Tony Finch wrote:
Martin Burnicki <[email protected]> wrote:
I agree, but as I've tried to point out above I think a better project design
would have been to use TAI instead of GPS time. PTP works natively with TAI,
and you can easily convert between he two scales.
I don't understand this paragraph. The three timescales you mention have
totally different formats:
TAI: YYYY-MM-DD T HH:MM:SS
PTP: SSSSSSSSSS
GPS: WWWW SSSSSS
They have different epochs:
TAI: 1958-01-01 T 00:00:00 Z
PTP: 1970-01-01 T 00:00:00 Z
GPS: 1980-01-06 T 00:00:00 Z
So I don't see how it makes sense to argue that PTP is more like TAI than
GPS time.
In the strict scientific sense I agree.
However, in practice, and for "current" dates, you can convert each of
them to a number of seconds since the epoch, and for practical purposes
the difference is just an integral number of seconds.
For example, the IEEE Std 1588-2008 says:
"The PTP epoch is 1 January 1970 00:00:00 TAI, which is 31 December 1969
23:59:51.999918 UTC"
However, the time stamps used in the PTP packets are just binary numbers
of seconds, and you have to apply an integral number of seconds to
convert this to UTC.
Similarly, the comments in the NIST leap second file say:
# The second column shows the number of seconds that
# must be added to UTC to compute TAI for any timestamp
# at or after that epoch.
Even though the number of seconds between TAI and GPS timestamps is
constant, if you use all the scales TAI/GPS/UTC in an application you
have to do more different conversions than if you had only TAI and UTC.
Just imagine a PTP grandmaster, controlled by a GPS receiver which
outputs raw GPS time.
Tony, I'm sure you know all of the above, but I just wrote this to make
clear what I meant.
Martin
--
Martin Burnicki
Senior Software Engineer
MEINBERG Funkuhren GmbH & Co. KG
Email: [email protected]
Phone: +49 (0)5281 9309-14
Fax: +49 (0)5281 9309-30
Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
Geschäftsführer/Managing Directors: Günter Meinberg, Werner Meinberg,
Andre Hartmann, Heiko Gerstung
Web: http://www.meinberg.de
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