HARCO is the name given to certain variants of the Decca Navigator radio position‐fixing system, in the particular context of the navigation of civil transport aircraft: an environment which is making increasingly insistent demands upon the navigational accuracy and flexibility of each individual user of the airspace, and upon the ability of the navigational equipment to operate with the highest possible degree of automation. An indication of the navigational capability required for an aircraft operating in high density upper airspace and in terminal areas is given by the operational specification issued in April 1961 by Eurocontrol, the European organisation for the safety of air navigation, which the Harco system is designed to meet. The terms of the specification, summarised in Table I, are self‐explanatory, with the possible exception of item (a): this is a reference to the fact that radio aids of the ‘point‐source’ type, unless used in conjunction with an airborne digital computer, tie the airways layout to the actual positions of the ground stations or beacons. If additional routes or airways are required or if an airway has to be moved, one or more stations therefore have to be sited or re‐sited and this can impose acute problems especially in or near an urban area.
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