lml@lancaironline.net Mailing List Archive

WAAS corrected GPS are all the rage. It offers: 1) takes GPS from 6.0m to 3.2m Horizontal accuracy with 95% confidence. 2) promise of newly written vertical LPV guided approaches. 3) promise of maybe getting "Sole means" en route blessing. It's true WAAS improves accuracy: http://www.navcen.uscg.gov/Ftp/gps/status.txt http://users.erols.com/dlwilson/gpswaas.htm http://waas.stanford.edu/metrics.html http://www.gpsinformation.org/dale/dgps.htm http://www.edu-observatory.org/gps/gps_accuracy.html http://gpsinformation.net/exe/iono-day.gif Ionosphere temp http://www.igeb.gov/sa/diagram.shtml http://www.montana.edu/places/gps/lres357/slides/GPSaccuracy.ppt http://www.montana.edu/places/gps/lres357/slides/GPSstatus.ppt and I did find 9 airports with a LPV DA: http://rob.com/airports/M/G/J/ http://rob.com/airports/M/S/L/ http://rob.com/airports/F/D/K/ http://rob.com/airports/G/A/I/ http://rob.com/airports/O/K/C/ http://rob.com/airports/S/L/O/ http://rob.com/airports/J/Y/O/ http://rob.com/airports/H/E/F/ http://rob.com/airports/O/S/H/ Two of them with lower mins! so one does get greater capability today with WAAS corrections. But this way over budget, way late government project also came with a mild threat by Administrator Blakey at the AOPA convention: "if you are slow to equip [with WAAS], there will be folks who will say there aren't enough users in the system and push to move the money away from developing and maintaining procedures for all those airports I talked about." http://www.faa.gov/newsroom/speeches/Blakey/2004/speeches_blakey_041021.htm Users of http://www.freeflightsystems.com/ the first WAAS corrected GPS, started complaining about losing position reports. Then CNX80 (second WAAS corrected gps) users also started noticing failed in flight position reports. When I started playing with a gps18, it worked fine on the ground, but when I turned away from the southern sky it would freeze its position. http://rob.com/lancair/flights/kirk/1gps.gif What's going on here? While I'm not saying GPS is junk http://www.gtwn.net/~keith.peshak/SatNavStatus.htm because we have been using it reliably for years, but let's look closer at the WAAS corrections: with 25 ground reference stations http://gpsinformation.net/exe/waas-coverage-dale.gif transmitting corrections to (Inmarsat IIIs: POR (Pacific Ocean Region) http://www.lyngsat.com/tracker/inmar3f3.shtml or AOR-W (Atlantic Ocean Region-West) http://www.lyngsat.com/tracker/inmar3f4.shtml These satellites are 22,300 miles above the equator vs 12,600 miles mostly over our head. If we look at the angle of the satellite TV dishes in our neighborhood, they point low on the horizon to get a satellite above the equator. That one signal is going twice as far, through more ionosphere. And unlike your satTV dish, the antenna must be omni directional, making it impossible to achieve any forward gain. When we lose this one signal, we lose the whole position solution. So is going from 6.0m to 3.2m with 95% accuracy worth the increased loss of availability? Wouldn't we increase safety by providing some information rather than no information? (this is a big one for the FAA) Perhaps with some new antenna technology, my point will be mute, but the 3 WAAS receivers I've tried, and all unlocked without southern sky. Heck, this new magic antenna will increase the reliability of wsi and xm weather service, as they face the same weak signal challenge, and are unavailable in an uncorrected way. (the weather service, Merlin, failed because they couldn't make an antenna that worked) Is accepting the added risk of a more complex system worth it? FADEC vs mags EFIS vs HSI WAAS vs GPS without corrections. Bonus question: is a TSOed C129A receiver with Receiver Autonomous Integrity Monitoring (RAIM) better than a 12 channel all in the sky receiver? BTW, CFS has been offering barro corrected GS on *all* approches for years, without WAAS or a TSOed GPS.