Radio direction finder and beacon

Ground plane referenced triplet receiving or transmitting antenna arrays (figure 2), are used to measure the direction of propagation of radio signals. The arrays may be optimised for high accuracy and high integrity elevation measurements over selected angular sectors, notably at low angles in the presence of ground reflections. Various receiving arrangements may be embodied in equipment compatible with transmitting, reflecting and transponder based systems; and may be used with identity and range information therefrom, to determine the height of objects. A particular array is described, and included in a system (figure 6), for monitoring the height of aircraft near to airfields. This system may "stand alone" or be fully integrated with standard secondary surveillance radar equipment in the "Airfield Management" or wider "Air Traffic Control" system. A transmitting arrangement is also described which will reduce the cost of standard I.L.S. glidepath installations.