Feature: GPS Modernization
Professional Surveyor Magazine - February 2004
Bryn Fosburgh and Bruce Peetz
Technology changes, impacting the way we work. In the early 1980s, when the Global Positioning System (GPS) first became available to surveyors, no one knew it would become such an integral part of a surveyor's toolbox. The first GPS receivers for surveyors were as large as suitcases and had to be transported in trucks - and satellite signals were only available during a few windows of time each day. In comparison, with receivers the size of cell phones and signals available 24/7 in most parts of the world, the GPS system today seems "space-age." And it is.
In space—11,000 nautical miles (20,200 km) from Earth—the original ten Block I satellites launched from 1978-1985 have been replaced by 24 Block II satellites launched from 1989-1997 and five Block IIR (Replenishment) satellites launched since 1997. Today's 29-satellite constellation includes 20 Block II and nine Block IIR satellites. And today, the satellite constellation is looking at significant advances due to a government-funded program called GPS Modernization.
GPS Modernization offers significant benefits to surveyors, mappers and construction workers— as well as other civilian users of the system, including aviation, marine and automobile navigation, agriculture and asset management. Increased accuracy and robustness of measurements, enhanced productivity and the potential for extensive advances in receiver technology lie just ahead. It's an exciting time for GPS users.
To understand where GPS modernization is taking us, it helps to look at where we've been—and why. Here we'll focus on the civilian segment of the GPS Modernization process.
The GPS Constellation
Developed in 1973 by and for the United States Department of Defense (DOD), the GPS system provides positioning, timing, and navigation signals to both military and civilian users worldwide. Each satellite sends out two signals—L1 and L2—each of which includes pseudo-random codes used to gain positioning, timing and navigation information. These codes enable GPS receivers to track several different satellite signals at the same time, enabling calculations that provide precise positioning information anywhere on earth at any time.
Traditionally the L1 signal, broadcast at 1575.42 MHz, has the C/A or commercially available (also known as coarse acquisition) code; the L2, at 1227.6 MHz - and its P/Y code - has been reserved for military use.
Initially, commercial survey receivers were single frequency, using only the L1 - civilian -- signal for observations. These single-frequency receiver measurements required post-processing of the data in order to achieve survey-accuracy positioning. Within a decade, however, the scientific and surveying community developed the capabilities to use the encrypted military L2 signal; in order to do this, innovative ways were developed to use information gained from the L1 code to acquire the most robust use of the L2 signal. This allowed dual-frequency receivers to measure the time of arrival for two signals and correct for the ionospheric and tropospheric errors that accumulate over distance. Dual-frequency receivers were initially reserved for long-distance measurements—and then, as technology advanced, for real-time measurements.
While this way of using L2 works, it also results in noisier measurements than those gained from using the L1 signal directly. And that makes it harder to do things over longer distances. So to get more robust answers with dual-frequency receivers, a better option was needed: enter GPS Modernization.
M = Modernization
In 1998, the U.S. Government formally announced a modernization effort to extend the capabilities of the GPS system. GPS Modernization will add two new signals and increased signal power for civilian users over several years - as well as advanced features and new signals for military use.
The first new civilian signal will add a new commercial code—called Civil Signal or CS -- to L2, referred to as the L2C signal. This will enable receivers to access two clean signals for the error correction required for advanced Real-Time Kinematic (RTK) functionality. L2C will also include a more sophisticated code that modern receiver technology is capable of utilizing, potentially providing more robust results. The L2C signal will be broadcast at a higher power level than the current L2 signal, though still a bit weaker than L1.
The first satellite with this sophisticated technology—Block IIR-M (Modernized)—is currently scheduled to launch in 2004, with the exact date dependent on the DOD launch schedule. While built earlier, Block IIR-M satellites have been "modernized" with the new civilian code as well as two new military codes; the DOD plans to modernize up to 12 Block IIR satellites currently in storage. It's the first in a series of well-planned steps forward in satellite technology.
