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2\18 ESA's ARTEMIS satellite reaches geostationary orbit - from total loss
to full recovery
Part 1 of 2

Paris, 18 February 2003
Information Note	
Nx 04-2003

ESA's ARTEMIS satellite reaches geostationary
 orbit - from total loss to full recovery
=============================================

In the late afternoon of Friday 31 January, a final trim manoeuvre 
nudged ARTEMIS into its assigned position in geostationary orbit, 
completing a most remarkable satellite recovery operation which had 
lasted 18 months.

The unusual route taken by ARTEMIS to get to geostationary orbit was 
long and hard, and beset with unfamiliar problems.  But the mission 
was saved by the skills of a dedicated team of engineers and other 
specialists from the European Space Agency, Alenia Spazio, the prime 
contractor, Telespazio, responsible for satellite operations at the 
Fucino control centre, and Astrium, which designed the ion propulsion 
system, and by the use made of this experimental system, which had not 
been designed for such a task.  The ion propulsion system - originally 
on board ARTEMIS to control small motion around its nominal position - 
was the key to climbing the final 5000 km to reach geostationary 
height.

Due to a malfunction in its upper stage, Ariane 5 left ESA's
telecommunications satellite ARTEMIS in a lower than intended 
elliptical orbit. The apogee (maximum distance from Earth) was only 17 
487 km, far short of the targeted geostationary transfer orbit with an 
apogee at 35 853 km. A team of ESA and industry specialists responded 
vigorously with a series of innovative control procedures to rescue 
the spacecraft. Daring manoeuvres were executed and these proved not 
only very successful but also highly efficient. Using almost all of 
the available chemical propellant, ARTEMIS managed to escape the orbit 
in which it had to contend with the deadly Van Allen belts and safely 
reach a circular orbit at an altitude of 31 000 km only a few days 
after launch.

A long haul to geostationary orbit 
----------------------------------
Since then, the rescue efforts have continued unabated using the four 
ion engines mounted on the satellite redundantly in pairs. These novel
engines, instead of conventional chemical combustion engines, use 
ionised Xenon gas. They were originally designed only to control the 
satellite's inclination by generating thrust perpendicular to the 
orbital plane. The rescue operation however required thrust to be 
generated in the orbital plane to push the satellite to final 
geostationary orbit. This could be realised by rotating the satellite 
in the orbital plane by 90 degrees with respect to its nominal 
orientation.

Taking optimum advantage of the spacecraft flight configuration, new
strategies were developed not just to raise altitude but also to 
counter the natural increase in orbital inclination. To implement 
those new strategies, new onboard control modes, a new station network 
and new flight control procedures had to be put in place.

The new concept for steering the ion propulsion engines included 
entirely new control modes never before used on a telecommunication 
spacecraft, as well as new telecommand and telemetry and other 
data-handling interface functions. In all, about 20% of the original 
spacecraft control software had to be modified.  Thanks to the 
reprogrammable onboard control concept, these modifications could be 
loaded by uplinking to the satellite software "patches" amounting in 
total to 15 000 words, the largest reprogramming of flight software 
ever done on a telecommunications satellite.

By the end of December 2001 work on the new software had been 
completed, and it was subsequently validated using the spacecraft 
simulator as testbed. With the characterisation of the four engines 
all preparatory activities were completed and on 19 February 2002 the 
orbit-raising manoeuvre was started using only the ion propulsion 
system.

From the start of orbit-raising operations spacecraft controllers had 
to respond to many kinds of unforeseen situations, since the new 
strategy could only be tested realistically on the spacecraft itself. 
Unlike traditional pre-flight acceptance testing, no testbed is 
available to exactly replicate the current scenario.

Thanks to the extreme flexibility and the redundancy inherent in the
system design, steady progress in the orbit-raising process was
maintained, albeit at a lower rate than would theoretically be 
possible.  ARTEMIS - through dogged operation of its ion engines with 
their very modest thrust of only 15 milli-Newton - climbed on average 
at a rate of 15 km per day: like a small boat with one propeller 
pushing a big cargo ship!


Payload tests and performance
-----------------------------
Several months passed between arrival in the parking orbit and
commencement of orbit-raising manoeuvres. That time was used to carry 
out commissioning and payload performance verification.

In November/December 2001 payload tests were performed. These tests 
could only be done every fifth day, when the ARTEMIS feeder link 
antenna beam "illuminated" ESA's test station in Redu (Belgium). 
Further constraints arose from the fact that some payload frequencies 
can be used only when ARTEMIS is at, or close to, its nominal orbit 
position.

Nevertheless, enough opportunities were found to demonstrate that all
payloads (S-band and Ka-band and optical data relay, navigation and 
L-band mobile payload) were available and that their performance was 
in line with pre-launch results. In other words, that they fully 
complied with specifications.

Correct operation of the closed-loop tracking system for the Ka-band
inter-orbit antenna was also demonstrated. The antenna acquired a 
signal transmitted from Redu and maintained the link automatically 
while ARTEMIS drifted slowly across the sky.

World premiere while still far from its working position 

The most spectacular event was the demonstration of SILEX operations.

Following successful initial commissioning using ESA's optical ground
station on Tenerife, the optical link was established between ARTEMIS 
and SPOT 4. On 30 November 2001, for the first time ever, image data 
collected by a low-flying spacecraft were transmitted by laser to a 
(quasi-) geostationary satellite and from there to the data processing 
centre in Toulouse.

In total, 26 attempts were made to establish the optical link and all 
26 were successful. The link was never lost before the preprogrammed 
point in time. Link quality was almost perfect: a bit error rate 
better than 1 in 109 was measured. This means that 1 bit at most is 
received erroneously per 1 000 000 000 bits transmitted.

(continued)

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