The 1090 Megahertz Riddle (second edition)

A Guide to Decoding Mode S and ADS-B Signals
By: Junzi Sun (junzis.com)

Mode S elementary surveillance

Mode S Elementary Surveillance (ELS) provides a set of basic functionalities that are supported by the Mode S transponders. These basic functionalities include the reporting of aircraft identity, altitude, transponder capability, and flight status. The set of BDS codes included in ELS are 1,0, 1,7, 2,0, and 3,0.

This message is designed to report the data link capability of the Mode S transponder installed. In Table 1.1, all fields in this report are shown.

Data link capability report (BDS 1,0), MB field
FIELD MSG MB BITS
BDS Code [0001 0000] 33-40 1-8 8
Configuration flag 41 9 1
Reserved [00000] 42-46 10-14 6
Overlay Command Capability (OCC) 47 15 1
Reserved for ACAS 48 16 1
Mode S subnetwork version number 49-55 17-23 7
Transponder enhanced protocol indicator 56 24 1
Mode S specific services capability 57 25 1
Uplink ELM average throughput capacity 58-60 26-28 3
Downlink ELM throughput 61-64 29-32 4
Aircraft identification capability 65 33 1
Squitter capability subfield (SCS) 66 34 1
Surveillance identifier code (SIC) 67 35 1
Common usage GICB capability report 68 36 1
Reserved for ACAS 69-72 37-40 4
Data terminal equipment (DTE) status 73-80 41-56 16

In the data link capability report, the first eight bits indicate the BDS number 1,0, which is 0001 0000 in binary format. The definitions of the other fields are:

1) ACAS related bits are shown as follows:

MSG     MB      Coding
--------------------------------------------------------------
48      16      0: ACAS failed or on standby
                1: ACAS operating
--------------------------------------------------------------
69      37      0: Hybrid surveillance not operational
                1: Hybrid surveillance fitted and operational
--------------------------------------------------------------
70      38      0: ACAS generating TAs only
                1: ACAS generating TAs and RAs
--------------------------------------------------------------
72|71   37|36   0|0: RTCA/DO-185 (pre-ACAS)
                0|1: RTCA/DO-185A
                1|0: RTCA/DO-185B or EUROCAE ED 143
                1|1: Reserved for future versions

2) Valid values for Mode S subnetwork version number are from 0 to 5, corresponds to compliance to a different version of ICAO documentations. Numbers 6 to 127 are currently unassigned. Numbers 0 to 5 can be interpreted:

0:  Subnetwork not available
1:  ICAO Doc 9688 (1996)
2:  ICAO Doc 9688 (1998)
3:  ICAO Annex 10, Vol III, Amdt 77
4:  ICAO Doc 9871 (Ed 1), RTCA DO-181D, EUROCAE ED-73C
5:  ICAO Doc 9871 (Ed 2), RTCA DO-181E, EUROCAE ED-73E
>5: Reserved for future use

3) Overlay Command Capability indicated whether the transponder supports BDS overlay (Data Parity).

4) When Transponder enhanced protocol indicator bit is set to , the transponder is a Level 5 transponder. Value indicates a Level 2 to 4 transponder.

5) When Mode S specific services capability bit is set to , at least one Mode S specific service is supported.1.

6) Aircraft identification capability indicates availability of identification.

7) When Squitter capability subfield bit is set to , both BDS 0,5 and 0,6 registers have been updated in the past 9 to 11 seconds.

8) Surveillance identifier code determines whether the transponder has the surveillance identification code capability.

9) Common usage GICB capability report is set to every time the GICB capacity report (BDS code 1,7) is changed. Register 1,7 is sampled every minute to check for changes.

Common usage GICB capability report (BDS 1,7)

This message is designed to report Common Usage Ground-initiated Comm-B (GICB) capabilities. The fields in this report are shown in Table 1.2. In this table, each bit indicates whether the corresponding service is available from the transponder.

A bit is set to when the corresponding register has a valid input that has been updated with the required rate. This means that the same aircraft would respond with different GICB reports due to the availability of the source data.

