[time-nuts] GPS jump

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Fri Oct 10 23:20:23 EDT 2014

>> 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?
>>> 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) 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.

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?

Take care. Thanks, Brian Inglis

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