May 1961 Popular Electronics
[Table of Contents]People old and young enjoy waxing nostalgic
about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through
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NASA 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
By Ken Gilmore
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:
- 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"
Lunar color photos Underwood & Underwood
Fig. 1. Each concentric ring shown represents the lunar area
capable of reflecting radar signals of specific millisecond
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.
It began 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
Listening for Echoes.
When 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.
If 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.
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
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.
This antenna has been used in both the Venus planetary radar
contact and moon-mapping activity of MIT's Lincoln Laboratory.
This computer at Lincoln Laboratory analyzed 10,000,000 computations
to confirm existence of weak radar echoes from Venus.
Intensity and time duration of the radar returns from Venus
matched these two curves prepared by the computer.
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 unmistakable echoes.
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.
this system has been further improved, detailed maps of the surface
of Mars and Venus may be possible.
Posted August 5, 2013