SETI League Technical Manual -- Accessories
Tom Clark's Totally Accurate Clock
There are number of interesting things which a SETI enthusiast can do with a high precision time and frequency standard. We use them to calibrate or control our receiver frequency coverage, determine accurate local sidereal time (and hence right ascention of our drift-scan radiotelescopes), and to provide a common time reference for correlating long-baseline interferrometry observations. Unfortunately, the technologies used in the past for accomplishing these goals, Cesium or Rubidium frequency standards and Hydrogen masers, are priced beyond the reach of most experimenters. Recently, three lower cost alternatives have presented themselves.
Dr. Thomas A. Clark (W3IWI), the NASA scientist whose name is virtually synonymous with very long baseline interferrometry, has been grappling with the time and frequency standard problem for his geodesy experiments. By deriving timing information from the atomic clock-controlled Global Positioning Satellites, he is able to produce a relatively low cost time and frequency standard with long term accuracy rivaling a Hydrogen maser.
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Tom Clark's Totally Accurate Clock project is providing SETI League members with a low-cost precision time and frequency standard. The circuit board shown here interfaces via an RS-232 connection to a personal computer's serial port. The metal box seen mounted above the circuit board is a Garmin GPS, with its patch antenna on top. In addition to providing accurate position information and both Greenwich Mean Sidereal Time (GMST) and Local Mean Sidereal Time (LMST) to atomic-clock accuracy, the unit can be used to slave a precision crystal oscillator to 1e-09 (one part in a thousand million) accuracy.
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Tom Clark's Totally Accurate Clock, or TAC (the acronym happens to be his initials) produces a 1 PPS output exhibiting 30 - 50 nSec precision and 30 nSec rms accuracy, even with Selective Availability implemented in the GPS system. At 1-day averaging, the unit produces a standard frequency accurate to one part in ten to the twelfth. This is equivalent to Cesium or Rubidium standards, at a cost of about $800. Better still, Tom has made hardware and software details available in the public domain. And as a special bonus, Tom has written SHOWTIME.EXE, a nifty program for displaying GMT time, sidereal time, and user location on a PC screen. The program, which is driven from Tom's GPS-based clock, is especially useful for radio astronomy applications, and is being distributed free of charge.
For news concerning the availability of the TAC-2 kit, now in production, see the TAPR website.
Alternatively, a related commercial product from SETI League corporate member Trimble Navigation Ltd., a precision crystal oscillator slaved to a GPS clock, is depicted below.
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Armed with a Thunderbolt GPS-Disciplined Clock donated by Trimble Navigation Ltd., SETI League president Richard Factor achieves frequency accuracy approaching that of an atomic clock, from anywhere in the world. See this press release for further details.
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Finally, a construction article describing a similar amateur project appears in the July 1998 issue of QST, the journal of the American Radio Relay League.
