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

David J Taylor david-taylor at blueyonder.co.uk
Tue Dec 9 05:13:35 EST 2014

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



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.

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Email: david-taylor at blueyonder.co.uk 

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