9
$\begingroup$

On several dates, my ground station received some airborne operational status ADS-B messages (TC=31) where the version number was reported as 3. As far as I know, the only valid versions are 0, 1, and 2.

I don't think it was just a garbled reception because

  1. I received more than 500 messages of the same type, all reporting 3 as the version number.
  2. The messages came from three USAF aircraft of the same type, KC-10A Extenders.

Here is a sample message, in raw form: 8DAE022EF8000000206143635C24

The dates on which I saw the messages:

  • 2018-04-13
  • 2019-02-16

The aircraft associated with the messages:

What's the most likely reason behind these messages having this (AFAIK non-existent) version number? A test? A misconfigured piece of equipment (it strikes me as unlikely that the version number can be set to 3, but maybe I'm wrong). Is it a bug in the decoder I'm using?

[Edited 2019-02-16 to include other aircraft I've received the anomalous messages from.]

|improve this question
$\endgroup$
  • 1
    $\begingroup$ When you receive these messages, do you receive a steady stream of other messages from the same 24 bit address? $\endgroup$ – DeltaLima Feb 19 '19 at 22:05
  • $\begingroup$ BTW: these aircraft appear to be Travis AFB based aircraft from markings in available photos (60AMW, to be precise). I poked the wing through the contact form on the Travis AFB website, requesting a reply, and will respond if/when I get one. $\endgroup$ – UnrecognizedFallingObject Feb 21 '19 at 3:37
5
$\begingroup$

I've decoded the message with my own decoder. I don't claim it is bug free, but it is designed to follow the 1090ES MOPS (ADS-B Minimum Operational Performance Standards) very closely and gives warnings when messages deviate from the standard. I forced decoding conforming to version 2 of the MOPS, which is the latest standard. 

HEX: 8DAE022EF8000000206143635C24
Aircraft Operational status message (airborne format)
    Capability Class:
        TCAS op.          false
        1090ES IN         false
        Reserved [13,14]      0
        ARV rep. cap.     false
        TS report cap.    false
        TC rep. cap. lvl      0
        UAT IN            false
        ADS-R rsvd flag   false
        Reserved              0
    Airborne Operational Mode (OM)
        TCAS RA active    false
        IDENT             false
        Reserved [ATC]    false
        Single Antenna    false
        SDA               0
        Reserved [33-40]  32
     MOPS version     3
     NIC supp. A      false
     NACp             1
     GVA              1
     SIL              0
     SIL supp.        true
     NIC baro         0
     Heading ref.     True North
     Reserved [56]    true

The 1090ES MOPS version is indeed indicated as version 3, which is not yet published. Also, the ME-bits 35 and 56 are set to 1. In version 2 of the 1090ES MOPS these are reserved bits, so they shall be 0.

Version 3 of the 1090ES MOPS is not yet published, so transponders should not use it.

Perhaps these messages are from an experimental transponder, which does not conform to the MOPS.

|improve this answer
$\endgroup$
  • $\begingroup$ Transponders with ICAO addresses AE* are usually those used in military planes. Military planes often tamper with the payload of the transmissions. My guess is that they set the version to 3 to make sure that any receiver following the specifications ignores these transmissions. $\endgroup$ – affitz Feb 20 '19 at 8:47
  • $\begingroup$ In addition, there's no way to know how to decode the message correctly. From version 0 to 1/2, the format of operational status messages changed entirely which might also happen in version 3. $\endgroup$ – affitz Feb 20 '19 at 8:49
1
$\begingroup$

According to the FAA's Advisory Circular AC20-165B, numbers higher than 2 are allowed:

A.2.34 Version Number.

The applicable TSO Minimum Operational Performance Standard (MOPS) level is communicated through the version number, which is fixed at the time the ADS-B equipment is manufactured. Version 2 applies to ADS-B equipment that meets MOPS documents RTCA/DO-260B with corrigendum 1 or RTCA/DO-282B with corrigendum 1. ADS-B equipment outputting version 2 or higher is required by § 91.227.

Version 3 ADS-B Out avionics meet the requirements defined in DO-260C or DO-282C; although since you are referring to operational status messages my guess is that you are talking about 1090ES messages (i.e. compliant with DO-260C).

|improve this answer
$\endgroup$
  • 1
    $\begingroup$ But DO-260C is not yet published :-) $\endgroup$ – DeltaLima Feb 19 '19 at 21:55
  • 1
    $\begingroup$ Working on it ;) $\endgroup$ – nodapic Feb 24 '19 at 1:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.