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Minimum Aviation System Performance Standards (MASPS) for the Interoperability of Airborne Collision Avoidance Systems (CAS)

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Organization: EUROCAE
Publication Date: 1 September 2020
Status: active
Page Count: 68


Detect and Avoid

These MASPS protect the correct operation of systems that generate RAs for the avoidance of imminent collision.

DAA contains a Remain Well Clear (RWC) function and a collision avoidance function. Some implementations might exclude one of these functions. The DAA collision avoidance function is a CAS and is referred to as CAS in this document. While CAS are the main subject of these MASPS, there are also requirements relating to RWC to ensure full interoperability between CAS on other aircraft and RWC. In the context of this document, DAA is not referenced since it might contain both functions. The document will reference the two functions distinctly as RWC and CAS.

RWC and other systems can interact with CAS, for example: by giving alerting or guidance affecting manoeuvres on a collision avoidance timescale; or by tracking aircraft using active interrogation and reply on 1030/1090 MHz. These MASPS include requirements on these systems to ensure satisfactory performance of CAS. Interoperability between RWC and systems other than CAS, including between RWC of different design, is not addressed.

Determining the extent to which stand-alone RWC and other systems can share the signals and spectrum used by CAS is the responsibility of the appropriate governmental agencies. For example, these MASPS require RWC to monitor ADS-B and 1030/1090 MHz in order to ensure complementary manoeuvre guidance, but these provisions do not imply permission to use active interrogation on 1030 MHz other than to support a collision avoidance function.

NOTE 1: TCAS I (DO-197A) has its own interference limiting provisions that differ from those of TCAS II.

Aircraft Types

These MASPS address systems installed on aircraft that typically operate in managed and unsegregated airspace, in numbers and at densities broadly comparable with those that exist today. They are not intended to facilitate a radical increase in aircraft density nor the widespread operation of novel aircraft types. For example, these MASPS are not intended to provide guidance on collision avoidance systems installed on very small Remotely Piloted Aircraft Systems that are confined to operations close to the surface.

Transponders, Passive CAS and Operational Coordination Messages (OCM)

Much of the functionality discussed in these MASPS will require transponders that comply with DO-260C/ ED-102B and DO-181F/ ED-73F or later [30], [31], [32], [33]. (DO-260C/ ED-102B and DO-181F/ ED-73F will be jointly2 referred to as the New Transponder MOPS, and transponders complying with these MOPS will be referred to as New Transponders.) Equipping CAS with such New Transponders is essential to enable the full operation of some CAS (e.g. section However, even the New Transponders will not deliver the full functionality described in these MASPS. This issue is discussed below.

Current CAS are Active CAS that coordinate their RAs by sending Mode S interrogations on 1030 MHz and transponder replies on 1090 MHz. Passive CAS are introduced and discussed in these MASPS. These systems are defined as CAS that do not use active interrogation for surveillance. Surveillance is likely to depend on a variety of devices and approaches, but it is assumed here that they will make full use of ADSB. It is also assumed they will use ADS-B to coordinate RAs; these MASPS describe the contents and protocols for Operational Coordination Messages (OCMs). However, the New Transponder MOPS do not provide for the broadcast of OCM. The implementation of OCMs is contingent on further work to define their operational requirements, and to confirm that the frequency of OCM transmissions can be contained within the limits considered acceptable.

These MASPS treat Passive CAS as a future concept that has not been reviewed for operational use. These MASPS do not serve the purpose of a MOPS for Passive CAS, nor comparable detailed design documents; the functional, safety and performance requirements for Passive CAS are not addressed.

