Many thanks !! Best Regards, Mike Durrant VH-FQF
On 13/09/2011, at 11:39 AM, Mike Borgelt <[email protected]> wrote: > > In the interests of mis - information being quashed: > > I was right about GPS vs Pressure altitude(PA). Tim advised me he had GPS and > Pressure Altitude transposed in his spreadsheet. So his analysis of flight > records now agrees with the theoretical analysis in my article and my manual > analysis of the IGC files that a few very helpful people sent me. Thanks > again guys. As we fly gliders mostly in summer when it is warmer than ISA, > most of the time GPS altitude is greater than PA. Not necessarily so in > winter on wave flights. > > If you find this isn't so it may be because you have an early GPS receiver in > the FR with some heavy filtering (early Colibris in particular or weird > processing of GPS and PA) or your pressure sensor in the FR is off > significantly. > > It also looks to me like the pressure altitude is the anomaly in Geoff > Vincent's Flarm. I'd need a few more files to be sure. > > If you want to compare GPS and PA do remember to take the offsets into > account before takeoff and after landing. In this case the FRs that don't > automatically start on movement or climb are probably better. Pressure varies > from day to day and the pressure sensor in the FR is fixed to 1013 Hpa > reference pressure. > > If you have an early Colibri don't attempt to use the GPS altitude for final > glide calculations. Some other FRs seem to show gaps in the GPS altitude > record or in the case of one recent FR design the GPS receiver seems to be > one optimised for 2D navigation and ground vehicle dynamics with maybe some > dead reckoning of GPS altitude under some circumstances . You can't really > use that GPS altitude for final glides either. > The CAI 302 traces seemed to be quite good with only the odd gap(Garmin > module), the EW Microrecorder seems to have an excellent GPS receiver and I > would expect FLARM GPS data to be very good also as the GPS module is one > where the user can set it for flight dynamics and full time 3D navigation. > Always assuming you have a good antenna location with no shielding by the > airframe or your body. > > This is one of the reasons we use our own GPS module in the B500 and now B800 > where we use GPS altitude for final glides. We know how the GPS is set up. > > The glider cares about geometric altitude for how far it can glide, not PA. > PA is a requirement for airspace compliance. In my experience given that the > surface pressure will change during the day and the errors inherent in using > cockpit statics as well as other problems in electronic pressure sensing, GPS > altitude is superior to PA for final glides. Do remember to give yourself a > margin as your glider may not really glide as well as assumed in the polar in > your electronic final glide computer. In some B800 configurations you'll get > a real time display of how well it glides against the assumed polar on every > glide. > > See the article on www.borgeltinstruments.com > > > > Mike > Borgelt Instruments - manufacturers of quality soaring instruments since 1978 > ABN: 75532924542 > phone 0746 355784 > fax 0746 358796 > cellphones 0428 355784 > 0429 355784 > email: [email protected] > website: www.borgeltinstruments.com > P.O.Box 4607 Toowoomba East, 4350 > Queensland Australia > > _______________________________________________ > Aus-soaring mailing list > [email protected] > To check or change subscription details, visit: > http://lists.internode.on.net/mailman/listinfo/aus-soaring _______________________________________________ Aus-soaring mailing list [email protected] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
