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Can anyone shed any light as to what this is (circled in white)?

enter image description here

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This is a military test transponder, not a real aircraft. The callsign GAFTT08 stands for German Air Force Test Transponder 08. When you zoom in on the map, you can see the reconstructed position move in a pattern around the fixed true position:

GAFTT08
(adsbexchange.com)

As far as I can tell, the military does not publicly say what exactly they are doing here. The only sources I could find are from forums:

GAFTTxx = German Air Force Test Transponder

(flugzeugforum.de)

German Air Force Test Transponder. So not an aircraft, not a drone. Don’t know why it is online all the time, but it is. Sometimes over Würzburg, sometimes over Ingolstadt.

(forum.scramble.nl)

Additionally, the squawk code of 7777 is used in Germany for fixed test transponders according to Wikipedia (thanks to ymb1 for pointing this out):

7777 US, Germany, Belgium, Netherlands: Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE)

(List of transponder codes - Wikipedia)

This is also confirmed by the following ICAO definition:

Code 7700 is reserved for recognizing an aircraft in emergency. (Codes 7711 to 7717 and 7721 to 7727 are reserved for SAR operations and code 7777 for monitoring the ground transponder.)

(ICAO - SSR CODE ASSIGNMENT SYSTEM, emphasis mine)

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    $\begingroup$ Test transponders are used to verify the accuracy of radar data. They are known to always be in a fixed position (the movement you see is probably due to inaccuracy of ADS-B Exchange). Controllers or technicians will periodically check that the reply from the test transponder is within a defined range of the expected position. $\endgroup$
    – 60levelchange
    Mar 8, 2020 at 19:57
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    $\begingroup$ It is not moving in a pattern, the multilateration system used to calculate its position has pattern in it's position errors. $\endgroup$
    – DeltaLima ♦
    Mar 8, 2020 at 20:49
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    $\begingroup$ @J.Hougaard & DeltaLima Thanks, I clarified in the answer what the movement is. $\endgroup$
    – Bianfable
    Mar 9, 2020 at 7:37
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It is a test transponder for the German Air force. According to Wikipedia, it is a test for the friend or foe identification system. So, basically they are testing technology to identify aircraft, so they can aviod friendly fire in battle situations. It is not a real aircraft.

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    $\begingroup$ I don't see "test transponder" or "7777" mentioned anywhere in that Wikipedia article. Could you clarify what makes you think it has anything to do with an IFF system? $\endgroup$
    – 60levelchange
    Mar 9, 2020 at 8:47
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    $\begingroup$ @J.Hougaard scroll down -> squawk code -> scroll down .. find "7777 US, Germany, Belgium, Netherlands Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE)" $\endgroup$
    – eagle275
    Mar 9, 2020 at 13:01
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    $\begingroup$ This should be the wiki page linked - people shouldn't have to follow the breadcrumbs. $\endgroup$
    – JPhi1618
    Mar 9, 2020 at 20:40
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    $\begingroup$ @eagle275 Still doesn't appear to have anything to do with an IFF system $\endgroup$
    – 60levelchange
    Mar 10, 2020 at 12:01
  • $\begingroup$ And it does not have. Military transponders are different. This is purely for ATC (even if used by military too). $\endgroup$
    – Vladimir F Героям слава
    Mar 10, 2020 at 13:24

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