The 1090 Megahertz Riddle (second edition)

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

Airborne collision avoidance system

Background

Airborne Collision Avoidance System (ACAS) is a system to reduce the risk of mid-air collisions and near mid-air collisions between aircraft. There are three types of ACAS systems according to , which are:

  • ACAS I: Gives traffic advisories (TA), without recommending manoeuvres.

  • ACAS II: Gives traffic advisories (TA) and resolution advisories (RA) only in vertical directions.

  • ACAS III: Gives traffic advisories (TA) and resolution advisories (RA) in both horizontal and vertical directions. ACAS III is not currently implemented

This chapter will focus on ACAS II. The ACAS II is a system that utilizes the aircraft transponder, which interrogates the Mode C and Mode S transponders of nearby aircraft. When threats are detected, corresponding alerts are given to the pilots.

Currently, the most common implementation of ACAS II is Traffic alert and Collision Avoidance System (TCAS) II version 7.1, which was initiated by EUROCONTROL. It has been mandatory for aircraft in Europe since 2015. ACAS II works independently of the navigation system, FMS, and ATC. No input from these systems is considered for producing the alerts.

In ACAS II, TA and RA are triggered when certain thresholds to the closest point of approach (CPA) are passed. The thresholds depend on the altitude, speed, and heading of the aircraft. The examples of the TA and RA regions are illustrated in Figure 1.1.

ACAS with Mode C transponders

ACAS system uses Mod C only all-call interrogation (see Figure [fig:mode_s_inter_mode] in chapter [chap:intro]) to detect aircraft that are only equipped with Mode A/C transponders. In this case, the ACAS system initiates a sequence of interrogations with increasing power.

The interrogation pulse is slightly different from a Mode A/C-only all-call interrogation. A special \(S_1\) pulse (also known as "whisper-shout") is designed to reduce interference. The \(S_1\) pulse is inserted 2 \(\mu s\) (\(\pm 0.15 \mu s\)) before the \(P_1\) pulse.

The reply should be sent with the following rules:

  • With both \(S_1\) and \(P_1\) above the minimum triggering level (MTL), no reply will be generated.

  • With both \(S_1\) and \(P_1\) at MTL, the transponder will respond to 10% of the interrogations.

  • With \(P_1\) at MTL and \(S_1\) at MTL - 3 dB, the transponder will respond to 70% of the interrogations.

  • With \(P_1\) at MTL and \(S_1\) at MTL - 6 dB, the transponder will respond to 90% of the interrogations.

ACAS with Mode S transponders

For Mode S transponders, ACAS system employs a three-phase process, which includes the phases of detection, surveillance, and coordination.

For the detection phase, ACAS passively listens to Mode S only all-call replies (DF=11). These all-call replies are usually generated as a result of ground SSR interrogations or as spontaneous replies. In this process, aircraft with Mode S transponders in the vicinity are discovered. ACAS may also listen to extended squitter messages (Downlink Format 17, ADS-B) to detect other aircraft.

Once an aircraft is determined to be within the ACAS surveillance range, and within 10,000 ft of the own ACAS aircraft, ACAS will initiate a short air-air interrogation (UF=11) to acquire the range. The interrogation rate is defined as:

  • Once every five cycles: when the target aircraft maintains in the surveillance range.

  • Once every cycle: when the target aircraft is within 3 NM or with time to closest approach less than 60 s.

The surveillance interrogation is stopped when all the following conditions are met:

  • A reply (DF=11) is received

  • Both aircraft are below 18,000 ft.

  • Target aircraft is more than 3 NM and 60 s away from the closest point of approach.

In Tables 1.1 and 1.2, fields for ACAS surveillance interrogation and reply messages are listed.

ACAS surveillance interrogation (uplink), UF=0
FIELD MSG BITS
Uplink Format UF 1-5 5
Reserved 6-8 3
Reply length RL 9 1
Reserved 10-13 4
Acquisition AQ 14 1
Data selector DS 15-22 8
Reserved 23-32 10
Address parity AP 33-56 24
ACAS surveillance reply (downlink), DF=0
FIELD MSG BITS
Downlink Format DF 1-5 5
Vertical status VS 6 1
Cross-link capability CC 7 1
Reserved 8 1
Sensitivity level SL 9-11 3
Reserved 12-13 2
Reply information RI 14-17 4
Reserved 18-19 2
Altitude code AC 20-32 13
Address parity AP 33-56 24

Once the target aircraft is within the RA region (a threat), ACAS initiates the coordination interrogation (UF=16). In this step, resolution information are transmitted and received through coordination replies (DF=16). Information that is included in both coordination messages is shown in Tables 1.3 and 1.4.

