Hal Murray wrote:
2) along with the gps time signal, information is broadcast to relate
gps time to utc. whenever there's a step change in these corrections
(such as on midnight 31/12/2008), how does the unit react? 2.1) in
particular, how quickly does the time code reflect the change, and is
the 1pps affected in any way?
Check the data sheet on the unit. It probably has a section describing the
leap second stuff.
It is rarely beyond slogan level, if even mentioned in the datasheets.
The manual rarely says much useful on the subject either. It happends
from time to time that receivers freezes on leap seconds only for the
stupid reasons it was not tested by the vendor. A simple single sat GPS
emulator would have helped to trigger the bug, which is sufficient for
timing receivers and should work for position receivers too.
They happen infrequently enough that you can usually get the info by other
means. I'm sure the next one will be announced here.
There won't be one next new years even, it just became official
yesterday. That was expected.
GPS works on GPS time. The satellites tell you the offset to UTC. I expect
that offset kicks over at the magic time, but I wouldn't be surprised by
bugs. Somebody may have data from watching the last time we had one. (I was
watching a NMEA unit. It inserted the leap second at midnight GPS time
rather than midnight UTC.)
OUPS!
The PPS is tied to GPS time. I don't expect any quirks from leap seconds.
If only listening to PPS and don't care about the time associated with
it, you are safe. GPS time or UTC time should work equally well.
I think you should fetch the GPS ICD 200 document and read up on the
details in the signal structure on how UTC is represented in
relationship to the GPS signal. Then imagine all the bugs there can be.
There are (at least) three interpretations of when leap-seconds may be
inserted:
1) At the end of every month.
2) At the end of every quarter.
3) At the end of every half-year.
GPS is operated under the assumption that case 3 holds.
There exist equipment assuming that case 2 holds.
The actual definition allows for case 1, making case 3 the primary
preference and case 2 the secondary preference.
Lovely mess, isn't it?
Anyway, if I don't recall it incorrectly (OK lazy to check), the GPS
signal actually indicate WHEN then upcomming leap second will occur.
This reduces to a up-comming leap second flag out of the GPS OEM board
which can trigger pre-maturely execution of leap-second algorithm on the
timing receiver, as we have seen in the Z3801A for instance.
Cheers,
Magnus
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