[time-nuts] 10MHz LTE-Lite - PPS accuracy?

gigneil at gmail.com gigneil at gmail.com
Wed Dec 10 10:29:29 EST 2014


I don’t know if everyone is aware, but the USB PPS out is basically useless on both my units - at least 200us off if not more.

Make sure you’re using the PPS OUT and not trying to measure on the DCD of the USB int, as is easy to be drawn to do.


From: David J Taylor
Sent: ‎Tuesday‎, ‎December‎ ‎09‎, ‎2014 ‎7‎:‎35‎ ‎AM
To: Discussion of precise time and frequency measurement

From: Dave Martindale

In my case, the LTE-Lite had been operating for at least a week before I
made my "accumulate mode" measurement, and the Thunderbolt had been
operating for at least a month.  But both antennas were in poor locations -
not bad enough to lose lock any time I was watching, but nowhere close to a
clear view of most of the sky.  I never saw the 1 PPS disappear while I was
watching it.

I wonder if your LTE-Lite ever finished its survey and switched into
1D/position hold mode?  A GPS operating in 3D mode can indeed fail to get a
position fix with 5 satellites being received, if they have bad geometry
(e.g. all are in the same plane in space) because the solution will have
horrible DOP values.  But a timing-mode GPS in position hold mode knows its
own (antenna) position, and only needs one visible satellite to continue to
provide timing outputs.

We don't know how the LTE-Lite's disciplining algorithm is tuned.  If
frequency stability was considered to be more important that timing, the
algorithm may limit the maximum frequency offset that can be used to
correct a timing error.  Watching the scope output in real time, I can see
the time offset between the two 1 PPS pulses change with time, but it
always changes rather slowly, so the maximum frequency difference I've seen
is quite small.  (I no longer have the equipment set up, so I can't provide
a quantitative number).

- Dave


Thanks for that background.  I'm sure Said must be on holiday (or unwell) 
otherwise he would have chipped in!

Mine did finish the survey - eventually - and I saw this as the positions 
which were emitted being identical, and also that the survey light was 
extinguished.  But at the moment the Lock OK light is out, and the survey 
light is out.  Four satellites are showing at strength 27 or above, but no 
position is being emitted.  PPS is present, about 170 ns late compared to 
the Rapco 1904M.  The other GPS receivers are showing normal lock and a 
positional output.

I suspect you are correct about the algorithms - the device being optimised 
for frequency rather than timing.

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

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