The radio astronomy team of Von Russel Eshelman, Robert C. Barthle, and Paul B. Gallagher transmitted and detected the first radar wave reflections from the Sun.

As in military radar, radio astronomy encompasses “passive” and “active” branches. In the former, receive-only radio telescopes study radiation naturally emitted by celestial objects. In the latter, or radar, case, radio or microwave signals are transmitted from Earth, and echoes are received after reflection and scattering from astronomical targets. Active radar astronomy has the advantage that signals can be chosen specifically to transmit through gas clouds and resolve spatial structures inaccessible to visual telescopy. Its chief limitation arises from significant losses caused by progressive geometrical spreading and attenuation with range. Radio astronomy;the Sun[Sun]
Sun, the
[kw]Radio Astronomers Transmit Radar Signals to and from the Sun (Early 1959)
[kw]Astronomers Transmit Radar Signals to and from the Sun, Radio (Early 1959)
[kw]Radar Signals to and from the Sun, Radio Astronomers Transmit (Early 1959)
[kw]Sun, Radio Astronomers Transmit Radar Signals to and from the (Early 1959)
Radio astronomy;the Sun[Sun]
Sun, the
[g]North America;Early 1959: Radio Astronomers Transmit Radar Signals to and from the Sun[05980]
[g]United States;Early 1959: Radio Astronomers Transmit Radar Signals to and from the Sun[05980]
[c]Astronomy;Early 1959: Radio Astronomers Transmit Radar Signals to and from the Sun[05980]
[c]Science and technology;Early 1959: Radio Astronomers Transmit Radar Signals to and from the Sun[05980]
Eshelman, Von Russel
Barthle, Robert C.
Gallagher, Paul B.

Since the late 1930’s, numerous radar Radar investigations of meteors were conducted with what were essentially military systems. Nevertheless, only after World War II were the first deliberate efforts made to send and receive radar echoes specifically to and from other bodies in the solar system. As in military and air traffic control radar units, astronomical radars include a transmitter and antenna array to radiate short pulses of electromagnetic energy, a small fraction of which returns as echoes to the antenna following reflection from a solid body, or ionized gas cloud. The distance from array to reflector can be determined directly from one-half the measured elapsed travel time multiplied by the speed of light.

Assessment of total radar capability to detect a given distant target can be made via the radar equation. The radar equation is a simple measure of radar performance, defined in terms of transmitted power, signal-to-noise ratio, antenna gain, antenna area, target reflector cross section, and antenna-target distance. The signal-to-noise ratio for a given radar reflector depends on the specific array geometry used, transmitted peak power, and the receiver frequency bandwidth. Whereas the former two factors are more or less fixed by hardware, the last can be chosen to exclude ambient noise without rejecting echo energy.

The target area of a reflector, measured in square meters, is its effective radar cross section, usually different from its simple geometric area. Because of its symmetrical shape, a true sphere, for example, exhibits the same radar cross section at all aspect angles, and in the so-called optical regime (where sphere radius is much larger than incident wavelength), the radar cross section is independent of wavelength. The simplest radar reflection to interpret is so-called specular (mirrorlike), or planar reflection, defined as reflection from a sufficiently smooth interface according to the Lord Rayleigh roughness criteria. Greater surface roughness causes some impinging power to be more randomly (“diffusely”) scattered or lost in other than the straightline direction of propagation.

In 1945, the first radar echoes were detected from the Moon. F. J. Kerr Kerr, F. J. , in England and Australia, published the first theoretical computations defining the kinds of radar power and antenna required for attempts to detect radar echoes from specific regions of the sun. As established by visual astronomy, the sun is divided into photosphere, chromosphere, and corona. The chromosphere is basically a thin, ionized gas layer overlying the photosphere. The corona is the outer part of the solar atmosphere, which can, at times of high solar activity, extend great distances outward from the sun.

