Aircraft height discrepancy

[superspotter]

Hi Guys;
I have both an SBS-1 and Airnav running side by side as I am getting to grips with the Airnav but one thing keeps jumping out and that is the difference in height readouts between the two sysstems. For example, I have just had an Emirates 777 arrive into Manchester (I am situated 8.5 miles out under the approach to 23Right) and directly overhead on the SBS the Emirates was showing 2400 feet but on the Airnav it was around 500 feet higher. This is the same for every other comparison I have checked. I feel quite confident that the SBS is giving me the correct height and that the Airnav is out, any suggestions?

[ACW367]

You need to go to the smartview tab next to the network tab.  Put in the code for your nearest airport and tick your 'Auto Set QNH' button.  It then takes QNH details from the METAR for that airfield.  Remember if you move to other airfields overseas to set the METAR for that airfield.

[superspotter]

Hmm. OK, I guessed somebody would tell me that! I checked that setting in the preferences and it is currently set for the MAN QNH so.........
SBS may be wrong??

[superspotter]

Thinking about this a bit more, QNH is the setting for AMSL and as MAN is approx 265 above sea level then, okay, it would be about right so that get me thinking as to how the SBS is deciding on the QNH?

[tarbat]

SBS may be wrong??

Correct.  SBS is wrong, since it makes no adjustment for QNH below the transition altitude.

Thinking about this a bit more, QNH is the setting for AMSL and as MAN is approx 265 above sea level then, okay, it would be about right so that get me thinking as to how the SBS is deciding on the QNH?

The SBS doesn't decide on the QNH.  It doesn't do any QNH adjustments.

Radarbox on the other hand correctly adjusts the altitude below the transition altitude using the QNH from the airport you designate as the nearest.

[superspotter]

Thanks Tarbat, it's clever this little Airnav box!!

[Tonka]

The QNH adjustment is only good for aircraft flying on the QNH. Aircraft at 2400' on the approach would surely be using QFE so applying the QNH would give an error, would it not?

[AirNav Support]

Commercial Aircraft will most likely be using QNH not QFE. QFE is used more for GA aircraft in circuits.

What the aircraft sends out via Mode-S reply is the 1013mb height. So regardless whether they are using QNH, QFE you will get the 1013mb height. If you have an airport in your smart view settings it will then translate it into QNH height if below the transition altitude.