Radio Measurements in Space
May 1967 Electronics World
first thing I learned (or re-learned) in reading this article is
that in 1967, "Hertz" had only recently been assigned as the official
unit of frequency. According to Wikipedia, International Electrotechnical
Commission (IEC) adopted it in in 1930, but it wasn't until 1960
that it was adopted by the General Conference on Weights and Measures
(CGPM) (Conférence Générale des Poids et Mesures). Hertz replace
cycles per second (cps).
May 1967 Electronics World
[Table of Contents
People old and young enjoy
waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May
1959 through December 1971.
As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
The next thing that happened
was that I was reminded of how images such as the op-art tracing
of antenna oscillation that are routinely generated today by sophisticated
software, required huge amounts of setup time and trials to yield
just a single useful and meaningful image using actual hardware.
The third thing was, wow, 1967 was 45 years ago, and that was
nine years after I was born. Ouch.
Radio Measurements in Space
By Joseph H. Wujek, Jr.
Scheduled for an early
launch is a satellite to be used for radio astronomy purposes only.
An array of space antennas having 750-foot elements will be used.
This unusual op-art tracing was made by a portion
of an antenna designed for the Radio Astronomy Explorer satellite.
The photo was made in a thermal test chamber with a small light
bulb attached to the end of about 35 feet of the antenna. The antenna
was allowed to swing free in the chamber to give engineers an insight
into what the normal deployed pattern would be after the momentum
When the brilliant Scots
physicist James Clerk-Maxwell (1831-1879) published his classic
"A Treatise on Electricity and Magnetism" in 1873, very little was
known about the nature of electromagnetic (EM) radiation. Although
Maxwell predicted the existence of EM waves, it was not until after
1885 that high-frequency EM waves were generated in the laboratory.
Heinrich Hertz (1857-1894) is generally acknowledged to be the first
to generate these waves and was recently honored by having the unit
of frequency - "hertz" - named for him. The theoretical work of
Maxwell and the subsequent experimental research of Hertz thus paved
the way for the technology which we now know as radio. We use the
term "radio" here to include that region of the EM spectrum which
extends from a few hertz to the edge of the infrared region, which
is about 1000 gigahertz (1 million megahertz or 1 terahertz ).
With the development of radio communications in the twentieth
century, major emphasis was placed on gaining a better understanding
of the nature of radio propagation and noise. Measurements of radio
propagation and noise characteristics were, and continue to be,
made with international cooperation. The National Bureau of Standards
(NBS) of the U.S. Department of Commerce guides this effort in the
United States with technical coordination maintained among NBS,
other government agencies, universities, and industry.
A natural outgrowth of propagation and noise studies was the detection
of radio-frequency noise from deep space. Until the recent advancements
in space technology, measurement of space r.f, signals was confined
to the ground or to those altitudes accessible to aircraft. This
was, of course, also true of r.f. propagation studies. While ground-based
and aircraft measurements have contributed much to our understanding
of these phenomena, measurements from space vehicles enhance these
results. Since the earth's atmosphere acts to severely attenuate
certain r.f, frequencies, a measurement of r.f. signal strength
taken above the atmosphere provides added information regarding
the source, strength, and character of these signals.
Scale model of the Radio Astronomy Explorer satellite, world's
first satellite devoted exclusively to radio astronomy.
Fig. 1. Graph of noise from solar activity at 2.8 GHz showing
the last complete eleven-year cycle. Right now solar activity
is on upswing and new peak should occur around 1969.
Fig. 2. The STEM (Storable Tubular Extendable Member) principle.
Fig. 3. The principles of gravity gradient stabilization.
science of radio astronomy has also benefited from space r.f. measurements.
It has been known for some time that stars, galaxies, and some planets
emanate EM waves. The star nearest earth, our sun, exhibits increased
flare, or sunspot activity, on a somewhat regular basis. In particular,
the occurrence of these flares increases to a maximum every eleven
years (Fig. 1). Radio communications in certain frequency bands
are severely affected during such increased solar activity.
By studying the nature of the r.f. emanations of the sun and
other stars, scientists are able to better understand the energy
processes which occur in these bodies. The solar flares, which are
believed to be reactions similar to those of a fusion or hydrogen
bomb, release enormous amounts of energy. Swarms of charged particles
and EM waves are discharged from these reactions. The earth is about
93 million miles or 8 light-minutes from the sun, yet some of these
particles and waves find their way through the atmosphere and ultimately
reach the earth. In an earlier article ("Radiation Measurements
in Space", August 1966) we showed how energetic particles are detected
and measured. Here we will discuss systems used to measure r.f.
energy in space.
Instruments used to measure radiation in the EM spectrum are called
"radiometers". Many different kinds of radiometers exist; the type
used will depend on the portion of the spectrum to be measured.
In this article we shall be concerned only with radio-frequency
Radiometers have been used in space experiments
from the very beginnings of space exploration. These systems generally
consist of an antenna, an amplifier, and a telemetry readout system.
