Hi Mike,Sorry for the delay in responding. For a while now I have been carrying 4 GPS units/loggers in my PIK 20B, focusing mainly on high altitude wave flights. These are an EW MicroRecorder (as primary IGC logger), Flarm (back-up logger for comps), Garmin Map 76S (navigational aid) and an iPAQ PDA with blue-tooth GPS or Binatone PNA with inbuilt GPS using SeeYou mobile as flight computer. Occasionally an LX 20 substitutes for the Garmin. I download the four IGC format files following each flight using SeeYou then examine the files using a text editor to establish the low-point and high-point values from the B records using a common time basis. The individual flight data is entered into an Excel spreadsheet where I can compare the pressure altitude max, min and gain of height values for the 4 devices (actually 3 because the PDA/PNA devices have no PA data) and the GPS data for the 4 devices, plus the average and std. dev. comparing PA and GPS. In part, this study was undertaken to build up some data which might be used to seek modification of the current IGC position on the requirement for a 3% margin for new altitude records - also adopted by GFA for Australian altitude records of course.
Now, to get back to Flarm. One data set in particular (Excel data summary file attached) showed the Flarm GPS values for the low point (around 5000 ft) being about 300 ft lower than the average of the the 4 GPS values, with the other three showing remarkable consistency, as well as with the PAs. Given that Flarm appears to use GPS altitude as an essential element of its proximity warning algorithms this rang some serious bells with me as quite a lot of our comp flying would be in that altitude vicinity. The Flarm difference at high altitude (some 26,000 ft) was around 150 ft.
Having re-visited this data I must point out that the Garmin PA values are "calibrated" using the GPS values and in the B records the two values are reported as identical. Obviously this skews the overall figures somewhat. Also in this particular flight the EW GPS was non-operational due to a antenna cabling problem. Never-the-less I was concerned enough about the Flarm discrepancy to mention it - which is where we came in!
Regards, Geoff V At 01:42 PM 5/09/2011, you wrote:
At 10:32 PM 28/08/2011, you wrote:Hi Chris,Your GPS hint is a tad misleading given that the unit has a native internal GPS that seems to satisfy the needs of a great many Flarm owners. While we are on the subject of Flarm can anyone definitively confirm whether the Flarm vertical separation algorithms rely on GPS or pressure altitude data. I have some very interesting empirical results that suggest Flarm GPS altitude data can be seriously out of step with other GPS derived data - although not necessarily out of step with other Flarm units.Regards, Geoff VHi Geoff, Would you care to elaborate on this? Private reply if you don't want to go public on it. Regards Mike Borgelt Instruments - manufacturers of quality soaring instruments since 1978 phone Int'l + 61 746 355784 fax Int'l + 61 746 358796 cellphone Int'l + 61 428 355784 email: [email protected] website: www.borgeltinstruments.com _______________________________________________ Aus-soaring mailing list [email protected] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
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