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Altitude encoding for ADS-B

Posted: Sat May 24, 2014 9:59 am
by milairuk
How is the altitude encoded in the ADS-B signal ? Is it based on the standard pressure setting of 1013.2HPa or fed from the encoding altimeters ? I've noticed the height that I lose landing A/C at EGSS varies with barometric pressure, so I'm guessing the ADS-B comes from a set standard that the pilots have no input to ??

Any one know the definitive answer please ?

John

Re: Altitude encoding for ADS-B

Posted: Sun May 25, 2014 4:12 am
by agw
It can be either the barometric altitude, which is based on the pressure setting of 1013.25mb, or the geometric altitude, which is the height above the elipsoid. I think most aircraft transmit the former. If you're using a raw feed then VRS will decode both but it doesn't distinguish between them on the website. However, there's a ticket on the list of things to do to report on which altitude and speed type the aircraft is transmitting, I'm planning on doing that for the next release.

There's also a ticket to find the nearest and latest METAR report for an aircraft and adjust reported altitudes below transition level accordingly, but that's not going to be in the next release :)

Re: Altitude encoding for ADS-B

Posted: Sun May 25, 2014 9:56 am
by milairuk
Thanks Andrew, this does make sense of my log now ! I'm using an RTL dongle running on a RPi running Dump1090 which I feed to FR24. But I much prefer VRS on my home PC's as I can customise it so much more. With all your tickets to do I wonder if you get any sleep ;)

Keep up the good work, look forward to the next release.

John

Re: Altitude encoding for ADS-B

Posted: Tue May 27, 2014 1:27 am
by agw
There's a world of difference between something going on the list and it actually getting done :) If I'm not working then I can get stuff done on VRS but these past couple of months I've been quite busy with work so things have slowed down a bit.

Out of interest I implemented the display of speed, altitude, vertical rate and track types so that I could see what was being reported. I've yet to see an aircraft transmit anything other than barometric altitudes, but some do report geometric vertical rates. For the most part though those displays are going to be pretty boring.

Re: Altitude encoding for ADS-B

Posted: Tue May 27, 2014 6:22 pm
by andrew
At least a decoder (RTL1090) displays "intent change" and selected autopilot altitude.
I don't know if raw feed sends such data, if yes can VRS use them?