Provision is made for Passive CAS in these MASPS so that the present generation of CAS, e.g. ACAS Xa and Xu, can include the capability to transmit OCMs and thus coordinate with future Passive CAS. It is understood that Passive CAS could use any of many different schemes to coordinate RAs between themselves; the fact that a scheme based on the use of OCMs is standardised here is not intended to imply the use of a particular link. However, OCMs are likely to be the only method for the present generation of CAS to coordinate RAs with Passive CAS; any coordination mechanisms that are not provided now will result in an inability to coordinate with legacy Active CAS, resulting in limitations in the RAs that future Passive CAS can generate. These MASPS have been designed for 1030/1090 MHz. If other frequencies, e.g. 978 MHz for UAT, are used for exchange of information, all the message exchanges required for coordination need to be assured.

CAS Configurations and Constraints These MASPS contain requirements governing many different potential CAS configurations. However, just because a configuration is enabled by these MASPS does not mean it will be allowed in any particular airspace. Configurations include whether or not the CAS:

• has 1030/1090 MHz interrogation/reply capability

• can transmit OCMs (specific ADS-B messages that contain the same pairwise RA complements as TCAS Resolution Messages, which are interrogations on 1030 MHz that coordinate RAs),

• can receive OCMs,

• is fitted in conjunction with a Mode S transponder, which might or might not meet the New Transponder MOPS, and

• is "junior".

NOTE 2: The concepts of "senior", "junior" and "peer" are defined in the Glossary (section 1.4), and explained in section 3.5.3.

Not all these different configurations are permissible. Those that are not permissible include:

• a junior CAS that is not always able to generate horizontal RAs, e.g. a junior ACAS Xa, (see R3.69, section;

• a junior CAS in conjunction with a transponder not complying with the New Transponder MOPS or later, because it would be unable to announce itself as junior resulting in the rules for coordination breaking down (see R3.20, R3.21, section 3.4.1);

• a transponder or transceiver that will be used for coordinating RAs which has an aircraft address consisting of 24 zeroes or 24 ones (see R3.4, section 3.1); and

• a senior Active CAS that issues only horizontal RAs and lacks the logic to calculate vertical avoidance manoeuvres, resulting in an inability to coordinate vertical RAs with TCAS (see NOTE 2).

NOTE 3: The senior Active CAS will disable the ability of TCAS to reverse its RAs when it has the lower relative aircraft address. To mitigate this hazard, it is required to coordinate vertical RAs with TCAS so that the TCAS RA can be reversed if necessary, (see section and R3.11 and R4.10 in section 4.6.2).

There are also configurations that, although permissible, would fail to achieve the full functionality described in these MASPS and that can be expected to deliver degraded performance in some circumstances.

• Where it is determined that an Active CAS can be paired with a transponder that does not comply with the New Transponder MOPS, the Active CAS will be indistinguishable from TCAS resulting in inappropriate or incorrect coordination in some circumstances. The consequences are discussed in these MASPS.

• It was noted in section 1.3.3 that failure to field transponders that transmit OCMs would hinder the development of Passive CAS, which will depend on the exchange of OCMs.

• The acceptability of fielding Passive CAS that need to receive an OCM for coordination with Active CAS would need to be demonstrated (see the last two paragraphs of section because of the likelihood of a large population of Active CAS that cannot transmit OCMs. For example, consider an encounter between a Passive CAS that requires OCMs to coordinate and an ACAS Xu that cannot transmit OCMs. In this example, coordination would not be possible; thus, it would not be permissible for the Passive CAS to generate either vertical or horizontal RAs, and CAS protection would be provided solely by the Active CAS.

2 While it is assumed that manufacturers will develop transponders complying with these two MOPS jointly, it is possible that transponders will be developed that comply with DO-260C/ ED-102B but not comply with DO-181F/ ED-73F and vice versa. This complication is not discussed in these MASPS.

3 R3.69 refers to non-vertical RAs rather than horizontal RAs, thus not precluding (for example) speed change RAs.

Document History

September 1, 2020
Minimum Aviation System Performance Standards (MASPS) for the Interoperability of Airborne Collision Avoidance Systems (CAS)
SCOPE AND ASSUMPTIONS Detect and Avoid These MASPS protect the correct operation of systems that generate RAs for the avoidance of imminent collision. DAA contains a Remain Well Clear (RWC)...