Since its corona is very hot (106 Kelvins), it was necessary first to consider carefully the effects of refraction-bending, scattering, and collisional attenuation losses for proposed radar sounding signals. In addition to Kerr, Soviet experts, and later noted radio astronomer Ronald Bracewell Bracewell, Ronald , theoretically examined, in 1955, the maximum expected penetration of radar waves, as a function of frequency and assumed raypath, into the ionized solar corona as limited by magnetic field and absorption. This is considerably more complex a computation than is required to parameterize the radar equation.

A theoretical formula by Baumbach and Allen (1947) was used by Kerr and Bracewell to predict the varieties and properties of radar wave propagation paths through the magnetized ion clouds of the corona. An optimum frequency of a difference of 25 to 30 megacycles per second was determined. It was not until mid-1958, however, that sufficiently precise and powerful radar units became available, first at the Lincoln Laboratories Lincoln Laboratories of the Massachusetts Institute of Technology (MIT) and shortly thereafter at Stanford University.

In early February, 1958, Robert Price Price, Robert and others at MIT made two attempted observations of reflected radar signals from Venus, which was expected to return an echo 5 million times weaker than that returned from the Moon. Although planetary echoes were made on this occasion, the returned echoes were too weak compared to ambient noise level to make detection clear and repeatable. In contrast to the near-exclusive focus of nascent radar astronomy on nearby planets, in early 1959, Von Russel Eshelman, Robert C. Barthle, and Paul B. Gallagher of Stanford University’s Radio Physics Center made the first successful radar contact with the sun.

The Stanford experiment was undertaken at a frequency of 25.6 megacycles per second. The radar system included 40 kilowatts transmitted by deploying a broadside array of four rhombic-shaped antenna elements covering more than 5.6 hectares of ground. The radar transmissions were pulsed alternatingly on and off at thirty-second intervals for fifteen minutes, slightly less than the theoretical round-trip travel time for radar signals between the earth and the sun. The received signal was amplified, filtered, frequency-modulated, and recorded on magnetic tape. The observed reflected radar echoes were much lower in amplitude than the solar and cosmic background noise levels and required additional careful filtering and other data processing to ensure clear detection.

The taped records were converted from analog to digital form at the high sampling rate of 4,096 samples per second, and the absolute (positive) amplitude values of these sampled signals binned together and summed for periods of one second. This widely used bin-sum process is equivalent to one-second integration. The sums were then cross-correlated, a mathematical technique to measure similarity between two curves, with a unit square wave of one-minute period, in an attempt to bring out any specific periodicities at this frequency. If the received solar echoes were an ideal square wave of one-minute period, in the absence of noise, cross-correlation or similarity curves would be perfectly triangular in shape with symmetrical straight-line sides.

Since the solar echoes received were not perfect square waves nor noise-free, but were noisy envelopes of many echo square waves arriving over a group of adjacent time delays, the triangular correlation curve appeared jagged and truncated. The amount of truncation served as a direct measure of the depth of penetration of radar signals into the solar corona, experimentally confirming that the corona was not a single or uniform specular reflector. The effect of solar and cosmic background noise on the triangular correlation curves was to generate further fluctuations from the ideal signal response. Only when the triangular curve sufficiently exceeded the magnitude of noise fluctuations could a radar echo signal be identified definitively as present.

To confirm further their data analysis for a definitive source of solar echoes (and not statistical artifacts), the Stanford team made two additional test records on numerically simulated test data. An initial tape encompassed only received radar band noise, while a second tape contained the same noise, plus a one-cycle-per-second square wave signal approximately one hundred times weaker than the noise. Processing both tapes as above, the triangular similarity curves were obtained. This permitted the experimenters to conclude safely that the signal-to-noise ratio of the received radar echoes from the sun was between a difference of 21 and 25 decibels, so that solar echo energy over the measurement period was a difference of 23 decibels less than the combined solar and cosmic ambient noise.

Although no direct measurement of the sun’s radar cross section was made during the 1959 experiment, Barthle subsequently made an indirect cross section estimate in 1960. Using empirical relations for the cosmic radio noise background versus frequency at 26 megacycles per second, an approximate noise level of 3 × 10’s watts per cycle per second was computed. The radar echo power received by the Stanford antenna was estimated to be 1.5 × 10-21 watts per cycle per second. Comparing correlated amplitude curves versus time, the Stanford results first indicated that radar energy was reflected apparently from a solar depth interval of between 1 and 3 solar radii.

