Algorithm ETA Calculation

ETA Calculation Algorithm

The ETA (Estimated Time of Arrival) calculation engine provides accurate arrival time predictions for all active flights. The system uses aircraft performance data, route geometry, wind estimation, and TMI delay factors to compute ETAs every refresh cycle.

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For Traffic Managers

What ETA Tells You

The ETA displayed in PERTI represents the runway threshold crossing time (wheels-on time), not gate arrival. Key indicators:

Prefix	Meaning	Confidence
E	Estimated	Standard calculation
P	Proposed	Pre-departure, using filed times
C	Controlled	Flight has EDCT/CTA assigned
A	Actual	Flight has arrived

Confidence Levels

ETAs include a confidence score (0.0 - 1.0) based on flight phase:

Phase	Confidence	Why
Final approach (<50nm, descending)	0.95-0.97	Position and speed well known
Descent phase	0.92-0.94	Route mostly flown
Enroute with SimBrief data	0.92	Detailed performance data available
Enroute (standard)	0.88-0.90	Default aircraft performance
Climbing	0.82-0.85	Still establishing profile
Taxiing	0.75-0.78	Departure time uncertain
Pre-filed	0.65-0.70	Based entirely on filed data

Note: Higher confidence values in each range apply when using parsed route distance (more accurate than GCD).

How ETAs Are Used

• GDP Planning: ETAs populate demand charts to identify arrival congestion
• TMI Scoping: Determine which flights are affected by time-based tiers
• Compliance Monitoring: Compare actual arrivals against projected

When ETAs Change

ETAs recalculate every ~15 seconds. Significant changes occur when:

• Flight departs (taxiing → airborne transition)
• Route amendments are detected
• Aircraft accelerates/decelerates significantly
• Altitude changes indicate climb/descent phase transition

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For Technical Operations

Monitoring ETA Health

Key Metrics to Watch

-- ETA calculation statistics from last refresh
SELECT 
    eta_method,
    COUNT(*) AS flights,
    AVG(eta_confidence) AS avg_confidence
FROM dbo.adl_flight_times
WHERE eta_last_calc_utc > DATEADD(MINUTE, -5, GETUTCDATE())
GROUP BY eta_method;

Expected eta_method Distribution (V3.2):

Method	Description	Expected %
V3_ROUTE_SB	SimBrief + parsed route	10-20%
V3_ROUTE	Parsed route distance	30-50%
V3_SB	SimBrief cruise data	5-10%
V3	Standard calculation	30-50%

V3.2 Note: Prefiles now receive ETA calculations using GCD/route distance even without position data.

Common Issues

Symptom	Likely Cause	Resolution
All ETAs NULL	Procedure not running	Check daemon status
Low confidence scores	Many pre-files	Normal during low traffic
ETAs far in future	GCD used instead of route	Route parsing backlog
ETA method = V3 (all)	SimBrief parsing disabled	Check sp_ParseSimBriefDataBatch

Performance Troubleshooting

Target execution time: < 500ms for full batch

-- Check ETA step timing in refresh stats
SELECT 
    step8_trajectory_ms,
    etas_calculated
FROM (
    SELECT TOP 10 *
    FROM dbo.adl_run_log
    ORDER BY run_utc DESC
) recent;

If > 1000ms, check:

1. Index health on adl_flight_times
2. Aircraft performance profile coverage
3. Temporary table cleanup

Key Tables

Table	Purpose
adl_flight_times	Stores calculated ETA and related times
aircraft_performance_profiles	Climb/cruise/descent speeds by aircraft
adl_flight_stepclimbs	SimBrief step climb data

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For Developers

Algorithm Overview

ETA calculation uses a multi-phase approach:

Phase 1: Build work table with flight data
Phase 2: Load SimBrief step climb speeds (if available)
Phase 3: Lookup aircraft performance profiles
Phase 4: Calculate time-to-destination by flight phase
Phase 5: Apply TMI delays (EDCT/CTA)
Phase 6: Update adl_flight_times

Core Calculation Logic

Distance Sources (V3.2 Priority Order)

1. Route Distance to Destination (route_dist_to_dest_nm) - Position-based remaining route distance
2. Parsed Route Distance (route_dist_nm) - Total route from waypoints
3. Great Circle Distance (gcd_nm) - Fallback, straight-line origin to destination
4. Current Position to Destination (dist_to_dest_nm) - GCD from current position

V3.2 Prefile Support: For prefiles without position data, the system uses route_total_nm or gcd_nm to estimate flight distance.

