For a number of years helicopter operations will mainly take place at operating altitudes between 500 and 2000 ft. Due to the inaccuracies of existing pressure altimeters vertical separation less than 1000 ft. cannot be applied and therefore maximum use will have to be made of lateral and longitudinal separation possibilities if the development of helicopter operations in high density areas is not to be impeded. As far as possible helicopter operations will be kept apart from fixed-wing traffic but this may be difficult to achieve in terminal areas where departing and arriving fixed-wing aircraft are at low altitude and the available airspace is restricted.

It is, therefore, essential that the navigational aid on which air traffic control base safe separation should be one which enables the helicopter pilot to adhere strictly to his pre-determined track and also provides him with an accurate measurement of progress along track. Quite apart from the air traffic control aspect, since the helicopter flies low, frequently in poor horizontal visibility and low cloud-base conditions, accurate track keeping is essential from the standpoint of avoiding obstructions or high ground in the vicinity of the route. Accurate and solid coverage over a wide area at low altitude is therefore an important requirement of any helicopter aid system, particularly in a high traffic density area such as western Europe.