Radar Explores the Moon
May 1961 Popular Electronics
is currently collecting a phenomenal amount of data on the planet Mars. No small part of the effort is to determine
whether sending humans to inhabit Mars would be feasible, or even at all possible. In order for it to be even possible
for a long-term stay, it would be necessary for consumable resources to be accessible by Earth Martians. Discovering
water ice would be the pièce de résistance since water is heavy and therefore very expensive to transport across
vast reaches of space. Another key bit of data needed is frequency and size of meteor strikes on the surface since
that figures directly into survivability. Long before we had the capability or even need to do that for Mars, NASA
was doing the same sort of investigation on our moon (as opposed to one of Mars' two moons,
Phobos and Deimos). The resolution of telescopes, all ground-based in the day, was good enough to perform site selection surveys in the x-y plane, but altitude data could only be inferred via estimations based on shadow lengths along the terminator (night/day line of demarcation)
and sideways glances of peaks and valleys. That was not good enough for planning a human expedition to
the surface, so engineers and scientists came up with a radar mapping technique to obtain z-axis data.
That effort is reported here in this May 1961 edition of Popular Electronics. More information
was needed prior to actually launching a Moon lander mission, including sending unmanned orbiters and
later manned orbiters to gather enough data to culminate in the highly successful Apollo 11 mission in
1969 - just 8 years later.
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Radar Explores the Moon
Electronic engineers have developed unique methods to determine the roughness of the lunar surface
man has not yet set foot on the surface of the moon, Mars, or Venus, his radar "fingers" have already reached out
and touched these solar-system neighbors of ours.
We are getting important new information from our first
electronic interplanetary trips. Scientists recently learned for example, that Venus and other planets are not quite
as far away as we had thought. Further striking new advances in radar equipment and spectacular progress in the
techniques of analyzing and interpreting radar echoes may soon bring answers to questions such as:
Lunar color photos Underwood & Underwood
- How rough is the surface of the moon?
- Does Venus, eternally covered by its thick mantel of clouds, rotate as does he earth - and if so, how fast?
Does it have mountains, seas, icecaps, and forests?
- Is it true that the sun has a highly variable atmosphere of charged particles which as some scientists theorize,
reaches out beyond the earth?
- What does the dark side of Mercury look like?
- How dense are the ionized particles known to exist in space? how thick is the cosmic dust endlessly drifting
through the universe?
- After years of speculation, what are the mysterious "canals" of Mars?
Fig. 1. Each concentric ring shown represents the lunar area capable of reflecting radar signals of specific
millisecond time delay.
Fig. 2. As the moon rotates, it also librates, so that radar echoes vary above or below their radiated microwave
frequency. Shaded section at right represents a certain frequency band which is located above radiated frequency.
Fig. 3. Combining both time delay and frequency selection tells us that the radar echo came from a fixed area
of the lunar surface.
Lunar radar echoes are long in duration since the same narrow pulse must be reflected from a spherical surface.
The echo can be easily divided up into segments to match those in Fig. 1.
The new branch of electronics seeking answers to these and related questions,
radar astronomy, is even younger than the more widely known radio astronomy - itself no old-timer. Radio astronomy,
touched off in 1931 when Karl Jansky discovered the strange radio noises of the heavens, consists mainly of listening
to the static generated by ionized gas clouds and stars throughout the universe. In radar astronomy, on the other
hand, we send out the signals ourselves, then receive and record the echoes when they come back.
in 1946 when the Army Signal Corps managed to bounce the first signal off our closest heavenly neighbor, the moon.
For the next decade, electronic techniques and equipment improved-radar transmitters became more powerful, the receivers
used to detect the extremely weak echoes more sensitive. Finally, in early 1959, a group of scientists at MIT's
famous Lincoln Laboratory made radar contact with Venus. Less than a year later, workers at Stanford University
got echoes from the sun - over 90 million miles away.
Until recently, all we could do with these radar contacts
was measure distance. We couldn't "see" much detail because radar beams, like light beams, spread out as distance
increases. The "tightest" beams are about one minute (1/60th of a degree) wide. Also, the fineness of detail which
the beam can distinguish depends on its diameter at the target. When the target is at interplanetary distances,
the beam spreads thousands of miles. Thus it has about the same resolving power - detail-seeing ability - as the
human eye. You can see only the barest detail on the moon with the unaided eye, and planets are simply pinpoints
of light. They look about the same to radar. Optical telescopes, on the other hand, can see far finer detail.
For some time, researchers have been looking for a way to improve radar's resolving power. The obvious
method - narrowing the beam - wasn't practical. Even small improvements would require tremendously large antennas.
Finally, scientists at Lincoln Laboratory came up with an ingenious plan for improving radar's resolution - without
narrowing the beam at all. With the new method, radar resolution now almost equals telescopic resolution on the
moon, and far surpasses it at interplanetary distances.
Listening for Echoes.
we look at the moon, it seems to be a flat disc in the sky, although it is really a sphere. The center of the sphere
is closest to us, the outer edges farther away.
