| 摘要 |
1513129 Doppler beacon STANDARD TELEPHONES & CABLES Ltd 30 Sept 1975 [8 Oct 1974] 43516/74 Heading H4D In a Doppler beacon utilizing an array of antennae sequential energized to simulate a moving source and produce a Doppler shift at a remote receiver corresponding to the bearing of the receiver with respect to the line of the array-e.g. as described in Specifications 1,225,190, and 1,234,541-errors may be produced at the receiver due to (a) multipath components and (b) transients in the receiver filter as the energization sequence finishes and/or starts. To mitigate these errors the Doppler frequency is tracked by a filter 4 having a variable frequency, continuously running, oscillator 4B controlled by a feedback loop from a sampler and frequency discriminator 4A to operate at a mulitiple, e.g. 4 times of the frequency being tracked. The feedback loop also includes a sample and hold circuit 5 which is controlled (6) to hold the control input to the oscillator 4B at the value pertaining at the end of the previous scan for an initial, e.g. 50% portion of the sequence. The frequency of the oscillator is monitored as the Doppler frequency. The first 25% of the Doppler frequency may be blocked (dotted box) to further reduce errors. |
