DOT plan adds second satellite signal

Global Positioning System suppliers are concerned about the cost-benefit trade-off of a Transportation Department plan that would add a second civil signal to the GPS satellite system.

Earlier this month DOT requested industry input about whether it should proceed with the option for a second civil signal as part of a $1.3 billion Air Force satellite upgrade contract awarded to Rockwell earlier this year. Designated L5, the second civil signal - to go along with the existing civil (L1) and military (L2) signals - potentially could improve the accuracy and availability of GPS for satellite receivers and GPS-based applications once it becomes available - some time around 2005. The option would cost about $28 million, DOT said.

However, the cost associated with taking advantage of the dual signals may outweigh the benefits to users, vendors said recently.

"Anything that would improve the accuracy of GPS is always welcome," said Jim White, spokesman for Magellan Systems Corp., San Dimas, Calif. "But we have to look at how that's achieved and what kind of expense it may add to the system."

DOT sees several potential benefits of a second civil signal. In particular, vendors might use that signal to improve the ability of their systems to calculate problems created by naturally occurring interference in the Earth's ionosphere. Sunspots and other activity can delay satellite signals to varying degrees, which, in turn, throws off the accuracy of the reading.

With two frequencies it would be easier to identify and correct for that delay, said George Wiggers, acting director for the radio navigation and positioning staff at DOT. Also, if a stable enough frequency is found, L5 could act as a backup signal for L1, which has proven vulnerable to interference, Wiggers said.

Additionally, users could expect even better accuracy once the Defense Department turns off the Select Availability feature that presently skews the L2 signal. The net result could be a globally available signal "well below the 10-meter accuracy," Wiggers said, which is close to the accuracy presently provided by costly local area differential systems. "That could provide some benefits to many of the user communities," he said.

However, DOT does not want to proceed with this contract option before getting feedback from industry, Wiggers said. "What we are asking is, if we put it on there, how would they use it," Wiggers said. "What are the benefits for having it on there?"

The second signal clearly would be important for some applications, particularly land surveying, automobile navigation and other areas where users need extremely accurate measurement. For example, for a land surveyor, a 25-meter error - not unusual for applications with a differential system - might land a reference point on the wrong side of the street, vendors said.

However, many corners of the GPS market "are driven by cost, and the cost will go up," said Len Kruczynski, director of strategic relationships at Ashtech Inc., Sunnyvale, Calif.

Other vendors agreed.

"Going to a dual-channel receiver certainly is going to add expense," White said. "For some users that is not a significant matter, but for many others - especially in the consumer GPS market - it is. We would be looking at adding expense and diminishing the size of the market if we do that."

Even the ability to correct for ionospheric interference is less of an issue than it was several years ago because the industry has learned techniques to work around the problem, vendors said.

"If this frequency had been available five years ago, a lot less money would have been spent in [research and development] in making use of the satellites," said Charlie Trimble, president and chief executive officer at Trimble Navigation.

It is possible the second signal would provide a more robust GPS environment "but that's nine years away, and we are going to have done an awful lot [more research]," Trimble said.