Altitude for target

[aja]

Is the altitude displayed for each ADS-B aircraft a barometric, geometric of QNH corrected.
If corrected how is this done?

[Oblivian]

Mr FAA FAQ knows all..

In fact, ADS-B reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best – this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS (Global Positioning Satellites) as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.

ADS-B does not report vertical or horizontal airspeed. Instead, ADS-B reports horizontal and vertical velocity relative to the Earth. This velocity is useful for air traffic control functions and ADS-B applications. Airspeed can be provided by other aircraft sensors.

[aja]

Mr FAA FAQ knows all..

What altitude is therefore displayed on the targets shown on Flightradar24 - the geometric or the barometric?
I take it that either is not the actual altitude that ATC are using to control and separate traffic and the altitude used in an ATM system is QNH corrected.

[Oblivian]

The ADS-B data transmitted are defined in the relevant standards and certification documents (e.g. EASA AMC 20-24 for ADS-B in Non-Radar Airspace or CS-ACNS for “ADS-B out”). They include (amongst others) the following:

Aircraft horizontal position (latitude/longitude)
Aircraft barometric altitude (will be the same as for the SSR)
Quality indicators
Aircraft identification:
Unique 24-bit aircraft address
Aircraft identification
Mode A code (in the case of CS ACNS for “ADS-B Out”)
Emergency status
SPI (special position indicator) when selected

It would seem the latter

[seriewoordenaar]

Are ADS-B reported barometric altitudes geopotential or geometric?

Anyone still alive and clicking on this 2-year old thread?

For a project I need to calculate the orthometric altitudes of flying aircraft using ADS-B data.
The reported ADS-B altitudes are barometric. However, it is more complicated than that - there are two types : 1) expressed as geopotential altitude, and 2) expressed as geometric altitude. I understand how and why. Both are described in standard ICAO Doc 7488/3, but I can't find some reference that states which is used in ADS-B.
And what about reported altitudes below Transition Altitude and Transition Level? Are these QNH-corrected altitudes also expressed as geopotential or geometric altitudes?
Without this knowledge, it's not possible to calculate the orthometric altitude!
No guesswork please, preferably back up your answer with an official reference.

[rueckwaertsflieger]

.... Both are described in standard ICAO Doc 7488/3, but I can't find some reference that states which is used in ADS-B.
And what about reported altitudes below Transition Altitude and Transition Level? Are these QNH-corrected altitudes also expressed as geopotential or geometric altitudes?
Without this knowledge, it's not possible to calculate the orthometric altitude!
No guesswork please, preferably back up your answer with an official reference.

In http://adsb-decode-guide.readthedocs...ion.html#ads-b clearly is desribed that ADS-B outputs both pressure altitude as well as GPS "Height above ellipsoid" HAE. ADS-B extended squitter is message type aka downlink format 17 = DF17. There are many ADS frames existing, with the B in ADS-B using DF17 messages only. In the ADS-B frame being 112 bits long, bits 33 to 37 in the data section or the payload's first bits declare the type code of the message. There is one type reporting pressure height (numbers 9 through 18) another type (20 through 22) reporting HAE.
Be aware that typical output of ADS-B receivers are something different. The Basestation protocol type "SBS1" reports pressure altitudes only. http://woodair.net/SBS/Article/Bareb...ocket_Data.htm
Pressure height is calculated height above pressure level 1013.25 hPa or QNE, thus no QNH correction is used.

[seriewoordenaar]

Thanks for the additional info.

In the mean time I found out that barometric altitude is a minimum requirement for Extended Squitter and thus ADS-B. The ellipsoid height is not used by ATC, but if present, there is no way to see the difference as I'm relying on Plane Plotter's data output. Furthermore, barometric implies geopotential heights as the standard atmosphere is described as such. And in another reference it is stated that altitudes always refer to 1013.25 mbar (QNE), thus QNH is nowhere applied in the ADS-B data stream. To quote :
To avoid human errors, barometric altitude transmitted by aircraft to ground stations
always makes reference to the ISA. Depending on whether the aircraft is flying above or below the
transition altitude, barometric altitude so received will be automatically corrected by the ATM
automation system based on local QNH before displaying to ATC.

In other words, QNH is only applied onboard and by ATC on the ground, but not transmitted in the ADS-B data stream.
No messing with TA and TRL needed!
All this makes the transformation to orthometric altitudes straightforward :
0) The ADS-B altitude data is barometric geopotential referring to QNE only.
1) Transform barometric geopotential altitudes to geometric (apparent orthometric).
2) Apply QNH to all the altitudes.
Et voilą.