Engineers battle to overcome antenna problem on SOHO

Posted: June 22, 2003

One of the world's premiere Sun-watching observatories has suffered a glitch that threatens to hamper its future studies of our nearest star.

An illustration of the SOHO mission to study the Sun. Credit: NASA/ESA
The joint NASA/ESA Solar and Heliospheric Observatory was launched into space in 1995, and has since collected information and images of the Sun and its sweltering surroundings for almost eight years. But a recent problem with the high-gain antenna's pointing mechanism has ground controllers scratching their heads for a solution.

A spacecraft maneuver on June 18 confirmed an earlier reading the SOHO fault monitoring system had detected -- that the high-gain antenna was not moving as it should. Extensive testing since then has narrowed the problem to the horizontal axis of the dish-shaped antenna's movement.

Because similar attempts to move the antenna failed in two different strings, engineers have determined that the source of the problem is likely the Moog-built mechanical drive motor or gear assembly that physically steers the antenna.

The communications beam provided by the high gain antenna is about 14 degrees wide, but can be expanded to 25 degrees using still-functioning motors to allow for longer periods of contact with SOHO.

An artist's concept of the SOHO orbit. Credit: NASA/ESA
The operations team were able to guide the sluggish antenna toward what officials call the "sweet spot" in space that allows for the least interruption in communications through the high gain antenna, which requires pointing in the vicinity of Earth to make contact.

The current position of the dish antenna would only limit communications during 19 days out of every three months. The first communications cutout is expected to begin late this week.

SOHO orbits around a LaGrange point located about a million miles toward the Sun from the Earth where the gravitational pull from both bodies is equal, forming a stable point in space.

Although this latest development has no direct impact on the health and safety of the spacecraft itself, science operations will be cut by at least some amount because the now-fixed high gain antenna communications swath will not be able to send or receive data or commands.

During these periods of communications blackout through the high gain system, ground controllers will still be able to control the spacecraft and assess its status and health through omni-directional low gain antennas. But SOHO officials believe they will be unable to receive science data during these blackouts.

SOHO's antennae configuration is shown in this illustration. Credit: NASA/ESA
Engineers are unable to ascertain whether the problem can be resolved in any fashion, partly because the exact nature of it is still unknown. Ideas have been floated around to further minimize the scientific loss, including altering the craft's orbit around its L1 LaGrange point if there is enough fuel available.

Now in its eighth year, SOHO has far outlived its original mission baseline of two years. Propellant was originally loaded aboard SOHO to provide for six years of operations, but the launch and orbital injection were accurate enough to leave enough fuel on-board for several decades, ESA says.

Two subsequent mission extensions until 2003, then most recently until March 2007, will keep the spacecraft operational so it can observe the Sun during a complete 11-year solar cycle.

SOHO was manufactured by Matra Marconi Space -- now Astrium -- under contract from the European Space Agency. NASA has been responsible for the launch of the craft aboard an Atlas rocket, and also for mission operations.

It was five years ago this week that the ground team inexplicably lost contact with SOHO for about six weeks before finally regaining communications with the observatory. The craft again suffered another problem in late 1998 when its final gyroscope failed and engineers had to devise a software patch to allow for continued science operations in a gyro-less mode. Other relatively minor periodic science interruptions have also occurred throughout SOHO's mission.

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