Latitude and longitude can point to a place. A useful position report must also tell rescuers when that place was true, how reliable it is and what the vessel has done since.
A position has a time
A vessel moving at ten knots travels a nautical mile in six minutes. A position copied correctly but reported without a time can therefore create a growing search area. Rescue coordinators need to know whether coordinates are current, observed a few minutes ago or taken from an older log entry.
“Last known position” is useful precisely because it acknowledges this distinction. When paired with course, speed and time, it allows planners to estimate where a vessel might now be. Wind, current, damage and human decisions make that estimate uncertain, so updates matter.
Accuracy is not the same as precision
A display may show many decimal places without guaranteeing that the underlying fix is accurate. Satellite geometry, antenna problems, poor installation, manual transcription or a failed sensor can all affect the result. Repeating what the display shows is appropriate; implying a level of certainty that is not present is not.
Coordinates must also be associated with the correct geographic datum. WGS 84 is standard in most modern marine navigation, but older charts and equipment can use other datums. A mismatch can move the plotted point by a significant distance.
Identity gives the point meaning
Rescue services rarely act on coordinates in isolation. They connect the position to a vessel name, call sign, MMSI, description and nature of distress. Two alerts near each other may refer to one casualty, two vessels, or a relay. Clear identity helps coordinators merge reports without confusing them.
Movement changes the problem
A disabled vessel drifting in wind and current, a yacht sailing toward shelter and a person in the water all move differently. Search planners use the last known position together with time, environmental information and the characteristics of the object to estimate a search area. That calculation is specialised SAR work; it is not simply drawing a circle around two numbers.
Digital does not mean infallible
DSC, EPIRBs and other digital systems can transmit identity and position quickly. Their usefulness depends on correct registration, connected navigation data and functioning equipment. A stale position, an unregistered beacon or an incorrect MMSI can consume time while responders reconcile the alert with other information.
Voice follow-up can add what a compact alert cannot: people aboard, vessel condition, intentions, hazards, drift and changes since activation. This is why digital alerting and human communication complement one another.
What rescuers need to understand
The operational question is not only “Where were you?” It is “Where should we look now, for what, and with what degree of confidence?” A well-contextualised position reduces ambiguity and helps a rescue coordination centre choose suitable resources.
Sources and further reading
- IMO: International Aeronautical and Maritime Search and Rescue Manual
- U.S. Coast Guard Navigation Center: maritime distress systems
- Cospas-Sarsat: beacon registration
Editorial note: This is a system-level explanation, not a distress-message procedure. Follow current equipment documentation, approved training and the instructions of the responsible rescue authority. National arrangements vary. Last fact-checked: 1 September 2026.
