GPS World, December 2014

DIRECTIONS 2015 GPS What It Takes to Make a Gold Standard Colonel William T Bill Cooley U S Air Force Director Global Positioning System L ast year in my Directions article I emphasized the commitment made by the U S Government to ensure GPS signals are available to all users and I shared some of our excitement in the GPS Directorate regarding the modernized capabilities we are developing and fielding This year Id like to share with you progress weve made in the past 12 months provide an update on the modernization initiatives and challenge civil users and receiver companies to innovate and accelerate these modernized capabilities for users worldwide This past year has been productive for the GPS program The most visible progress was the addition of four new Boeing built GPS IIF satellites to the GPS constellation bringing the total number of available satellites from 36 to 39 SVN 33 was safely disposed in October 2014 or the number would be 40 These additions also reduced the average age of the satellites on orbit from 111 to 103 years This years GPS launch tempo had not been matched since the early 1990s TABLE 1 lists the current satellites in the constellation by block Perhaps the most exciting aspect of the GPS satellite constellation is the ever improving performance As I mentioned last year the 2008 Standard Positioning Service SPS Performance Standard issued by the Office of the Secretary of Defense codifies our commitment to civil users Among other attributes that make GPS the gold standard for positioning navigation and timing PNT the SPS requires a signal in space SIS user range error URE of 40 meters or less for every healthy satellite The SIS URE is the difference between a This years GPS launch tempo had not been matched since the early 1990s Col Bill Cooley GPS satellites navigation message for example ephemeris data and satellite clock correction data versus the truth for example satellite transmit antenna location and satellite clock offset from GPS time While the commitment of the U S Government is four meters or less the actual average performance over the past year has been 068 meters and in the past quarter has been an impressive 063 meters While this is admirable continued modernization efforts will allow us even better performance A significant contributor to the errors experienced by GPS receivers are ionospheric delays that can be eliminated only with knowing the characteristics of the ionosphere free electron density in the region roughly 100 1000 kilometers above the Earths surface or by using two signals at different known frequencies While systems like Federal Aviation Administration Wide Area Augmentation System WAAS and the U S Coast Guard National Differential GPS NDGPS provide a modeled approximation of the ionosphere the new L2C and L5 civil signals on the GPS IIR M GPS IIF and soon to launch GPS III satellites enable GPS receivers to directly measure and eliminate the ionospheric delays altogether thereby delivering on the GPS modernization program first announced in 1999 These new signals began pre operational Civil Navigation CNAV message broadcast on 28 April 2014 with the L2C signal set healthy and L5 set unhealthy until sufficient monitoring capability is established With CNAV now on the air civil users should take advantage of it My challenge to commercial receiver companies and innovators is to incorporate the modernized signals in future receivers and continue to improve user experience and GPS performance Currently 14 L2Cbroadcasting satellites are in the constellation and by early 2016 we expect to have 19 on orbit and transmitting L2C 7 GPS IIR Ms and 12 GPS IIFs GPS modernization is well on its way from a signal in space perspective receiver manufacturers and innovators must bring new improved products and solutions to users Less visible but real progress modernizing the GPS Enterprise is underway with the next generation of GPS satellites ground control and user equipment segments The first GPS III satellite and the newly developed navigation payload have been delayed approximately two years from the original planned delivery of the completed GPS III satellite of October 2014 But in September GPS World December 2014 www gpsworld com 28