TAI via GPS

[Markus Kuhn]

"Joseph S. Myers" wrote on 1999-12-04 12:14 UTC:
> Do you happen to know what GPS receivers are available that provide user
> access to TAI (or some other form of the leap second offset information
> transmitted in GPS; ideally all the parameters from Table 2-11 (UTC
> Parameters) of the GPS Standard Positioning Service Signal Specification
> 2nd Edition)?  This doesn't seem to be the sort of feature manufacturers
> of GPS receivers advertise, but it's of more interest to me in a GPS
> receiver than the route storage features that get promoted.

I don't have much experience with current low-cost GPS receivers and
their serial port protocols. When we played in Erlangen around with NTP,
we had a Mainberg GPS167 time receiver, i.e. a GPS receiver that was
especially designed for reference clock applications. And even that one
did *not* provide TAI, or the current UTC-TAI or UTC-GPS difference.
It's output format looked suspiciously compatible to that of other
Meinberg DCF77 receivers, which naturally have no clue about TAI.

More info on Meinberg receivers is available on

http://www.meinberg.de/english/index.htm

including the full manuals.

Also the IRIG-A and IRIG-B US military standard time telegram formats
supported by some GPS reference clock receivers only provide UTC and do
not allow you to reconstruct TAI.

I have seen output from some Trimble GPS receivers that contained the
GPS time as a (week, second) pair, which can be converted easily into
TAI, but I am also not sure, how much of the leap second announcement
you can access here. Trimble clocks such as the Thunderbolt on

  http://www.trimble.com/oem/om_timng.htm

support the Trimble Standard Interface Protocol (TSIP) via RS232 as
documented on

  http://www.trimble.com/support/files/thunder.htm

Even there, they don't mention whether TAI is available.

A bit more information is available on

  ftp://ftp.trimble.com/pub/tracking/pla400-TAIP-Reference.pdf

which describes the Trimble ASCI Interface Protocol (TSIP). It *does*
provide in the TM message a 2-digit "GPS/UTC Time Offset" field that
indicates the number of seconds, but there is no leap second
announcement field.

Conclusions: If you look long enough, you might get TAI extracted out of
a good GPS receiver, but not all GPS receivers support this and even for
those that do, the leap second announcement is not necessarily available
or well supported, or the documentation is very obscure.

No wonder, hardly anybody is using TAI in practice and everything is UTC
based.

Markus

-- 
Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK
Email: mkuhn at acm.org,  WWW: <http://www.cl.cam.ac.uk/~mgk25/>