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When viewing ADS-B data from sites like FlightAware and Flightradar24 (and also from a personal antenna), the landed altitudes vary greatly from one aircraft to another. What accounts for this variance?

For example, given a runway at 500 ft. elevation above sea level, 3 aircraft landing within a couple of minutes of each other show landed altitudes of 500 ft., 600 ft., and 750 ft. (AMSL)

I should also point out that the ADS-B data points are the same across the tracking websites and personal antenna, and the lat/lon values show landed positions - so there is no variance between the tracking sites.

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  • $\begingroup$ Just as a reference, the operative FAR is 14 CFR § 91.217 which states that the altitude transmitted must be referenced to 29.92 inhg and that ads-b and mode c must share a data source. So I'm not sure what would account for the discrepancy. $\endgroup$
    – TomMcW
    Commented Jun 3, 2022 at 2:22
  • $\begingroup$ I was going to say perhaps the messages aren't that frequent - but ADSB transmits every second (faa.gov/about/office_org/headquarters_offices/avs/offices/afx/…) .. $\endgroup$
    – Mr R
    Commented Jun 3, 2022 at 4:57

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The aircraft do not send out altitude when they are on the ground.

On the ground, aircraft send out the surface messages, which combine position, speed and heading.

The airborne message formats combine position and pressure altitude in one message, and speed vector and geometric altitude in another.

The lowest altitude you see is the last altitude decoded by the receiver before the aircraft touched down. Unless the receiver has very good line of sight to the final approach and touchdown area, and is not suffering from garbling (overlapping of messages received from various aircraft at the same time), the last altitude you will may be actually be quite high above the runway.

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  • $\begingroup$ However the planes do transmit their position while on ground, so shouldn't the altitude also be receivable all the way untill the system shutoff? In case the altitude transmission stops with weight on wheels event, last transmission should be very close to field elevation (ambient pressure taken in account?). $\endgroup$
    – Jpe61
    Commented Jun 3, 2022 at 9:43
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    $\begingroup$ @Jpe61 the transmission of altitude seizes with WoW. The last altitude transmitted is typically at or 25ft above the field (after applying QNH correction). But transmitted does not equal received. At low altitude, there is often not a direct line of sight between the transmitting antenna and the receiver antenna. Also, messages get lost due to collision with other messages being received at the same time. $\endgroup$
    – DeltaLima ♦
    Commented Jun 3, 2022 at 11:27
  • $\begingroup$ But how does the ground position keep updating? $\endgroup$
    – Jpe61
    Commented Jun 3, 2022 at 12:37
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    $\begingroup$ @Jpe61 Ground position keeps update through multipath (reflections), diffraction etc. but not at a rate of 1/seconds. Flight radar 24 is very good at connecting the dots with lines, but really on many airports the surface tracks are not that frequently updated. 10 second between updates would already result in at >100ft error at 700 fpm descent rate, but is barely visible in the ground track. $\endgroup$
    – DeltaLima ♦
    Commented Jun 3, 2022 at 20:18
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    $\begingroup$ Thanks @DeltaLima that makes sense 👍 $\endgroup$
    – Jpe61
    Commented Jun 3, 2022 at 21:39
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Aircraft DO transmit altitude while on the ground... look at the ADSB data that comes from the aircraft. If the data hasn't been edited, in the "alt" column you can see clearly altitude IS being transmitted even though the aircraft is on the ground and displaying GND status. The data can also include the corrected altitude and the pilot entered correction pressure that is supposed to yield the correct height above sea level. That resultant varies quite a bit also.

As to why there are altitude variations among aircraft simultaneously on the ground at the same time... varying typically about 200 feet, boggles the mind. It's hard to imagine that a digitally monitored analogue device that is supposed to be calibrated (once a year?), could get so out of whack. But, there's another contributing factor.

Older and less expensive transponders output altitude data in 100 foot increments (newer are 25 ft and a few at 10 ft resolution); this means that when the first pulse of 100 ft altitude appears, you are probably only about 51 feet when the transponder transmits the message. This step resolution transmission of altitude is usually smoothed by some method of averaging. But what do you do when an aircraft has the right altimeter setting but displays -100 feet when you're at 16 feet above sea level? And then you move to another aircraft on the ground 16 ft above sea level and it too has the correct altimeter setting but it reads 50 feet?

For municipal and County owned airports, they may opt to buy a service like WebTrak from Envirosuite so aircraft can be accurately tracked locally. But, in our area, Envirosuite/WebTrak solves the above problem by wiping out all ground data and substituting one of two values that correspond with the elevation of the airport (Geodetic or GPS). But one sin leads to another.

This clean up means that altitude data has to be 'adjusted' all the way up to about 800 feet. This process erases all the altitude differences of aircraft on the ground. I'm currently looking for data on normal variations of altimeters. I guess we could blame it on Cro-Magnon technology of pressure cans moving a spring, but what are the alternatives? Radar altimeters are very expensive. GPS has satellite time delays and depends on ellipsoids and Geoids, none of which are universally accepted. That still leaves us without a satisfying explanation.

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