03 February 2006
United States Updates Global Positioning System Technology
New GPS satellite ushers in a range of future improvements
Washington –A major upgrade of the U.S. Global Positioning System (GPS), and a range of upcoming improvements to the system that provides positioning, navigation and timing data to U.S. military and civilian users around the world were announced by a Department of Commerce official January 25.
Deputy Secretary David Sampson told a January 25 media forum that the Air Force launched in September 2005 the first in a series of next-generation satellites to augment the GPS which tens of millions of people worldwide use freely every day.
The satellite, declared operational in December 2005, broadcasts a second civilian signal called L2C that improves accuracy and reliability when combined with the original signal.
L2C was designed for commercial use. It is transmitted with a higher effective power to improve the performance of GPS receivers in urban areas and indoors. And it takes less energy to receive the signal, an important feature for battery-powered devices like mobile phones.
“We have a presidential mandate to improve the civilian capabilities of GPS to ensure U.S. technological leadership and competitiveness,” Sampson said.
“By continuously upgrading, we're improving civilian GPS performance while promoting its commercial acceptance and use,” he added.
GPS technology is in everything from cars, boats and airplanes to cell phones, wristwatches and computers. It is used in navigation, farming, mining, construction, surveying, taxicab operations, logistical supply-chain management, package delivery and science and technology applications.
Geologists use GPS to measure expansion of volcanoes and movement along fault lines. Ecologists use GPS to map differences in a forest canopy. Biologists can track animals using radio collars that transmit GPS data. Geographers use GPS to define spatial relationships between features of the Earth's surface.
GPS also contributes to public safety by preventing traffic accidents and reducing response times for ambulances, firefighters and other emergency services.
THE EARLY DAYS OF GPS
Like the Internet, which arose from a 1969 research project of the U.S. Defense Department, GPS began as a military research project in the 1960s and 1970s. The idea was to fly atomic clocks on satellites and use the data for navigation.
The system has several components: a constellation of 24 NAVSTAR satellites (operated by the U.S. Air Force) in Earth orbit with atomic clocks aboard, ground stations that control the system, five on-orbit spare satellites and receivers for users.
GPS satellite launches started in 1978, and second-generation satellites were launched beginning in 1989. The system became fully operational in 1995, with a signal for military users and a less-accurate signal for civilians, but the commercial market had begun to open up more than a decade earlier.
In 1983, Soviet jet interceptors shot down a Korean Air civilian airliner carrying 269 passengers that had mistakenly entered Soviet airspace.
Because crew access to better navigational tools might have prevented the disaster, President Ronald Reagan issued a directive guaranteeing that GPS signals would be available at no charge to the world when the system became operational. The commercial market has grown steadily ever since.
In 2004, President Bush issued an updated policy that keeps civilian GPS free of direct user fees.
HOW GPS WORKS
GPS satellites transmit signals to equipment on the ground. GPS receivers need a clear view of the sky, so current technology is used mainly outside and does not work well in mountainous areas or near forests or tall buildings.
Each GPS satellite transmits data that indicate its location and the current time. All GPS satellites synchronize operations so these repeating signals are transmitted at the same instant. Ground stations precisely track each satellite's orbit.
GPS satellites transmit signals on two main carrier frequencies -- L1 and L2. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others.
The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it calculates its position in three dimensions.
The accuracy of a GPS-determined position depends on the receiver. Most hand-held GPS units have 10-meter to 20-meter accuracy. Other receivers use a method called differential GPS (DGPS) for much higher accuracy.
DGPS requires one roving receiver and one receiver fixed at a known location nearby. Observations made by the fixed receiver are used to correct positions that the roving units record and are accurate to less than 1 meter.
When the GPS system was created, the Defense Department inserted timing errors into its transmissions to limit the accuracy of nonmilitary GPS receivers to 100 meters. This “selective availability” was eliminated in May 2000.
INTERNATIONAL DEVELOPMENT
Today, research is under way in the United States, Australia, France, the United Kingdom and Japan into “ubiquitous” positioning. The system would work everywhere, would be available all the time with a high level of precision and an acceptable cost, but it is still a long way from reality.
In the meantime, other countries -- including Russia, the European Union, Japan and China -- are developing their own international satellite navigation systems.
The Russian system, GLONASS (for Global Navigation Satellite System), is a radio satellite navigation system whose satellites began entering service in 1983. The system, operated for the Russian government by the Russian Space Forces, was complete in 1995.
Like GPS, the GLONASS constellation consists of 24 satellites -- 21 operating and three on-orbit spares. Because of troubled economic conditions in Russia, only about 14 satellites are now operating, according to media reports.
The Russians developed an advanced GLONASS satellite with an operational life of seven years and launched a three-satellite block of the new version on December 26, 2004. An even more improved GLONASS satellite, with reduced weight and an operational life of 10 to 12 years, is due to enter service in 2008.
In a 2005 joint venture with Russia, the Indian government agreed to share development costs of the improved GLONASS satellites and launch two of them from India. With this help, the Russians propose to have GLONASS operational again by 2008 with 18 satellites, and by 2010 with 24 satellites.
The European Union (EU) is building an alternative to GPS and GLONASS. The proposed Galileo positioning system will be a 30-satellite satellite navigation system that should be operational by 2010.
Galileo is intended to give users access to greater precision than is now available, according to the EU, and improve coverage of satellite signals at higher latitudes. Only one of Galileo’s planned four navigation services will be available at no charge to users.
Since 2003, several countries have joined the project – China (which is investing $296 million), Israel, Ukraine, India, Morocco, Saudi Arabia and South Korea.
Japan also plans to build a regional three-satellite positioning system called the Quasi-Zenith Satellite System (QZSS) that will supplement and be interoperable with GPS. The first satellite launch is scheduled for 2008, the second and third in 2009.
QZSS could improve regional service for positioning, timing and navigation users in Japan and surrounding areas, where mountainous terrain and population density sometimes make GPS unavailable.
China is also developing an independent navigation satellite system. The Twinstar Rapid Positioning System, or Beidou Navigation System, consists of two satellites in geosynchronous orbits.
Two satellites were launched in 2000, and China plans to complete the system with a second pair, media reports say. Another satellite was put into orbit in 2003. China is also associated with the EU Galileo system.
FUTURE GPS
Over the last decade, the United States has implemented several improvements to GPS service, including new signals for civil use and increased accuracy and integrity for all users.
Among these improvements are new satellite signals for civilian use – L2C, L5 and L1C.
L2C, available now, will improve as newer satellites are added to the GPS constellation. It will also be interoperable with Japan’s QZSS.
L5 will be available after the next improved GPS satellite launches later this year. L5 will transmit at a higher power than current civil GPS signals and have a wider bandwidth. It will be compatible with Galileo, GLONASS and QZSS. Its lower frequency may improve indoor reception.
L1C arose from an agreement on GPS and Galileo signed by the United States and EU member states to have a compatible and interoperable signal on the L1 frequency. L1C will have an advanced design and be broadcast at a higher power level.
“Global GPS sales have surpassed $20 billion a year,” said the Commerce Department’s David Sampson, “and will keep on growing at a healthy rate, according to industry estimates.”
More than 95 percent of GPS units sold, he added, are sold for civilian use.
“All of this extraordinary development and growth,” Sampson said, “is the result of consistent government policies that encourage civilian and commercial use of GPS.”
Additional information about the GPS is available on the Web sites of the National Oceanic and Atmospheric Administration and the U.S. Navy.
(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)
