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
