
What about the other 1%? How much variation can the time pulse have? If
it's really 60ns it's much better than I need.

============================

Luke,

A first approximation would be to assume a Gaussian distribution of the 
error, although there is a systematic component as well due to the system 
clock sampling inside the ublox module itself.  Its likely not to have any 
excursions of the order of 1 microsecond.  You will be aware that there will 
be a period after the device is first switched on where it will be acquiring 
the satellites and may have no PPS output, or PPS not locked to UTC.  It may 
also be outputting GPS time seconds (UTC without all the leap-seconds).

There are now some units which are a little better than the 6M, such as the 
7Q, available ready-built as an add-one for the Raspberry Pi.  I did play 
with a number of GPS modules when getting NTP working on the Pi, and quite 
honestly at the 0.1 microsecond level it's difficult to see any difference 
between them!

Ublox 7Q module:
  http://ava.upuaut.net/store/index.php?route=product/product&path=59_60&product_id=95

Raspberry Pi NTP:
  http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html

Quite possibly the Raspberry Pi  might form the basis for the sort of unit 
you want, with the advantage of programmability, but with not quite the 
accuracy of a 10 MHz PIC.

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