Time to Closest Point of Approach (TCPA) (TCPA)
The predicted time remaining until two moving objects reach their calculated closest point of approach if their motions remain unchanged.
TCPA is derived with closest point of approach from relative position and velocity, using radar plotting, ARPA, AIS or other tracking. A positive value normally indicates the CPA is ahead in time; a negative value indicates it has already passed, subject to equipment convention.
The calculation assumes steady course and speed and is sensitive to target manoeuvres, own-ship motion, tracking quality and sensor data. A large CPA or long TCPA does not remove the need for repeated assessment, and display values must not replace the collision regulations or visual and radar observation.
Key Concepts:
- Closest Point of Approach (CPA): The minimum distance between two objects on their current trajectories.
- Time to Closest Point of Approach (TCPA): The time remaining until the CPA is reached.
Importance of TCPA:
- Collision Avoidance: Helps in assessing potential collision risks and determining when and how to take evasive action.
- Navigational Safety: Assists in maintaining safe distances between vessels or aircraft, especially in congested or constrained environments.
Practical Example:
- Ship A is moving at 20 knots on a heading of 45 degrees.
- Ship B is moving at 15 knots on a heading of 135 degrees.
- The current positions of the ships are known.
Interpretation:
- Positive TCPA: Indicates the time remaining until the CPA.
- Negative TCPA: Indicates that the CPA has already occurred.
Usage in Systems:
- Radar Systems: Display TCPA and CPA to help operators make informed decisions.
- Automatic Identification Systems (AIS): Use TCPA to provide collision warnings.
