euclid The Time Traveler's Clock, User Manual

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When GNSS time signals are available, the quartz, silicon, and mechanical clocks are
kept synchronized to satellite time.
Silicon Clock Advantages and Deficiencies
One of the redundant clocks contained in the TTC is an integrated-circuit chip (Dallas
DS3231) which contains a temperature-corrected quartz oscillator of its own and an
extremely-low power microprocessor which maintains correct time, even if it’s main
power supply is lost, using a small backup battery. The lifetime of the backup battery is
at least ten years, and it is only used when the normal power supply is not available.
This integrated-circuit clock is not as accurate as an atomic or GPS clock, but it can serve
as a timing reference if all else is lost, and it is a redundant clock to supplement the
other quartz, atomic, and mechanical timekeeping systems. The integrated-circuit clock
in the Dallas DS3231 chip is specified to be accurate to within one minute per year;
DS3231 chips used in the TTC are individually tested and calibrated to achieve
significantly improved timekeeping, on the order of seconds per year.
Figure 11: Real-time Clock with DS3231 Silicon Integrated Circuit
Atomic Reference Advantages and Weaknesses
TTCs use atomic frequency standards based upon the oscillations of cesium or rubidium
atoms. These are the most precise reference standards available that can be used in
portable equipment. The “Miniature Atomic Clock” (MAC) and “Chip-scale Atomic
Clock” (CSAC) technology was developed by Jackson Laboratories.
The current state-of-the-art devices are cesium CSAC modules such as the one used in
the latest MCB version of the TTC. The signal from this module, a 10 MHz reference, is
supplied to the GNSS timing receiver so that if satellite signals are not available, the
receiver will go into “holdover” mode with the precision of an atomic clock. The cesium
CSAC modules have an accuracy of 1 x 10 -10 , which is equivalent to a few milliseconds
per year. In the TTC, this signal serves as the reference to not only the GNSS receiver,
but also to the mechanical chronometer by way of the servo synchronization system.
Atomic clocks require continuous electrical current to maintain their accuracy.
However, if power is restored after a temporary loss, the atomic clock module will re-
establish its operation according to the last stored tuning data and once again begin to
supply a stable reference signal to the GNSS receiver. During the time that the atomic
reference is lost, the GNSS receiver will revert to its own internal quartz reference,
which is precise to 1 x 10- 7 , or 8 milliseconds per day.