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

Choices:  
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 is  B: A fluttery, irregular fading  

Correct answer: B — A fluttery, irregular fading

1) Why B is correct
- Libration fading is caused by reflections from many points on the Moon’s rough surface, each with a slightly different Doppler shift. The combined signals add and cancel unpredictably, producing fluttery, irregular fading and a “warbly” tone on CW. This rapid, random QSB is the hallmark of libration fading in EME.

2) Why the other choices are incorrect
- A: A slow change in the pitch of the CW signal is due to overall Doppler drift as Earth–Moon radial velocity changes over minutes, not libration fading.
- C: A gradual loss of signal as the sun rises is associated with ionospheric/D-layer effects on HF paths, not lunar reflections (EME).
- D: A frequency offset of the echo (higher or lower) is plain Doppler shift. It can be hundreds of hertz at VHF/UHF, not just “several hertz,” and it is not “fading.”

3) Useful formulas and rules of thumb
- One-way Doppler shift:
  $$\Delta f = f_0 \frac{v_r}{c}$$
  where $f_0$ is the operating frequency, $v_r$ is radial velocity, and $c$ is the speed of light.
  - Example at 144 MHz with $v_r \approx 1000\ \text{m/s}$:
    $$\Delta f \approx 144\times10^6 \times \frac{1000}{3\times10^8} \approx 480\ \text{Hz}$$
- Libration fading arises from Doppler spread (many slightly different $\Delta f$ from different lunar facets). Rule of thumb:
  - At 144 MHz: frequency spreading often tens of hertz.
  - At 432 MHz: can be on the order of 100+ Hz.
- Contrast:
  - Slow pitch change = overall Doppler drift (minutes).
  - Fluttery, irregular QSB/warble = libration fading (rapid, random).