GPS World, April 2014

THE SYSTEM PNT Advisory Board Hears Air Force CNAV Plan The U S National Space Based Positioning Navigation and Timing PNT Advisory Board published the minutes of its December 2013 detailing a report from Air Force Space Command on the road ahead for implementation of the GPS Civil Navigation CNAV message on L2C and L5 The subject has stirred some controversy of late particularly between the U S Departments of Transportation DoT and Defense DoD and DoT is currently seeking public comments on the plan The minutes relay the gist of General Whelans CNAV remarks as follows CNAV has been under discussion for a considerable time Currently L2C and L5 signals are being transmitted but without a navigation message AFSPC is working hard to activate these messages as soon as possible One of the reasons for the delay is that additional time was needed to complete testing prior to activation Testing began in late summer 2013 and based on initial test results a way ahead has been plotted Current plans are to begin initial broadcasting in the spring of 2014 CNAV uploads will occur twice weekly The signal will meet GPS Standard Positioning System SPS standards but may not achieve current accuracy levels until full implementation in late 2014 CNAV live sky testing occurred in June 2013 and was conducted in cooperation with civil industry and international partners The two week test series included independent assessment and verification The tests identified four errors that required action The first which was addressed in real time related to implementation of the test series The second required improvement to the tools suite which should be totally integrated into the ground segment by December 2014 The third and fourth errors required patches to satellite software All four issues are now regarded as closed DOT Speaks A subsequent presentation from the Department of Transportation did not directly mention CNAV according to the meeting minutes but did include this update on civil signal monitoring taken from the meeting minutes DOT is responsible for performance monitoring of GPS civil signals The International Bright New IIF Aloft The latest GPS IIF satellite was successfully launched into orbit on February 21 GPS IIF 5 will replace the vintage 1997 spacecraft known as GPS IIA 28 in Plane A Slot 3 of the constellation IIA 28 will go into a reserve role in the network for the remainder of its useful life This is the first of three GPS launches planned through July to replace aging craft in the constellation The IIF generation will form the backbone of the GPS space fleet for the next 15 years providing greater navigational accuracy through improvements in atomic clock technology a new more robust third civil signal L5 for commercial aviation and safety of life applications a second civil signal L2C for the dual frequency GPS receivers and improved anti jam capabilities for military and civil users around the world Committee on GNSSs ICGs transparency principle states that Every GNSS provider should publish documentation that describes the signal and system information the policies of provision and the minimum levels of performance offered for its open service Currently this is only done on GPS L1 C A signals Performance standards for L2C and L5 have not yet been established The crucial function of signal service monitoring is to verify that commitments to GNSS performance are being met Additionally monitoring improves the situational awareness for GNSS operators and provides assurance that any civil service failure is detected and resolved promptly Opposing Activation At the close of 2013 a departing DOT assistant secretary wrote a letter to the Air Force opposing activation of the CNAV signal in April 2014 In March DOT opened a 30 day comment period on the proposed CNAV message on L2C and L5 The comment period closed on April 4 after press time for this magazine so no results are yet known The GPS Block IIF satellites are built by Boeing Earlier IIF satellites were launched in 2010 2011 2012 and 2013 All 12 satellites in the GPS IIF series have completed production The Air Force plans to launch the remaining IIF satellites by 2016 www gpsworld com April 2014 GPS World 15