Hi John
Confusing isnt it
Aha that's understood, but what is sent to the RunwayHD/Skydemon display to indicate the height of the conflicting aircraft compared with the aircraft I am flying, if it is based on the baro sensor on the PAW then is your reference 1013.2 (same as Mode S).
As I mentioned, it depends upon what is the conflicting aircrafts reference .
If a packet is received via ADS-B - we compare to the barometric altitude
If a packet is received via P3I - we compare to the GPS altitude
Only the difference altitude is supplied to the navigation device, so it is irrelevent whether we used barometric or GPS for the calculation, so long as we use a common reference. The Navigation device is effectively told +/- nnnnFT relative to our position.
The way I see it is there are three different heights to consider.
The actual height the aircraft is flying based on its own altimeter set to QNH local aerodrome.
The Flight level broadcast by ADSB based on 1013.2 datum
The altitude based on GPS
Well almost, there are in fact only two heights to consider, Barometric and GPS, and for both of these we have a local reference for comparison, from our local Barometric Pressure Sensor, and our local GPS
How do you normalise all these differing datums so that it makes sense to the pilot i.e if he sees a conflict 200 feet above, how does he know whether this 200 feet is relative to his altimeter the altitude shown by the ModeS display (the TRT shows the FL) or the RunwayHD GPS height.
This has already been calculated in the comparison. So if the Navigation reports traffic at +500ft, this was either calculated by comparing Barometric(remote) versus Barometric(local), or GPS(remote) versus GPS(local).
Ultimately, 500ft is 500ft, no matter how it was calculated.
It only seems to make sense if everyone uses the same reference datum - i.e 1013.2
What is important is that when making the difference calculation, ensure you are using the same reference at both ends, whether that is either GPS or Barometric.
Just to confuse things a little further, ADS-B also has the capability to supply GPS altitude in its data stream, although that appears to be rarely used from transmission data I have looked at (so lets not even go there)
Thx
Lee