[time-nuts] June 30 2015 leap second

Jim Lux jimlux at earthlink.net
Sat Jan 10 20:00:48 EST 2015

On 1/10/15 1:25 PM, Hal Murray wrote:
> jimlux at earthlink.net 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


latest version has stuff about security and more discussion about the 
various kinds of time (UT1, TAI, GMT, UTC, etc.)

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