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January 21, 2009 > TechKnow Talk: GPS technology: getting from here to there

TechKnow Talk: GPS technology: getting from here to there

Forty years ago, when the TechKnow Guy learned to drive, directions might have included phrases such as, "Turn left just past the grocery store" or "Look for an old barn and an elm tree on top of a hill." Today, the savvy traveler simply keys in the destination address and follows colorfully-illustrated directions, updated continuously until arrival.

This is made possible by the Global Positioning System (GPS). This navigational satellite "constellation" was built for the U.S. military, but was made available for use by everyone worldwide in 1983, partially as a response to the shooting down of Korean Air Lines Flight 007, when a navigational error caused it to enter prohibited airspace over the Soviet Union. The current constellation, sufficiently accurate to guide us through city streets, was completed in 1995.

Each satellite is roughly the size and mass of an automobile and is powered by solar panels and on-board batteries. The complete constellation consists of 24 satellites orbiting about 12,500 miles (20,200 km) above the Earth, though at any given time there are also several operating spares in orbit. Each satellite is assigned to one of six orbital paths or "planes," with each plane separated by 60 degrees, providing full 360 degree coverage of the entire planet.

Each satellite orbits the Earth every 12 hours, and passes over the same spot on the ground approximately every 24 hours, as the Earth rotates into the same position beneath it. Each satellite continuously broadcasts a radio-frequency signal (at 1575.42 MHz), containing information on which satellite it is, its precise position, and the exact time.

The U.S. Air Force monitors and operates these satellites, using a network of antennas spread around the globe. Signals are sent to the satellites to synchronize their extremely accurate clocks, and to make fine adjustments to their positions. The GPS ground control operations are headquartered at Schriever Air Force Base near Colorado Springs.

The portion of the system familiar to most of us is the receiver. The critical components of the receiver are an antenna to receive the satellite signal and a clock. As the constellation moves above, a receiver with a clear view of the sky can "see" at least five satellites at all times, and often more. This contact must be "line of sight." Though trees, overpasses, and other relatively small obstructions will not impede the very long wavelength satellite transmissions, large buildings and the Earth itself do block the signal.

To calculate an accurate position, receivers must contact and receive data from at least four satellites. The receiver compares the time the signal was sent to the time it was received. Since the signal travels at the speed of light, a simple calculation determines the distance between the satellite and the receiver. This is done for each of the four or more satellites from which the receiver is in contact.

Knowing its distance from a satellite, the receiver mathematically constructs a sphere of that radius from the position of the satellite. Repeating the process for each satellite, these spheres will intersect at only one location, the position of the receiver on the surface of the Earth (or in the case of an airplane, above the surface). The more satellites involved in the calculation, the better the potential accuracy of the result. The current GPS system is capable of providing a position accurate to within about 10 feet, more than sufficient for getting around by car.

To achieve this accuracy, the satellites use extremely precise, and extremely expensive, atomic clocks. As these would be unaffordable for a consumer receiver, the ordinary quartz clock in the receiver resets itself during use. Since it can only be at one place at a given time, the spheres must always intersect at a single point. By examining the slight inaccuracies in these intersections, it mathematically infers its deviation from the proper time based on the orbiting atomic clocks, and corrects itself accordingly. This is one of several very clever design features in GPS.

Inaccuracies are introduced by a number of factors. For example, various atmospheric conditions can influence the signal. Signals from satellites positioned lower on the horizon travel through much more air than those more directly overhead. Solar flares may also disrupt the signal. There is also the potential for the signal to reflect off large buildings or natural features prior to reaching the receiver, a circumstance which would increase the distance and time the signal traveled. Any slight perturbation in orbital path also introduces error.

GPS designers and operators have become very sophisticated in correcting for these and other sources of error. So much so that the Air Force deliberately introduced a random inaccuracy of about 100 feet into the GPS signal for civilian use. They encrypted the most accurate signal, reserving it for military use only.

The intent was to frustrate efforts to use GPS to guide enemy missile strikes or for other operations. This inaccuracy was turned off in 2000, and everyone now receives unadulterated positioning information. GPS satellites do, however, continue to transmit an encrypted signal at a different frequency for military use.

GPS technology is increasingly being used for non-navigational purposes. It has become a huge aid in surveying and map-making. Receivers have also been placed along earthquake faults to accurately measure earth movement over time. Because the system delivers highly accurate time information, it is used in scientific research as well.

GPS satellites are designed for an 8-10 year life, though some have been in service for as long as 16 years. As old satellites fail, they are replaced with more sophisticated models. As upgrades to both on-orbit and receiver components continue, additional civilian capabilities are becoming available, including access to more signal channels offering greater accuracy.

A host of receiver-based services beyond simple navigation are also being offered. Examples include the ability to locate the nearest Thai restaurant, gas station, hotel, or casino as well as guided tours of cities, historical districts, or scenic areas. In addition, GPS receiver technology is now available from several cell phones service providers.

A new generation of GPS satellites, commonly called GPS III, has been contracted by the Air Force and is under development, with initial launches anticipated about 2014. This constellation will provide much stronger signal strength, making it more difficult to jam, and will have the capability to turn off the signal to localized geographical areas, both features useful for military operations.

For civilians, it will provide at least three signal channels and greatly improved accuracy. The ultimate positioning accuracy of GPS III available for civil use will be in the range of about 5 feet.

The ability to accurately pinpoint location anywhere on Earth and navigate a ship, airplane, or car with confidence to unfamiliar destinations is nearly taken for granted today. But only a few decades ago it was science fiction fantasy. Given the tremendous benefits it offers to millions of people, GPS has been described as the world's greatest free utility.

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