AFFORDABLE TIMEKEEPING IN THE FIELD

Not all astronomical research is done in lavishly equipped observatories by professionally trained astronomers paid by money from the government purse! On a nightime drive down the Cooma Road you may have seen the bizzare sight of a group of strangely-dressed people setting up telescopes and radio gear by the roadside. These amateur observers contribute valuable data on star and asteroid occultation timing, supernova and comet discovery and other projects without needing official funding. Because of the increased availability of affordable, high-performance CCD cameras and other equipment, there are many ways in which amateurs can contribute to the science of astronomy. Professional astronomers welcome this collaboration.

But “time is the essence”! How do you measure the time of events when you are in the field away from electric power, computer networks and standard time systems? Until recently, Australian occultation observers and astro-surveyors used HF (short-wave) radio time signals which gave UTC (Coordinated Universal Time the sucessor to the old Greenwich mean time) with an accuracy of better than 5 milliseconds and which could be received on a simple short-wave radio. With the closure of the Australian standard time and frequency station VNG, observers are finding that more distant stations such as WWVH Hawaii and WWV Colorado have large and variable propagation delays and may be too weak to be reliable.

A very brief history of official time dissemination in Australia

In the 18th century astronomy and maritime navigation demanded increasing accuracy in timekeeping. Observatories for astronomical purposes had been established at Dawes Point when the first fleet arrived in 1788 and at Parramatta in 1821. However the definition and maintenance of precise time for maritime, scientific and civil use in the early days of the colony of NSW really started with the establishment of Sydney Observatory in 1858. Time dissemination was by way of conventional maritime time-balls and eventually much use was made of the electric telegraph which was introduced into Australia in 1860s. The social aspects of timekeeping in Australia are engagingly addressed by a book by Graeme Davidson... reference [1].

After the second world war, radio time signals were generated at the Commonwealth Solar Observatory later called Mt Stromlo Observatory and transmitted by Navy radio transmitters VHP (on 44kHz LF) and VIX (on three HF frequencies).

In 1964 radio VNG a standard frequency and time transmitter was established at Lyndhurst, Victoria by the Post-Master General’s Department (later to become Telecom Australia then Telstra). Frequencies of 4.5, 7.5 and 12.0 MHz were used. Over the next 18 years VNG was to provide the Australia with a valuable source of ‘medium accuracy’ time to about a millisecond.

VNG was closed down in 1987 when the Australian government disposed of the Lyndhurst site. An organisation called the VNG Users Consortium was formed and, by the concerted efforts of numerous volunteers, the station was re-established at the Civil Aviation Authority transmitter site at Llandilo near Penrith NSW transmitting on frequencies of 2.5, 5.0, 8.638, 12.984 and 16.0MHz. This endeavour is described in reference [2]. The service continued until 2002 when budget considerations lead to its final closure. The station ceased transmission shortly before midnight on 31 December 2002 after providing Australia with radio time signals for 38 years. The last second marker was at 23:43:43 UTC.

A Replacement Precise Time System for Field Use

The committee of the VNG Users Consortium, whilst lamenting the closure of the station, decided that the HF dissemination of time signals nolonger made sense in the face of the decreasing costs and superior performance of GPS satellite timation equipment. Field requirements dictate a low-cost, portable, compact, precise-time system which can provide absolute time traceable to UTC to a precision of about a millisecond.

Commercial GPS timation receivers are still beyond the means of many users and do not necessarily provide time signal outputs which are exactly suited to such work. Commodity hand-held GPS units are optimized for navigation and (because of display readout delays and other problems) do not keep adequately precise time. Accordingly it was proposed that a suitable GPS timation receiver and time signal generator be designed and a prototype built and exhaustively tested.

This work is being undertaken by Gary Hovey, an engineer at Mt Stromlo Observatory and a member of the VNG UC committee who has designed and built several other GPS timation units for astronomical installations at Mt Stromlo and in Antarctica. The committee envisages a design which will be produced both in kit form for assembly by interested parties and as a commercially finished unit for field use. All information including microcontroller code will be freely available. If we succeed in obtaining substantial government funding for this project the intellectual property rights for the circuit design and the ownership of the source code will be transferred to the Commonwealth of Australia.

What does it do and how does it work?

The GPS Time Signal Generator consists of a commercially available GPS satellite receiver module which has been interfaced to an 8-bit microcontroller chip and a few other circuits. It forms a self-contained precise-time UT clock and time-signal generator and is designed to have very low power consumption to permit extended field use. No computer nor other device is necessary for its functioning. A standard magnetic-mount active antenna is used to receive the GPS signals.

Visual, audible and electrical time signals are provided on the UT second with accuracy traceable via the GPS standards to the US Naval Observatory. When switched on, the unit commences processing the L-band CA-code signals from whichever GPS satellites which are visible. As soon as the GPS receiver has obtained a stable navigation and time fix from the GPS satellite constellation, the unit enables its time signal outputs and display. Thereafter a number of checks are made continuously to ensure that the generated time and time signals are valid. The time signals and display are locked to the Universal Coordinated Time (UTC) timescale. Detailed specifications can be found in a separate document [3].

Transit of Venus at Woodford, NSW

In 1884 the transit of Venus was observed at Woodford in the Blue Mountains, NSW near the local pub (now Woodford Academy) which was owned by a prominent Woodford business man Alfred Fairfax. He was a keen observer himself and owned an excellent 120mm refractor with achromatic optics made by Dr Ing. Schroeder in Germany. This was made available to the NSW Surveyor General for the transit and a telegraphic connection established to Sydney Observatory to get precise time traceable to astronomical timescales. This Fairfax telescope has since been partially refurbished at the Power House museum under the guidance of Nick Lomb of Sydney Observatory.

On June 8 this year the Woodford Academy together with National Trust of Australia staged a re-enactment of this event and 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.

References
[1] The Unforgiving Minute: how Australia learned to tell the time (An insightful look at timekeeping in Australia from a social perspective) Graeme Davidson, 1993, OUP. Melbourne. ISBN 0-1955-3496-4
[2] Keeping Australians in tune and on time. Electronics Australia, June, 1993, pp26–29
[3] VNG-uc GPS Time Signal Generator: Preliminary Specifications. Gary Hovey, 30/6/2004

Issued by Gary Hovey for the VNG Users Consortium 30/June/2004