The 1090MHz Riddle

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

Track and turn (BDS 5,0)

Within the BDS 5,0 message, five different types of aircraft states are given, mostly related with the turns:

  • roll angle

  • true track angle

  • ground speed

  • track angle rate

  • true airspeed

The 56-bit MB field is structured as follows:

+---------------------------------------+------+------+
| FIELD                                 | MB   |N-BITS|
+=======================================+======+======+
| Status                                |  1   |  1   |
+---------------------------------------+------+------+
| Sign, 1 -> Use two's complement       |  1   |  1   |
+---------------------------------------+------+------+
| Roll angle                            |  3   |  9   |
|                                       |      |      |
| range = [-90, 90] degrees             |      |      |
|                                       |      |      |
| LSB: 45/256 degree                    |  11  |      |
+---------------------------------------+------+------+
| Status                                |  12  |  1   |
+---------------------------------------+------+------+
| Sign, 1 -> Use two's complement       |  13  |  1   |
+---------------------------------------+------+------+
| True track angle                      |  14  |  10  |
|                                       |      |      |
| range = [-180, 180] degrees           |      |      |
|                                       |      |      |
| LSB: 90/512 degree                    |  23  |      |
+---------------------------------------+------+------+
| Status                                |  24  |  1   |
+---------------------------------------+------+------+
| Ground speed                          |  25  |  10  |
|                                       |      |      |
| range = [0, 2046] knots               |      |      |
|                                       |      |      |
| LSB: 2 knots                          |  34  |      |
+---------------------------------------+------+------+
| Status                                |  35  |  1   |
+---------------------------------------+------+------+
| Sign, 1 -> Use two's complement       |  36  |  1   |
+---------------------------------------+------+------+
| Track angle rate                      |  37  |  9   |
|                                       |      |      |
| range = [-16, 16] degrees             |      |      |
|                                       |      |      |
| LSB: 8/256 degree / second            |  45  |      |
+---------------------------------------+------+------+
| Status                                |  46  |  1   |
+---------------------------------------+------+------+
| True airspeed                         |  47  |  10  |
|                                       |      |      |
| range = [0, 2046] knots               |      |      |
|                                       |      |      |
| LSB: 2 knots                          |  56  |      |
+---------------------------------------+------+------+

When a parameter is signed (SIGN = 1), the two’s complement should be used to calculate the value. The value can be calculated as follows:

(-2**{Number of bits parameter} + value) * LSB.

The number of bits for roll angle and track rate is 9, and for true track angle 10. An example calculation with a signed parameter is given in (Airspeed and Heading) https://mode-s.org/decode/ehs/bds60-airspeed.html.

An example:

MSG:  A000139381951536E024D4CCF6B5
MB:           81951536E024D4

---------------------------------------------------------------------------------
MB BIN:   1 0 000001100 1 0 1010001010 1 0011011011 1 0 000000100 1 0011010100
---------------------------------------------------------------------------------
STATUS:   1
SIGN:       0
ROLL:         12 (x45/256)
---------------------------------------------------------------------------------
STATUS:                 1
SIGN:                     0
TRACK ANGLE:                650 (x90/512)
---------------------------------------------------------------------------------
STATUS:                                1
GROUND SPEED:                            219 (x2)
---------------------------------------------------------------------------------
STATUS:                                             1
SIGN:                                                0
TRACK ANGLE RATE:                                      4 (x8/256)
---------------------------------------------------------------------------------
STATUS:                                                           1
TRUE AIRSPEED:                                                      212 (x2)
---------------------------------------------------------------------------------
FINAL:      2.1 deg      114.3 deg       438 kt      0.1 deg/s     424 kt
---------------------------------------------------------------------------------

Of course, all fields are not always available in each of DBS 5,0 message. For the information that is not available, status bits are set to 0.

Site maintained by @junzis. Build with LaTeX, Pandoc, and GitHub