Common usage GICB capability report (BDS 1,0), MB field
FIELD MSG MB BITS
0,5 Extended squitter airborne position 33 1 1
0,6 Extended squitter surface position 34 2 1
0,7 Extended squitter status 35 3 1
0,8 Extended squitter identification and category 36 4 1
0,9 Extended squitter airborne velocity information 37 5 1
0,A Extended squitter event-driven information 38 6 1
2,0 Aircraft identification 39 7 1
2,1 Aircraft registration number 40 8 1
4,0 Selected vertical intention 41 9 1
4,1 Next waypoint identifier 42 10 1
4,2 Next waypoint position 43 11 1
4,3 Next waypoint information 44 12 1
4,4 Meteorological routine report 45 13 1
4,5 Meteorological hazard report 46 14 1
4.8 VHF channel report 47 15 1
5,0 Track and turn report 48 16 1
5,1 Position coarse 49 17 1
5,2 Position fine 50 18 1
5,3 Air-referenced state vector 51 19 1
5,4 Waypoint 1 52 20 1
5,5 Waypoint 2 53 21 1
5,6 Waypoint 3 54 22 1
5,F Quasi-static parameter monitoring 55 23 1
6,0 Heading and speed report 56 24 1
Reserved for aircraft capability 57 25 1
Reserved for aircraft capability 58 26 1
E,1 Reserved for Mode S BITE (Built In Test Equipment) 59 27 1
E,2 Reserved for Mode S BITE (Built In Test Equipment) 60 28 1
F,1 Military applications 61 29 1
Reserved 62-80 30-56 17

We can use the following message as an example:

MSG:      A0000638FA81C10000000081A92F
MB:               FA81C100000000
MB BIN:   11111010100000011100000100000000000000000000000000000000

From the MB field, we can identify that following bits are set to :

Bit: 1, 2, 3, 4, 5, 7, 9, 16, 17, 18, 24

Hence, BDS codes supported by the transponder are:

BDS05 BDS06 BDS07 BDS08 BDS09     < ADSB
BDS20                             < ELS
BDS40 BDS50 BDS51 and BDS60       < EHS

Try it out Using pyModeS, we can decode the GICB information as:

import pyModeS as pms

msg = "A0000638FA81C10000000081A92F"
capabilities = pms.commb.cap17(msg)

A list of supporting BDS code will be returned.

Aircraft identification (BDS 2,0)

Similar to an ADS-B aircraft identification message, the callsign of an aircraft can be decoded from BDS 2,0 messages. The structure of the MB field is defined in Table 1.3.

Aircraft identification (BDS 2,0), MB field
FIELD MSG MB BITS
BDS Code [0010 0000] 33-40 1-8 8
Character 1 41-46 9-14 6
Character 2 47-52 15-20 6
Character 3 53-58 21-26 6
Character 4 59-64 27-32 6
Character 5 65-70 33-38 6
Character 6 71-76 39-44 6
Character 7 77-82 45-50 6
Character 8 83-88 51-56 6

The first 8 bits indicates the BDS code 0010 0000 (2,0 in hexadecimal). Each of the callsign characters is represented by 6 bits. We first need to convert the binary numbers to decimals. Each decimal value corresponds to the index of the letter in the following character map:

#ABCDEFGHIJKLMNOPQRSTUVWXYZ##### ###############0123456789######

It is worth noting that this character mapping is the same as the one used for ADS-B identification, which also corresponds to a part of the ASCII code map.

The following is an example on how to decode the callsign from a sample BDS 2,0 message:

MSG:  A000083E202CC371C31DE0AA1CCF
MB:           202CC371C31DE0
-----------------------------------------------------------------------------
MB BIN:  0010 0000 001011 001100 001101 110001 110000 110001 110111 100000
HEX:        2    0
DEC:                 11     12     13     49     48     49     55     32
CHR:                 K      L      M      1      0      1      7     [SPACE]
-----------------------------------------------------------------------------
ID:   KLM1017

Try it out Using pyModeS, we can decode the callsign in this message as:

import pyModeS as pms

msg = "A000083E202CC371C31DE0AA1CCF"
callsign = pms.commb.cs20(msg)

Callsign KLM1017_ will be returned by the previous function since the space is replace by _ in pyModeS.

