Reflectors Would Support Secondary GPS 3 Applications

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WASHINGTON — Several U.S. government agencies are awaiting U.S. Air Force approval for a plan to include laser reflectors on the service’s next generation of GPS navigation and timing satellites, a $200 million investment advocates say would support a variety of scientific and mapping applications.

NASA has agreed to fund the project, entailing the installation of laser retro-reflectors on the second block of GPS 3 satellites. The reflectors would be used to determine more precisely the orbital location of the satellites, data that, when combined with GPS position-location and timing information, would improve the quality of various secondary GPS applications, according to U.S. government officials.

Among the agencies advocating, besides NASA, are the U.S. National Geospatial-Intelligence Agency (NGA), the National Oceanic and Atmospheric Administration and the U.S. Geological Survey. NASA is taking the lead because it has experience with the technology and because the reflectors would support agency efforts to model the Earth’s gravity field and improve its understanding of the Earth’s shape and distribution of mass, NASA spokeswoman Stephanie Schierholz said in an emailed response to questions.

The NGA, whose functions include mapmaking and collecting, analyzing and distributing satellite imagery, would use the information for more accurate time references and maps, NGA spokeswoman Karen A. Finn said in an emailed response to questions.

Plans call for integrating the laser retro-reflectors beginning with the second block of GPS 3 satellites under development by Lockheed Martin Space Systems of Denver. The GPS 3 program plan calls for launching eight Block 3A satellites starting in 2014, and beginning deployment of the upgraded Block 3B satellites, which would carry the retro-reflectors, in 2016.

The request to include the retro-reflectors on the GPS 3 satellites was submitted in 2005, which was too late to integrate the hardware on the first block of satellites, Air Force Space Command spokeswoman Christina J. Sukach said in an emailed response to questions. The request is under review in an interagency forum to determine whether it meets policy guidelines for secondary payloads aboard the GPS 3 satellites, the statement said. Assuming that condition is met, the retro-reflectors will be added to the GPS 3 requirements documents for further evaluation, the statement said.

According to NASA, the laser retro-reflectors would cost $500,000 each, plus ongoing support for tracking and analysis. The total cost would be $200 million through the lifetime of the GPS 3 constellation, Schierholz said.

NASA Administrator Charles Bolden informed the Air Force in June 2010 that the agency would provide equipment for the project and related tracking and analysis data to NGA.

NASA would tap the Jet Propulsion Laboratory in Pasadena, Calif., and the Goddard Space Flight Center in Greenbelt, Md., to develop the reflectors. Those centers have expertise in developing GPS receivers and laser ranging instrumentation for scientific research applications.

The retro-reflectors would reflect laser signals beamed from the ground back to their sources, revealing the satellites’ precise orbital location based on the time it takes for the light to make the round trip. Satellites tend to drift from their prescribed orbits over time, and this drift reduces the accuracy of certain high-precision GPS applications such as measuring the movement of the Earth’s tectonic plates. With better orbital location data, scientists can correct for the satellites’ drift.

The concept of combining GPS data with laser retro-reflector measurements is not new. Two GPS 2 satellites were equipped with reflectors in 1994, and similar hardware is either being included or planned for inclusion in the Russian Glonass and European Galileo satellite navigation systems, according to U.S. government officials.

The addition of retro-reflectors is part of a “commitment … to keep GPS the gold standard of [global navigation satellite systems] worldwide,” Col. Harold Martin, head of Positioning, Navigation and Timing (PNT) Command at Air Force Space Command, said here during a recent meeting of the government’s PNT advisory board.

 

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