Hi.
Since we've been discussing UTC vs GPS time and we are about to
experience a leap second event about 8 hours from when I send this, I
thought it might be helpful to show what's about to go on.
UTC has used leap seconds since 1972 to synchronize timestamps to the
rotational motion of the Earth. A leap second is added to try to keep
the UTC clock readout within 1 second of midnight when the 0 degree
longitude crosses the physical midnight point in the Earth-Sun system.
They don't add or subtract anything from the passage of time in terms of
elapsed seconds. The leap second (similarly to a leap day) is an
instruction to make the seconds part of the HH:MM:SS clock readout
(timestamp) count up one higher than usual, so proper GPS and UTC clock
readouts this evening (from my time zone) will run like this:
GPS Elapsed Time since 1980-01-06
GPS timestamp
UTC timestamp
Comment
weeks 1851
seconds 262813 2015-07-01 00:00:13
2015-06-30 23:59:58
weeks 1851
seconds 262814 2015-07-01 00:00:14
2015-06-30 23:59:59
weeks 1851
seconds 262815
2015-07-01 00:00:15
2015-06-30 23:59:60 leap second
weeks 1851
seconds 262816 2015-07-01 00:00:16
2015-07-01 00:00:00
weeks 1851
seconds 262817 2015-07-01 00:00:17
2015-07-01 00:00:01
GPS, on the other hand, is actually a count of elapsed weeks and seconds
within a week since it's epoch of 1980-01-06 00:00:00 UTC. It can also
be represented as a timestamp, and should be converted to a timestamp
without considering leap seconds. This puts it 15 seconds ahead of UTC
now, and 16 seconds ahead in about 8 hours when the new UTC leap second
is applied.
Grace and peace,
Jim
--
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Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
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