The problem of landing an aeroplane under conditions of low visibility has been a subject for speculation for the last twenty years, but it is only within the last ten years that it has been given any serious practical attention. This practical attention has been forced by the necessity for aircraft to maintain schedule regardless of weather conditions in competition with other forms of transport.

Although certain schemes involving radio were amongst the earliest proposals for a solution of the problem, the first practical work appears to have been concerned with methods of a more mechanical or optical nature, including a proposal to deposit the fog by high tension discharge.