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Traffic Alert and
Collision Avoidance
System (TCAS)
Program Office
Airborne Collision Avoidance System X
(ACAS X) Overview
By: Mike Castle
Date: June 10-11, 2014
Federal Aviation
Administration
Mission Need (TCAS II Performance Review)
Performance monitoring assessment shows that TCAS works as
intended but alerts during many normal, safe operations
Federal Aviation
Administration
2
Challenges for TCAS II in the Future
Federal Aviation
Administration
3
TCAS Upgrade Challenges
Federal Aviation
Administration
4
ACAS X Introduction
Airborne Collision Avoidance System
• ACAS X – An interoperable expansion of a
family of aircraft collision avoidance systems
developed for use in NextGen airspace
• Provides the same general role as TCAS II:
– Surveillance of nearby aircraft
– TA/RA Generation
– Coordination with other aircraft collision avoidance
systems
• Supports New Capabilities:
– Leverages Additional Surveillance Sources
– Intended for multiple types of host aircraft
– Tunable for Reduced Separation Operations
Federal Aviation
Administration
5
ACAS X Program
Airborne Collision Avoidance System (ACAS X)
• FAA initiated formal research
in 2009
– Decision theoretic safety logic
– Flexible surveillance tracker
• Benefits
–
–
–
–
Enables reduced separation
Fewer unnecessary alerts
Extends to new user classes
Easier to adapt to changing
airspace
Federal Aviation
Administration
6
ACAS X Threat Logic
Federal Aviation
Administration
7
ACAS X Alerting is Different Than TCAS
Step 1: Is there a hazard?
Step 2: Climb or descend?
Step 3: What altitude rate?
Intruder
Legacy
TCAS
Predicted “miss”
Climb
Rate 1
Rate 2
Rate 3
Own
Descend
Rate 4
Alert if time to closest
approach and projected miss
distance are below thresholds
Choose sense that
maximizes miss distance
Step 1: State Distribution
Step 2: Look-Up Table
•0 0 0 0 0 1 0 1 0 1 0 1 1 0 0 0 0 0 0
1110101101111100011
0011101100011011001
1110011101110110110
1010010000110010111
1011111011001011110
0111111011011010000
0100010111011000101
1100100001111000110
ACAS X
Estimate ~10M states based on
beliefs about own and intruder
dynamics
For each state estimation,
look up expected cost
related to available actions
Choose the lowest vertical
rate predicted to achieve
required separation
Step 3: Choose action
with lowest cost
No Alert
0.8
Level-off
0.1
Descend
0.9
Climb
0.2
Choose the action resulting
in the lowest cost
Federal Aviation
Administration
8
How ACAS X and TCAS Alerts are Modified
Assumed behavior
Thresholds
Rules
• Tau = 40 sec 35 sec
Legacy
TCAS
•Zthr = 600 ft 400 ft
•ALIM = 300 ft 250 ft
Change assumptions of own
and intruder aircraft behavior
Establish new alert criteria
Dynamics
Offline costs
1
A
•0.1
ACAS X
B
2
•0.5
A
•0.9
B
•0.3
Online costs
•1
•0.6
•0.4
Change existing
pseudo code
A
B
3
•0.7
Modify weights of belief states
and state transitions
•
•
•
•
•
NMAC (-1)
Alert (-0.01)
Reversal (-0.01)
Strengthen (-0.009)
Clear of conflict (0.0001)
Change the reward values
for alerting parameters
Alert inhibit altitude
Used actual parameters
live on aircraft
Federal Aviation
Administration
9
TCAS Logic Development
Legacy TCAS Development Cycle
Logic(pseudo
code)
Encounter Model
Performance
Metrics
Simulation
Evaluation
•manual pseudocode revision
• Human effort focused on pseudocode
• Time-consuming process
• Many parameters require tuning
• Unlikely to be optimal
Federal Aviation
Administration
10
ACAS X Logic Development
