wardialer
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Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 10:41 am

How come most online flight tracking systems like Flightradar24 or ADSBExchange shows altitudes either as being 0 feet or On Ground when aircraft is on the ground or after it has landed?

I thought the altimeter settings are based off the elevation of the airport in MSL as instead of AGL.

For example, if an aircraft is at a field elevation of 800 feet, then I would assume that the altimeter is set to 800 feet and 0 feet.
Am I correct?

So, would these online flight tracking systems use the Altitudes based on AGL?
 
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Starlionblue
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 11:58 am

IIRC the transponder always uses standard QNH, so you'll get a difference.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
wardialer
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 12:56 pm

My question would be, do these flight trackers use AGL or MSL that we see on the flight tracking systems such as ADSB-Exchange or FR24?

So when we see an aircraft on the ground on these flight tracking systems, why it is not displaying the actual airport altitude?

Instead, it would display as 0 feet or "On Ground"
 
GalaxyFlyer
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 1:47 pm

Because it’s on the ground and the altimeter reading is not irrelevant to the display. The trackers are following the flight, not the altimeter reading.
 
IAHFLYR
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 3:54 pm

Though not a flight tracker per say this site does show the correct altitude in MSL.

http://www.airportviewer.com/airport/KIAH

Go into settings and you can bring up a few items for the data block.....pretty good site to look at from time to time when airports are busy especially, though that is not happening as much now days and into the foreseeable future. :(
Any views shared are strictly my own and do not a represent those of any former employer.
 
wardialer
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 7:44 pm

Ok, I went to that web page from the last posted response, and does look good. Thanks for that.

If you go into the ADSB Exchange tracking and click on an aircraft and from the left panel you will see all the data which is quite interesting. I hovered my mouse over where it says Altitude and the description says that is an The uncorrected pressure derived height of the aircraft above mean sea level (based on barometric pressure).

So, what does uncorrected pressure derived height mean? Is this still measured in AGL or MSL?
Please explain this as I am not really understanding this.
 
GalaxyFlyer
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Re: Aircraft altitudes on Flight Tracking systems?

Sat Mar 21, 2020 8:04 pm

It means MSL pressure altitude, that is, set 29.92” or 1013mb and read it. So, an airport at sea level with a barometer (altimeter setting) of 28.92 would read 1,000’ pressure altitude based on standard atmosphere.

Height, in aviation, typically means above ground level, QFE or RadAlt, not altitude which means above MSL. They used some confusing terminology that muddies it up.
 
wardialer
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Re: Aircraft altitudes on Flight Tracking systems?

Sun Mar 22, 2020 7:29 am

So let us make this a bit more easier.

The flight tracking systems uses the same measurements of altitude as what the aircraft's altimeter measures in the Primary Flight Display?

That is what I wish to know.

And what do they mean by "uncorrected"?

Does it mean that the flight tracking system assumes that there is no baro adjustments and the baro setting is always kept at 29.92?
 
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Starlionblue
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Re: Aircraft altitudes on Flight Tracking systems?

Sun Mar 22, 2020 7:46 am

wardialer wrote:
So let us make this a bit more easier.

The flight tracking systems uses the same measurements of altitude as what the aircraft's altimeter measures in the Primary Flight Display?

That is what I wish to know.

And what do they mean by "uncorrected"?

Does it mean that the flight tracking system assumes that there is no baro adjustments and the baro setting is always kept at 29.92?


Trackers use transponder data for altitude. AFAIK transponders always work on standard QNH (1013.25 or 29.92 depending on where you are). It is thus "uncorrected" for local barometric pressure.

As you say, no baro adjustments.

What we see on the PFD its corrected below transition, and uncorrected (standard) above.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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zeke
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Re: Aircraft altitudes on Flight Tracking systems?

Sun Mar 22, 2020 11:01 am

wardialer wrote:
How come most online flight tracking systems like Flightradar24 or ADSBExchange shows altitudes either as being 0 feet or On Ground when aircraft is on the ground or after it has landed?

I thought the altimeter settings are based off the elevation of the airport in MSL as instead of AGL.

For example, if an aircraft is at a field elevation of 800 feet, then I would assume that the altimeter is set to 800 feet and 0 feet.
Am I correct?

So, would these online flight tracking systems use the Altitudes based on AGL?


These websites are not actually displaying the data from the aircraft, the data from the aircraft goes to the receiver, where it is processed and a fraction of the received data is uploaded to the cloud.

I think what is happening at the receiver is when each ADS-B data packet (which contains a lot of data) includes an “aircraft on ground” bit the altitude is sent as being zero. In reality runways are not level, you can have 100’ difference in height between the ends.

Transponders transmit pressure altitude normally independently of what is displayed in the cockpit. Below the transition altitude pilots set the local QNH on the altimeter sub-scale, and the ATC radar also accounts for QNH below the transition altitude, the transponder is still sending out pressure altitude. Above transition level pilots and ATC are on standard QNH, everyone is using pressure altitude.
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IAHFLYR
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Re: Aircraft altitudes on Flight Tracking systems?

Sun Mar 22, 2020 3:35 pm

zeke wrote:
the ATC radar also accounts for QNH below the transition altitude


Exactly, we put the local altimeter into the RADAR software system locally and change it whenever the altimeter changes for airspace below FL180.
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