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fhorinek
GPS height consistently wrong by +50m
I checked this during a 2hr flight yesterday, comparing to a Bluefly vario and my mobile phone. The GPS height is consistently wrong by +50m. A friend of mine has reported the same issue. FW140 installed.
I don't think its wrong - the device is using WGS84 as earth model, which gives the height to your current location.
I understand that WGL84 is just a model and therefore inaccurate. But from a flight instrument packed with excellent hardware and software capabilities I'd expect some smarter usability. Eg. height calibration to the map automatically before takeoff. Or at least an option for manual adaption, similar to QNH (would this make sense? in a certain region, is the WGL84 accuracy changing a lot?)
How are other flight instruments handling this?
We are flying at Gerlitzen, often the TRA is not active, then you must stay 300m AGL maximum. With 50m uncertainty this will not work.
I found a similar bug for XCsoar (45m difference), where the root cause assumption is "misapplied geoid correction on the incoming GPS data", probably due to mapping from feet to meters. And someone eles comments "The gps system sends the correct altidute and xcsoar adds the WGS84 correction once more". Are you sure you have no bug here in the GPS data handling? See https://github.com/XCSoar/XCSoar/issues/525
Oh, that is interesting - I was not aware that GPS chips might be able to perform the necessary correction automatically. However, we are consistently too high, I am guessing that points more towards no correction at all, contrary to that problem where it seems to be applied twice?
So, the "bug" would be the missing correction from "above-geoid-height" to locally accepted MSL? I was always hoping that it would be an "automatic" fix once the AGL option is added. As you kind of said, GPS height doesn't really matter, correct QNH and AGL measurement does.
However STRATO does not show the correct GPS hight at the place it is located. All my other flight instruments and even my 200€ Garmin fitness watch shows the correct value. +50 is also the value i have at my location.
