[time-nuts] GPS jumps of -13.7 us?

brian bjones0 at mindspring.com
Tue Jan 26 21:15:04 EST 2016


This may be a dumb question, but I am new to using LadyHeather with my
Thunderbolt (previously it had been slaved to driving a clock display
but that broke, which made me get around to installing a serial card in
my linux box)...

Are these "events" something I should be able to see in LH?  If so what
should I be looking for?     I see a pair of off-setting events in the
graphs (a sharp negative spike in DAC Voltage & PPS, followed later by
a sharp positive spike of the same magnitude),  but not sure if I am
seeing what I want to see, of if this is actually what I am looking
for.

Off to study the LH Manual some more!

73,
Brian
KD4UYP


On Wed, 2016-01-27 at 01:15 +0100, Magnus Danielson wrote:
> Hi,
> 
> On 01/26/2016 11:46 PM, Martin Burnicki wrote:
> > Paul Boven wrote:
> > > Hi everyone,
> > > 
> > > Has anyone else seen GPS time jump by -13.7 usec today?
> > > I just heard from several geographically quite distributed radio
> > > observatories that they have seen their GPS receiver(s) jump
> > > compared to
> > > their in-house standards.
> > 
> > We were able to track this down today at Meinberg.
> > 
> > The problem was that some satellites were sending invalid UTC
> > correction
> > parameters. The UTC correction parameter set not only contains
> > current
> > leap second information but also coefficients (A0, A1, WNt, tot)
> > for a
> > polynomial used to compensate the fractional difference between GPS
> > time
> > and UTC(USNO).
> > 
> > Normally A0 (the constant offset) is very small, close to 0. WNt
> > and tot
> > give the week number (truncated to 8 bits) and second-of-week for
> > which
> > A0 is valid, i.e. the reference time for the time correction
> > parameters.
> > WNt is currently about 89.
> > 
> > Today the faulty satellites sent about 13.7 microseconds for A0,
> > and WNt
> > as well as tot were both 0. So when the GPS receiver updated its
> > UTC
> > correction parameters from a faulty satellite the UTC correction
> > jumped
> > from close to 0 to about 13.7 microseconds, which let the UTC time
> > step,
> > and when the GPS receiver received the UTC parameter set from a
> > healthy
> > satellite the UTC time stepped back.
> > 
> > We have recorded a few of the faulty subframe words. If someone is
> > interested I can provide more detailed information. However, I'm
> > currently out of the office and don't have the information here
> > right now.
> 
> You know I want more details. ;-)
> 
> I did see the report you guys sent to the USCG, good work there and
> good 
> report.
> 
> I also some another report showing issues with other SVNs.
> 
> Cheers,
> Magnus
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