[time-nuts] Lost GPS lock or 1PPS recently?

Scott McGrath scmcgrath at gmail.com
Tue Sep 4 14:09:43 EDT 2018

My TrueTime DC-XL has lost lock since yesterday as has my Z3805 and my car’s onboard GPS will not lock since the 2’nd.  I need to get about a mile from home before Car’s nav system reports ready.

There has been a great deal of repair work on the local cable system so this is almost certainly related to that

But it proves my point about the fragility of GPS

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On Sep 4, 2018, at 12:31 PM, John Sloan <jsloan at diag.com> wrote:


GPS jamming and spoofing isn't really my area of expertise, but it's something I worry about, not just for its impact on geolocation and navigation applications, but also because GPS has become critical as a high precision timing reference in the telecommunications realm, which *is* my area of expertise.

Yesterday (2018-09-03) afternoon (about 22:00UTC, 16:00MDT) I noticed one of my three home-made GPS-disciplined NTP servers had lost its GPS lock. After some forensics on my part, this (2018-09-04) morning (about 16:00UTC, 10:00MDT) I replaced the amplified antenna, and the device reacquired its lock. I figured it was just an antenna failures; this is an amplified filtered antenna so it has active electronics.

Then just an hour or so later, I noticed one of my commercial GPS-disciplined NTP servers (TimeMachines) had lost the GPS 1PPS timing signal, but indicated it still had GPS lock. (I question now what this actually means in the context of this particular device). As a troubleshooting step, I power cycled the device, and it reacquired 1PPS. But as I did that, the second commercial GPS-disciplined NTP server (Uputronics) right next to it lit up with a red warning on its display, indicating it had lost GPS lock. A minute or so later it also reacquired lock and indicated 1PPS, with no action on my part.

All of these devices are completely independent, have different software (and probably hardware), have separate amplified antennas sitting side by side in the window of my home office, and are not all on the same electrical outlet (but may be on the same household circuit).

I lit up the LCD display on my little GPS monitoring tool I built that runs Lady Heather 24x7 and see on the graphical display sudden jumps of reduced timing accuracy of a factor of 10^2 (from nanoseconds to hundreds of nanoseconds) in the recent past. But I’m thinking this can also be caused just by the dynamic satellite geometry, and might be normal. It’s not like I watch this graph all the time (even though it does sit right in front of me on my desk).

No clue what's going on in my suburb near Golden Colorado. But I’m a little freaked out. Trying to figure out which rule, [1] It’s something stupid I’ve done, or [2] I am not unique, to apply.


J. L. Sloan            Digital Aggregates Corp.
+1 303 940 9064 (O)    3440 Youngfield St. #209
+1 303 489 5178 (M)    Wheat Ridge CO 80033 USA
jsloan at diag.com        http://www.diag.com <http://www.diag.com/>
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