CPA / TCPA Tool: Closest Point of Approach & Relative Motion

CPA • TCPA • Relative Motion • Range Rate • Bearing Rate • BCR / BCT • Trial Manoeuvre • Multi-Target Guard Screen

CPA / TCPA Tool: Closest Point of Approach & Relative Motion

Calculate closest point of approach and time to CPA from true-motion vectors. Simple mode solves one target from own-ship course/speed plus target bearing/range/course/speed. Advanced adds multiple targets, user-defined CPA/TCPA guard limits, bow-crossing range/time, relative-bearing rate, passing side, trial own-ship course/speed, a radar-style relative-motion plot and a trial-manoeuvre sensitivity matrix.

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Need much more depth? Click Advanced above.Advanced adds multiple targets, user-defined CPA/TCPA guard limits, BCR/BCT, bearing rate, passing side, trial course/speed, relative-motion graphics and a Target 1 manoeuvre matrix.

Fast6 primary inputs

Core answerCPA + TCPA

Bridge-safe framingGeometry only, no COLREG verdict

Fast CPA / TCPA Screen

Use true bearing, true course and speeds on a consistent reference basis. Constant course and speed are assumed for both vessels.

Own ship true coursedegrees true.

Own ship speedknots.

Target true bearingdegrees true from own ship.

Target rangenautical miles.

Target true coursedegrees true.

Target speedknots.

Own Ship & Motion Reference

Own ship true course

Own ship speed

Vector reference basis

Prediction horizonminutes used for plots and guard screening.

User Guard Screen

These are operator-entered screening limits only. They are not COLREG, IMO or company-policy defaults unless your own procedures specify them.

CPA guard limitNM.

TCPA guard horizonminutes.

Target Register

All target bearings are true bearings from own ship at the calculation instant. All motion vectors must use the same ground- or sea-stabilized basis selected above.

Bearing °T	Range NM	Course °T	Speed kn

Trial Own-Ship Manoeuvre

Trial own coursedegrees true.

Trial own speedknots.

Navigation Readiness

True bearing / range source verifiedRadar/AIS/manual data source and timestamp understood. Own motion vector verifiedCourse and speed reference basis is correct. Target motion vector verifiedTrue course/speed is not stale or mixed-reference data. Target tracking is steadyRecent acquisition/manoeuvre/tracking disturbance considered. Visual/radar lookout maintainedCPA/TCPA is not being used as the sole collision-risk input. COLREG context reviewedVisibility, vessel status, aspect and applicable rules assessed separately. Guard limits match bridge procedureUser-entered CPA/TCPA screening limits are deliberate. Trial manoeuvre checked against all trafficOne target is not considered in isolation.

Navigation limitation: this tool is a mathematical planning/training cross-check, not an ARPA, radar, AIS or collision-avoidance system. It assumes constant course and speed and cannot determine COLREG give-way/stand-on responsibility, safe speed or whether an avoiding action is compliant. Use all available means, maintain a proper lookout and continuously verify the effect of any manoeuvre.

Current compliance context: COLREG Rules 5–8 require proper lookout, safe speed, use of all available means to determine collision risk and appropriate action to avoid collision; Rule 7 specifically warns against assumptions based on scanty information. IMO resolution MSC.192(79) requires radar target tracking to provide motion prediction after steady-state tracking, recognizes CPA/TCPA alarms using user-set limits, and notes that accuracy may be significantly reduced during or shortly after acquisition, own-ship manoeuvre, target manoeuvre or tracking disturbance. The tool therefore treats CPA/TCPA as predicted geometry only and user guard limits as screening criteria, never universal safety thresholds.

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