*Question E3A02:
What characterizes libration fading of an EME signal?*  

*Correct Answer is  B: A fluttery, irregular fading*  

### AI Response: (may contain errors)  

Libration fading is a phenomenon that affects Earth-Moon-Earth (EME) communications. Here's a breakdown of the correct answer and why the other choices are incorrect:

### Correct Answer: B: A fluttery, irregular fading

- **Explanation**: Libration fading is caused by the movement of the Moon and the irregularities on its surface. As the Moon moves, the path length of the reflected signal changes slightly, causing rapid, irregular variations in signal strength. This results in a fluttery, irregular fading effect on the received signal.

### Why the Other Choices are Incorrect:

- **A: A slow change in the pitch of the CW signal**: This describes a Doppler shift effect, not libration fading. Doppler shift occurs due to the relative motion between the Earth and the Moon, causing a change in frequency, not the fluttery fading characteristic of libration.

- **C: A gradual loss of signal as the sun rises**: This is unrelated to libration fading. A gradual loss of signal due to the sun rising might be associated with ionospheric conditions or other environmental factors, not the Moon's movement.

- **D: The returning echo is several hertz lower in frequency than the transmitted signal**: This again refers to a Doppler shift, where the frequency of the signal changes due to relative motion. Libration fading is about signal strength variation, not frequency change.

### Summary

Libration fading is specifically characterized by its fluttery, irregular nature due to the Moon's movement and surface irregularities, distinguishing it from other phenomena like Doppler shift or environmental signal loss.