Aircraft Altitude Reconciliation

[Mechanik]

I’m looking for some help understanding altitudes by survey vs. barometric pressure with ADS-B, GPS, and FlightRadar24. I’m sure this is simple and I have some ideas on the issue but would would like to hear a definitive answer.

for simplicity, assume I’m at 3,000 ft per USGS and cell phone data. I see an aircraft reported by FR24 as 6,000 barometric altitude. Ok, it’s 3,000 feet above me.

Now I watch the aircraft land at a nearby airfield. The FR24 altitude drops to zero. However, the airport is at 2,000 feet elevation per survey. If the aircraft is zeroed at the airport, was it really at 8,000 feet when I observed it? That would put it 5,000 feet above.

Thanks in advance for your thoughts.

 

[Mendel]

Altitudes and elevations are always measured relative to a defined level. The technical term for this is "datum". The most commonly used datum is WGS 84, which refers to the World Geodetic System. GPS uses this datum. It corresponds to what laypeople think of as "mean sea level" (MSL), though hydrographers don't.

Airliners can also measure their altitude via a radar altimeter. It directly measures the distance to the ground, which results in the "altitude above ground level" (AGL).

Typically, an aircraft altimeter is barometric. It measures the (static) air pressure around the aircraft and relates it to the air pressure on the ground (which is another datum). Obviously, the weather influences this measurement, which is why every major airport employs a metereologist, issues weather reports almost hourly, and requires pilots to listen to them.
Pilots must adjust their altimeter to the proper pressure datum to have the correct altitude. Finding on landing that the runway is further down than you thought, or worse, that the ground is higher up, can really ruin a pilot's day.

Article:

Aircraft pressure altimeters indicate the elevation of the aircraft above a defined datum. The datum selected depends on the barometric pressure set on the altimeter sub-scale. Sound altimeter setting procedures are an essential tool in ensuring safe separation from the ground and from other aircraft.

QNH - The pressure set on the subscale of the altimeter so that the instrument indicates its height above sea level. The altimeter will read runway elevation when the aircraft is on the runway.
Airfield QNH is obtained by correcting a measured QFE to sea level using ISA regardless of the temperature structure of the atmosphere. As your altimeter is calibrated using ISA, it will indicate altitude correctly at the airfield reference point. At other altitudes, the indicated altitude is likely to be in error, depending on the temperature of the atmosphere.

Source: https://www.skybrary.aero/articles/altimeter-pressure-settings

For fast-moving jets, adjusting the altimeter to local weather conditions is impractical. It's more important that everyone has the same idea of what their altitude is, so they all use the same datum, even though that may mean their actual altitude is off.

Article:

Altitude
The vertical distance of an object measured from mean sea level.

Flight Level (FL)
A surface of constant atmosphere pressure which is related to a specific pressure datum, 1013.2hPa, and is separated from other such surfaces by specific pressure intervals.
Altitude above sea-level in 100 feet units measured according to a standard atmosphere. Strictly speaking a flight level is an indication of pressure, not of altitude. Only above the transition level (which depends on the local QNH but is typically 4000 feet above sea level) are flight levels used to indicate altitude; below the transition level feet are used.
e.g. FL250 = 25,000 feet above mean sea level when the pressure at sea level is 1013.2 mb.

Source: https://www.skybrary.aero/articles/altitude-flight-level-and-height

Regarding the FR24 issue:

Article:

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.

Source: https://www.airliners.net/forum/viewtopic.php?t=1443209

 

[flarkey]

ADSB Exchange allows us to download kml files with three different altitude references, which I think are defined as follows....

1. The geometric altitude = which is the height above the WGS86 datum and uses gps to calculate alt
2. Baro+AVG = pressure alt using the average pressure in that region of the world
3. Uncorrected pressure alt = pressure alt using the 1013 mb standard.

 

[Mick West]

The geometric altitude = which is the height above the WGS86 datum and uses gps to calculate alt

Which is the one I use for Google Earth. I think you are mixing your geoids with WGS86 though. There's WGS84 and EGM96, no WGS86.

