[SatNews] The Department of Defense (DoD) has announced that U.S. Air Force Space Command will begin broadcasting Civil Navigation (CNAV) messages on all operational GPS satellites capable of transmitting the L2C and L5 signals.
L2C and L5 are the first of several new civil capabilities being added to GPS as part of the GPS modernization program announced in 1999. The L2C signal is designed to meet commercial needs and L5 meets safety-of-life transportation requirements.
"We have been working in partnership with the U.S. Department of Transportation (USDOT) to enable early delivery of two more civilian frequencies from the GPS satellite constellation," said Maj. Gen. Robert E. Wheeler, DoD deputy chief information officer, C4 and Information Infrastructure Capabilities. "These new CNAV messages will enable manufacturers to develop and test advanced civil receivers and make for a more robust Position, Navigation and Timing (PNT) solution available to the civilian public. We do not anticipate any GPS satellite outages or legacy degradations as a result of the pre-operational deployment of these frequencies, and those currently using the GPS Standard Positioning Service should not be impacted," he added.
Artistic rendition of a GPS III satellite.
Image courtesy of Lockheed Martin.
The implementation will take place in two phases. First, on April 28, 2014, the initial broadcast of CNAV message-populated L2C and L5 signals will occur at a reduced data accuracy and update frequency compared to the legacy GPS signals in wide use today. Second, in December 2014, CNAV data updates will increase to a daily rate, bringing L2C and L5 signal-in-space accuracy on par with the legacy signals. However, derived position accuracy cannot be guaranteed during the pre-operational deployment of the frequencies. These pre-operational signals are primarily used to test various equipment and should be employed at the users' own risk; not used for safety-of-life or other critical purposes.
The Air Force will broadcast L2C messages with the health bit set "healthy," as was the case during a June 2013 test. L5 messages will be set "unhealthy," but as greater experience with the L5 broadcast and implementation of signal monitoring is achieved, this status may change upon review. The public will receive ample notification before any decision to set the L5 health bit to "healthy."
"The U.S. Department of Transportation is pleased with the collaborative effort and work of the CNAV tiger team, formed between the Office of the Secretary of Defense, Air Force Space Command, and the U.S. Department of Transportation, to address concerns about implementation of a pre-operational CNAV capability on the GPS L2C and L5 signals," said Greg Winfree, assistant secretary for research and technology at USDOT.
