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European Space Agency

Press Release

SOHO's antenna anomaly: things are much better than expected

2 July 2003
 
After a number of tests and new insights, SOHO engineers now say
there will be no 'blackout' periods for SOHO science data. 
 
High-rate transmissions 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). The loss of signal occurred
on a 26-metre station of NASA’s Deep Space Network (DSN). 

Until 30 June 2003, however, the spacecraft continued beaming down
its science data, which were successfully picked up by larger
34-metre DSN stations (when available). In addition, dumping on-board
recorder data during these contacts has further reduced data losses
so far. 
 
On 30 June 2003, the 70-metre DSN station in Madrid, Spain,
successfully transmitted high-rate science data through SOHO's
omnidirectional on-board low-gain antenna. SOHO normally uses this
antenna only for low-rate telemetry in emergencies, and the antenna
does not need to be repointed. 
 
 
Successful switch
 
Even better, when high-rate telemetry was lost on 1 July 2003, during
a 34-metre station pass, engineers successfully switched SOHO into a
medium-rate telemetry mode, using the low-gain antenna. In medium
rate, all real-time science telemetry can be downlinked during
station passes. However, on-board recorder dumps are not possible in
this mode. 
 
The relatively late occurrence of the initial loss of contact means
that the effective SOHO’s HGA antenna beam width is larger than
anticipated. Also, since the 34-metre stations are much quieter than
the smaller stations, you can use them for longer time periods than
expected. Being able to transmit science data through the on-board
low-gain antenna using 70- and 34-metre stations therefore means that
there will be no hard blackout periods for SOHO science data, given
sufficient ground station resources.
 
 
Minor losses
 
However, 34- and 70-metre stations are in higher demand than the
26-metre stations that SOHO normally relies on. Minor data losses are
therefore inevitable every day during the 2-3 week periods. “We’re
now talking only moderate fractions per day every day during the 2-3
week periods,” says Bernhard Fleck, ESA’s SOHO Project Scientist. 

SOHO scientists expect full high-rate telemetry coverage, even on
26-metre stations, to resume on or about 14 July 2003. To achieve
this, they will make the spacecraft roll 180º around its Sun-pointing
axis in a manoeuvre currently planned for 8 July 2003.
  
For more information, please contact:
Dr. Bernhard Fleck, 
ESA SOHO Project Scientist
Tel: +1 301 286 4098
Fax: +1 301 286 0264
E-mail: bfleck{at}esa.nascom.nasa.gov

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