Google Earth uses KML 2.2 which uses the WGS84 ellipsoid geoid for Lat and Lon (geodetic not geocentric), and the EGM96 Geoid for height (i.e. the EGM96 geoid gives the location of sea level)

 

[Mick West]

Now I watch the aircraft land at a nearby airfield. The FR24 altitude drops to zero.

It's not gradually dropping to zero. It gradually drops to the altitude of the airfield, then after the plane lands it jumps to zero. Watch planes land at a high airport like Denver:
https://www.flightradar24.com/2023-07-20/17:34/20x/39.83,-104.59/12

 

[Mechanik]

This is great! Thank you all for the primer. I had not occurred to me that aircraft had multiple altitude settings. Makes sense, though.

So in my case, the aircraft was probably 3000 ft above me. If I had kept watching FR24 as it descended, instead of clicking around through multiple aircraft, FR24 would probably have shown a gradual descent to 2000 ft and then a reset to zero. Onboard the aircraft, they would have seen a descent from 4000’ to zero. Again, makes perfect sense.

I was watching a circling firefighting spotter plane which turned into a brilliant white cylinder when the rising sun hit it. Now I know what an airplane looks like when it’s reflecting sunlight 3000’ above and about 3 miles distant!

 

[Mick West]

So in my case, the aircraft was probably 3000 ft above me. If I had kept watching FR24 as it descended, instead of clicking around through multiple aircraft, FR24 would probably have shown a gradual descent to 2000 ft and then a reset to zero. Onboard the aircraft, they would have seen a descent from 4000’ to zero. Again, makes perfect sense.

No, aircraft altimeters show the altitude above Mean Sea Level (MSL). Onboard the aircraft they will see the altitude go from whatever they were at down to whatever the runway is at.

They can also have an AGL (Above Ground Level) display, either from the GPS + Terrain map, or from a radar altimeter. But the main altimeter is MSL.

 

[Mendel]

No, aircraft altimeters show the altitude above Mean Sea Level (MSL). Onboard the aircraft they will see the altitude go from whatever they were at down to whatever the runway is at.

That depends.

Article:

The aircraft altimeter barometric sub-scale must be set to the appropriate setting for the phase of flight. These are:

Flight level. Standard pressure setting (1013 hPa) is set when flying by reference to flight levels at or above the transition level;

Altitude. Regional or airfield pressure setting (QNH) is set when flying by reference to altitude above mean sea level at or below the transition altitude;

Height. Altimeter pressure setting indicating height above airfield or touchdown (QFE) is set when approaching to land at airfield where this procedure is in use. Note that this setting is not used in other portions of the flight (climb, cruise and initial descent).

Source: https://www.skybrary.aero/articles/altimeter-setting-procedures

The altimeter only displays MSL altitude when the altimeter is set to QNH.

QFE is very important for things like the minimum decision altitude, at which a pilot must abort the landing if a safe outcome is not assured (e.g. not stabilized on the glide path, or runway not in sight).

 

[Mechanik]

This is all very helpful. Thank you.

 

[Mick West]

The altimeter only displays MSL altitude when the altimeter is set to QNH.

QFE is very important for things like the minimum decision altitude, at which a pilot must abort the landing if a safe outcome is not assured (e.g. not stabilized on the glide path, or runway not in sight).

All commercial and probably most GA flights in the US are going to use QNH though.

 

[Mendel]

All commercial and probably most GA flights in the US are going to use QNH though.

No, most commercial flights (that don't involve low-level work, like crop dusting or power line inspections) take place above 4000 ft and therefore use flight levels, which are pressure levels and not altitudes (unless the weather conditions fit exactly). For such a flight, during most of its duration barometric altitude and GPS altitude will typically not align, except somewhat approximately.

for simplicity, assume I’m at 3,000 ft per USGS and cell phone data. I see an aircraft reported by FR24 as 6,000 barometric altitude. Ok, it’s 3,000 feet above me.

It would be on FL 60, and probably not exactly 3000 ft. above.