GPS satellites tell us where we are, but what tells them where they are?
By Jeff Salton
November 1, 2009
Global Positioning System (GPS) devices have permeated society to the point where millions of us rely on them daily for directions, locations and traffic avoidance (if only they could tell me where I left my car keys). GPS satellites send signals to a receiver in your handheld or car-based GPS navigator, which calculates your position on the planet 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. But have you ever thought 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 order to know where the satellites are, you have to know the orientation of the Earth very precisely."
Sounds easy when you say it quickly, but it’s not. Space isn’t conveniently marked out in grid lines like on a football field where it would be easy to determine Earth’s position. Making it even harder, everything in space is constantly on the move. The Earth even wobbles as it rotates due to the gravitation pull (tides) of the moon and the sun. Ma says even minor things like shifts in air and ocean currents and motions in Earth's molten core all influence our planet's orientation.
Stars too close
Just as you can use landmarks to find your place in a strange city (before you got your GPS), astronomers use landmarks in space to position the Earth. Stars have been used throughout history to navigate on Earth, but "for the extremely precise measurements needed for things like GPS, stars won't work, because they are moving, too," says Ma.
We need objects so remote that their motion is not detectable – they need to be far enough away for this to occur but also bright enough to be seen. Things like quasars, which are typically brighter than a billion suns, can be used. Scientists believe these objects are powered by giant hungry 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.
Thankfully for us, most quasars lurk in the outer reaches of the cosmos, over a billion light years away (one light year = 6 trillion miles), and are therefore distant enough to appear stationary to us. NASA says our entire galaxy, consisting of hundreds of billions of stars, is about 100,000 light years across.
A collection of remote quasars, whose positions in the sky are known precisely, forms a map of celestial landmarks that helps 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 of 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 and was officially recognized as the fundamental reference system for astronomy by the IAU in August this year.
Despite the brilliance of quasars, they can’t be seen with a conventional telescope because they’re just too far away. Ma and his team had to use a special network of radio telescopes is used, called a Very Long Baseline Interferometer (VLBI) to locate the quasars.
The larger the telescope, the better its ability to see fine detail, called spatial resolution. A VLBI network coordinates its observations into the equivalent of a single telescope as large as the entire network. VLBI networks have spanned continents and even entire hemispheres of the globe, delivering the power of a telescope thousands of miles in diameter. For instance, the VLBI observations reduced uncertainties in position to angles as small as 40 microarcseconds, about the thickness of a 0.7mm 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.
Sensitive to noise
VLBI networks are so sensitive they can detect 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.
Noise can also be generated by changes in the structure of the quasars themselves, which can be seen because of the extraordinary resolution of the VLBI networks, according to Ma.
The technique is set to become even more accurate when the European Space Agency (ESA) launches Gaia in 2012. This satellite will observe about 500,000 quasars and will likely provide the next ICRF update around 2020.
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