Next-Generation GPS – Global positioning inches toward a makeover

HERE I AM– at least to within two meters or so, the current civilian resolution of GPS.

By Steven Ashley – Scientific American

Some 20 million people now regularly use Global Positioning System (GPS) technology, relying on signals emitted by 24-plus U.S. NavStar satellites orbiting the earth (20,000 kilometers up) at any one time. GPS geolocation proved indispensable during the Afghan and Iraq wars. Every day shipping firms track delivery trucks while backcountry trekkers pack handheld GPS units that guide them through pathless wilderness. Motorola, Nextel and other firms are building cell phones fitted with GPS chipsets. One company is even designing a tiny, implantable GPS sensor.

From some perspectives, there does not seem to be much room for improvement in ubiquitous GPS. Yet in fits and starts, U.S. officials have begun planning the next generation of satellite navigation technology, known as GPS III (the current system is the second generation). The driving forces are better accuracy and reliability, concern about more effective signal-jamming techniques, alternative geolocation services, and new, more sophisticated applications, such as intelligent highway and traffic-safety systems.

Soon the U.S. Air Force is expected to request proposals for two-year development contracts worth up to $25 million. Initial launch of a GPS III satellite may occur as early as 2010. Competitors for the multibillion-dollar program–Boeing and the combination of Lockheed Martin and Spectrum Astro–have indicated their interest.

Per Enge, director of Stanford University’s GPS Laboratory, sees three "megatrends" in the near-term evolution of GPS technology. The first is frequency diversity, which in fact is already being addressed as aging GPS II satellites are replaced periodically. When completed, the constellation of modernized orbiters will furnish civilian users with three new positioning signals. It will, moreover, provide U.S. armed forces with two additional signals that, being higher power, can better resist jamming. The extra frequencies afford redundancy to help fight timing errors resulting from ionospheric refraction of GPS signals, Enge states.

The second big trend concerns overcoming radio-frequency interference (RFI). "GPS broadcasts are extremely low power–equivalent to that of five lightbulbs," Enge explains. "With received power levels of 10-16 watt, the signal can be easily overwhelmed by nearby radio emitters." GPS receivers cut through the noise by matching the phase of the received ranging code with a replica code stored locally. When the wave phases align exactly, the receiving unit can use the timing of the signals as a precise reference and hence locate itself accurately. When deployed, so-called RFI hardening will permit the GPS receiver to double-check its calculations by keeping tabs on television and other terrestrial broadcast signals, which also employ this type of coding and emanate from well-known antenna sites.

Enge’s third GPS megatrend revolves around the installation of "integrity machines–systems that guarantee that the positioning error is smaller than a stated size." In July the U.S. Federal Aviation Administration brought online an enhanced-reliability GPS signal technology for guiding civil aviation. Called the Wide Area Augmentation System, the concept was developed by the FAA in cooperation with researchers at Enge’s Stanford lab and elsewhere. Employing what are known as differential GPS techniques, the system obtains updated error-correction information from communications satellites in geosynchronous orbit. The revised data derive from ground-based reference receivers that monitor incoming GPS broadcasts and characterize the degree of distortion. "The fact that a geolocation signal had a two-meter error yesterday says nothing about today," Enge says.

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