1429814 Radio navigation ELLIOTT BROS (LONDON) Ltd 9 July 1974 [26 Oct 1973] 49931/73 Addition to 1,388,620 Heading H4D In a navigation aiding system in accordance with the main Specification 1,388,620, a craft interrogating a relatively fixed station during a synchronizing period decodes its own interrogating signals and maintains constant the time period between the start of the synchronizing period in craft time and the time when it decodes its own interrogating signal, while the fixed station, responding to the craft by transmitting time synchronizing signals, maintains constant the time from a predetermined point in the time allocation slot to the time it decodes its own transmitted response signal. These time periods are chosen to be equal so that cancellation of errors to a first order, due to variable delays in the signal processors in the craft and on the ground respectively, is stated to be effected. The airborne and ground equipment are modified so that the craft receiver (Fig. 2) and ground station (Fig. 3, not shown) may detect the occurrence of their own transmitted signal by the introduction of couplers 5 and 9 and sample mixer 7 so that a signal transmitted from aerial 6 is also detected in receiver 8 as a received response.
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