As a consequence of these results, regular radar studies of the sun were initiated by MIT’s Lincoln Laboratories as well as other institutions. MIT employed a large cross-polarized array at El Campo, Texas, with the first extended observations in April, 1961, using a more powerful continuous-wave transmitter broadcasting at approximately 38.25 megacycles per second. Observations of solar radar returns were made as the sun passed across the antenna’s beam area. The improved depth resolution of this array detailed further finer scale structure of the solar interior.

Strong solar noise bursts have occasionally decreased radar echo reception when noise frequencies overlap with those of radar, so that additional cross-correlational and deconvolutional (inverse waveform) filtering techniques were necessary to ensure reliable echo waveform detection. Doppler spreading of typically 30 to 40 kilocycles per second indicates strong plasma waves. The Doppler shift is the amount of change in the measured frequency of a (radar) wave caused by the relative motion of reflector with respect to transmitter.

The data from MIT’s tests provided direct estimates of the sun’s radar diameter. It was first recognized here that the sun’s apparent radar cross section fluctuates notably with time between 0.5 and 2 degrees angular width. Similar variations in depth of radar penetration were noted (between 1.1 and 1.5 solar radii). Overactive coronal regions, such as the characteristic long streamers, were shown to have typically high ion particle densities approximately ten times higher than those previously predicted by the theoretical models of Kerr and Bracewell.

As a result, other more comprehensive models for solar radar propagation were developed, incorporating phenomenological models for random scattering and random reflection, as well as refraction, which notably advanced the state-of-the-art in the theory of wave scattering as a whole. The success of the Stanford radar experiment gave renewed impetus to use of radar for planetary exploration. Following its conjunction in 1961, Price and Gordon H. Pettengill Pettengill, Gordon H. of MIT, and researchers at Jodrell Bank, RCA Laboratories, and the Soviet Union all obtained sufficiently strong radar echoes from Venus with reasonably good mutual agreement. In June, 1962, the Soviet Union reported radar detection of Mercury, with subsequent radar measurements of Mars and Jupiter reported in spring and fall, 1963. The mandatory minimization of all Earth-end radar system noise brought notable engineering improvements in quieting electronic receiver and antenna noise, as well as increasing sophistication in signal modulation and detection algorithms. Radio astronomy;the Sun[Sun]
Sun, the

• Eshelman, V. R., R. C. Barthle, and P. B. Gallagher. “Radar Echoes from the Sun.” Science 131 (1960): 329-332. Gives a brief semitechnical description of the Stanford experiment setup and results.
• Kerr, F. J. “On the Possibility of Obtaining Radar Echoes from the Sun and Planets.” Proceedings of the IRE (Institute of Radio Engineers) 40 (1952): 660-662. This paper, together with Kerr’s tutorial published a few months before the Stanford experiment, accurately estimates antenna and radiated power requirements for Earth-based detection of solar radar reflections.
• NATO Advanced Study Institute on Solar System Radio Astronomy. Solar System Radio Astronomy. Edited by Jules Aarons. New York: Plenum Press, 1965. Shows the rapid development of radio telescopy and astronomical radar in the years immediately following the Stanford experiment.
• Page, R. M. The Origin of Radar. New York: Greenwood Press, 1979. A most comprehensive and readable introduction to the development of basic radar concepts and instrumentation.
• Proakis, John G., and Dimitris G. Manolakis. Introduction to Digital Signal Processing. New York: Macmillan, 1988. The best introduction to correlational, multiplexing, and other basic filtering techniques employed in radio astronomy.
• Stix, Michael. The Sun: An Introduction. 2d ed. New York: Springer, 2002. A more advanced undergraduate text, requiring a second-year physics and mathematics background. Repays careful study.
• Wentzel, D. G. The Restless Sun. Washington, D.C.: Smithsonian Institution Press, 1989. Accurately presents an elementary-level account of key concepts and methods with numerous illustrations.

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