Question
What characterizes libration fading of an EME signal?
Answer Options
- A) A slow change in the pitch of the CW signal
- B) A fluttery, irregular fading
- C) A gradual loss of signal as the sun rises
- D) The returning echo is several hertz lower in frequency than the transmitted signal
Correct Answer: B
Explanation
Libration fading is a major characteristic and challenge of Earth-Moon-Earth (EME) communication. It is caused by the very slight ‘wobbling’ motion of the Moon as seen from Earth (a phenomenon called libration). This motion changes the reflection points on the Moon’s surface relative to the ground stations. Since the Moon’s surface is rough, the radio wave reflections arrive at the receiving station slightly out of phase over time.
This continuous phase change causes rapid, deep, and irregular changes in the signal strength at the receiving station. Therefore, libration fading is characterized as a fluttery, irregular fading of the EME signal. This type of fading requires the use of specialized digital modes, like Q65, that are highly optimized to decode signals that repeatedly dip below the noise floor.
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