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Looking at a plane tracker, I found something where the altitude is crazy high. Could it be a plane masking identity?

enter image description here

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  • $\begingroup$ There is an AEROSTAR up right now on boarder of Canada at 50,300 ft. What can go over 100,000 ft? $\endgroup$
    – Josh
    Aug 24, 2023 at 21:50
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    $\begingroup$ a bad altimeter setting can go over 100,000 feet. unfortunately the registration and ICAO codes are cut off in the screenshot, so it's hard to go back and see. $\endgroup$ Aug 24, 2023 at 23:13
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    $\begingroup$ @tedder42 Registration could be SU-BAV, which is an Egyptian military C130. Would be nice if we had a timestamp though. $\endgroup$
    – Chris
    Aug 25, 2023 at 2:10
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    $\begingroup$ @Chris I've seen Egyptian AF C130s with this exact issue many times, so I think you're absolutely correct $\endgroup$ Aug 25, 2023 at 14:26
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    $\begingroup$ I guessed this was about the Egyptian C-130s just from the title. I've seen them flying into BWI with my own eyes while they're reporting these erroneous altitudes. Whatever the failure is, it's not just a one-off - it happens with multiple different aircraft. $\endgroup$
    – nobody
    Aug 30, 2023 at 1:08

2 Answers 2

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Without knowing the details of the flight, it looks like one of the bits in the altitude transmission is set wrong. In the coding of altitiude, the transponder uses the so-called Gillham code. This is a modified Gray code which changes only one bit between successive values.

I suspect the D2 bit is driven by a cable which either has a loose connection or a break and is intermittently shorted to ground. In this specific case this would lead to an altitude of 8300 ft to be transmitted as 117200 ft.

The transponder sends altitude encoded in 12 bits, logically grouped in four groups of three bits: [D4,D2,D1] [A4,A2,A1], [B4,B2,B1] en [C4,C2,C1]. (The transmission sequence of bits is a different order then depicted). D4 is not used for altitude, 11 bits are sufficient.

8300 ft is coded as (0)00 011 010 100, with D2 being the bold (MSB) bit. 117200 ft is (0)10 011 010 100.

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  • $\begingroup$ Interesting!, can you elaborate on how a short would cause an altitude of 8300 ft to become 117200 ft? $\endgroup$
    – ROIMaison
    Aug 25, 2023 at 8:31
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    $\begingroup$ @ROIMaison 8300 ft is 000 1101 0100, with D2 being the leftmost (MSB) bit. 117200 ft is 100 1101 0100. The fact that the color coding of the track indicates jumps up and down makes me believe this is very likely correct answer. You can look more at gray codes on avionictools.com/graycalc.php $\endgroup$
    – vidarlo
    Aug 25, 2023 at 8:39
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    $\begingroup$ I've noticed this exact issue many times with Egyptian air force C130 flying over Europe, but I'm not sure if it's always the same aircraft. $\endgroup$ Aug 25, 2023 at 14:30
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    $\begingroup$ Amazing domain knowledge ! $\endgroup$
    – Fattie
    Aug 27, 2023 at 15:34
  • $\begingroup$ I'm looking right now at SU-BKS doing the same thing intermittently so it's definitely not a specific-aircraft issue (maybe a manifacturing defect on the whole batch of transponders?). It's alternating between 104300ft and 21200ft, so it's again the leftmost bit that is changing $\endgroup$ Sep 6, 2023 at 8:52
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The altitude record for a subsonic fixed wing aircraft is 76,000 ft (Perlan II).

It is not possible that both the altitude and the ground speed are correct. Lots of things fly at that ground speed. Without knowing anything about the flight, it's quite probable the altitude is wrong.

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  • $\begingroup$ Is there any possible way a C130 could be flying at 117,200 ft? $\endgroup$
    – Someone
    Aug 25, 2023 at 1:33
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    $\begingroup$ @Someone No. Everything about the design is wrong. $\endgroup$ Aug 25, 2023 at 1:34
  • $\begingroup$ What was wrong with my edit then? $\endgroup$
    – Someone
    Aug 25, 2023 at 3:32
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    $\begingroup$ @Someone Edits really aren't intended to be used to wordsmith somebody else's answer. When there's a typo in a post, sure. When a link can be added or updated, great. When the originator isn't a native English speaker and an edit can improve the readability of a question or answer, then yes. But "improving" somebody's choice of words, not so much. Which is why the original author retains the ability to reject edits. When one disagrees with an answer's content, the way to register that is a down-vote. But Stack Exchange doesn't use edits (the way Wikipedia does) to correct another's work. $\endgroup$
    – Ralph J
    Aug 25, 2023 at 4:27
  • $\begingroup$ @RalphJ - just to clarify for those of us who are newer, I don't think you meant that it is out of line to have a different opinion on wording, but that if it changes meaning even slightly it is best to discuss it in the comments rather than editing. And downvoting if I understand correctly is for disagreeing with the entire premise of the answer not just disagreeing with some of the specific wording. The exception would be community wikis which have a different methodology. At least that's how I understand it. $\endgroup$ Aug 25, 2023 at 18:33

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