CPA / TCPA
Closest Point of Approach distance and time from own-ship and target course+speed. The core radar plotting relationship for COLREG compliance.
Current range
5.08nm
Bearing to target
61.8°T
CPA
2.40nm
TCPA
10.3min
Formula, assumptions, and limits
Flat-earth plane geometry for distances < 20 nm; converts lat/lon to north/east nm using cos(own lat). For long-range CPA use ECDIS ARPA instead. Positive TCPA = future; negative = past/diverging.
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| Current relative position vector | nm | derived | |
| Relative velocity = target − own | kn | course/speed | |
| Closest Point of Approach distance | nm | result | |
| Time to CPA | min | result |