Page:Airmobility 1961-1971.pdf/163

and a better weapons system. With the strong urging of the combat commanders, the Army decided to procure an "interim" system for immediate requirements while it sorted out the problems of the "ultimate" system.

The history of the use, lack-of-use, and misuse of DECCA has many important lessons for future developers of airmobile equipment.

It was recognized by the earliest planners that one of the limitations of the airmobility concept would be operating at night and under periods of extreme low visibility. Research and Development offered many possible options to improve the helicopter's capability under these conditions, but all were expensive and complex.

The British had perfected a low-level radio navigational aid known as DECCA which essentially used three low frequency ground radio stations to propagate a series of hyperbolic curves which could be translated by a cockpit instrument into a position fix. Accuracy depended on the spread of the stations, the distance from the station, and the weather conditions. Because of the low frequency, one of the attractive features was its low altitude capability. This contrasted to the line-of-sight limitation of omnidirectional radio navigation aid and Tactical Air Navigation used by the Air Force. The Army tested several versions of the DECCA system and decided it had enough advantages to warrant its installation on command helicopters and lead aircraft.

A DECCA chain had been installed in Vietnam and in the early 1960's, the Army took over its supervision and maintenance. A big disadvantage in the DECCA system was the requirement for special maps printed with the hyperbolic grid and a reluctance by the user to take the time and effort to develop confidence in the system. Its use was further complicated by the resistance of the Air Force to accept a position report in instrument weather from a DECCA read-out as a positive fix. Many senior officers were dissatisfied with the accuracy and reliability of the DECCA system at night and eventually the DECCA died from lack of use and misuse. The requirement for a secure, accurate means of low-level navigation remained.

On 28 September the 1st Cavalry Division was notified that an increasing enemy buildup in the I Corps Tactical Zone might require that the III Marine Amphibious Force be reinforced. The