Question E2D06:
Which of the following is a method for establishing EME contacts?  

Choices:  
A: Time-synchronous transmissions alternating between stations  
B: Storing and forwarding digital messages  
C: Judging optimum transmission times by monitoring beacons reflected from the moon  
D: High-speed CW identification to avoid fading    

Correct Answer is  A: Time-synchronous transmissions alternating between stations  

To work EME (Earth-Moon-Earth), stations typically use strict, alternating time slots so each knows when to transmit and when to listen. This is essential because signals are extremely weak and the round-trip delay is long.

Why A is correct (Time-synchronous transmissions alternating between stations)
- EME uses “sequencing”: one station transmits during defined time slots (e.g., even minutes) while the other listens, then they swap.
- Precise timing (GPS/NTP) is used to align sequences, especially with weak-signal digital modes like JT65 or Q65.
- This avoids overlap, allows deep integration/decoding, and accommodates the ~2.5 s round-trip delay.

Why the others are incorrect
- B: Storing and forwarding digital messages is a satellite/packet technique, not how you establish real-time EME QSOs.
- C: “Monitoring beacons reflected from the moon” is not a standard or reliable method. EME conditions are assessed via moon position (common window), path loss, sky noise, degradation index, etc., not by moon-reflected beacons.
- D: High-speed CW does not avoid fading. Libration and Faraday rotation cause QSB; EME favors slow CW or robust digital modes to maximize SNR, not high-speed CW.

Useful formulas and rules of thumb
- Round-trip delay:
  $$t_{\text{rt}} \approx \frac{2D_{\text{moon}}}{c} \approx \frac{2 \times 384{,}400\ \text{km}}{299{,}792\ \text{km/s}} \approx 2.5\ \text{s}$$
- Free-space path loss (one-way) increases with frequency and distance:
  $$\text{FSPL(dB)} = 32.44 + 20\log_{10}(f_{\text{MHz}}) + 20\log_{10}(d_{\text{km}})$$
  For EME, effective loss is even higher due to lunar surface reflection/scattering.
- Operational rules:
  - Use time-synchronous alternating sequences (even/odd minutes).
  - Ensure a common moon window (both stations have the Moon above the horizon).
  - Expect and plan for deep QSB; use narrowband, low-rate modes (JT65, Q65, slow CW).