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Since October 28 and extending through November 13, MIT and Russian scientists have been testing a new technique to map the sky--specifically the ionosphere, or upper atmosphere.
The ionosphere, a highly variable medium found at an altitude of 100 to 1,000 kilometers, affects all radio arth or into space. The new work is important because it could lead to an inexpensive technique to map the ionosphere continuously on a global sacle.
"With continuous coverage you could call up a map of the ionosphere much like weather forecasters now call up weather maps," said John C. Foster, assistant director of the Haystack Observatory and a principal investigator for the work. Such an ability would greatly aid scientists' understanding of the ionosphere.
The current collaboration involves scientists from MIT's Haystack Observatory, Moscow State University and the Polar Geophysical Institute in Murmansk. In addition, scientists from the US Air Force have set up a similar experimental apparatus that will allow them to compare their approach to that of the Russians.
The technique, known as ionospheric radio tomography, involves a satellite that sends radio signals through the ionosphere to receivers located at intervals on the ground. By analyzing the radio signals once they reach Earth, scientists can determine variations in the density of the electrically charged gas that makes up the ionosphere. From there, they can map these variations to get the general structure of the ionosphere, including small-scale local phenomena.
The technique could lead to global maps of the ionosphere because the receivers involved are small and portable, and they could be distributed around the world. In contrast, the large radar facilities currently used to produce images of the ionosphere-there are four such facilities in the world, including the Millstone Hill Research Radar at Haystack-are "much more expensive to build and operate," Dr. Foster said, precluding a large world-wide network.
For the experiment now underway-officially known as the Russian-American Tomography Experiment (RATE)-the scientists placed four receivers provided by the Russians in a north-south line along the northeastern US and eastern Canada. They are located in Robervale, Quebec; Jay, VT; Nashua, NH, and Block Island, RI. (The antenna for the receiver in Nashua is actually on the roof of the Russians' apartment building there.)
A satellite also provided by the Russians flies over these sites, and sends down radio signals every hour to all four receivers simultaneously. The data are then sent to the Haystack Observatory (and later to Moscow), where they will be analyzed to produce an image of the ionosphere using mathematics developed by the Russians.
The scientists will then compare the images produced via the experimental tomographic technique to actual images of the ionosphere made over the same period from the Millstone Hill radar facility. By doing so, they will be able to determine the accuracy of the tomographic technique, and they can use the resulting data to improve the satellite hardware and refine the mathematics.
"The goal is to improve the images from the tomographic technique so that they more closely resemble those from the [Millstone Hill] radar," Dr. Foster said.
Concurrently, the US Air Force scientists are conducting a similar experiment with US receivers at the same four sites and a US satellite overhead. "So we'll be intercomparing the Russian and US techniques, in addition to comparing the resulting data from both to the Millstone images," Dr. Foster said. The results will be interesting because the two countries have approached ionospheric radio tomography using mathematical techniques "that are completely different," he said.
Over the coming year, "the MIT scientists will be going to Moscow and Murmansk to participate in the analysis and interpretation of the data collected from the campaign," Dr. Foster said. They also plan to write a paper on the work, sharing drafts via e-mail.
RATE is funded by the NSF. A total of four Russian and eight MIT scientists are involved in the experiment. The Russians from Moscow State University are Professor Vyatcheslav E. Kunitsyn (who is also a principal investigator) and Dr. Elena Andreeva; those from the Polar Geophysical Institute are Professor Evgeny Tereshchenko (another principal investigator) and Dr. Boris Khudukon.
Those MIT scientists most directly involved in the work are Dr. Foster and Dr. John Holt at Haystack. In addition, three technicians are at the remote field sites. They are Mr. Steve Sawicki (at the Canadian site), Mr. Chris Farrell (in Vermont), and Mr. Alex Carson (in Rhode Island). Other Haystack scientists involved are Dr. Dwight Sipler, Mr. Tab Gallardo, and Mr. Aaron Pailes.
A version of this article appeared in the November 10, 1993 issue of MIT Tech Talk (Volume 38, Number 13).