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A new generation of Airborne Collision Avoidance Systems: ACAS X
Introduction• Single slide on “why” a new CAS
– ACAS X to replace TCAS II
• Discuss the organizational context – Who does what
• How ACAS X differs from TCAS II• Use made of ADS-B • Finally, the design question:
the use to make of “ADS-B only” tracks
Why?
• We have a collision avoidance system: TCAS II• Been around for some 25 years• Global mandate
– Almost all passenger carrying aircraft are required to be equipped
• Not fit for future ATM environment• Not easy to modify
Who?
• The FAA does everything: R&D, regulator, ANSP• RTCA gives policy advice to the FAA
– SC meetings have to be open to the public
USA Europe
Develops FAA, or ….. SESAR/Eurocontrol, or ….
Standardises RTCA, SC-147 EUROCAE, WG-75
Regulates FAA EASA
US perspective• TCAS II (ACAS X) was (is) developed by FAA• When ready, system is handed to RTCA SC-147
for standardization– Development does not stop– Process is consensual– Standards reviewed very widely before publication
• When MOPS agreed, FAA (cert) issues a TSO• Manufacturers build what the TSO says• FAA certifies that they have done so• FAA publishes (or does not) a Rule: “Carry TCAS”
(In parentheses)
• The level of detail in the specifications varies– True for both TCAS II and ACAS X
• Collision avoidance logic is specified in absolute detail at a very low level– The ACAS X specification is executable
• Each time change is required the FAA team will redefine the logic & SC-147 will revise the MOPS– Freely available (with the MOPS), but who owns it?– The logic is not GFE
Transatlantic cooperation for ACAS X• SESAR validating ACAS X for Europe
– It could work in the USA but not in Europe • WG-75 and SC-147 work jointly
– RTCA and EUROCAE MOPS are technically identical– Exception: Autoflight TCAS
• EASA and FAA cert coordinate closely– Do not (always) publish technically identical TSOs
• The “Rules” are different!
Differences between TCAS II and ACAS XA
• Operational concept:ACAS XA = TCAS IIbut does it better
• The design approach to the collision avoidance logic is profoundly different– but selects from the same template of actions
• Tracking algorithms have been improved• Use of ADS-B information
What is ADS-B?
• Each aircraft broadcasts it position and identity regularly and frequently
• Mandated from 2020• Risk of spoofing always been recognised
– Early discussions of encryption - rejected
Use of ADS-B• Original TCAS II tracks aircraft by active use of SSR
– Has a big impact on the 1030/1090 radio frequencies• New improved TCAS II uses ADS-B to avoid this
– Signal strength used as rough measure of range– When close, ADS-B must be “validated” actively– When really close, active tracking takes over
• ACAS X will use the ADS-B data in the collision avoidance logic – if and only if it is validated
The design issue• Track can be formed using ADS-B from aircraft
for which active validation is not possible– We know of these aircraft from their ADS-B alone
• Four options were identified– Ignore these tracks –do not display– Display these tracks but generate no alerts
• Using distinct symbology (because no RA)!!?– Generate TAs but not RAs (The current choice)– Generate RAs
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