In a ground station providing guidance to aeroplanes, microwave energy is transmitted from a reference aerial and a vertical array of radiating elements along which the signal is commutated. The frequency of the transmitted signal supplied to the array is offset positively or negatively according to the direction of scan with respect to the signal fed to the reference aerial. The doppler shift between the signal from the reference aerial and the signal from the array as detected in an aircraft indicates elevation of the aircraft. Reversal of the frequency applied to the array is arranged to occur at a crest or a trough of the doppler frequency, so that discontinuities or cusps are not produced in the doppler signal at this time.
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