iFly GPS Forum

A quick, hopefully not dumb, question: what does this instrument display?  I can't find an explanation in the manual.  There are lots of altitudes, including GPS (which this is not), barometric ("corrected" or not), geometric, etc.

I basically want to know if it's showing me what ATC is likely seeing on their display as my aircraft's altitude, which I believe is my Mode C barometric altitude (pressure altitude I think) corrected by some nearby airport's barometer reading.  That (pressure?) altitude is obatined by the transponder from the "blind" altitude encoder, which in my plane is very old and subject to "drift".

The iFly Ind.Alt instrument is consistently showing an altitude that is about 250' higher than the iFly GPS Altitude instrument (which it is obtaining from my SkyRadar ADS-B In unit with WAAS, so I trust that it's pretty accurate). My "real" instrument panel altimeter is indicating altitudes reasonably close to that GPS altitude.  But this iFly Ind.Alt instrument is considerably higher.  I'm just trying to figure out what it's telling me.

If I can put an underline on what you were saying: if the SkyRadar is supplying its own barometric altitude in the data stream to iFly, then iFly is likely showing that, and it has no certain relationship with what ATC would see as my altitude.  But if it's coming from my transponder (via SkyBeacon's sniffing of my transponder output, and then SkyRadar pulling it out of the ADS-B Out stream) then it possibly would. 

I am making an assumption here that ATC does not use altitude from my ADS-B output, but instead still uses Mode C transponder data. If ATC does use ADS-B data to display my altitude, then I'm concerned that this Ind.Alt I'm seeing is in fact what ATC is seeing, in which case it is very wrong.  I should probably contact ATC and ask what they're seeing for my altitude, huh?

It could also be that there's a bug in iFly and it's applying an incorrect pressure correction, or where none is needed (e.g. SkyRadar is supplying an altitude number that does not need it).  Does anyone else out there use this Altitude instrument (aka "Ind.Alt" in it's title box) and does it agree more or less with GPS altitude in your aircraft?

It would be good if someone from iFly would weigh in on this.

Yeah, I'm beginning to wonder what this instrument is good for and whether I should have it displayed.  But I'd like to hear from iFly what goes into that number.

[Update: as I sit here at home, I brought up iFly on my phone to ponder my instrument layout and possibly removing the Ind.Alt instrument.  It currently displays "unavail" in that box.  I tapped on it to see what would happen and it brought up a dialog box that says "No Pressure Altitude (PA).  The Altimeter's Indicated Altitude requires ownship 'Pressure Altitude' (PA), fed from an ADS-B device that provides this reading.  Not all ADS-B devices provide PA."  It further says that it will "flash current GPS Altitude for reference", which I assume is what it does while it waits for an ADS-B PA source.

So, based on this tidbit of info, it seems to me that either SkyRadar is sending an incorrect PA, or iFly is applying too big a correction to it.  Also, this PA that SkyRadar is sending to iFly: is it self-derived or is it coming from the transponder?  If the latter, then that was what is/was causing me worry. ...And now that I look on the SkyRadar website and in the very thin manual, nowhere does it mention altitude sensing, so it must be getting it from the SkyBeacon, which is getting it from the transponder.  It seems to me then that it's more likely that the PA being given to iFly is off because of this daisy-chain of encoder-to-transponder-to-SkyBeacon-to-SkyRadar-to-iFly.]