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Chronometer Error
Introduction
Inasmuch as chronometers are never reset aboard ship, an accumulated
error may become quite large. Such an error is unimportant, though, if an
accurate record is kept of the error. The most accurate check on the
chronometer and other timepieces is by comparing the radio time signal
broadcast by radio station WWV and other stations listed in Radio
Navigational Aids (Pub No. 117) with the chronometer time.
Time Ticks
Since 1 January 1973, the broadcast time signals (UTC) have differed
from GMT by amounts up to ±0.7s. The difference arises because the
times given in the navigational tables depend on the variable rate of
rotation of the Earth, while the broadcast time signals are now based on
an atomic time scale. Step adjustments of exactly 1 second are made to
the time signals as required (normally at 24th on December 31 and June
30) so that the difference between the time signals and GMT may not
exceed 0.9s. For those who require GMT to an accuracy better than 1s, a
correction (DUT) is coded into the transmitted time signal. GMT accurate
to 0.1s is obtained by applying DUT to the transmitted time signal; that is,
GMT = UTC + DUT
Naval radio stations transmit time signals (on seven different frequencies)
for the 5 minutes immediately preceding certain hours GMT. The DUT
correction is given in Morse code in the final 9-second pause prior to the
long dash.
Each second in the time signal is marked by the beginning of a dash; the
end of the dash has no significance. Beginning at 5 minutes before the
hour, every second is transmitted except the 51st second of the 1 st minute,
52nd second of the 2nd minute, 53rd second of the 3rd minute, 54th
second of the 4th minute, 29th second of each minute, the last 4 seconds.
of each of the first 4 minutes, and the last 9 seconds of the last minute.
The hour signal after the 0-second break (59m 60s) consists of a longer
dash than the others. For clarity, the system of dashes are shown
graphically in the accompanying table on the next page.
5-15
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