Earth–Moon–Earth communication
Moon bounce enables two-way radio via lunar reflection.
Earth–Moon–Earth communication (EME), often called Moon bounce, works by sending radio waves from a transmitter on Earth toward the Moon, where they reflect off the lunar surface and return to a receiver on Earth. This creates the longest possible communication link between any two points on the planet.
The idea of using the Moon as a passive reflector was first proposed in 1940 by W.J. Bray of the British General Post Office. He calculated that with available microwave transmitters and low-noise receivers, it should be possible to beam signals to the Moon and back, and that at least one voice channel might be achievable.
During World War II, in 1943, German engineers working with radar equipment noticed an unusual echo. Dr. Ing. W. Stepp described it in *Der Seewart* magazine: a strong "perturbation" that appeared about two seconds after the transmitter was switched on, lasted for several pulses, and faded gradually after the transmitter was turned off. It only occurred when the antenna pointed east, and disappeared if the antenna was moved, reappearing about two seconds after returning to the original direction. Stepp concluded they had detected the Moon rising behind clouds, and that the gradual fading was due to the Moon moving out of the narrow, horizontally aimed beam as it rose.
After the war, dedicated efforts to bounce radar waves off the Moon began. The first successful attempt took place on January 10, 1946, at Fort Monmouth, New Jersey, by a team code-named Project Diana, led by John H. DeWitt. Less than a month later, on February 6, 1946, a Hungarian group led by Zoltán Bay succeeded as well. The subsequent Communication Moon Relay project led to practical applications, including a teletype link between Pearl Harbor, Hawaii, and U.S. Navy headquarters in Washington, D.C. Before communications satellites, this link was revolutionary because it avoided the instability and problems of ionospheric propagation.
The advent of communications satellites in the 1960s made the technique obsolete for official use, but radio amateurs adopted it as a hobby. The first amateur Moon bounce communication took place in 1953, and enthusiasts worldwide still use it. In 1970, Australian radio ham Ray Naughton (VK3ATN) of Birchip, Victoria, achieved Australia's first Moon bounce using a home-built 250-meter-wide, 30-meter antenna and a 100-watt signal.
- First proposed
- 1940 by W.J. Bray of the British General Post Office
- First successful attempt
- January 10, 1946, by Project Diana at Fort Monmouth, New Jersey
- Second successful attempt
- February 6, 1946, by a Hungarian group led by Zoltán Bay
- First amateur radio moon bounce
- 1953
- Average round-trip delay
- 2.56 seconds
- Distance range
- 384,400 km average
- Doppler shift at 144 mhz
- 300 Hz at moonrise and moonset
Lore & Background
The use of the Moon as a passive communications satellite was proposed by W.J. Bray of the British General Post Office in 1940. It was calculated that with available microwave transmission powers and low-noise receivers, it would be possible to beam microwave signals up from Earth and reflect them off the Moon, with at least one voice channel thought possible. Radar reflections off the Moon were received and recognized as such in 1943 during German experiments with radio measurement equipment, as reported by Dr. Ing. W. Stepp in Der Seewart magazine. Stepp noted a 'perturbation' that appeared about two seconds after switching on the transmitter and disappeared correspondingly later after switching it off, occurring only when the antenna was aimed east, and disappearing upon a major change of direction. The first successful attempt specifically for bouncing radar waves off the Moon was carried out at Fort Monmouth, New Jersey, on January 10, 1946, by a group code-named Project Diana, headed by John H. DeWitt, followed less than a month later by a second successful attempt by a Hungarian group led by Zoltán Bay. The Communication Moon Relay project that followed led to more practical uses, including a teletype link between the naval base at Pearl Harbor, Hawaii and United States Navy headquarters in Washington, D.C. The development of communication satellites in the 1960s made this technique obsolete, but radio amateurs took up EME communication as a hobby; the first amateur radio moon bounce communication took place in 1953. Radio ham Ray Noughton (VK3ATN) of Birchip, Victoria, Australia accomplished Australia's first moon bounce with a home-built 250 m wide, 30 m antenna with a 100 watt signal in 1970. NASA scientists, having ridiculed his DIY project claiming at least a kilowatt would be required, invited him to the U.S., all expenses paid, to learn from his grassroots engineering. The effect has been put to artistic use by composer Pauline Oliveros in her 1987 work Echoes from the Moon, and in 2024 German musician Hainbach experimented with moonbounce and created an audio plug-in to reproduce the effect.
Reader's Guide
EME presents significant challenges to amateur operators interested in weak signal communication. Amateur frequency bands from 50 MHz to 47 GHz have been used successfully, but most EME communications are on the 2 meter, 70-centimeter, or 23-centimeter bands. Common modulation modes are continuous wave with Morse code, digital (JT65, Q65, FT8) and, when link budgets allow, voice. Recent advances in digital signal processing have allowed EME contacts with powers in the order of 100 watts and a single Yagi–Uda antenna. World Moon Bounce Day, June 29, 2009, was created by Echoes of Apollo and celebrated worldwide as an event preceding the 40th anniversary of the Apollo 11 Moon landing. A highlight was an interview via the Moon with Apollo 8 astronaut Bill Anders. The University of Tasmania in Australia with their 26-meter dish were able to bounce a data signal off the Moon which was received by the Dwingeloo Radio Observatory in the Netherlands, setting a world record for the lowest power data signal returned from the Moon with a transmit power of 3 milliwatts. The second World Moon Bounce Day was April 17, 2010, coinciding with the 40th anniversary of the conclusion of the Apollo 13 mission. In October 2009 media artist Daniela de Paulis proposed to the CAMRAS radio amateur association based at the Dwingeloo Radio Observatory to use Moon bounce for a live image transmission performance, leading to the first image transmission via the Moon using the open source software MMSSTV, a technique she called 'Visual Moonbounce'.
Did You Know?
- The average round-trip delay for EME is 2.56 seconds.
- A data signal was bounced off the Moon with a transmit power of only 3 milliwatts, about 1,000th of the power of a flashlight lamp.
- The Doppler shift at 144 MHz is 300 Hz at moonrise and moonset.
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