Helitrak: a helicopter-tracking receiver system for television outside broadcast links

Abstract

Helicopters are used by BBC Television for certain live outside broadcasts which require an elevated platform for a camera or for a radio-link mid-point repeater. Video signals are communicated from the helicopter to the ground using microwave radio links, and the signals are received by a narrow-beamwidth dish antenna which must be kept pointing accurately towards the aircraft. In the past, this receiving antenna has always been pointed manually, but in some circumstances the performance of the radio links can be limited by the ability of an operator to keep the antenna on track.

Such a limitation can be avoided by using an automatic tracking system; this report describes the approach taken by Research Department to develop the "HELITRAK" tracking receiver system. Unlike some of the proprietary counterparts, this is a set of equipment which facilitates automatic tracking when applied to an existing radio-link receiver. It is specially tailored to the requirements of a video transmission system and needs only one conventionally transmitted radio-link signal. It has the potential to increase the reliability and the maximum range of helicopter links, and to improve operational efficiency.

The text covers the development of the tracking system, detailing the operational requirements and the design options. The system uses a novel method of tracking by detecting the direction of incoming RF signals, using higher-order mode synthesis in the antenna feed. Operational experience (from use in live outside broadcasts) and system limitations are noted, and areas for further improvements are identified.

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