moonbounce or Earth-Moon-Earth (EME)

Moonbounce, also called Earth-Moon-Earth (EME), is a form of wireless communication in which the moon is used as a passive satellite.

Moonbounce, also called Earth-Moon-Earth (EME), is a form of wireless communication in which the moon is used as a passive satellite. To the uninitiated, this sounds a little like science fiction, but it has been done and continues to be done by experimentally-inclined amateur radio operators.

There are several challenges and difficulties inherent in moonbounce operation. One of the most troublesome for two-way communication is the fact that the moon's distance introduces lag time. The moon is approximately 250,000 miles away from the earth, and radio waves travel at 186,282 miles per second. A signal sent to the moon does not return until 2.7 seconds have elapsed. If two people are engaged in a conversation and one person asks a question, that person cannot expect a reply until at least 5.4 seconds later (the answer must travel to the moon and back, as must the question).

Besides propagation delay, the path loss to and from the moon is considerable.  The moon is a relatively poor reflector of electromagnetic rays at any wavelength, including radio waves.  Its surface is irregular, and it scatters, rather than focusing, reflected energy. Because of this, sophisticated equipment is necessary to successfully bounce a signal off the moon and hear it return.

Another problem with moonbounce communication is libration fading and Doppler shifting. The moon does not always present exactly the same face; it "wobbles" a few degrees back and forth.  This "wobbling," called libration, produces a constant change in every component of any signal reflected from the moon.  The returned signal consists of the sum total of countless rays that have bounced off mountains, boulders, crater walls, and other lunar features.  The relative phase of these components rapidly fluctuates because of libration, so any signal returning from the moon is "fluttery" and distorted.

Amateur-radio moonbounce generally requires the following:

  • A sensitive receiver with a narrowband filter

  • A transmitter capable of operating on at least one amateur band above 144 MHz, and capable of producing 1500 watts of continuous radio-frequency output

  • An antenna with high directivity and gain, capable of being rotated in both the azimuth and elevation planes

  • A location in which the moon can be seen without obstruction for extended periods

  • A location in which humanmade radio noise is minimal

  • Neighbors who will tolerate the presence of a large antenna and the proximity of a high-power radio transmitter

  • A neighborhood without ordinances or covenants prohibiting large antennas and/or high-power radio transmitters

  • Operating skill and patience

This was first published in September 2005

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