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

Dave Martindale dave.martindale at gmail.com
Tue Dec 9 08:53:09 EST 2014

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

On Tue, Dec 9, 2014 at 5:13 AM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:

> I work with Said at Jackson Labs.  I've been reading the time-nuts
> discussion for a few years, but rarely chime in.  I saw this discussion and
> wanted to make a couple points.
> * The LTE Lite time accuracy specification corresponds with the Skytraq GPS
> receiver's specs page which I have attached.  The specification is for the
> output directly from the GPS receiver available on the LTE Lite Eval
> Board's JP1 connector pin 12.  This specification assumes optimal antenna
> placement and thermal conditions, and position hold mode. It is also an RMS
> (1-sigma) measurement not a peak-to-peak measurement.
> * The GPSDO-generated 1PPS on the LTE Lite Eval Board's J1 connector has a
> phase offset to the GPS raw 1PPS that is shown in the PJLTS message (2nd
> field).  The GPSDO functions to drive this phase offset to zero.  But at a
> given time--especially shortly after power up--the offset may 100 ns or
> more.
> Keith
> ==============================================
> Keith,
> Thanks very much for chiming in, as it has resolved what we are seeing,
> particularly your second comment.
> One thing I do notice is that the device appears less sensitive than some
> other GPS devices I have.  Perhaps "sensitive" isn't the correct word, but
> looking at the NMEA output it seems to indicate bursts of "no/invalid
> position" a lot more often than I would expect.  This is shown by the all
> the signal strength bars being grey rather than some of them being blue.
> I've also seen times when five or more satellites are above strength 29,
> and yet there is no position shown.  This also seems to stop the generation
> of the PPS output, which would be not so good when driving an NTP server.
> I am wondering whether this is due to overly stringent criteria being set
> for a "position found", at least for my location and antenna location, and
> if this is the case, whether there is any chance of relaxing those
> criteria. I'm guessing not, as the device will not accept any serial input.
> You will have gathered that my main interest is time rather than
> frequency, and it seems that other GPS devices give PPS outputs which are
> nearer to UTC but they have considerably more jitter.  I'm only seeing this
> on the 'scope - likely my PCs would bother with a microsecond either way.
> Cheers,
> David
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
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