Transponder and GPS issue

[AGLyme]

My 2 year old Hdx sky view system in my flight design worked well until... my 2 year transponder check. The technician got into the transponder screen and clicked on a trig model by mistake... I went flying and I received errors and my pal pulled me up on flightaware and said my light sport showed 200 knots. When I landed I searched this forum and found that the wrong transponder was selected in the “Transponder setup” screen. I then selected the transponder “Dynon SV-GPS-2020”. Error message went away. All was well for a couple of hours of flying until I received the message “XPNDR error” in flight. I flew the next day and the error message did not reappear. However I went flying today and I pulled up the flights on flightaware and the altitude showed 2,700 when I know I plugged in 3,000 in the AP... and on the return trip, the flightaware altitude revealed 3,700 feet when I know I plugged in 4,000 feet. I checked the Baro before both flights.
I am stupid on this stuff but how can I get the system back to normal such that the gps and transponder are in synch again and revealing correct gps height values. Speed values look right on the money.
Thank you in advance.

 

[RV6-KPTW]

Re Altitude: flightaware shows pressure altitude, not gps altitude. Correct to indicated altitude with baro for your own peace of mind but still won’t match gps altitude.

 

[airguy]

Right - the Flightaware altitude will rarely match your actual flight altitude, don't let that make you think you have a transponder error.

 

[DBRV10]

Yes, as mentioned above, they rarely match because the flight aware data is predicated on 1013HPA or millibars. Your QNH was likely 1023 or whatever that is in inches.

 

[AGLyme]

RV6, Airguy, DBRV10, thank you did not know that... let's hope the Class D and C folks give me a pass...; )... seriously, very good to know. I took a photo of the error message that came up in flight...

 

[DBRV10]

Yes, your QNH at 30.37 is 1028mb or 15 above 1013, which would be 450 feet different. The GPS height is within range so thats probably close to the mark.

No idea what your error is from off the top of my head. Consult the pilot guide or install manual.

 

[airguy]

GPS altitude is also based on the average earth surface, not from mean sea level. "zero GPS altitude" will not correspond to what we call MSL.

 

[kellym]

My 2 year old Hdx sky view system in my flight design worked well until... my 2 year transponder check. The technician got into the transponder screen and clicked on a trig model by mistake... I went flying and I received errors and my pal pulled me up on flightaware and said my light sport showed 200 knots. When I landed I searched this forum and found that the wrong transponder was selected in the “Transponder setup” screen. I then selected the transponder “Dynon SV-GPS-2020”. Error message went away. All was well for a couple of hours of flying until I received the message “XPNDR error” in flight. I flew the next day and the error message did not reappear. However I went flying today and I pulled up the flights on flightaware and the altitude showed 2,700 when I know I plugged in 3,000 in the AP... and on the return trip, the flightaware altitude revealed 3,700 feet when I know I plugged in 4,000 feet. I checked the Baro before both flights.
I am stupid on this stuff but how can I get the system back to normal such that the gps and transponder are in synch again and revealing correct gps height values. Speed values look right on the money.
Thank you in advance.

Time to get whoever did the certification back, after you make sure for the install manual that your transponder is configured correctly.
Then put transponder in test mode, so the ADAHRS doesn't get screwed up during the test. Tell him not to touch the Dynon settings because he obviously doesn't know what he was doing. He shouldn't charge you because he obviously screwed it up. I make sure I am present during test, and I call out the altitude readings while he runs down the pressure and notes corresponding altitudes. Saves him time and ensures I know it is reading correctly.
Kelly

 

[swatson999]

GPS altitude is also based on the average earth surface, not from mean sea level. "zero GPS altitude" will not correspond to what we call MSL.

Not quite. GPS position/time is computed in Cartesian coordinates (X, Y, Z, t), and then converted to whatever you need. Nominally, that's (lat, lon, alt, t). But there are quite a few different "alt" conversions. You're talking about HAE, or height above ellipsoid. The ellipsoid is an approximation to the overall shape of the earth, and there are many, many different ellipsoids, but for most users, the GPS uses the WGS84 reference ellipsoid.

That's then *corrected* to HAG, or Height Above Geoid, where the geoid is the gravitational equipotential surface that approximates what MSL would be. That's actually what it's showing you when it shows you MSL.