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GSFC XMM-Newton GOF Status Report #165: XMM-NEWTON NEWS #83

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_/            XMM-NEWTON NEWS #83   ---    28-Oct-2008            _/
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               XMM-Newton Science Operations Centre at
                ESA's European Space Astronomy Centre,
  P.O. Box - Apdo. 78, 28691 Villanueva de la Canada, Madrid, Spain


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XMM-Newton to Restart Observing Next Week

On Saturday 18 October ESA lost contact with the XMM-Newton X-ray
observatory (see http://www.esa.int/esaSC/SEM268RTKMF_index_0.html).
XMM-Newton uses different ground stations depending on which gives
the strongest signal with one of the spacecraft's two antennas which
point in different directions. On Saturday evening XMM-Newton was
approaching perigee passage with the instruments in safe
configurations and communicating normally with the Santiago ground
station in Chile. After the spacecraft moved out of visibility from
Santiago it was expected to be picked-up by ESA's Villafranca ground
station in Spain about an hour later. The usual time-tagged command
had been loaded on-board to change the operating antenna to the one
pointing towards Villafranca. However, the telemetry signal from the
spacecraft was not detected at the expected time and standard
recovery procedures did not re-establish contact.

These activities were repeated the next day, but the problem was
still present even when other ESA ground stations were used. This
confirmed that the loss of contact was related to either an on-board
problem or a catastrophic event in orbit. On Monday 20 October
images of the track of XMM-Newton against the night sky taken by
amateur astronomers in Germany's Starkenburg observatory showed that
the satellite was still in one piece. Other ground based telescopes
at Zimmerwald in Switzerland of the astronomy institute of the
University of Bern and the ESA Space Debris telescope at Tenerife
(Canary Islands, Spain), the German FGAN radar near Bonn and NASA
and US Space Surveillance network also observed the satellite and it
was possible to confirm that it was in its expected orbit.

The next step on Tuesday 21 October was to use the more powerful ESA
35 meter ground station at New Norcia (Western Australia) which was
pointed in the direction of XMM-Newton using a special radio-science
mode. A weak signal was detected from the spacecraft helping confirm
suspicions that the antenna switch was stuck in an intermediate
position. Engineers at ESOC, supported by European Industry and
experts from other ESA sites, attempted to command the spacecraft,
but the commanding threshold was still not reached by around 8 dB.
Consequently ESOC declared a spacecraft emergency and requested
support from NASA's Deep Space Network Goldstone antenna. Due to its
location Goldstone provides visibility of the spacecraft when it is
very close to the Earth so allowing a higher signal power at the
spacecraft. ESOC sent commands that moved the antenna switch back to
its last working position and then managed to obtain radio contact
with the spacecraft with ESA's 15-metre ground station in
Villafranca on Wednesday 22 October around 18:10 Central European
Summer Time (CEST).

Since then XMM-Newton is safe and fully under control by the mission
control team at ESOC. There were no unexpected events during the
4-days without normal communications. Currently there are no plans
to move the antenna switch and investigations involving ESOC, ESTEC,
and industry experts are continuing. Until the failure mode is
better understood, we do not plan to use the available backup switch
and instead are concentrating on operational modes that do not
require the use of these switches:

* XMM-Newton will re-start observations on Monday November 3 using
   only one antenna as a temporary measure. Due to the favourable
   celestial geometry this will allow coverage of nearly all the
   orbit, expect for around 6 hours near perigee. The coverage
   available on antenna will slowly decrease over the next months,
   such that in around 6 months nearly all the orbit will only be
   visible with the second antenna.
* In parallel two operational concepts are being investigated which
   should allow nominal operations to resume by the end of November.
   The first concept envisages using both transponders simultaneously
   with each providing a signal to its own antenna. The second
   concept assumes that the transponders will be switched-on and -off
   to only power the antenna pointing towards the required ground
   station. Both concepts avoid having to use the antenna switch
   again and have advantages and disadvantages which will be
   evaluated in the next weeks before the deciding on the way

A.N. Parmar
XMM-Newton Mission Manager

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