ACAS coordination interrogation (UF=16)
FIELD MSG BITS
Uplink Format UF 1-5 5
Reserved 6-8 3
Reply length RL 9 1
Reserved 10-13 4
Acquisition AQ 14 1
Reserved 15-32 18
Message, U MU 33-88 56
Address parity AP 33-56 24
ACAS coordination reply (DF=16)
FIELD MSG BITS
Downlink Format DF 1-5 5
Vertical status VS 6 1
Reserved 7-8 2
Sensitivity level SL 9-11 3
Reserved 12-13 2
Reply information RI 14-17 4
Reserved 18-19 2
Altitude code AC 20-32 13
Message, V MV 33-88 56
Address parity AP 89-112 24

Specific fields in above messages are defined as follows:

  • Reply length (RL): 1 bit, defines the required reply format: requires a reply with DF=0, while requires reply with DF=16.

  • Acquisition (AQ): 1 bit, contains code that controls the content of RI field in the reply.

  • Data selector (DS): 8 bits, indicates the BDS code of the MV content in reply with DF=16.

  • Vertical status (VS): 1 bit, indicates whether the aircraft is airborne () or on the ground ().

  • Cross-link capability (CC): 1 bit, refers to the capability of reply DF=16 upon request of UF=0. When this 1-bit field is set to , the cross-link is supported. Otherwise, the field is set to .

  • Sensitivity level (SL): 3 bits, represents the sensitivity level of the ACAS system. indicates the ACAS is inoperative. Other values indicate the sensitivity level.

  • Reply information (RI): 4 bits, indicates the type of reply to interrogating aircraft. For ACAS message, valid values are 0 and from 2 to 4. Other values are not part of the ACAS:

    0000: No operating ACAS
    0010: ACAS with resolution capability inhibited
    0011: ACAS with vertical-only resolution capability
    0111: ACAS with vertical and horizontal resolution capability

  • Altitude Code (AC): 13 bits, encodes the altitude of the aircraft. It can be decoded according to section [sec:alt_code].

Message fields of ACAS coordination messages

Message U-definition (MU) and Message V-definition (MV) are transmitted in ACAS coordination interrogation and reply messages, respectively. MU is used to transit resolution, ACAS broadcast, and RA broadcast. MV is used for reply purposes.

MU (U-definition)

MU of ACAS resolution message

When ACAS resolution information is transmitted in the UF=16 message, the first 8 bits of MU, U-definition subfields (UDS), is set to 0011 0000 (UDS=3,0). The corresponding fields are indicated in Table 1.5.

UF=16, MU for ACAS resolution messages, UDS=3,0
FIELD MSG MU BITS
U-definition subfield 1 [0011] UDS1 33-36 1-4 4
U-definition subfield 2 [0000] UDS2 37-40 5-8 4
Reserved 41 9 1
Multiple threat bit MTB 42 10 1
Cancel vertical RAC CVC 43-44 11-12 2
Vertical RAC VRC 45-46 13-14 2
Cancel Horizontal RAC CHC 47-49 15-17 3
Horizontal RAC HRC 50-52 18-20 3
Reserved 53-55 21-23 3
Horizontal sense bits HSB 56-60 24-28 5
Vertical sense bits VSB 61-64 29-32 4
Aircraft address MID 65-88 33-56 24

The significance of fields are listed as follows:

  • Multiple threat bit (MTB): 1 bit, indicates whether multiple threats are present.

  • Vertical RAC 1 (VRC): 2 bits, contains vertical resolution advisory complement information:

    00: No vertical RAC information
    01: Do not pass below
    10: Do not pass above
    11: Not assigned

  • Cancel vertical RAC (CVC): 2 bits, cancels previously sent VRC:

    00: No cancellation information
    01: Cancel "Do not pass below"
    10: Cancel "Do not pass above"
    11: Not assigned

  • Horizontal RAC (HRC): 3 bits, contains horizontal resolution advisory complementary information:

    000: No information
    001: Other ACAS sense is turn left; do not turn left
    010: Other ACAS sense is turn left; do not turn right
    101: Other ACAS sense is turn right; do not turn left
    110: Other ACAS sense is turn right; do not turn right
    other: Not assigned

  • Cancel horizontal RAC (CHC): 3 bits, cancels previously sent HRC:

    000: No cancellation information
    001: Cancel "Do not turn left"
    010: Cancel "Do not turn right"
    other: Not assigned

  • Horizontal sense bits (HSB): 5 bits, uses Hamming code with an extra parity bit to detect errors (up to 3 bits) in CHC and HRC fields.

  • Vertical sense bits (VSB): 4 bits, uses Hamming code with an extra parity bit to detect errors (up to 3 bits) in CVC and VRC fields.

