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