ESA/NASA’s solar watchdog, SOHO, is back to full operation after its predicted
9-day-long high-gain antenna blackout. Engineers and scientists are now
confident that they understand the situation and can work around it in the
future to minimise the data losses.

Since 19 June 2003, SOHO’s high-gain antenna (HGA), which transmits high-speed
data to Earth, has been fixed in position following the discovery of a
malfunction in its pointing mechanism. This resulted in a loss of signal through
SOHO’s usual 26-metre ground stations on 27 June 2003. However, 34-metre radio
dishes continued to receive high-speed transmissions from the HGA until 1 July 2003.

Since then, astronomers have been relying primarily on a slower transmission
rate signal, sent through SOHO’s backup antenna. It can be picked up whenever a
34-metre dish is available. However, this signal could not transmit all of
SOHO’s data. Some data was recorded on board, however, and downloaded using
high-speed transmissions through the backup antenna when time on the largest,
70-metre dishes could be spared.

SOHO itself orbits a point in space, 1.5 million kilometres closer to the Sun
than the Earth, once every 6 months. To reorient the HGA for the next half of
this orbit, engineers rolled the spacecraft through a half-circle on 8 July
2003. On 10 July, the 34-metre radio dish in Madrid re-established contact with
SOHO’s HGA. Then on the morning of 14 July 2003, normal operations with the
spacecraft resumed through its usual 26-metre ground stations, as predicted.

With the HGA now static, the blackouts, lasting between 9 and 16 days, will
continue to occur every 3 months. Engineers will rotate SOHO by 180 degrees
every time this occurs. This manoeuvre will minimise data losses. Stein Haugan,
acting SOHO project scientist, says "It is good to welcome SOHO back to normal
operations, as it proves that we have a good understanding of the situation and
can confidently work around it."

Related articles

* SOHO’s antenna anomaly: things are much better than expected
* Antenna anomaly may affect SOHO scientific data transmission

More information

* More about SOHO
* The very latest SOHO images
* ESA Science homepage


As often occurs in life, there is bad news and good news. High-rate
transmissions (used to send its images and data) from the Solar and Heliospheric
Observatory (SOHO) were initially interrupted on 27 June 2003. The interruption
was expected due to a recent malfunction in the pointing mechanism of the
spacecraft’s high-gain antenna (HGA). For 2-3 week periods every 3 months, its
high gain antenna will not be able to transmit any data back to Earth.

However, SOHO engineers found that medium-rate transmissions can be sent through
its two omni-directional, low gain antennas when larger NASA Deep Space Network
receiving stations are available. On the largest stations, with 70 meter dishes,
even high-rate transmissions are possible. Medium-rate transmissions contain
real-time science data, but does not have the capacity to dump on-board
recordings of earlier gaps in contact.

Given this newfound capability, SOHO expects to experience only moderate data
losses every day during the recurring 2-3 week periods. Full transmission is
expected to return about 14 July 2003 until the next orbital problem period,
expected to start on 22 September 2003. Scientists are quite relieved as SOHO is
the only spacecraft that can effectively monitor certain solar storms, which can
generate effects felt on Earth.