ACAS active resolution advisory (BDS 3,0)

The BDS 3,0 message is used to report resolution advisories (RA) generated by ACAS equipment. The structure of the MB field is defined in Table 1.4.

ACAS active resolution advisory (BDS 3,0), MB field
FIELD MSG MB BITS
BDS Code [0011 0000] 33-40 1-8 8
Active resolution advisories 41-54 9-22 14
Resolution advisory complements record 55-58 23-26 4
RA terminated 59 27 1
Multiple threat encounter 60 28 1
Threat type indicator 61-62 29-30 2
Threat identity data 63-86 31-56 26

1) The first 8 bits show the BDS code 3,0 or 0011 0000 in binary format.

2) The 14-bit active resolution advisories (ARA) indicate the characteristics of the RA generated by the ACAS associated with the transponder. To decode the information, we need to decode MB bit 9 (MSG bit 41) and MB bit 28 (MSG bit 60). Table 1.5 shows how to interpret these two bits.

BDS 3,0 MB bits 9 and 28
MB:9 MB:28
0 0 No RA has been generated
0 1 Multiple threats, RA is intended to provide vertical separation blew some threats and above some other threats
1 0 Only one threat
0 1 Multiple threats, RA is intended to provide vertical separation in the same direction

MB bits 16-22 are reserved for ACAS III. When MB:9=1, the meanings of MB bits from 10 to 15 are shown in Table 1.6.

BDS 3,0 MB bits 10 to 15 (MB:9=1)
MB Value
10 0 RA is preventive
1 RA is corrective
11 0 Upward sense RA has been generated
1 Downward sense RA has been generated
12 0 RA is not increased rate
1 RA is increased rate
13 0 RA is not a sense reversal
1 RA is a sense reversal
14 0 RA is not altitude crossing
1 RA is altitude crossing
15 0 RA is vertical speed limit
1 RA is positive

When MB:9=0 and MB:28=1, the meanings of MB bits from 10 to 15 are shown in Table 1.7.

BDS 3,0 MB bits 10 to 15 (MB:9=0 MB:28=1)
MB Value
10 0 RA does not require a correction in the upward sense
1 RA requires a correction in the upward sense
11 0 RA does not require a positive climb
1 RA requires a positive climb
12 0 RA does not require a correction in the downward sense
1 RA requires a correction in the downward sense
13 0 RA does not require a positive descend
1 RA requires a positive descend
14 0 RA does not require a crossing
1 RA requires a crossing
15 0 RA is not a sense reversal
1 RA is a sense reversal

) The resolution advisory complements (RAC) record consists of 4 bits, and each bit has the following meaning:

MB:23=1   Do not pass below
MB:24=1   Do not pass above
MB:25=1   Do not turn left
MB:26=1   Do not turn right

3) A RA terminated bit indicates whether previously generated RA has been terminated.

4) Threat type indicator contains 2 bits with the following meaning:

00    No identity data in threat identity data
01    Threat identity data contains a Mode~S transponder address
10    Threat identity data contains altitude, range, and bearing
11    Not assigned

When the threat type indicator is 01, MB bits 31-54 contain the 24-bit Mode S transponder address and the last two bits are set to zero.

When the threat type indicator is 10, MB bits 31-56 is divided into three different segments.

  • MB bits 31-43 contain the 13-bit threat altitude code. It is the same structure as the altitude code from section [sec:alt_code].

  • MB bits 44-50 contain the most recent threat range from ACAS. When it is 0, no range estimation is available. When it is 127, the range is greater than 12.55 nautical miles. For other values, the range is calculated as \((n-1) / 10 \pm 0.05\) nautical miles.

  • MB bits 51-56 contain the most recent estimated bearing of the threat aircraft, relative to their own heading. When it is 0, no estimation is available. Values larger than 60 are not assigned. For other values (from 1 to 60), the bearing range is calculated as \([6(n-1), 6n]\) in degrees.


  1. The additional specific services are BDS codes other than 0,2, 0,3, 1,0, 1,7-1,C, 2,0, and 3,0↩︎

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