Dead Reckoning
Dead Reckoning (DR) is the technique of working out a ship position by starting from a known position and advancing it by the course steered and the distance run (speed multiplied by time). It uses no outside references — no GPS, no landmarks — only the heading and speed of the ship.
The DR position drifts from the true position over time because it takes no account of current, leeway from wind, or steering error. So it is continually corrected by position fixes from GPS, radar or visual bearings; the DR carries the estimated position forward between fixes and provides a sanity check on the electronics.
Dead reckoning is the fallback that keeps a ship navigable when the electronics fail, and the discipline that stops blind trust in GPS. A navigator who keeps a good DR knows roughly where the ship is even without a fix, and notices immediately if a GPS position looks wrong.
How does dead reckoning differ from a GPS fix?
A GPS fix is an actual measured position; a dead-reckoning position is calculated from course, speed and time since the last known position. DR is used to estimate position between fixes and as a backup if position sources fail.