Radio Measurements in Space
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
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. All copyrights are hereby acknowledged.
Electronics World articles.
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.
third thing was, wow, 1967 was 45 years ago, and that was nine years
after I was born. Ouch.
See all the available
Electronics World articles.
Radio Measurements in SpaceBy 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 had decreased.
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.
The 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 systems.
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
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.
While 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
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 bursts.
4. To obtain an EM map of our galaxy (the Milky Way) in the
frequency range from 400 kHz to 10 MHz.
5. To 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.