Active Galaxy NGC 4428
Credits: NASA/ESA, Jeffrey Kenney (Yale University),
Elizabeth Yale (Yale University)
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Deep-space missions require precise navigation, in particular when
approaching bodies such as Mars, Venus or a comet. How precise?
It’s necessary to pinpoint a spacecraft 100 million kilometres from
Earth to within 1 km. To achieve this level of accuracy, ESA experts use
‘quasars’ – the most luminous objects known in the Universe – as
beacons in a technique known as Delta-Differential One-Way Ranging, or
delta-DOR
.
Quasars are fascinating objects that can emit 1000 times the energy of
our entire Milky Way galaxy. This prodigious luminosity originates from a
region only about the size of our Solar System. They are fuelled by
supermassive black holes – which might be billions of times as massive
as our Sun – feeding on matter at the centre of their host galaxies.
The image shows one such quasar galaxy, NGC 4438, 50 million light-years from Earth.
Because quasars are extremely bright and distant, they can be used as reference points for spacecraft navigation.
In the delta-DOR technique, radio signals from a spacecraft are received
by two separate ground stations, one, say, in New Norcia, Australia and
one in Cebreros, Spain, and the difference in the times of arrival is
precisely measured.
Next, errors due to the radio signals passing through Earth’s atmosphere
are corrected by simultaneously tracking a quasar – the coordinates of
which are precisely known.
“For delta-DOR to work, the quasar and the spacecraft should be within
10º as seen from Earth,” says Markus Landgraf, from ESA’s Mission
Analysis team.
“There are around 200 000 quasars known in the Sloan Digital Sky Survey,
and almost any of them are potential candidates to be used in delta-DOR
tracking.”
Once the location of the spacecraft derived from the ground stations is
compared to the known location of the quasar, engineers can apply
corrections, delivering a significantly more accurate fix on its
position.
“Quasar locations define a reference system. They enable engineers to
improve the precision of the measurements taken by ground stations and
improve the accuracy of the direction to the spacecraft to an order of a
millionth of a degree,” says Frank Budnik, a flight dynamics expert at
ESA.
Using the results of the delta-DOR processing together with the range and Doppler measurements,
which are also derived from the spacecraft signals received on ground,
ESA can achieve an accuracy in spacecraft location of just several
hundred metres at a distance of 100 000 000 km.
More information
Source: ESA
