That global positioning system (GPS) that guides your car
and keeps you on the map is itself guided by the positions of some of the brightest,
strangest objects in the universe — quasars.
GPS satellites send signals to a receiver in GPS
navigators down on Earth. These in turn calculate your position based on
the location of the satellites and your distance to them.
Millions have come to rely on the precision of this system
for a multitude of purposes, but to provide accurate position readings, the GPS
system itself has to have a point of reference.
"For GPS to work, the orbital position, or ephemeris,
of the satellites has to be known very precisely," said Chopo Ma of NASA's
Goddard Space Flight Center in Greenbelt, Md. "In order to know where the
satellites are, you have to know the orientation of the Earth very
precisely."
This can be a tricky prospect because from our perspective
on Earth, "everything is always moving," Ma said. For example, Earth
wobbles as it rotates due to the gravitational pull from the moon and the sun.
And even seemingly minor movements, such as shift in air and ocean currents and
motions in the Earth's molten core, all influence the orientation of the
planet.
To get a stable reference point, GPS systems have to turn to
the denizens of space.
Quasar collection
Stars might seem like the obvious signposts to use, since
their positions seem so steady to us here on Earth. But they won't work for GPS
"because they are moving too," Ma explained. The sun, for example, is
circling the center of our Milky Way galaxy, taking our solar system along with
it.
What are needed for the job are objects so remote that any
motion they have is undetectable from Earth. They also need to be bright enough
to be seen over the vast distance of space.
Quasars, which burn brighter than a billion suns, turn out
to be the perfect candidates.
Quasars are thought to be powered by giant
black holes feeding on nearby gas. Gas trapped in the black hole's powerful
gravity is compressed and heated to millions of degrees, giving off intense
light and/or radio energy.
Most quasars
lurk in the outer reaches of the universe, over a billion light years away and
so are distant enough to appear stationary from Earth's perspective.
A collection of remote quasars, whose positions in the sky
are precisely known, forms a map of celestial landmarks in which to orient the
Earth. The first such map, called the International Celestial Reference Frame
(ICRF), was completed in 1995. It was made over the course of four years and
based on the analysis of the positions of about 600 objects.
Ma recently led a three-year effort to update this map and
improve its precision.
New and improved
Called the ICRF2, this new map uses observations of about
3,000 quasars with the Very
Long Baseline Interferometer (VLBI).
The VLBI is a special network of radio telescopes from all
over the world that together have the resolving power of a telescope thousands
of miles in diameter. (Though quasars burn intensely brightly, their enormous
distance makes them too faint to locate accurately with a conventional optical
telescope.)
The new quasar observations were able to reduce the
uncertainties in the quasar positions to angles as small as 40 microarcseconds
— about the thickness of a 0.7-millimeter mechanical pencil lead in Los Angeles as viewed from Washington, D.C. This uncertainty is five times better than the
original ICRF.
The observations from the VLBI network must be cleaned of
noise from changes in atmospheric pressure, humidity and changes in the
structure of the quasars themselves.
The IAU officially recognized the ICRF2 in August.
This quasar map not only helps to guide navigation on Earth,
but is also used to help navigate spacecraft on interplanetary missions.
Astronomers also use them to guide telescopes and trace the motion of stars and
other celestial objects to look for clues to their origin and evolution.
The next update to the ICRF might be done from space, with
the planned European Space Agency satellite
Gaia that will observe about half a million quasars. That mission, slated
for launch in 2012, will use quasars that are too dim in the radio part of the
spectrum to be seen by the VLBI network.
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