Radio communications technique
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
.
History
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]
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
them off the Moon. It was thought that at least one voice
channel
would be possible.
[1]
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", which "appeared, had a duration of several impulses, and larger impulse strength than the strongest nearby targets. It didn't appear until about two seconds after switching on the transmitter and disappeared (pulsatingly) correspondingly later after switching it off. But the rest of the echo image appeared and disappeared at the instance of switching the transmitter on/off. The 'perturbation' only occurred when the antenna was aimed to the east, and it disappeared immediately upon a major change of direction, but reappeared only about two seconds after rotating back to the original direction. Apparently we had detected the rising moon behind the clouds with the equipment. It explained the gradual disappearance of the impulses by the reflecting body slowly moving out of the strongly focussed, horizontally aimed beam, as it rises above the horizon."
[2]
It was not until the close of
World War II
, however, that techniques specifically intended for the purpose of bouncing radar waves off the moon to demonstrate their potential use in defense, communication, and
radar astronomy
were developed. The first successful attempt was carried out at
Fort Monmouth
, New Jersey, on January 10, 1946, by a group code-named
Project Diana
, headed by
John H. DeWitt
.
[3]
It was followed less than a month later, on February 6, 1946, by a second successful attempt, by a Hungarian group led by
Zoltan Bay
.
[4]
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.
In the days before
communications satellites
, a link free of the
vagaries
of
ionospheric propagation
was revolutionary.
The development of
communication satellites
in the 1960s made this technique obsolete. However,
radio amateurs
took up EME communication as a hobby; the first
amateur radio
moonbounce communication took place in 1953, and amateurs worldwide still use the technique.
Current EME communications
[
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]
A
single sideband
contact between IZ1BPN in
Italy
and PI9CAM at the
Dwingeloo Radio Observatory
, as received from PI9CAM's location. PI9CAM's transmission is shifted up in pitch slightly to compensate for IZ1BPN's transmission being shifted down by the
Doppler effect
. At the end of PI9CAM's transmission you can hear the echo of his signal returning from the
Moon
at a lower pitch due to the Doppler effect.
Amateur radio (ham) operators utilize EME for
two-way communications
. EME presents significant challenges to amateur operators interested in weak signal communication. EME provides the longest
communications path
any two
stations
on Earth can use.
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
) and when the link budgets allow, voice.
Recent advances in
digital signal processing
have allowed EME contacts, admittedly with low data rate, to take place 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 of the celebrations was an interview via the Moon with
Apollo 8
astronaut
Bill Anders
, who was also part of the backup crew for Apollo 11. The
University of Tasmania
in Australia with their 26-meter (85')
dish
were 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
flashlight
lamp. 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. As a result of her proposal, in December 2009 CAMRAS radio operator Jan van Muijlwijk and radio operator Daniel Gautchi made the first image transmission via the Moon using the open source software MMSSTV. De Paulis called the innovative technology "Visual Moonbounce" and since 2010 she has been using it in several of her art projects, including the live performance called OPTICKS, during which digital images are sent to the Moon and back in
real time
and projected live.
Echo delay and time spread
[
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]
Radio waves
propagate
in vacuum at the
speed of light
c
, exactly 299,792,458 m/s. Propagation time to the Moon and back ranges from 2.4 to 2.7 seconds, with an average of 2.56 seconds (the average distance from Earth to the Moon is 384,400 km).
The Moon is nearly spherical, and its radius corresponds to about 5.8 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 effective time spread of an echo amounts to no more than 0.1 ms.
Antenna polarization
for EME stations must consider that reflection from a smooth surface preserves
linear polarization
but reverses the sense of
circular polarizations
.
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. The diffused component may appear as significant noise at higher message data rates.
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 and reflecting lunar surface changes, signal components sometimes add and sometimes cancel, depending on their
phase
relationship, creating large amplitude fluctuations in the received signal. These "libration fading" amplitude variations are well correlated over the
coherence bandwidth
(typically a few kHz). The libration
fading
components are related to the time spread of reflected signals.
Modulation types and frequencies for EME
[
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]
VHF
UHF
Microwave
Other factors influencing EME communications
[
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]
Doppler effect
at 144 MHz band is 300 Hz at moonrise or moonset. The doppler offset reduces to around zero when the Moon is overhead. At other frequencies other doppler offsets will exist. At moonrise, returned signals will be shifted approximately 300 Hz higher in frequency. As the Moon traverses the sky to a point due south or due north, the Doppler effect approaches zero. By Moonset, they are shifted 300 Hz lower. Doppler effects cause many problems when tuning into, and locking onto, signals from the Moon.
Polarization
effects can reduce the strength of received signals. One component is the geometrical alignment of the transmitting and receiving antennas. Many antennas produce a preferred plane of polarization. Transmitting and receiving station antennas may not be aligned from the perspective of an observer on the moon. This component is fixed by the alignment of the antennas and stations may include a facility to rotate antennas to adjust polarization. Another component is
Faraday rotation
on the Earth-Moon-Earth path. The plane of polarization of radio waves rotates as they pass through ionized layers of the Earth's atmosphere. This effect is more pronounced at lower VHF frequencies and becomes less significant at 1296 MHz and above. Some of the polarization mismatch loss can be reduced by using a larger antenna array (more Yagi elements or a larger dish).
[5]
Gallery
[
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]
-
An array of eight Yagi antennas for 144 MHz EME at EA6VQ, Balearic Islands, Spain
-
A part of 144 MHz EME antenna array at WA6PY in California, U.S.
-
A dish antenna for microwave EME work at WA6PY, California, U.S.
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A dish antenna for UHF EME at I2FZX, Milan, Italy
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Amateur radio antenna array used for Earth?Moon?Earth communication on 144 MHz. Location Kilafors in central Sweden.
-
Amateur radio antenna array used for Earth?Moon?Earth communication on 144 MHz. Location Jader, central Sweden.
-
Amateur radio antenna array used for Earth?Moon?Earth communication on 144 MHz. Location Staffanstorp, southern Sweden.
See also
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References
[
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]
- ^
Pether, John (1998).
The Post Office at War
.
Bletchley Park Trust
. p. 25.
- ^
Stepp, Ing.
"cq-DL 7/79, p. 328"
.
PA4FWM's homepage
. Translated by de Boer, Pieter-Tjerk.
- ^
Butrica, Andrew J. (1996).
To See the Unseen: A History of Planetary Radar Astronomy
.
NASA
. Archived from
the original
on 2007-08-23.
- ^
"Bay, Zoltan"
.
omikk.bme.hu
. OMIKK
. Retrieved
2017-01-13
.
- ^
Larry Wolfgang, Charles Hutchinson, (ed),
The ARRL |Handbook for Radio Amateurs, Sixty Eighth Edition
, American Radio Relay League, 1990
ISBN
0-87259-168-9
, pages 23-34, 23-25,
External links
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