Dead Reckoning (DR)
From SKYbrary Wiki
Dead Reckoning (DR) is a method of navigation relying on estimating one's current track, groundspeed and position based on earlier known positions.
Although DR navigation is not much used in practice today, the principles involved may still be employed by pilots to estimate, for example, their ETA for a position ahead, or to estimate an alteration of heading to regain track.
DR navigation is also employed in some electromechanical navigation equipment.
Dead reckoning is the default mode in the event of failure of the input to some Flight Management System systems. This has the effect that heading and true airspeed (perhaps modified by previously encountered wind effects) are fed to the FMS in place of the track and groundspeed derived from Global Positioning System (GPS) or other navigation systems. This event, which may not be immediately evident to the pilot, results in serious degradation of navigational accuracy.