[time-nuts] GPS/UTC time

[Jim Lux]

On 7/3/15 9:45 PM, Brek Martin wrote:
>
>
> Hi Guys,
>
> I feel like I missed “The Big Thing” in time keeping land. I should have watched my Ublox LEA-5T.
> What is the difference if it is in the reporting mode for GPS or UTC time?
> If they skip a second UTC, surely the GPS time isn’t run incorrectly forever.
>


Who's to say which is "correct", UTC or GPS?

GPS time is derived from TAI; both are monotonically increasing, 
continuous, and constant rate.  These are nice attributes for something 
you're going to use for time stamping, or controlling.  No gaps, no 
jumps, etc.

UTC (and local civil time, and GMT, etc.) have leap seconds, to adjust 
the time scale to the motion of the Earth;  so that the sun is highest 
at noon (after accounting for the equation of time).

While that's somewhat convenient, I doubt anyone would really object to 
noon being a few tens of seconds away from the zenith crossing when 
standing on the line. TAI is ahead of UTC by 36 seconds.  They add a 
leap second every year and a half, so I guess in 100 years, we'll have 
drifted some minute or so away.  (the earth has slowed down in the last 
200 years.. a day is now 86400.0015 seconds long, although it's faster 
now than it was in the 70s, when it was 86400.003 seconds)

http://tycho.usno.navy.mil/leapsec.html

(the Japan earthquake in 2011 sped the earth up by 1.8 microseconds/day. 
  The Sumatra quake on 26 Dec 2004 had a bigger effect: 6.8 microseconds)