Earth–Moon–Earth communication (EME), also known as moon bounce, is a radio communications technique that relies on the propagation of radio waves from an Earth-based transmitter directed via reflection from the surface of the Moon back to an Earth-based receiver.
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 the available microwave transmission powers and low noise receivers, it would be possible to beam microwave signals up from Earth and reflect off the Moon. It was thought that at least one voice channel would be possible.
The "moon bounce" technique was developed by the United States Military in the years after World War II, with the first successful reception of echoes off the Moon being carried out at Fort Monmouth, New Jersey on January 10, 1946 by John H. DeWitt as part of Project Diana. 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, DC. In the days before communications satellites, a link free of the vagaries of ionospheric propagation was revolutionary.
EME communications technical details
As the albedo of the Moon is very low (maximally 12% but usually closer to 7%), and the path loss over the 770,000 kilometre return distance is extreme (around 250 to 310 dB depending on whether the VHF or UHF band used, modulation format and Doppler shift effects), high power (more than 100 watts) and high-gain antennas (more than 20 dB) must be used.
The Moon must be above the horizon in order for EME communications to be possible.
To determine "EME Path Loss" we need to know
- Moon distance from either the transmitting or receiving station
- Transmitter station output in watts, expressed as ERP [roughly transmitter power output (minus feedline loss) x forward antenna gain]
- Receive station gain (actual receiver gain minus feedline loss, x antenna gain)
- The operating frequency of the transmitter and receiver
Substituting into the free-space loss formula and converting to into :
Adding factors for reflection from the Moon results in
The standard radar path link formula is basis for EME path loss calculations
After including the factor for surface reflectivity it becomes
- where is the Moon's diameter
Since the diameter of the Moon is
The formula becomes
- , F = MHz, d = km
For some reason not specified, Josef has increased the loss by 3 dB producing:
Note that the distance from Earth to the Moon varies because the Moon's orbit is not perfectly circular, but somewhat elliptical with a mean radius of 390,000 kilometres (240,000 mi). This means there is an apogee (the largest distance) and a perigee (the shortest distance). In addition, the orbital plane precesses with a principal period of 18.6 years.
Depending on the position of the Moon with respect to Earth, apogee can be as much as 406,700 km, whereas perigee can be as little as 356,400 km.
- This translates to as much as 2.25 dB difference in path loss from apogee to perigee.
- The mean distance from Earth to Moon is given as 384,400 km.
- These calculations consider the fact that the Moon is only 7% efficient as a reflector, use the radar equation (which defines a two-way path-loss model) and the assumption that the Moon is a spherical reflector.
Echo delay and time spread
Propagation time to the Moon and back is therefore 2d/c
- d is distance (average distance at any given time)
- or about 2.4 s at perigee
- or about 2.7 s at apogee
- or about 2.56 s on average, but for message coding 2.25 s is used
The Moon is nearly spherical, and its radius corresponds to milliseconds of wave travel time. The trailing parts of an echo, reflected from irregular surface features near the edge of the lunar disk, are delayed from the leading edge by as much as twice this value.
Most of the Moon's surface appears relatively smooth at the typical microwave wavelengths used for amateur EME. Most amateurs do EME contacts below 6 GHz, and differences in the moon's reflectivity are somewhat hard to discern above 1 GHz.
Lunar reflections are by nature quasi-specular (like those from a shiny ball bearing). The power useful for communication is mostly reflected from a small region near the center of the disk.
The Moon is nearly spherical, with the reflection differential being the Moon's radius divided by the speed of light, ~5.8 ms (electromagnetic wave travel time).
The effective time spread of an echo amounts to no more than 0.1 ms.
At shorter wavelengths the lunar surface appears increasingly rough, so reflections at 10 GHz and above contain a significant Diffuse Component as well as a quasi-specular component.
The Diffuse Component is depolarized, and can be viewed as a source of low level system noise.
Significant portions of the Diffused Component arise from regions farther out toward the lunar rim. The median time spread can then be as much as several milliseconds.
In all practical cases, however, time spreading is small enough that it does not cause significant smearing of CW keying or intersymbol interference in the slowly keyed modulations commonly used for digital EME.
Faster message keying may encounter the Diffused Component as significant system noise.
