RSAA News of the Month: July 2004
Time and the Transit of Venus
RSAA engineer develops new portable timing system
The Transit
of Venus on June 8, 2004, saw the first astronomical use of what
may become the standard timekeeping apparatus for scientific and
engineering fieldwork. Developed by RSAA engineer, Dr
Gary Hovey, the device incorporates a low-power Global Positioning System
(GPS) receiver and some specially-designed circuitry to produce
time signals locked to the Universal Coordinated
Time (UTC) system with microsecond accuracy.
Accurate timing is vital to most astronomical observations, and to
telescope pointing and tracking control systems. A problem arises when
observations have to be made away from the observatories, remote from
electrical power, computer systems and time standards. Many such
observations, e.g. asteroidal occultations,
eclipses
and transits, are done by amateur astronomers, who used the time
signals from the Australian standard time and frequency radio station
VNG up to its closure at the end of 2003. The need now is for a
simple, cheap and reliable timing unit to replace the VNG signals for
astronomers, surveyors and others working remote from their base. You
can hear the last minute of VNG transmission via this MP3 file (your Web browser
will need to be appropriately configured).
An article by Dr Hovey including a brief history of time-keeping in
Australia and the importance of a replacement for VNG can be read
here.
Commercial GPS hand-held units are optimized for navigation and do not
keep time to millisecond accuracy. Dr Hovey's new system uses a
commercial GPS receiver interfaced to a small microprocessor chip and
a few special circuits. It forms a small, self-contained, highly
accurate UTC clock and time signal generator. It produces audible,
visual and electrical time signals, tied to the UTC system. Gary has
previously designed and built several GPS timing units for ANU telescopes at Stromlo, Siding Spring and
in Antarctica.
The prototype GPS Time Signal Generator as used for timing the 2004
Transit of Venus.
Top image: the complete
system. Lower images: close-ups of the circuit board
and the display.
The prototype was constructed for the committee of the VNG Users
Consortium (VNG-uc). The aim is to create a design that can be
produced either in kit form or as a finished unit. All information on
the design, including the microprocessor code will be freely
available.
Preliminary specifications of the GPS Time Signal Generator can be
seen here.
First "real" use of the prototype was timing the Transit of
Venus at a historical re-enactment at Woodford, in the Blue
Mountains. The 1884 transit had been observed by astronomers from Sydney Observatory,
from the vicinity of the old Woodford inn, now Woodford
Academy. On June 8, the Woodford Academy, together with National Trust of
Australia staged a re-enactment at which two surveyors, George
Baitch and Case Boseloper from the NSW Lands Department, observed the
transit using the prototype VNG-uc Time Signal Generator to provide
precise time. Everything worked!
Left:
Gary Hovey and a member of Woodford Academy
staff, waiting for Transit.
The prototype VNG-uc Time Signal
Generator is in the plastic container on the table.
Right:
Second Contact! Transit seen through a T2 theodolite as used at
Woodford.
Image by Mark Morrison, Newcastle.
For more information on the mechanisms and standards of time keeping,
check out the links on Markus Kuhn's Computer
Time Resources page.
For previous Monthly News items, click here.
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