Question E2D01:
Which of the following digital modes is designed for meteor scatter communications?  

Choices:  
A: WSPR  
B: MSK144  
C: Hellschreiber  
D: APRS    

Correct Answer is  B: MSK144  

Why B is correct:
- MSK144 is a WSJT-X mode purpose-built for meteor scatter. It uses minimum-shift keying with short symbols, strong FEC, and time-synchronized 15-second T/R periods, so it can decode very brief “pings” from meteor ionization trails that last only milliseconds.

Why the others are incorrect:
- WSPR (A): A beacon/propagation tool with very long transmissions, $T_{TX} \approx 110\,\text{s}$ per message. Far too slow for millisecond meteor bursts.
- Hellschreiber (C): A facsimile-style keyboard mode for HF; not optimized for short, bursty ionized-trail reflections.
- APRS (D): AX.25 packet for position/messaging on VHF/UHF networks; not designed for meteor-scatter characteristics.

Useful rules of thumb:
- Meteor scatter signals arrive as short bursts: $T_{ping} \approx 5\text{ ms} - 500\text{ ms}$.
- Choose modes with short symbol duration $T_s$ and strong FEC so that $T_s \ll T_{ping}$.
- Modern meteor-scatter modes: MSK144 (current standard). Legacy: FSK441.
- Common practice: time-synced even/odd 15 s sequences on VHF (e.g., 50 and 144 MHz).