DOD, DOT join forces to expand civilian use of GPS

The Defense and Transportation departments reached an agreement last month that ensures civilian users continued access to a second frequency from the Global Positioning System (GPS).

DOT officials called the agreement "essential" for applications such as the next-generation GPS-based air traffic control system. Also benefiting from the new signal will be applications that will create more accurate weather forecasting and the development of an earthquake warning system.

GPS is a DOD-created network of 24 satellites that individuals with widely available receivers can access to measure their exact position or that of an object anywhere on the globe or in the air. DOD uses a signal called L2 that can pinpoint positions within a range of five to 25 meters. A second signal for use by civilians called L1 is purposefully distorted for national security reasons and is accurate to 100 meters.

But the DOD/DOT agreement will allow civilian users to continue to access the radio frequency (RF) carrier emitted by the current generation of GPS satellites. Previous agreements between DOD and DOT only granted civilian users access to the L1 signal.

High-accuracy dual-frequency GPS receivers which can simultaneously receive the L1 and L2 signals will use the L2 carrier to correct for the delay on the L1 signal caused by the ionosphere which is the outer part of the Earth's atmosphere that slows down the signal thereby potentially causing an inaccurate position reading.

The new pact calls for development of a second civilian frequency for the next generation of GPS satellites known as Block IIF which are slated for launch in the next century. DOT officials said the DOD GPS joint program office has requested a proposal from Boeing North American the contractor for Block IIF for an alternative design providing a second civilian frequency.

Ensured access to the RF carrier from the L2 signal will allow the Federal Aviation Administration to move forward with its next-generation air traffic control system called the Wide-Area Augmentation System. WAAS will create a network of ground stations designed to make satellite positioning signals reliable enough for navigational use by aircraft.

Brian Mahoney the system engineering team leader for WAAS said the L2 carrier phase signal will allow for corrections of the ionospheric delay on the L1 signal providing aircraft using WAAS with the correction "we need in order to do precision approaches."

Brian Britigan the WAAS program manager for Hughes Aircraft Co.'s Command and Control Systems Division called guaranteed access to a second frequency essential to the success of the system. "DOD knew all along that when the FAA contracted for the WAAS design it needed a second frequency in order to perform corrections. In order to meet operational requirements we need two frequencies " he said.

William Strange a geodesist who serves as project chief for the National Oceanic and Atmospheric Administration's Continuously Operating Reference Stations GPS network said "We wouldn't be able to do anything without that signal."

Civilian users such as surveyors and earthquake researchers apply carrier phase corrections to help them obtain millimeter accuracy. NOAA uses carrier phase techniques to measure the water vapor in the atmosphere which could eventually lead to a significant improvement in forecasting severe weather.

Networks worldwide use these corrections to track the movement of tectonic plates in the Earth's crust - a use that is essential to the development of an earthquake early-warning system. Seth Gutman a systems analyst at NOAA's forecast research laboratory in Boulder Colo. said the ability to measure water vapor is contingent upon access to dual GPS frequencies with the L2 carrier phase signal performing well. If DOD does add a new civilian L5 signal to future GPS spacecraft "it could be used for improved ionospheric corrections " Gutman said.

Charles Trimble president of Trimble Navigation Ltd. said that from his perspective the "most significant part of the agreement is the stability it provides to users. [It] ensures them years in advance that they will have access to dual frequencies." Asked if industry can meet the demand for new receivers capable of handling the L5 signal Trimble answered "That's not going to happen until 2008 to 2010 and industry will have L5-capable receivers at least 10 years before then.