Speed Sources (Priority Order)

1. SimBrief TAS - From step climb data (simbrief_cruise_kts)
2. SimBrief Mach - Converted to TAS at altitude
3. Aircraft Performance Profile - From fn_GetAircraftPerformance
4. Hardcoded Default - 450 KTAS for jets

Time Calculation by Phase

-- Simplified phase-based calculation
CASE
    WHEN phase = 'arrived' THEN 0
    
    -- Final approach: use current groundspeed
    WHEN phase = 'descending' AND dist_to_dest < 50 THEN
        dist_to_dest / groundspeed * 60
    
    -- Descent: use descent performance
    WHEN phase = 'descending' THEN
        dist_to_dest / descent_speed * 60
    
    -- Cruise: cruise segment + descent segment
    WHEN phase IN ('enroute', 'cruise') THEN
        (dist_to_dest - tod_dist) / cruise_speed * 60
        + tod_dist / descent_speed * 60
    
    -- Climbing: climb + cruise + descent
    WHEN phase IN ('departed', 'climbing') THEN
        toc_dist / climb_speed * 60
        + (dist_to_dest - toc_dist - tod_dist) / cruise_speed * 60
        + tod_dist / descent_speed * 60
    
    -- Pre-departure: taxi + full flight + TMI delay
    ELSE
        15 + dist_to_dest / cruise_speed * 60 * 1.15 + tmi_delay
END

TOD/TOC Calculations

-- Top of Descent distance (3nm per 1000ft descent)
tod_dist_nm = (filed_altitude - dest_elevation) / 1000.0 * 3.0

-- Top of Climb distance (2nm per 1000ft climb)
toc_dist_nm = (filed_altitude - current_altitude) / 1000.0 * 2.0

Procedure Signature

EXEC dbo.sp_CalculateETABatch 
    @eta_count = @count OUTPUT,  -- Returns number of ETAs calculated
    @debug = 0                    -- Set to 1 for detailed output

Output Columns (adl_flight_times)

Column	Type	Description
eta_utc	DATETIME2(0)	Calculated ETA (runway time)
eta_runway_utc	DATETIME2(0)	Same as eta_utc
eta_epoch	BIGINT	Unix timestamp for API consumption
eta_prefix	CHAR(1)	E/P/C/A indicator
eta_confidence	DECIMAL(3,2)	0.00-1.00 confidence score
eta_method	VARCHAR(16)	V3, V3_SB, V3_ROUTE, V3_ROUTE_SB
eta_dist_source	VARCHAR(8)	ROUTE or GCD
tod_dist_nm	DECIMAL(10,2)	Calculated TOD distance
tod_eta_utc	DATETIME2(0)	ETA at top of descent
eta_wind_component_kts	INT	Estimated wind (+ tailwind, - headwind)
eta_tmi_delay_min	INT	TMI delay applied
eta_last_calc_utc	DATETIME2(0)	When ETA was last calculated

Integration Points

VATSIM Refresh
    │
    ├─► sp_ProcessTrajectoryBatch (inline ETA for trajectory)
    │       └─► Basic ETA for tier decisions
    │
    └─► sp_CalculateETABatch (primary ETA calculation)
            ├─► Uses fn_GetAircraftPerformance
            ├─► Uses SimBrief step climb data
            └─► Updates adl_flight_times

Performance Considerations

1. Batch Processing: All flights processed in single set-based operation
2. Temp Tables: Work table built once, reused across calculations
3. Index Usage: Ensure clustered index on flight_uid in flight_times
4. No Cursors: Pure set-based SQL for parallelization

Version History

Version	Date	Changes
V3.2	2026-01-13	Prefile ETA support - LEFT JOIN on position, distance fallback chain
V1	2025-12	Basic ETA calculation
V2	2026-01	SimBrief step climb integration
V3	2026-01	Parsed route distance integration
V3.1	2026-01	Added eta_dist_source tracking
V3.2	2026-01-13	Prefile ETA support using GCD/route distance

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Related Documentation

• Acronyms#timing--scheduling - ETA/ETD terminology
• Algorithm-Route-Parsing - Route distance calculation
• Algorithm-Trajectory-Tiering - Uses ETA for tier decisions
• GDT-Ground-Delay-Tool - Primary consumer of ETA data