  • Aircraft address (MID): 24 bits, contains the 24-bits aircraft transponder address of the interrogating ACAS aircraft.

MU of RA broadcast message

When UF=16 is used for RA broadcast, UDS is set to 0011 0001 (UDS=3,1). The corresponding fields are indicated in Table 1.6.

UF=16, MU for RA broadcast, UDS=3,1
FIELD MSG MU BITS
U-definition subfield 1 [0011] UDS1 33-36 1-4 4
U-definition subfield 2 [0001] UDS2 37-40 5-8 4
Active RAs ARA 41-54 9-22 14
RAC’s record RAC 55-58 23-26 4
RA terminated indicator RAT 59 27 1
Multiple threat encounter MTE 60 28 1
Reserved 61-62 29-30 2
Mode A identity code AID 63-75 31-43 13
Mode C altitude code CAC 76-88 44-56 13

The significance of fields are listed as follows:

  • Active RA (ARA): 14 bits, indicates the resolution advisory characteristics. It has to be interpreted together with the MTB field.

    • When ARA first bit (MSG bit 41) is and MTE is either or :

      Bit 42: RA is corrective () or preventive ()
      Bit 43: RA is downward sense () or upward sense ()
      Bit 44: RA is increased rate () or not ()
      Bit 45: RA is a sense reversal () or not ()
      Bit 46: RA is altitude crossing () or not ()
      Bit 47: RA is positive () or vertical speed limit ()
      Bit 48-54: Reserved for ACAS III

    • When ARA first bit (MSG bit 41) is and MTE is :

      Bit 42: RA requires a correction in the upward sense () or not ()
      Bit 43: RA requires a positive climb () or not ()
      Bit 44: RA requires a correction in the downward sense () or not ()
      Bit 45: RA requires a positive descent () or not ()
      Bit 46: RA requires a crossing () or not ()
      Bit 47: RA is a sense reversal () or not ()
      Bit 48-54: Reserved for ACAS III

    • When ARA first bit (MSG bit 41) is and MTE is , no vertical RA is generated.

  • RAC’s record (RAC): 4 bits, contains current active RACs that are received from other ACAS aircraft (if any). Each of the four bits in this field indicates the following RAC when set to . When a bit is set to , the corresponding RAC is inactive.

    Bit 55: Do not pass below
    Bit 56: Do not pass above
    Bit 57: Do not pass left
    Bit 58: Do not pass right

  • RA terminated indicator (RAT): 1 bit, indicates whether ACAS is currently generating RA in the ARA field or RA in the ARA field has been terminated.2

  • Multiple threat encounter (MTE): 1 bit, indicates whether multiple threats are currently being processed by the ACAS resolution. When MTE is set to , either one threat is being processed (ARA bit 41 sets to ) or no threat is being processed (ARA bit 41 sets to ). When MTE is set to , multiple threats are being processed.

  • Mode A identity code (AID): 13 bits, contains the Mode A identity code (sqwake code) of the reporting aircraft. It can be decoded according to section [sec:id_code].

  • Mode C altitude code (CAC): 13 bits, contains the Mode C altitude code reporting aircraft. It can be decoded according to section [sec:alt_code].

MU of ACAS broadcast message

When UF=16 is used for ACAS broadcast, UDS is set to 0011 0010 (UDS=3,2). The corresponding fields are indicated in Table 1.7.

UF=16, MU for ACAS broadcast, UDS=3,1
FIELD MSG MU BITS
U-definition subfield 1 [0011] UDS1 33-36 1-4 4
U-definition subfield 2 [0010] UDS2 37-40 5-8 4
Reserved 41-64 9-32 24
Aircraft address MID 65-8 33-56 24

The only information that is broadcast in the MU of this message is the 24-bit transponder address of the interrogating aircraft.

MV (V-definition)

Similar to MU in the UF=16 message, MV in the DF=16 message contains a few common fields. The corresponding fields are indicated in Table 1.8.

DF=16, MV for coordinated reply, VDS=3,0
FIELD MSG MU BITS
V-definition subfield 1 [0011] VDS1 33-36 1-4 4
V-definition subfield 2 [0001] VDS2 37-40 5-8 4
Active RAs ARA 41-54 9-22 14
RAC’s record RAC 55-58 23-26 4
RA terminated indicator RAT 59 27 1
Multiple threat encounter MTE 60 28 1
Reserved 61-88 29-56 28

We can see that the structure of the MV fields is similar to MU fields of RA broadcast (UDS=3,1) from Table 1.6. The interpretations of these fields are also the same as in section 1.4.1.2.


  1. RAC: Resolution Advisory Complement↩︎

  2. The Mode S transponder is still required to report RA 18 seconds after it is terminated by ACAS. Hence, the RAT field is used.↩︎

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