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2\18 ESA's ARTEMIS satellite reaches geostationary orbit - from total loss
to full recovery
Part 2 of 2

Ion propulsion to the rescue
----------------------------
After the hectic and exciting orbital recovery operations in the days
after launch, it was not easy to come to terms with the incremental
progress provided by the ion propulsion, and for those not involved in 
the satellite operations it must have seemed a monotonous and 
uneventful activity. Nothing could be further from the truth for the 
operators and engineers responsible for maintaining a steady rate of 
climb.

Since the new attitude control mode was commissioned in February, and 
the ion engines started to expand the orbit with an almost 
imperceptible thrust, the workload has been gruelling and almost every 
week has brought new problems to be solved. Although generally minor, 
these anomalies needed investigation and sometimes resulted in an 
interruption in effective thrusting, slowing progress.

In addition to careful monitoring and optimisation of the ion engine's
performance, the operators explored several different attitude control
techniques to orientate the spacecraft for the most efficient use of 
the impulse from the ion engines. The planning and sequencing of 
satellite mode changes, including regular updating of critical 
parameters, and the management of ground station contacts involved 
steady but considerable background tasks.

In October the satellite left the third and final eclipse season since 
its launch. During eclipse the earth's shadow hides the sun for some 
two hours each orbit and for reasons of power and attitude control the 
satellite has to be commanded from thrust mode to earth-pointing mode 
and the ion thrust turned off. These manoeuvres cost time and effort.


Final operations
----------------
With these difficulties behind them, the operators turned their 
attention to planning for the process of station acquisition in the 
geostationary orbit and initial operations on station.

At altitudes only a few hundred kilometres below the geostationary 
ring, it takes several weeks for the satellite to drift once around 
the Earth.  It is therefore important to avoid overshoot by tuning the 
drift rate to arrive at the designated station longitude (21.5 degrees 
East) just as the geostationary altitude is reached.

These orbital adjustments were made using small chemical propellant
thrusters, activated for the first time since launch.  The first 
thrust was performed successfully in December and two more in January, 
slowing the drift rate to a few degrees/day as the satellite made its 
last pass over Europe to arrive at its working position in 
geostationary orbit.

When the last manoeuvre was performed on 31 January it was an 
emotional moment. From the attitude control mode which had sustained 
the ion thrusting for so long, the satellite was turned to point to 
earth for normal operations and the ion thrusters themselves were the 
toast of the day. Ground controllers were able to stand down the 
network of ground stations around the world that had helped in 
commanding the satellite.

Now on station, ARTEMIS will function as originally planned and there 
is sufficient chemical propellant for 10 years' operation.

ARTEMIS arrives on station just when a significant community of users 
is waiting for it.   During its first few weeks in nominal orbit, an
exhaustive check-out of the ARTEMIS payloads has taken place using the
In-Orbit Test facilities at Redu, Belgium. All payloads are performing
well and the first optical link with SPOT-4 has also been established.

The satellite can now be made available to serve its first users: 
SPOT4, ENVISAT, EGNOS and EUTELSAT/Telespazio.  A preparatory test 
will also be made with NASDA's Earth observation mission ADEOS-II.  
Other users planning to use ARTEMIS in future include ESA's Automated 
Transfer Vehicle and Columbus elements of the International Space 
Station.

Not only has ARTEMIS clocked up a number of unique first-time 
applications during its recovery action - first optical inter-orbit 
satellite link; first major reprogramming of a telecommunications 
satellite; first orbital transfer to geostationary orbit using ion 
propulsion; longest ever operational drift orbit - but it will provide 
the promotional opportunity and stimulus for future European data 
relay services.  A promising future for this incredible mission!

For further information, please contact: 
Gotthard Oppenhäuser
Artemis Project Manager
ESA/ESTEC
Phone: 00 31 71 565 3168
Fax: 00 31 71 565 4093
e-mail:gotthard.oppenhauser{at}esa.int 

ESA Media Relations Service
Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690

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