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See Inside December 2008

How GPS Units Work

How handheld global positioning system devices can determine your position on Earth--even when indoors

This holiday season all kinds of products are coming equipped with GPS receivers to tell consumers exactly where on earth they are. The choices include dashboard navigators for cars, pocket navigators for humans, “golf buddies” that reveal the distance between a golfer and greens and sand traps, and, most prominently, cell phones. GPS transponders also now track paroled criminals, errant pets, migrating elephants and retreating glaciers.

Positioning satellites have been beaming signals for decades, but three converging factors are broadening the marketplace, according to Per K. Enge, a professor of aeronautics and astronautics at Stanford University. The size of the circuitry needed inside a receiver has shrunk. Circuit makers are selling that hardware to consumer electronics manufacturers for less than $5 a unit. “And Apple provided visibility,” Enge says, “by putting GPS into the iPhone.” Buyers loved it, proving there was a desire.

As handy as personal navigators might be, their accuracy has remained within five to 10 meters for years. They will improve only after satellites surrounding the planet are upgraded. The current satellites each send a single radio-frequency signal to civilian receivers, which then combine signals from four satellites to determine position. But the ionosphere can cause slight and random delays, introducing error into the receiver’s calculations. Next-generation satellites will each send three signals at slightly different frequencies, giving receivers the data they need to cancel out the delays, resulting in accuracy of less than 50 centimeters. The first three-frequency satellite is supposed to be launched in 2009.

Do civilians need such specificity? Perhaps not, although surveyors and scientists would. More enticing, Enge maintains, are systems that work better in large cities and indoors. GPS does not perform well in these places because buildings, roofs and walls block the incoming signals. So-called hybrid, or augmented, GPS receivers, which are now beginning to be deployed, also receive broadcasts from nearby television, cell phone or Wi-Fi (for “wireless fidelity”) transmitters that provide receivers with added data about position. These broadcasts can fill urban canyons and penetrate buildings. Similarly, smaller transmitters sprinkled throughout skyscrapers will eventually solve the long-standing “problem of the vertical,” which current GPS cannot do: determining not just that a visitor is at the Empire State Building, but whether he is on the seventh or 70th floor.

Did You Know ...
TRI AGAIN: Most colloquial descriptions of GPS say the technology uses triangulation to determine the position of a receiver on the earth. Mathematically speaking, the system uses trilateration. Triangulation determines position by measuring the angles of the triangles formed between an observer and three known points. Trilateration determines position by measuring the distance from an observer to three known points; the timing signals sent from GPS satellites (the known points) to a receiver determine those distances.

SATELLITE ENVY: More than 24 U.S. Navstar satellites make up the GPS system. Although it could provide service anywhere on the planet, some nations prefer independence. Russia has its own constellation called GLONASS; begun during the cold war, it was failing by the early 2000s but is being revived. Europe plans to complete its Galileo system by 2013. China is sending its Compass hardware aloft now. India wants to complete IRNSS by 2012—which would provide just enough satellites to cover its region. And Japan will float three satellites known as QZSS to augment GPS coverage over its territory.

Note: This article was originally published with the title, "Where On Earth You Are".

This article was originally published with the title "Where on Earth You Are."

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