Section 10.3: Meteor Scatter and Moonbounce #

Section 7.4 introduced two unusual paths, each stingy in its own way. A meteor trail lasts only a moment. The Moon stays in view for hours but returns only a tiny part of the energy sent to it. Each path calls for its own kind of digital mode: one fits a complete message into a fleeting opening, and the other recovers a message from deep noise.

Catching a Meteor Burst #

A meteor trail may support only part of a normal-length transmission. MSK144 answers that with a message frame just 72 milliseconds long, repeated over and over, so a brief burst has a chance to contain a complete decodable copy. Its name comes from minimum-shift keying (MSK), a smooth form of FSK.

Key Information: MSK144 is designed for meteor scatter communications.

Operators agree on the mode, frequency, and transmit periods. You may hear nothing but noise for most of a receive period. Then comes a ping, and a complete line of decoded text appears. The exchange moves forward when the needed message gets through, not when every repetition does. Major showers, such as the Perseids each August, bring pings far more often.

Recovering a Lunar Echo #

On an Earth–Moon–Earth (EME) path, the reflected signal may arrive too weak to hear at all. JT65 copes by carrying a compact, error-protected message as a slow sequence of audio tones, which an SSB transmitter translates to RF.

Key Information:

• JT65 can decode signals with a very low signal-to-noise ratio.
• JT65 uses multitone AFSK.

The 65 in the name counts tones: 64 carry data and one provides synchronization. It does not specify bandwidth or baud rate.

One receive period may still contain too little information for a decode. Q65 handles that by combining the evidence from several periods that carry the same message, which can recover a message that no single period reveals. Astrophotographers stack exposures of faint galaxies for the same reason. Despite the Q, Q65 does not use quadrature modulation; like JT65, it sends one of 65 FSK tones at a time.

Key Information:

• Q65 is designed for EME communications.
• Q65 differs from JT65 by averaging multiple receive cycles.

WSJT-X can average JT65 messages too. Q65 centers on averaging, though, and adds a newer error-correcting code and better tolerance of fast fading. Averaging combines only copies of the same message, so keep sending it until it gets through.

Take Opposite Turns #

Two stations transmitting together cannot hear each other’s lunar return. FT8’s solution works here too, with longer periods.

Key Information: A method of establishing EME contacts is time-synchronous transmissions alternating between stations.

Agree on a period length and who transmits first, then let synchronized clocks keep the sequence. For 60-second periods, the plan could be:

Keep the antennas on the Moon as it crosses the sky. Check the agreed period, and let successful decodes determine when to advance the exchange.