Altitude encoding for ADS-B

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milairuk
Posts: 34
Joined: Wed Apr 09, 2014 8:12 pm
Location: Chelmsford, Essex

Altitude encoding for ADS-B

Post by milairuk » Sat May 24, 2014 9:59 am

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

agw
Posts: 2239
Joined: Fri Feb 17, 2012 3:20 am

Re: Altitude encoding for ADS-B

Post by agw » Sun May 25, 2014 4:12 am

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 :)

milairuk
Posts: 34
Joined: Wed Apr 09, 2014 8:12 pm
Location: Chelmsford, Essex

Re: Altitude encoding for ADS-B

Post by milairuk » Sun May 25, 2014 9:56 am

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

agw
Posts: 2239
Joined: Fri Feb 17, 2012 3:20 am

Re: Altitude encoding for ADS-B

Post by agw » Tue May 27, 2014 1:27 am

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.

andrew
Posts: 20
Joined: Sat Apr 26, 2014 10:31 am

Re: Altitude encoding for ADS-B

Post by andrew » Tue May 27, 2014 6:22 pm

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?

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