On 1/10/15 1:25 PM, Hal Murray wrote:
[email protected] said:
Which is why we use TAI in the space business and don't fool with this
"Greenwich Mean Time" or "Coordinated Universal Time" which is
discontinuous and potentially non-monotonic.
Does the system clock on your PCs run on TAI or do they have a separate clock
for space applications?
Is that even a sensible question? Do you use PCs running traditional OSes?
A good question. We run traditional OSes (Tons of Suns running Unix in
the deep space network), but newer systems run Linux or Windows. That's
for terrestrial applications. Software conversions used between TAI and
"user time" on whatever platform, and leap seconds are handled in an
ad-hoc way (in the sense that there's no particular standardized way).
We distribute TAI time via a variety of timecodes (there's IRIG-B, for
instance) and other means (NTP, etc.) NTP is UTC, so it has to be
converted to TAI in software.
Lots of dedicated boxes that deal with time.
In flight, VxWorks and RTEMS: I have a lot more familiarity with the
latter. In flight, until recently, we don't ever convert from raw
spacecraft clock - SCLK which is just a free running counter driven from
some oscillator. Someone on the ground figures out the offset and rate
of the oscillator, and if you want something to occur at 12:34:53, you
convert that to SCLK and say "do this at time X" where X is a binary
number of some sort.
This is gradually changing.
That said, when a formatted time is used, I think CCSDS Unsegmented Time
Code (CUC) is most common, and it's TAI seconds and fractions of
seconds. That is, the basic time is in integer seconds, with some
multiple of 8 bits worth of fractional seconds (e.g. if you have 1 octet
of fine time, it would be in units of 2^-8 seconds, etc.).
At least that's what *I* am using these days
http://public.ccsds.org/publications/archive/301x0b4e1.pdf
latest version has stuff about security and more discussion about the
various kinds of time (UT1, TAI, GMT, UTC, etc.)
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