The second new civilian signal will be an entirely new signal: the L5 signal; broadcast at 1176.45 MHz, L5 will provide a higher power level than other signals. L5 will also have a larger bandwidth, enabling longer codes, which will make it easier to acquire and track weak signals. Because it was not feasible to put the L5 on the Block IIR-M satellites, a fourth generation of Block II satellites, called Block IIF, is currently being developed. These new Block IIF satellites will have all the modernized features of the IIR-M satellites, as well as the third civilian signal, L5. The first IIF satellite is slated to be launched in 2006.
So What's in it for Surveyors?
By adding the civilian L2C signal to the constellation, the DOD is making a strong statement to civilian users, especially surveyors. Surveyors have used the L2 signal more than any other civilian user in order to achieve centimeter-level accuracy for RTK surveying. By adding a civilian signal onto what was intended as a military channel, the DOD is recognizing the strength and importance of the civilian user community, of which surveyors form a very important group. Truly GPS can no longer be seen as primarily a military system.
While current receivers will remain effective—the Modernized GPS system has been designed to be fully backward compatible for existing and legacy GPS user equipment—new receiver technology entering the market today will enable surveyors to make use of the more advanced satellite capabilities as they become available.
And today? Since most surveyors use their equipment for years, those currently purchasing equipment can take these factors into consideration and protect their investment for many years to come. New solutions currently available can potentially enhance productivity substantially: today, it's important to ensure that equipment purchased—whether for field surveying or infrastructure use—provides the technology able to access the advanced capabilities of the new L2C signal as it becomes available tomorrow.
The Institute of Geological and Nuclear Sciences Limited (GNS) recently decided to purchase additional instruments to expand continuous global positioning system (CGPS) networks across the North and South Islands of New Zealand. GPS Modernization considerations were an important factor in their decision to purchase 40 L2C-ready Continually Operating Reference Station (CORS) GPS receivers.
"GNS needed a receiver suitable for remote, solar-powered sites, with low power consumption and a Linux-based Internet communications interface," said geophysicist Dr. John Beavan, one of the leaders of the GNS side of the project. "In addition, Trimble's next generation NetRS receivers will take advantage of the new civilian GPS signals, which make them ideal for long-term use."
The Ohio Department of Transportation made a similar decision to purchase L2C-ready CORS GPS receivers for the VRS (Virtual Reference Station) network they are building. Since reference stations in an infrastructure network often sit on a fixed site and aren't as easy to swap out as a rover receiver, surveyors and scientists investing in GPS infrastructure components gain even more from technology ready for GPS Modernization.
Surveyors are also likely to benefit from additional planned developments in space-based and other positioning technology in the future. Not only is GPS Modernization imminent, but the European Commission (EC) and European Space Agency's (ESA) Galileo program is also moving forward towards implementation towards the end of the decade. Both of these programs promise to enhance productivity and potentially add features and capabilities previously unforeseen. Aware of this, manufacturers are continually investing in research and development to be ready to utilize each technology as it becomes available.
In addition, new advances being developed by integrating inertial navigation systems (INS) with satellite-based systems such as GPS and Galileo will be able to provide users with optimum performance throughout a wide variety of environments and applications. GPS augmented with INS technology will potentially offer robust RTK positioning in situations where GPS satellite signals are obstructed in difficult environments such as urban high-rise or heavily forested areas. Manufacturers are researching products that integrate inertial technology, which will provide future benefits to surveyors as well as others.
Because of the time required to develop a new generation of satellites, the DOD is already planning system considerations through 2030. Currently, a team of scientists is researching the next generation of satellites, called GPS III. As currently conceived, there are few commercial improvements in GPS III over the IIF design. The main benefit for surveyors will be filling out the constellation with L5-equipped satellites, and continued replenishment of the system through 2030.
But, in keeping with the history of surveyors' use of GPS, who knows what advancements in receiver technology will occur before then - and what unforeseen benefits and new applications surveyors will gain. The sky's the limit. Literally.
is Vice President of Geomatics & Engineering at Trimble. Prior to Trimble, Fosburgh was a civil engineer with the Wisconsin Department of Transportation where he was responsible for coordinating the planning, data acquisition, and data analysis for statewide GPS surveying projects in support of transportation improvement projects.
Bruce Peetz is Vice President of Advanced Technology and Systems at Trimble. He has held a variety of positions at Trimble, including directing product development for both surveying and mapping.
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