[ntp:questions] DUT1 frolics
David L. Mills
mills at udel.edu
Tue Jan 9 15:54:30 UTC 2007
DUT1 is the UTC correction in units of .1 second determined by the
International Earth Rotation Service (IERTS) and disseminated to the
public by radio, satellite and telephone. It decrements every few months
or so. When DUT1 hunts the outskirts of -5 it increments one second and
a leap second is declared in UTC.
While tweaking the audio reference clocks I discovered a mystery. The
CHU broadcast signals show +0 for the DUT1 correction, as does the NIST
modem time. However, the WWV signal last I checked showed +1. Intrigued,
I checked the NIST web
http://tf.nist.gov/timefreq/pubs/bulletin/leapsecond.htm and found it
said the correction has been +1 since 28 September 2006. A polite note
from the CHU operators said the +0 is correct, but they received notice
only hours before it went into effect. Well, according to IERTS Bulletin
D http://hpiers.obspm.fr/eop-pc/ dated 15 December 2006, the DUT1 was
changed to +0 on 22 December, just in time for Christmas. Then, I
checked WWV again and found DUT1 +0. Apparently, I must have last
checked before 22 December. Life moves fast.
Curious, I looked for evidence why the IERTS declared when it did.
Looking at the daily data, the predicted DUT1 crossed .05 s on the way
down on 23 December midway between 0.1 and 0.0, so the criteria must be
round to the nearest decisecond. The scary part is the increased rate
DUT1 is dropping.
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