GPS World, April 2011

THE SYSTEM WAAS Official Again The Federal Aviation Administration FAA announced on March 18 that WAAS PRN 135 has resumed normal operations The WAAS team recently received the final report from Lockheed Martin on the failure of Galaxy 15 reported FAA GNSS program manager Leo Eldredge After a review of that report the team determined that the satellite was ready to be returned to operations The FAA said that PRN 135 is currently located at 120 W and enroute to its final destination of 1331 W but is now broadcasting operational corrections that can be used by both aviation and ground users including those in Northwest Alaska In April 2010 satellite operator Intelsat reported it had lost contact with PRN 135 named Galaxy 15 and it was drifting uncontrolled At that time the FAA reported that it would drift out of WAAS service within a few weeks Instead PRN 135 remained within a usable condition location although drifting east until December 2010 when it ceased operating On December 23 Intelsat reported that the power from the Galaxy 15 battery completely drained during its loss of Earth lock and the baseband equipment command unit reset as it was designed to do Shortly thereafter Galaxy 15 began accepting commands and Intelsat engineers began receiving telemetry in the operations center Intelsat determined that static electricity charge caused the initial failure and has uploaded new software to prevent the event from occurring again There are now three operational WAAS GEO satellites PRN 133 located at 98 W PRN 135 located at 1331 W currently at 120 W will arrive at 1331 W on or about April 4 2011 PRN 138 located at 1073 W The GPS community is concerned that desensitization overload due to strong signals outside of the GPS band may cause GPS receivers to operate in a non linear mode with reduced gain that is gain compression for the desired GPS signal Other receiver impairments may also arise as a result of the nearby strong signals The TWG has agreed to move forward with a combination of laboratory based and field based testing programs Field testing will be performed at outdoor test locations using transmitters filters and antennas similar to those that Light Squared plans to deploy in its commercial operations Other items of interest in the report Definition of Harmful interference at the GPS GNSS Augmentations L Band Receiver The TWG members have discussed a number of receiver parameters related to the definition of harmful interference In the FCC Rules harmful interference is defined as interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades obstructs or repeatedly interrupts a radiocommunication service operating in accordance with the ITU Radio Regulations Harmful interference affects different types of receivers in different ways The key factors that pertain to the functioning of GPS receivers and or whether service is degraded obstructed or interrupted are accuracy position velocity time availability ability to perform a given function coverage within what space can a function be performed integrity what is the probability that the results are correct and continuity what is the probability that a given function can be completed Metrics for harmful interference are developed from an understanding of the consequential relationship between negative impacts and receiver parameters which include effective C N 0 PVT accuracy time to first fix loss of lock cycle slips etc The signal conditions to be taken into account are defined in the GPS Standard Positioning Service SPS Performance Standard 4th Edition Interface Specifications ISs GPS policy and both the present and planned future signal environments will be considered Environmental and field conditions in which GPS receivers operate will also be considered It should be possible to assess interference impact up to that which includes harmful interference using metrics in terms of receiver parameters that include measurable changes in effective C N 0 as well as position accuracy time to first fix loss of lock cycle slips etc Related to this discussion is whether there is any margin that could be budgeted for terrestrial broadband operation and if so what that amount could be When considering systems guaranteed for safety of life operations there may be very little or no margin There is general agreement within the TWG that the device testing protocols should include changes in effective C N 0 and degradation of other key performance measures so as not to exclude data that might be relevant for the post See FIRST REPORT page 17 WEBINAR APRIL 21 Lightsquared and GPS Our Story So Far April 21 10 a m Pacific 1 p m Eastern Experts will discuss findings to date of the April 15 status report by the FCC Technical Working Group on LightSquared GPS Interference Speakers Logan Scott consultant in precision indoor navigation and GPS jammer location Walter Feller senior director of precision engineering Hemisphere GPS William Klepczynski Global Timing Services former ION AAAS Government Fellow Captain Joe Burns managing director Technology and Flight Test United Airlines Eric Gakstatter contributing editor for Survey GPS World Register free at www gpsworld com webinar GPS World April 2011 www gpsworld com 14