The term "chronometer" is also used to describe a marine chronometer used for celestial navigation and determination of longitude. The marine chronometer was invented by John Harrison in 1730. This was the first of a series of chronometers that enabled accurate marine navigation. From then on, an accurate chronometer was essential to open-ocean marine or air navigation out of sight of land. Early in the Twentieth Century the advent of radiotelegraphy time signals supplemented the onboard marine chronometer for marine and air navigation, and various radio navigation systems were invented, developed, and implemented during and following the Second World War (e.g., Gee, Sonne (a.k.a. Consol), LORAN(-A and -C), Decca Navigator System and Omega Navigation System) that significantly reduced the need for positioning using the onboard marine chronometer. These culminated in the development and implementation of global satellite navigation systems (GSN-GPS) in the last half of the 20th century. The marine chronometer is no longer used as the primary means for navigation at sea, although it is still required as a backup, since radio systems and their associated electronics can fail for a variety of reasons.