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-tay...@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-tay...@blueyonder.co.uk > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.