The 1090MHz Riddle

An open-access book about decoding Mode-S and ADS-B data

Aircraft Operation Status

Operation status message is introduced since the Version 1 of ADS-B. And there are also slight differences in the structure of Aircraft Operation Messages between Version 1 and 2.

To understand about these versions, first take a look at the ADS-B version chapter.

The operation status is transmitted with Type Code 31 (TC=31). The structure of the message (in Version 1) is laid out as follows:

+----------------------------------------+------+------+------+
| FIELD                                  | MSG  | MB   |N-BITS|
+========================================+======+======+======+
| Downlink Format = 17                   |  1   |      |  8   |
|                                        |  8   |      |      |
+----------------------------------------+------+------+------+
| ICAO Address                           |  9   |      |  24  |
|                                        |  32  |      |      |
+----------------------------------------+------+------+------+
| Type Code = 31                         |  33  |  1   |  5   |
|                                        |  37  |  5   |      |
+----------------------------------------+------+------+------+
| Subtype Code                           |  38  |  6   |  3   |
|                                        |      |      |      |
|  - 0: airborne                         |      |      |      |
|  - 1: surface                          |      |      |      |
|  - 2-7: reserved                       |  40  |  8   |      |
+-------------------+--------------------+------+------+------+
|                   |                    |  41  |  9   | 16   |
|  Airborne         |  Surface           |      |      |      |
|  capacity class   |  capacity class    |      |      |      |
|  codes            |  codes             |      |      |      |
|                   |                    |  52  |  20  |      |
|                   +--------------------+------+------+------+
|                   |                    |  53  |  21  | (4)  |
|                   |  Length/width      |      |      |      |
|                   |  codes             |      |      |      |
|                   |                    |  56  |  24  |      |
+-------------------+--------------------+------+------+------+
| Operational mode code                  |  57  |  25  |  16  |
|                                        |      |      |      |
|                                        |      |      |      |
|                                        |  72  |  40  |      |
+----------------------------------------+------+------+------+
| ADS-B version number                   |  73  |  41  |  3   |
|                                        |  75  |  43  |      |
+----------------------------------------+------+------+------+
| NIC supplement bit                     |  76  |  44  |  1   |
+----------------------------------------+------+------+------+
| NACp: Navigation accuracy category     |  77  |  45  |  4   |
|        - position                      |  80  |  48  |      |
+-------------------+--------------------+------+------+------+
| BAQ = 0           | Reserved           |  81  |  49  |  2   |
|                   |                    |  82  |  50  |      |
+-------------------+--------------------+------+------+------+
| SIL: Surveillance integrity level      |  83  |  51  |  2   |
|                                        |  84  |  52  |      |
+-------------------+--------------------+------+------+------+
| NIC-BARO          | TRK/HDG            |  85  |  53  |  1   |
+-------------------+--------------------+------+------+------+
| HRD                                    |  86  |  54  |  1   |
+----------------------------------------+------+------+------+
| Reserved                               |  87  |  55  |  2   |
|                                        |  88  |  56  |      |
+----------------------------------------+------+------+------+

Acronyms:

  • BAQ: Barometric Altitude Quality (always set to zero for airborne message subtype=1)

  • HRD: Horizontal Reference Direction

    • 0: True North

    • 1: Magnetic North

In ADS-B Version 2, most part of the message remains the same, we will only address the second half of the message, where the changes have been made.

+----------------------------------------+------+------+------+
| FIELD                                  | MSG  | MB   |N-BITS|
+========================================+======+======+======+
| Airborne          | Surface            |  57  |  25  |  16  |
| operational       | operational        |      |      |      |
| mode code         | mode code          |      |      |      |
|                   |                    |  72  |  40  |      |
+-------------------+--------------------+------+------+------+
| ADS-B version number                   |  73  |  41  |  3   |
|                                        |  75  |  43  |      |
+----------------------------------------+------+------+------+
| NIC supplement bit - A                 |  76  |  44  |  1   |
+----------------------------------------+------+------+------+
| NACp: Navigation accuracy category     |  77  |  45  |  4   |
|       - position                       |      |      |      |
|                                        |  80  |  48  |      |
+-------------------+--------------------+------+------+------+
| GVA               | Reserved           |  81  |  49  |  2   |
|                   |                    |  82  |  50  |      |
+-------------------+--------------------+------+------+------+
| SIL: Surveillance integrity level      |  83  |  51  |  2   |
|                                        |  84  |  52  |      |
+-------------------+--------------------+------+------+------+
| NIC-BARO          | TRK/HDG            |  85  |  53  |  1   |
+-------------------+--------------------+------+------+------+
| HRD                                    |  86  |  54  |  1   |
+----------------------------------------+------+------+------+
| SIL supplement bit                     |  87  |  55  |  1   |
+----------------------------------------+------+------+------+
| Reserved                               |  88  |  56  |  1   |
+----------------------------------------+------+------+------+

Acronyms:

  • GVA: Geometric Vertical Accuracy - GNSS position source, 95% vertical figure of merit (VFOM)

    • 0: unknown or > 150 meters

    • 1: < 150 meters

    • 2: < 45 meters

    • 3: reserved

  • SIL, NIC, NAC are also related to measurement uncertainty or accuracy.

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