How Does GPS Determine Your Location?
Many of us have been rescued from unfamiliar territory by directions from a Global
Positioning System (GPS) navigator. GPS satellites send signals to a receiver in
your GPS navigator, which calculates your position based on the location of the
satellites and your distance from them. The distance is determined by how long it
took the signals from various satellites to reach your receiver.
Where Are the Satellites?
The system works well, and millions rely on it every day, but what tells the GPS
satellites where they are in the first place?
"For GPS to work, the orbital position, or ephemeris, of the satellites has to be
known very precisely," said Dr. 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."
All of the Landmarks Are Moving
This is not as obvious as simply looking at the Earth – space is not conveniently
marked with lines to determine our planet's position. Even worse, "everything is
always moving," says Ma. Earth wobbles as it rotates due to the gravitational pull
(tides) from the moon and the sun. Even apparently minor things like shifts in air
and ocean currents and motions in Earth's molten core all influence our planet's
orientation.
Just as you can use landmarks to find your place in a strange city, astronomers
use landmarks in space to position the Earth. Stars seem the obvious candidate, and
they were used throughout history to navigate on Earth. "However, for the extremely
precise measurements needed for things like GPS, stars won't work, because they are
moving too," says Ma.
Finding Landmarks With Undetectable Motion
What is needed are objects so remote that their motion is not detectable. Only a
couple classes of objects fit the bill, because they also need to be bright enough
to be seen over incredible distances. Things like quasars, which are typically
brighter than a billion suns, can be used. Many scientists believe these objects
are 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 cosmos, over a billion light years
away, and are therefore distant enough to appear stationary to us. For comparison,
a light year, the distance light travels in a year, is almost six trillion miles.
Our entire galaxy, consisting of hundreds of billions of stars, is about 100,000
light years across.
Quasars as Celestial Landmarks
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 four years using painstaking analysis of observations on
the positions of about 600 objects.
Ma led a three-year effort to update and improve the precision of the ICRF map by
scientists affiliated with the International Very Long Baseline Interferometry Service
for Geodesy and Astrometry (IVS) and the International Astronomical Union (IAU).
Called ICRF2, it uses observations of approximately 3,000 quasars. It was officially
recognized as the fundamental reference system for astronomy by the IAU in August, 2009.
Making the ICRF Map
Making such a map is not easy. Despite the brilliance of quasars, their extreme
distance makes them too faint to be located accurately with a conventional telescope
that uses optical light (the light that we can see). Instead, a special network of
radio telescopes is used, called a Very Long Baseline Interferometer (VLBI).
The larger the telescope, the better its ability to see fine detail, called spatial
resolution. A VLBI network coordinates its observations to get the resolving power
of a telescope as large as the network. VLBI networks have spanned continents and
even entire hemispheres of the globe, giving the resolving power of a telescope
thousands of miles in diameter. For ICRF2, the analysis of the VLBI observations
reduced uncertainties in position to angles as small as 40 microarcseconds, about the
thickness of a 0.7 millimeter mechanical pencil lead in Los Angeles when viewed
from Washington. This minimum uncertainty is about five times better than the ICRF,
according to Ma.
These networks are arranged on a yearly basis as individual radio telescope
stations commit time to make coordinated observations. Managing all these coordinated
observations is a major effort by the IVS, according to Ma.
Removing the Noise of Earth's Motion
Additionally, the exquisite precision of VLBI networks makes them sensitive to many kinds
of disturbances, called noise. Differences in atmospheric pressure and humidity caused
by weather systems, flexing of the Earth's crust due to tides, and shifting of antenna
locations from plate tectonics and earthquakes all affect VLBI measurements. "A
significant challenge was modeling all these disturbances in computers to take them
into account and reduce the noise, or uncertainty, in our position observations," said Ma.
Another major source of noise is related to changes in the structure of the quasars
themselves, which can be seen because of the extraordinary resolution of the
VLBI networks, according to Ma.
ICRF Maps, Spacecraft Navigation and Astronomy
The ICRF maps are not only useful for navigation on Earth; they also help us find our
way in space -- the ICRF grid and some of the objects themselves are used to
assist spacecraft navigation for interplanetary missions, according to Ma.
Despite its usefulness for things like GPS, the primary application for the ICRF maps
is astronomy. Researchers use the ICRF maps as driving directions for telescopes.
Objects are referenced with coordinates derived from the ICRF so that astronomers
know where to find them in the sky.
Also, the optical light visible to our eyes is only a small part of the electromagnetic
radiation produced by celestial objects, which ranges from less-energetic, low-frequency
radiation, like radio and microwaves, through optical light to highly energetic,
high-frequency radiation like X-rays and gamma-rays.
Astronomers use special detectors to make images of objects producing radiation our eyes
can't see. Even so, since things in space can have extremely different temperatures,
objects that generate radiation in one frequency band, say optical, do not necessarily
produce radiation in another, perhaps radio. The main scientific use of the ICRF maps
is a precise grid for combining observations of objects taken using different frequencies
and accurately locating them relative to each other in the sky.
Astronomers also use the frame as a backdrop to record the motion of celestial objects
closer to us. Tracing how stars and other objects move provides clues to their origin
and evolution.
Updating Celestial Landmarks and ICRF Maps
The next update to the ICRF may be done in space. The European Space Agency plans to
launch a satellite called Gaia in 2012 that will observe about a half-million quasars.
Gaia uses an optical telescope, but because it is above the atmosphere, the satellite
will be able to clearly see these faint objects and precisely locate them in the sky.
The mission will use quasars that are optically bright, many of which are too dim in
radio to be useful for the VLBI networks. The project expects to have enough
observations by 2018 to 2020 to produce the next-generation ICRF.
ICRF2 involved researchers from Australia, Austria, China, France, Germany, Italy, Russia,
Ukraine, and the United States; and was funded by organizations from these countries,
including NASA. The analysis efforts are coordinated by the IVS. The IAU officially
adopts the ICRF maps and recommends their occasional updates.
Bill Steigerwald
NASA Goddard Space Flight Center
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| This is an artist's concept of a quasar (bright area with rays) embedded in the center of a galaxy. Image by T. Pyle (SSC) NASA/JPL-Caltech |
| A sky map of the 295 defining sources of ICRF2. The dashed line represents the ecliptic and the solid line is the galactic plane.
Image by Dave Boboltz, USNO. |
| A radio telescope at the Kokee Park Geophysical Observatory, NASA's VLBI station in Hawaii, one of the most active
sites in the global geodetic/astrometric VLBI network. Image by US Navy / PMRF. |
| Historical Geology in the News |
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