Collision Detection (TCAS) in Stratux Radar Display

The Stratux Radar Display implements a simplified TCAS (Traffic Collision Avoidance System) that detects potential collisions It analyzes relative positions and velocities to classify threat levels. This provides basic collision avoidance suitable for general aviation.

How It Works

1. Input Data

• Traffic: GPS position, altitude, course, speed, vertical speed
• Own Aircraft: GPS position, altitude, course, speed, vertical speed

2. Coordinate Transformation

GPS coordinates are converted to Cartesian coordinates using a reference point between both aircraft. Velocities are transformed to x,y components for vector calculations.

3. Tau Calculation (Time to CPA)

The system calculates time to Closest Point of Approach (CPA):

Horizontal:

• Relative position and velocity vectors
• Time to CPA tau is calculated from relative motion as well as distance at CPA

Vertical:

• Relative altitude and vertical speed
• Time to vertical convergence vertical tau is calculated by altitude difference and vertical speeds.

4. Threat Classification

Multi-level thresholds determine threat levels:

Priority	Condition	Time	Distance	Altitude
RA (1)	Resolution Advisory	≤ 25s	≤ 0.2 NM	≤ 600 ft
TA (2)	Traffic Advisory	≤ 40s	≤ 0.3 NM	≤ 1000 ft
Collision (3)	Potential Collision	≤ 180s	≤ 1.5 NM	≤ 2000 ft
No Collision (4)	No Threat	-	-	-
Unclear (0)	Incomplete Data	-	-	-

5. Decision Logic

Both horizontal AND vertical conditions must be met for each threat level. The system uses a safety factor (1.2) for conservative estimates.

6. Visualization

Depending on the threat level, aircraft will be display in different colors or shading, e.g for the OLED 1.5:

• RA: Red
• TA: Yellow
• Collision: Black
• No Collision: Green, thin outline
• Unclear: Hollow, thin outline

E.g. for epaper normal mode(we only have black and white):

• RA: Large aircraft arrow, full black
• TA: Normal size aircraft arrow, full black
• Collision: Normal size aircraft arrow, hollow with thick outline
• No Collision: Hollow aircraft symbol, thin outline
• Unclear: Hollow aircraft symbol, medium outline

6. Audio indication

Depending on the threat level, aircraft will be spoken with different attributes. Some examples:

• RA: "Alarm, traffic 10 o'clock, +200 feet"
• TA: "Advise: traffic 10 o'clock, -600 feet"
• Collision: "traffic 10 o'clock, +400 feet"
• No Collision: no audio output. If no aircraft in collision mode is left audio "Clear of conflict" will be announced
• Unclear: "traffic 10 o'clock, +600 feet"

Basically less audio output will be spoken compared version 2. All traffic without a collision risk will not be spoken (altough it is displayed). For every threat level a timeout is defined after which (if the threat persists) the audio is repeated for this aircraft. Timeout for RA is 10 seconds, TA timeout is 30 seconds, collision traffic 60 seconds, unclear 30 seconds.

Limitations

• Assumes straight-line motion (no maneuver prediction), updated aircraft information (what ADSB-out typically does every second) will be taken into account
• Dependent on GPS accuracy
• Simplified flight path model
• Not a replacement for professional TCAS