The Satellite Tracking Keyhole Problem: A Parallel Mechanism Mount Solution

A narrow beam antenna system employing two axes for tracking a subsynchronous satellite has a key hole where it cannot adequately track. This keyhole is caused either by mechanical constraints or position slew rate limitations of the antenna mount. The addition of a third axis can provide the mechanism with an extra degree of freedom so that the keyhole can be avoided. This serial connection of linkages means that the revolute joint errors and linkage flexure cause additive pointing errors. Extremely rigid joints and linkages are used to minimise the pointing errors and the result is a large expensive mounting system. This paper proposes the use of a modified Stewart platform or parallel linkage mechanism for use as a 'keyholeless' antenna mount. The joint and linkage errors are not additive and the structure is relatively light. The conventional six spatial coordinates can be mapped to the six controlled prismatic joint linkages. Only two degrees of freedom are required to aim the antenna. The remaining four degrees of freedom are used to avoid the singularities of the mechanism and to maximise the rigidity of the mounting system.