When we bounce a radar signal off the moon, a strange thing
happens. We send out a pulse of one length, and get back a much longer echo. This is because part of the signal
bounces off the center (which is closest) , part bounces off some distance away from the center, and so on, all
the way out to the edge. It is as though we were getting a series of echoes from a number of different targets,
each slightly farther away than the preceding one, so that all blend and overlap into one long echo.
we select a small part of the signal, say the first 1/10th that returns, we know this is the echo from the moon's
center. The next 1/10th will be from an area surrounding the center slightly farther from us, and so on. By selecting
various parts of the echo, we can isolate echoes from various ring-shaped portions of the moon's surface - see Fig.
1. The drawing to the right is an idealized representation of how such an elongated return echo might look. The
various parts of the echo are from the numbered circular portions on the moon's surface.
We have now narrowed
down the portion of the moon's surface from which the echoes are coming, but for the system to be really useful
in mapping the satellite by radar, we must narrow it down still further. And in the moon's slight natural movement,
Lincoln Laboratory scientists found the key to doing just this. Although the face of the moon does not rotate with
respect to the earth - this is why we always see the same side - it does wobble. Scientists call this wobbling "libration."
As the moon librates, it turns slightly in one direction, then back in the opposite direction, and so on.
As it librates in one direction, one outer edge is moving toward earth; the other, away from us. Now, as a radar
beam strikes the entire surface of the moon, the echo which bounces off the side coming in our direction is slightly
raised in frequency, due to the Doppler effect. (This is the effect discovered many years ago which seems to make
a train whistle change in pitch as the train approaches, then passes you.) The signal coming from the receding side
is lowered in frequency. The center, which remains at a constant distance, returns an echo at the same frequency
as the original signal
With various portions of the moon's surface returning echoes of different frequencies,
by tuning in only echoes of one frequency and rejecting the rest, we can listen to echoes from any part individually.
See Fig. 2. (The order reverses, but the principle remains the same as the moon librates back in the opposite direction.)
By tuning only for one frequency, and at the same time selecting only one part of the returning pulse, both
of our selection systems are in operation. The only echo we receive is from the two small spots on the moon where
the two patterns overlap - see Fig. 3.
Mapping the Moon.
This antenna has been used in both the Venus planetary radar contact and moon-mapping activity of MIT's Lincoln
This computer at Lincoln Laboratory analyzed 10,000,000 computations to confirm existence of weak radar echoes
Intensity and time duration of the radar returns from Venus matched these two curves prepared by the computer.
Using this technique, scientists have made a rough radar map of the moon.
Radar pictures made in this way will never be as sharp as telescopic pictures of the moon that we have had for years.
But the system still has tremendous value. It can give a pretty good idea of how rough certain parts of the moon's
surface are - essential information for a space ship landing.
The primary value of the new radar mapping
technique lies in the fact that while the resolving power of a light telescope diminishes with distance, the resolving
power of the new radar telescope does not diminish with distance. Since the Doppler and time delay effects used
are functions of the size and speed of rotation of a planet, and not its distance from earth) this radar system
will resolve features easily and accurately on Mars, Venus, Mercury, or any other planet our radar is strong enough
to reach. The moon is now serving to calibrate and test the system so scientists can compare results with known
terrain. Soon, radar eyes will be turned on the planets.
The planets are millions of miles away and radio
energy loses strength rapidly as it travels through space. It follows the inverse square law, which means that if
you double the distance a signal has to travel, you end up with not one half, but the square root of the power you
had before. Since a radar signal has to travel two ways - out to the target and then back - the strength of its
signal diminishes more rapidly. It varies inversely as the fourth power of the distance. In other words, when you
double the radar distance, you get back only 1/16th the power.
Mapping the Planets? To reach the planets
and get a return echo, we need tremendous amounts of transmitted power - all that we can generate. Even at maximum
power, the echo that comes back is not a sharp pip, easily received and spotted. All we get from outer space is
a lot of noise. Somewhere buried in that hash, we hope, is the signal we want. But the signal may be as much as
10,000 times weaker than the hash which is drowning it out.
Scientists are managing to solve the problem
of detecting weak signals by sending out a string of pulses, rather than one pulse, lasting - in the case of the
Venus contact, for example - just under five minutes. The return signal - a lot of hash with some echoes mixed in
- is fed to a computer which electronically adds up all the areas where the pulses ought to be. Since the returning
pulses are regular, and the noise is irregular, theoretically the pulses will add up faster than the noise. This
theory actually works in practice. The computer, after making as many as 10,000,000 separate computations, has spotted
Radar astronomy, with its new techniques and improved equipment, will soon set out
on what may be the most spectacular job of its career: mapping the planets. And when man himself actually leaves
on his first trip, his travels will be far safer, surer, and more valuable, because the fingers of radar astronomy
will have already paved the way for him.
Working actively in the field of radar astronomy is the Stanford Research Institute, Menlo
Park, Calif. This 142·foot antenna, erected by Stanford in northern Scotland, will be used for moon-echo studies
similar to those being made by Lincoln Laboratory.
Using techniques described in this article, scientists at the Lincoln Laboratory have made
this radar map of the moon.
When this system has been further improved, detailed maps of the surface of Mars
and Venus may be possible.
Posted August 5, 2013