To get around, satellites sailing through space use the same
tools that ancient mariners used to navigate the inhospitable
oceans — the stars. However, soon, instead of sending back
details of their position to experts here on Earth,
spacecraft will be able to calculate and adjust their course
all by themselves. ESA now has special software that it will
test on its SMART-1 mission, due for launch in Spring 2003.

The first sailors used stars as reference points to find out
where they were going. Generally, satellites use a ‘star
tracker’ instrument that allows them to maintain their
orientation in space using groups of stars as a reference
point. The spacecraft’s location in space is calculated
using radio signals transmitted between the spacecraft and
Earth. Experts process all these data here on the ground.
Once that is done, they work out what adjustments are
needed to the satellite’s course for it to reach its
proper destination. However, all of this is set to change
following the introduction of new technologies during the
SMART-1 satellite mission.

There is a more ‘intelligent’ system that allows the
spacecraft to determine its orientation autonomously. The
data obtained by a new generation of star trackers, called
‘star mappers’, are processed and then compared with a map
of the sky directly on-board the spacecraft.

However, the real revolution in spacecraft navigation is
OBAN, a new technology due to be tested on SMART-1. It
will allow the spacecraft to determine autonomously its
precise location in space, without being tracked by ground
stations. This is the most important step towards a new
generation of self-controlled spacecraft.

“Only once a spacecraft is able to know exactly its
position in space will it be able to decide by itself the
best trajectory to follow to reach a defined target. That
target maybe a planet or other celestial body, for
example.” says Andrea Marini, the engineer in charge of
the experiment. “Spacecraft will ‘understand’ where they
are located. They will be able to decide how to get to
their target and monitor their own course all the time.
This allows us to save precious fuel. It also allows the
spacecraft to correct its path almost continuously. And
the need for intervention from the ground is reduced,
with all the associated costs.”

It took scientists more than 10 years to develop OBAN.
During the mission they will run the OBAN software on
Earth, combining and processing the data coming from
the satellite’s star mapper and AMIE. AMIE is a special
camera that takes images of near-by asteroids and
planets. In the future the OBAN package will be installed
on-board spacecraft. However, SMART-1 will use traditional
navigation methods alongside OBAN, allowing ground
teams to test the accuracy of OBAN against their own
calculations.

Once the experiment is successfully completed, OBAN may
well allow spacecraft travelling all over the Solar
System to set their own courses. If they had only lived
to see it, the navigators on-board the old ships that
once sailed the Seven Seas on Earth would probably
approve of this independence.

USEFUL LINKS FOR THIS STORY

* More about SMART-1
http://sci.esa.int/smart

* More about OBAN
http://spdext.estec.esa.nl/content/doc/bd/19645_.htm

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=10&cid=12&oid=30911&ooid=17950]
Artist’s impression of testing smart technology to set your
own course.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=10&cid=12&oid=30911&ooid=21885]
One of the earliest Moon maps, a naked-eye drawing by
English physician William Gilbert in 1603 but only
published in 1651 after the advent of the telescope. Four
centuries later we are still exploring!