EME Time Spreading does have one very significant effect. Signal components reflected from different parts of the lunar surface travel different distances and arrive at Earth with random phase relationships.
As the relative geometry of the
- transmitting station
- receiving station
- reflecting lunar surface
changes, signal components may sometimes add and sometimes cancel.
The dynamic addition and cancellation will create large amplitude fluctuations. These amplitude variations are referred to as Libration Fading. These Libration Fading amplitude variations will be well correlated over the Coherence Bandwidth (typically a few kHz). The Libration Fading components are related to the inverse of the time spread.
Current EME communications
Amateur radio (ham) operators utilize EME for two-way communications. EME presents significant challenges to amateur operators interested in working weak signal communications. Currently, EME provides the longest communications path any two stations on Earth can utilize for bi-directional communications.
Amateur operations use VHF, UHF and microwave frequencies. All 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 utilized by amateurs are continuous wave with Morse Code, digital (JT65) and when the link budgets allow, voice.
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 of the celebrations was an interview via the Moon with Apollo 8 astronaut Bill Anders. He was also part of the backup crew for Apollo 11. The University of Tasmania in Australia with their 26m dish was able to bounce a data signal off the surface of the Moon which was received by a large dish in the Netherlands - Dwingeloo Radio Observatory. The data signal was successfully resolved back to data setting a world record for the lowest power data signal returned from the Moon with a transmit power of 3 milliwatts - about 1,000th of the power of a strong flashlight filament globe. 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 visual artist Daniela de Paulis and the CAMRAS radio amateurs association based at Dwingeloo radio telescope (NL) developed a new application of Moonbounce, called Visual Moonbounce, which allows moonbouncing images using the MMSSTV software. The technology was applied to a live performance called OPTICKS during which digital images are sent to the Moon and back in real time and projected live.
Modulation types and frequencies optimal for EME
Other factors influencing EME communications
A single sideband contact between IZ1BPN in Italy and PI9CAM at the Dwingeloo Radio Observatory. IZ1BPN's transmission is shifted up in pitch slightly to compensate for PI9CAM's transmission being Doppler Shifted down. At the end of IZ1BPN's transmission you can hear the echo of his signal returning from the Moon, again pitched down by Doppler Shift.
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Doppler Effect at 144 MHz band is 300 Hz at Moonrise or Moonset. The Doppler Offset goes to around Zero when the Moon is overhead. At other frequencies other Doppler Offsets will exist. The 300 Hz offset is the average Doppler Offset for the 144 MHz band.
- At moonrise, returned signals will be shifted approximately 300 Hz higher in frequency due to the Doppler effect between Earth and the Moon.
- In the Northern Hemisphere, as the Moon traverses the sky to a point due south the Doppler effect approaches nothing. As the Moon sets, signals are shifted lower in frequency until at Moonset they are shifted 300 Hz lower.
- Doppler effects cause many problems when tuning into and locking onto signals from the Moon.
An array of 8 Yagi antennas for 144 MHz EME at EA6VQ, Balearic Islands, Spain
- Communication Moon Relay
- Information theory
- Lunar Laser Ranging experiment
- Meteor burst communications
- Passive repeater
- Radar Equation
- Pether, John (1998). The Post Office at War. Bletchley Park Trust. p. 25.
- Butrica, Andrew J. (1996). To See the Unseen: A History of Planetary Radar Astronomy. NASA. Archived from the original on 2007-08-23.
- NASA, Beyond the Ionosphere: the development of satellite communications
- http://www.k5rmg.org/tech/EME.html (another calculator)
- http://www.df9cy.de/tech-mat/pathloss.htm (gives formulas for EME path loss calculation)
- http://www.camras.nl site of CAMRAS radio amateurs association at Dwingeloo radio telescope
- http://echoesofapollo.com/moon-bounce/ World Moon Bounce Day - Echoes of Apollo
- http://www.wia.org.au/members/armag/2009/august/ Amateur Radio - August 2009 - Wireless Institute of Australia
- http://www.k3pgp.org/uhftveme.htm K3PGP - UHF TV reception via Earth–Moon–Earth communication
- http://internal.physics.uwa.edu.au/~agm/eme.html UHF TV carrier reception via moonbounce (EME)