Minimal data loss expected from SOHO antenna problem
BY STEPHEN CLARK
Posted: July 3, 2003
Despite what at first appeared to be a serious problem with the SOHO solar observatory, engineers have now discovered a way to save the vast majority of science data that was once believed to have been lost.
Ground controllers believe they have traced the problem to the dish-shaped antenna's drive motor or gear system that is responsible for turning the high-data-rate communications beam toward Earth to transmit information from its instruments as they study the Sun.
Because the problem likely lies within a mechanical system aboard the spacecraft, a software patch that could be uplinked to SOHO probably would not help the situation.
As expected, communications with the high gain antenna ceased on June 27 as the now-fixed beam slid away from Earth as SOHO orbited a million miles away from the planet at the LaGrange point L1, where the gravity from the Earth and the Sun are equal and effectively cancel each other out.
The team was still in contact with the 8-year old observatory through its omni-directional low gain antenna, which does not require precise pointing, but also cannot handle the same rate of communications and volume as the high gain antenna.
Under normal conditions, science data is picked up from SOHO by a 26-meter station in NASA's Deep Space Network. The 26-meter station lost the high gain antenna signal on June 27 as engineers had surmised, but a larger 34-meter station continued to receive science data for three more days until Monday, June 30.
Also, an even larger 70-meter NASA DSN station in Madrid, Spain, received high-rate science data via SOHO's low gain antenna last Monday.
On Tuesday, controllers were able to switch SOHO to a medium-rate telemetry mode. This allows the craft to downlink real-time science data from its low gain antenna when in view of a ground station.
Scientists are pleased with this latest development because it keeps scientific telemetry streaming down to Earth for the vast majority of the nearly three-week periods that officials once feared would be a total loss in terms of science data.
There will still be some cutouts in SOHO science coverage when Earth is out of contact with the high gain antenna because the larger diameter dish stations are committed to supporting a number of other missions currently throughout the solar system.
"We're now talking only moderate fractions per day every day during the 2-3 week periods," said ESA SOHO project scientist Bernhard Fleck.
Full-time communications with the high gain antenna is expected to be regained on about July 14 after a July 8 maneuver to roll the spacecraft 180 degrees.
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