⚓ CPA & TCPA Calculator – Closest Point of Approach for Radar Plotting

Calculate the Closest Point of Approach (CPA) and Time to CPA (TCPA) for maritime radar plotting. Evaluate collision risks using direct ARPA vectors or manual observations.

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Note: For educational and training purposes only. Do not use for navigation.

⚙️ How to Use

Use this tool for collision avoidance training and maneuvering board practice. Choose your input mode based on available data.

Mode A: Radar Plot

When to use: Use this mode when you are performing manual plotting from raw radar observations and need to deduce the target's true motion from two successive relative positions.

Select Mode A to practice manual radar plotting.
Enter Own Ship Course and Speed.
Provide two timed observations (Bearing and Range) of the target.
The calculator will recover the Target's True Course and Speed, then solve the relative motion triangle.

Mode B: ARPA Direct Vector

When to use: Use this mode when you already have the target's true course and speed from your ARPA or AIS and want to quickly calculate CPA, TCPA, and BCR.

Select Mode B if you already have the target's True vector from AIS or ARPA.
Input Own Ship and Target Course & Speed.
Input the instantaneous Bearing and Range to the target.

Note: This tool is for training and educational purposes only. Never rely on it for actual navigation or collision avoidance. Always comply with the COLREGs and use certified equipment.

📖 Definitions

CPA (Closest Point of Approach)

The minimum distance between your vessel and a target, assuming both maintain current course and speed.

TCPA (Time to CPA)

The time remaining until the CPA is reached. A negative TCPA means the CPA has already passed.

BCR (Bow Crossing Range)

The distance from your vessel to the point where the target's relative track crosses your heading line.

BCT (Bow Crossing Time)

The time at which the target will cross your heading line.

DRM & SRM

Direction of Relative Movement and Speed of Relative Movement define the apparent motion of the target relative to your own ship.

📐 Worked Example

Let's solve a simple crossing situation:

Own Ship: COG 000°T, 10 kn
Target: Bearing 045°T, Range 10 NM
Target Vector: COG 270°T, 10 kn

1. Both ships are closing at a 90° angle. Own ship moves North, target moves West.

2. The relative velocity resolves to a relative speed (SRM) of 14.1 knots (√2 × 10).

Result:

CPA = 7.07 NM

TCPA = 30.0 min

BCR = +10.0 NM (Target crosses ahead)

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