New satellite signals improve civilian use

The addition of two new civilian signals to navigation satellites widely used by federal agencies ranging from the Forest Service to the Census Bureau will provide greater accuracy and reliability with little or no increase in the cost of the receivers, according to top managers in the Global Positioning System industry.

The Defense Department-

developed GPS provides highly accurate position and navigation information based on triangulation from a constellation of 24 satellites. GPS was developed to serve the Pentagon's requirements for precise targeting and navigation, but one out of the two military frequencies on each satellite also provides less precise data to civilian users, including civilian agencies and private-sector users.

The decision to add two new signals for civilian users was announced by the Interagency GPS Executive Board, which jointly is led by representatives of DOD and the Transportation Department, which handles civilian-related GPS policy.

Charles Trimble, president of Sunnyvale, Calif.-based Trimble Navigation Ltd., a GPS receiver manufacturer, called the agreement "highly significant since it has [Vice President Al] Gore's name on it.... This is extremely positive because of [Gore's] clout.''

The agreement is expected to boost the value of GPS to the civilian community, Trimble said. Over the past decade the Pentagon has relegated civilian GPS users— who far outnumber military users— as second-class citizens with no real guarantees of availability or accuracy. The agreement reached last week provides civilian users with the guarantee of availability they need, Trimble said.

The additional signals will make it easier to correct for distortions induced by the iono-sphere as GPS radio waves travel from a satellite and through space to a receiver.

Trimble's company manufactures a wide line of professional-class receivers, and he does not envision it will cost much to deliver the added capability promised by the new civilian signals. GPS receivers run on firmware built into powerful processing chips, and adding the capability to handle the additional signals amounts to "tweaking the silicon.... Any additional cost [to receive and process the new signals] is minimal.... The cost of electronics is dropping 30 per cent a year,'' he said.

Todd Townsend, senior vice president for product development for Magellan Corp., agreed with Trimble that the new civilian signals will not drive up the cost of dual-frequency receivers. But Townsend also said that it will be well into the next century before the civilian community will gain any benefits.

The Air Force, which manages the GPS system, needs to build and launch a whole new class of satellites that can deliver the new signals. Townsend said that for many civil applications— such as surveying— users will need nearly a full constellation of 24 satellites to derive the promised accuracy and reliability, and it will take years to get the new signals fully deployed.

Mike Shaw, assistant for global positioning in the Pentagon's Office of the Undersecretary for Space, said that in his view "some users will be able to benefit from one satellite...while others will need the full constellation.'' The Pentagon plans to add the civil signals to GPS satellites built by Boeing North American Inc. and slated for launch in 2004. But, Shaw said, it may be possible to add the signals to satellites planned for launch as early as 2002.

Shaw said the new civil signals "will provide the robustness needed for safety-of-life'' systems, such as the Federal Aviation Administration's planned Wide-Area Augmentation System, which will provide enhanced GPS signals as navigation aids. The new signals, Shaw said, "are key to the FAA for WAAS.''

Earlier this year a report on critical infrastructures released by a presidential panel cautioned the FAA not to rely on GPS as its sole navigation system, due to concerns about potential jamming, redundancy and reliability. Townsend said the new civil signals will help make "GPS immune from narrowband, ground-based jamming.... If one signal is jammed, users can switch to another signal.''

Civilian users, such as surveyors who need centimeter-level accuracy, also stand to benefit from the new signals. Paul Spofford, a geodesist who also serves as the team leader for the National Geodetic Survey's GPS-based Continuous Operating Reference Stations, said the new signals should help reduce the heavy computational load needed to attain centimeter-level accuracy.


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