Slashdot Mirror
Discrepancy Detected In GPS Time
jones_supa writes that on Tuesday, 26th January, Aalto University's Metsähovi observatory located in Kirkkonummi, Finland, detected a rare anomaly in time reported by the GPS system (Google translation). The automatic monitoring system of a hydrogen maser atomic clock triggered an alarm which reported a deviation of 13.7 microseconds. While this is tiny, it is a sign of a problem somewhere, and does not exclude the possibility of larger timekeeping problems happening. The specific source of the problem is not known, but candidates are a faulty GPS satellite or an atomic clock placed in one. Particle flare-up from sun is unlikely, as the observatory has currently not detected unusually high activity from sun.
This is the second time a bug in the firmware of Motorola Oncore GPS receivers have manifested itself. There is a bug relating to a 32 bit wide bitmap, and DoD just took the GPS satellite numbered 32 out of the constellation and that seems to be the cause. I have data for two such receivers showing the anomaly and for one different receiver seeing no trouble at all.
Poul-Henning Kamp -- FreeBSD since before it was called that...
Lots of folks on the time-nuts mailing list have GPS-based systems to maintain not only precision time, but also precision frequency standards, and many of them saw and recorded this one.
Clock speeds are sensitive to the structure of the gravitational field. Maybe other aspects of the Universe.
GPS times have all sort of noise. Some geophysicists use this "noise" to figure things like the atmospheric temperature and density. The GPS signal wavefront bend slightly then. You can tomographically invert for spatial location of the travel time anomalies to locate temperature and density changes. There are papers on this every year at the American Geophysical Union meeting.
Microsecond size anonalies are huge and may have more mudane causes like software.
It looks like the actual problem was a bad data upload; Specifically, some satellites were transmitting incorrect parameters for UTC offset correction. https://www.febo.com/pipermail... is the posting from a gentlemen at Meinberg that has the details. http://www.usno.navy.mil/USNO/... has more information about the time offset parameters (A0 and A1) and how they interact with GPS and UTC time.
According to another message (https://www.febo.com/pipermail/time-nuts/2016-January/095686.html), PRNs 2, 6, 7, 9, and 23 got hit. It is interesting to note that the satellite that was taken out of service this morning (PRN 32) is not in this list. It looks like the decommissioning of PRN32 was quite possibly scheduled (see http://gpsworld.com/last-block...), and even if not, a failure of that specific satellite could not have caused multiple satellites to start broadcasting incorrect offset data.
I'm really looking forward to the postmortem on this.