[time-nuts] GPS jump

Bob Camp kb8tq at n1k.org
Sat Oct 11 08:51:46 EDT 2014


On Oct 10, 2014, at 11:20 PM, Brian Inglis <Brian.Inglis at SystematicSw.ab.ca> wrote:

>>> On Oct 9, 2014, at 7:43 PM, Jim Palfreyman <jim77742 at gmail.com> wrote:
>>>> We look after 5 separate hydrogen masers spread all over Australia and we
>>>> collect tic phases between the masers and the GPS.
>>>> On around ~Oct 7 we have noticed that the normal steady straight line (with
>>>> standard daily noise) took a noticeable downward turn - on all 5 masers.
>>>> On 2013-10-03 05:33, Jim Palfreyman wrote:
>>>>> Noticed an above average bump in our H-Maser vs GPS graphs - from sites
>>>>> all over Australia.
>>>>> Recent coronal mass ejection or US government shutdown not updating GPS?
>>>>> Anyone else seen it?
>>>> drop out gap between about 04.21-04.26 UTC?
>>>> clockstats.20131003:
>>>> 56568 15684.876 $GPRMC 042124 A ...
>>>> 56568 16004.862 $GPRMC 042644 A ...
>>>> peerstats.20131003:
>>>> 56568 15684.876 961a -0.000002270 ... 0.000005344
>>>> 56568 16004.862 961a -0.000013150 ... 0.000015721
>>>> loopstats.20131003:
>>>> 56568 15684.876 -0.000002270 0.899 0.000007071 0.000070 4
>>>> 56568 16004.862 -0.000013150 0.898 0.000008830 0.000114 4
>>>> Did anyone else who tracks H-masers notice this as well?

That’s a pretty small group 

>>>> Is it JPL making corrections?
>> Le 10 oct. 2014 à 03:09, Bob Camp a écrit :
>>> GPS is steered by the Air Force last time I checked.
>>> A really good place to check is the NIST Time and Frequency pages that show both real time and historical data for each GPS sat compared to NIST time:
>>> http://www.nist.gov/pml/div688/grp40/gpsarchive.cfm
>>> Hopefully it’s accessible via that link from a variety of locations.
>>> Since the NIST data is independent of the steering (two different outfits involved) it should not be vulnerable to a “our ground segment broke and we steered everything to match” sort of error.
> On 2014-10-09 23:06, mike cook wrote:
>>   I remember Jim reported a similar issue back in october last year:
>>   That dates are close enough to make you wonder if it is not part of some cycle.
> From: http://www.usno.navy.mil/USNO/time/gps/gps-info
> GPS system time is given by its Composite Clock (CC).
> The CC or "paper" clock consists of all operational Monitor Station and satellite
> frequency standards. GPS system time, in turn, is referenced to the Master Clock
> (MC) at the USNO and steered to UTC(USNO)

Because USNO is the official keeper of “time” in the US. NIST is the official for frequency.

> from which system time will not deviate
> by more than one microsecond. The exact difference is contained in the navigation
> message in the form of two constants, A0 and A1, giving the time difference and
> rate of system time against UTC(USNO,MC).
> Page also gives links to GPS time data ftp://tycho.usno.navy.mil/pub/gps/utcgps30.dat
> which shows a 2ns jump in UTC(USNO)-GPS smoothed over 2 days from Oct 7-8, but that
> appears normal; the <1ns differences from Oct 2-7 appear anomalous.

Consider in all the data that it *is* coming from fairly normal receivers. They use good survey grade stuff, but it’s a receiver you could buy off the shelf. Pops do happen

> Looking at the NIST 10 min data, from Oct 3-8 the gap between GPS samples and NIST
> closed about 1.5ns/day, dropping now to about .5ns/day: the graph shows the values
> sliding down to the right, and now levelling off about zero

> So are NIST and USNO steering each other?

All the data you see from NIST and USNO in terms of “what time is it” is the output of some *very* fancy filtering. They take a weighted set of inputs from a large number of sources. They also do a cross check through BIH to keep in synch worldwide. In the sense that BIH corrections get in the mix, everything is locked to everything. 

The GPS master clock is a similar thing. GPS master time (the ground clocks) are fed into a fancy software filter to come up with a local time estimate. That’s massaged to track USNO (more software). The ground estimate is used to look at and steer the satellite clocks through still another software filter. 

All of this stuff has crazy long memory in it, so a “once every three months” or even a “once a year” update of some sort is not at all out of the question. In a sense, leap seconds are a “once very rarely” correction to all this stuff. 
> -- 
> Take care. Thanks, Brian Inglis
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If there was no notice of a change on the various GPS / NIST / USNO / BIH web sites, I’d look for a paper at an upcoming conference. Depending on who did the bump (if there was one) and who was simply following the herd, that paper could pop up in a lot of different places.


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