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Workshop tackles minute Earth measurements

More than 100 scientists are attending a workshop at the Haystack Observatory that features the latest techniques and instrumentation necessary to measure the wobbles, wiggles and shakes of the Earth down to a metaphorical gnat's eyebrow.

The NASA-sponsored workshop, entitled "VLBI Geodetic Operations," focuses on one application of VLBI, or Very-Long-Baseline Interferometry. VLBI is a technique that uses a global array of radio-astronomy antennas to simultaneously observe distant radio sources such as quasars. Originally conceived to produce ultra-high resolution images of those distant radio objects, Haystack scientists extended the VLBI technique in the late 1960s to make ultra-high precision geodetic and geodynamic measurements.

With measurement precision approaching one centimeter anywhere on Earth, the geodetic-VLBI technique was the first to directly measure the slow but steady motion of the Earth's tectonic plates. For the last 15 years, VLBI measurements have acted as the cornerstone of Earthorientation and rotation measurements made by the US and other countries. In the last 10 years, the use of geodetic VLBI has extended to more than 20 countries worldwide and is still growing, coordinated and led primarily by NASA's Goddard Space Flight Center.

Haystack Observatory, located in the hills of Westford, MA, has been close to the center of nearly all geodetic-VLBI developments since its inception, and has continued to develop and refine the geodetic-VLBI technique and instrumentation. Haystack scientists are recognized as world leaders in this art, and their work has been freely distributed worldwide.

Modern measurements require the use of a variety of technologies including electronics that are far more stable than any that are commercially available, highly accurate hydrogen-maser frequency standards, recording systems capable of continuous recording at hundreds of megabits per second, and highly complex correlation-processing systems capable of sustained operation at Gigabit/sec rates. New systems currently under development for NASA and the US Naval Observatory will push well beyond these current limits, with the goal of global measurements with a precision of one millimeter by the end of this decade.

Freeman Dyson, a prominent physicist and well-known science-fiction writer at Princeton, observed a few years ago that "VLBI is the most international of sciences." This is clearly evident in the international character of the Haystack meeting attended by representatives of more than 15 nations. Though each observing station or antenna operates independently, the need for compatibility of observing schedules, parameters and equipment requires constant worldwide communication and coordination to a degree that transcends political boundaries.

Those attending the Haystack workshop will learn the latest instrumentation and techniques necessary to insure collection of the highest quality data at each of their respective sites. Among the attendees are representatives from Australia, Brazil, Canada, Chile, China, England, Germany, Italy, Japan, the Netherlands, Norway, Russia, South Africa, Spain, Sweden, Ukraine and the United States, each of whom can help improve the quality of this global effort.

A version of this article appeared in the November 16, 1994 issue of MIT Tech Talk (Volume 39, Number 12).

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