The amplifiers are usually of the frequency-selective variety so
as to amplify and pass only those frequencies of interest, while
all other frequencies are rejected. Some systems use several amplifiers
and/or antennas which are shared by means of automatic switching
controlled by a programmer subsystem. Ground commands may also be
used to select a particular channel when the payload is traversing
a given region of space.
As in the case of ground-based
systems, antenna design depends on the range of signal frequencies
to be gathered. Space radiometers have been developed which have
input sensitivities as low as 0.1 microvolt per meter. For some
perspective, remember that in order to obtain a good-quality TV
picture on most commercial receivers, a signal strength of 100 microvolts
per meter is required with a signal-to-noise ratio of at least 30
dB. Space systems can yield higher sensitivities because they are
far removed from high-level man-made signals and interference. These
higher sensitivities cannot, in general, be verified experimentally
in the laboratory due to the high level of surrounding interference.
Radio Astronomy Explorer Satellites
The first Radio Astronomy Explorer (RAE) satellite has been
tentatively scheduled for launch this year. This will mark the first
time a satellite has been designed and developed for radio astronomy
purposes exclusively. Due to be another first in space technology
is the array of antennas, each of which is 750 feet in length.
These antennas were first developed by The de Havilland
Aircraft of Canada, Limited. In addition to functioning as antennas,
the long tubular sections provide gravity gradient stabilization
of the spacecraft. The principle by which these rods are fabricated
is designated STEM, from the name Storable Tubular Extendable Member.
STEM devices have been used successfully on such space missions
as Gemini (16-foot antenna), the Canadian Topside satellite (60-foot
antennas), and the TRAAC satellite (60-foot gravity stabilizing
The STEM device consists of a strip of thin material,
usually stainless steel or beryllium-copper alloy, which has been
preformed to a tubular configuration. The strip is then wound on
a drum or compressed in telescope fashion into a canister. In the
case of the longer element lengths, a drive motor rotates the drum
to unfurl the STEM device (Fig. 2). The canister-version boom is
expanded by removing the canister lid, resulting in a jack-in-the-box
unfurling. An explosive bolt or squib is usually detonated by an
electrical signal to shear a pin or latch and thus open the canister.
the principles of antennas are familiar to all of us, the notion
of gravity gradient stabilization is perhaps not so familiar. The
physics involved here is not too different from the tightrope walker
who carries a long pole for balance. In the case of spacecraft stabilization,
the small difference in gravity over the length of the rod produces
a torque which tends to align the rod parallel to the gravitational
field, as shown in Fig. 3. The addition of more long rods to the
spacecraft produces more torque which yields a spacecraft attitude
which is stable with respect to earth.
Because of the great
length and thin walls of STEM devices, several problems appear with
their use. The vacuum of space is a cold void except when matter
is present to be heated by the sun's radiations. As a result, that
side of the STEM device which faces the sun is much warmer than
the side which looks away from the sun. Due to contraction and expansion
of materials with heating, the element tends to bend under these
temperature conditions. Thus, the tip of such an element of 300-foot
length, with 1/2-inch diameter and 0.002-inch walls, may deflect
more than 100 feet. The deflection may be reduced by using thicker
walls in the tubing, but if this is done, weight is also increased
- which is a great disadvantage in a good many space applications.
Testing of long STEM devices is also a problem since a low-gravity
environment is required. This is particularly a problem for the
longer elements. How does one create a low-gravity, high-vacuum,
sun-simulating environment for testing? The mechanical forces which
act during unfurling are quite complicated and testing is demanded.
Engineers at NASA's Goddard Space Flight Center have provided at
least a partial solution by using cameras and photographing the
trace created by a small lamp attached to the tip of the antenna.
Some very interesting light patterns are produced during such tests.
One of these is illustrated in the lead photograph on page 46.
The RAE will probably be assigned a three-stage improved
Delta launch vehicle with an over-all length of 91 feet. The first
stage Thor rocket develops 346,000 pounds of thrust. Recall that
jet engines, as used in commercial transports today, typically develop
16,000 pounds of thrust. The second stage develops approximately
7000 pounds of thrust, with the third stage (which carries the spacecraft)
producing about 2000 pounds thrust. It is anticipated that an orbital
altitude of about 300 kilometers (186 miles) will be used.
The RAE Mission
The mission of the
RAE satellite may be categorized by five scientific objectives.
1. To observe low-frequency radio storms on earth. These
storms are believed to be interactions between particles emanating
from the sun and earth's radiation belts.
2. To monitor
large radio noise sources, such as the constellation Centaurus A.
3. To study Jupiter, which is the only planet other than
the earth which is known to occasionally emit low-frequency noise
4. To obtain an EM map of our galaxy (the Milky
Way) in the frequency range from 400 kHz to 10 MHz.
gather data on low-frequency bursts of EM energy which emanate from
the sun. This data should provide added insight into the nature
of the sun's reactions.
IIn order to achieve orbit and deploy
the four 750-foot antennas, a sequence which will require about
two weeks will be initiated by ground command from Goddard Space
Flight Center, Greenbelt, Maryland.
The data gathered by
RAE satellites and their successors may provide space scientists
with enough information to formulate new theories concerning the
earth and its surroundings.
Posted February 16, 2012
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