Model-Based Optimization Approach
Encounter
Model
Performance
Metrics
Optimization
Logic
(table)
Encounter
Performance
Model
Metrics
Simulation
Evaluation
revision of performance metrics and models
• Human effort focuses on defining performance metrics
• Computers generate lookup table
• Optimal, robust logic
Federal Aviation
Administration
11
ACAS X Architecture
Federal Aviation
Administration
12
OPS
F&E
Program Segments and Work
Packages
Operational TCAS/ACAS
Freeze
Design
Codify
Requirements
Formalize
Standards
OpEval
Regulatory
Publication
ACAS Xu and Xp
Not part of this
investment
Federal Aviation
Administration
13
ACAS-X Tuning Process
Automated Tuning
Analyst Feedback
Surrogate Modeling
Results
Identify Top
Policies
Design Parameters
LLCEM EE: Pnmac Performance
-6
x 10
8
6
4
2
0
90
94
105
200
283
192
Run 11TCAS 7.1
…
Reversal
cost
Strengthening cost
Vertical rate change
cost
Modeled Acceleration noise
Dynamic Programming
Optimization
5 minutes
Aggregate
Results
Flight Testing
Results
D3 Interactive
Results
Viewer
Operational
Suitability
Results
Tables
Performance Result
Simulation
20 minutes
Surrogate Model Optimization
5 seconds
Inner Loop Cycle Time: ~20 Minutes
Preference
Elicitation
Analyst
Preferences
Analyst Loop
Cycle: Weekly
• ACAS-X tuning accomplished via supervised optimization
– Analysts specify initial objective function and automated tuning
performs search producing candidate logics
– Automated search is periodically interrupted and analyst preferences
incorporated via an updated objective function
Federal Aviation
Administration
14
Safety Results
EUPnmac
(100’)Performance
LLCEM EU(C):
-4
x 10
-4
x 10
1.5
EU Pnmac
(25’) Performance
LLCEM EU(S):
1
0.8
1
0.6
0.4
0.5
0.2
0
12(ADS-B)
Run 12
(ADS-B)
12(TCAS)
Run 12
(Active)
11(TCAS)
11(ADS-B)
Run 11
(ADS-B)
Run 11
(Active)
-6
x 10
0
TCAS 7.1
TCAS
12(ADS-B)
Run 12
(ADS-B)
12(TCAS)
Run 12
(Active)
11(ADS-B)
Run 11
(ADS-B)
11(TCAS)
Run 11
(Active)
TCAS 7.1
TCAS
LLCEM EE: Pnmac
EE Performance
8
6
4
2
0
12(ADS-B)
Run 12
(ADS-B)
12(TCAS)
Run 12
(Active)
11(ADS-B)
Run 11
(ADS-B)
11(TCAS)
Run 11
(Active)
TCAS 7.1
TCAS
EE: Run 12 performance improves upon Run 11, exceeds TCAS
EU: Run 12 performance exceeds TCAS
Federal Aviation
Administration
15
Overall Alert Rate
12
# of Encounters with Alerts
160000
140265
140000
120000
100000
118576
(85% TCAS)
98546
(70% TCAS)
86115
(61% TCAS)
94035
(67% TCAS)
80000
60000
40000
20000
0
Includes CSPO,
all military,
formation
Run 9
Run 11
2013
Flight
Test
1st Tuned for
Altimetry
Bias
Run 12
(ADS-B)
Run 12
(Active)
TCAS v7.1
1st Tuned for ADS-B
Federal Aviation
Administration
16
BACKUP
Federal Aviation
Administration
17
XA – Active Surveillance
XO – Operation Specific
XU – Unmanned Aircraft System
XP – Passive Surveillance
ACAS X Variants
ACAS XA
Useful
Segment 1
User
Group
Surveillance
Technology
Advisories
Current TCAS II
users (large aircraft)
Active radar
supplemented
with passive
Same as
current TCAS II
Active radar
supplemented
with passive
Procedure-specific
alerts for selected
aircraft, global
alerting against all
others
Potentially radar,
EO/IR, etc.
“Coordinated”
vertical advisories
Passive only
Reduced advisory
set
Users of specific
operations
ACAS XO
(e.g., CSPO, Formation
Flights, ASAS
Operations)
Unmanned aircraft
NextGen Concept
Maturation
ACAS XU
*ACAS XP +
(controlled airspace)
General aviation,
etc.
Federal Aviation
Administration
18
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