| 摘要 |
An airborne altimeter system indicates the instant when a descending craft equipped with that system reaches a predetermined operating altitude. A transceiver aboard the craft emits a continuous high-frequency sine wave toward the ground and isolates a low-frequency Doppler component from an incoming reflected wave due to ground echoes. The Doppler component is peak-detected and the resulting envelope signal is fed to two passive impedance networks having different transfer functions so that the magnitude ratio of their output signals is equal to a predetermined value as soon as the operating altitude is reached. A comparator derives the desired altitude indication from these output signals to actuate a utilization circuit. In order to make the magnitude ratio of the output signals of the impedance network substantially independent of the rate of descent of the craft, within a predetermined speed range, the envelope signal fed to each network may be weighted by filters of different attenuation characteristics inserted between the transceiver output and respective peak detectors lying in cascade with the two impedance networks.
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