Speedlogs is used on ships to measure and display the speed of the vessel.
Modern Logs are:
Induction - Their action is based on the ability of electromagnetic induction. With the help of a special magnet at the bottom of the vessel creates a magnetic field. The volume of water at the bottom, which is exposed to the magnetic field of the lag can be seen as a set of elementary conductors of electric current, which is induced e. d. p .: value of such e. d. s. It gives an indication of the speed of movement of the vessel.
Doppler - Their work is to measure the Doppler shift of the high frequency sonar signal sent from the ship and reflected from the bottom surface.
Satellite - Determine the speed of the change of coordinates.