Arecibo Observatory: From Ionospheric Radar to Planetary Observation

Arecibo Observatory stood in the mountains south of Arecibo in northern Puerto Rico and was initially called the Arecibo Ionospheric Observatory. Construction arose from a proposal made in 1958 by Cornell electrical-engineering professor William Gordon: transmit powerful radio waves upward and receive the weak backscatter from ionospheric electrons to investigate the upper atmosphere.

Gordon worked with Henry Booker and Ben Nichols on instrument parameters. Kenneth Bowles subsequently demonstrated the feasibility of incoherent-scatter detection in experiments in Illinois. Equipment on this scale could not serve only one problem, so design also considered planetary radar and radio astronomy. The U.S. Department of Defense's Advanced Research Projects Agency (ARPA) supplied funding, while Cornell organized construction and research.

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Site selection had to account for both celestial positions and terrain. A tropical location allowed planets to pass near the observable zenith region, while a 305-meter antenna required an economically feasible support system. Cornell engineers William McGuire and George Winter studied using a natural basin. After comparing several islands and regions, the team selected a karst depression in Puerto Rico.

The original concept was a fixed upward-pointing parabolic antenna with a central tower. Through ARPA adviser Ward Low, Gordon contacted a group studying spherical reflectors at the Air Force Cambridge Research Laboratories, and the design changed to a spherical dish with a movable feed. Cables from three towers suspended a platform; moving the antenna on it changed beam direction without rotating the giant dish. A line feed compensated for spherical-reflection aberration.

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Main construction took place between 1960 and 1963. Contractors divided responsibility for structures, antennas, concrete towers and electronics, while the U.S. Army Corps of Engineers supervised civil works. The roughly 300-ton platform was assembled at the bottom of the depression and raised to about 152 meters. The vast collecting area, suspended structure and transmitting equipment all had to meet design requirements together.

Before the large dish was complete, the team tested the 430-megahertz radar using a separate horn antenna. By October 1962, it could receive signals from the Sun at transit and lunar echoes; another antenna near Ithaca, New York also received the reflected lunar signals. The inauguration took place on November 1, 1963, with Cornell managing the observatory under an Air Force contract and Gordon as director.

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Once the dish entered use, its 430-megahertz radar soon detected ionospheric incoherent scatter. However, the first square-section line feed performed below expectations, with aperture efficiency of about 22%, meaning the large reflector was not being fully translated into effective receiving capability. Inauguration established observing conditions, but instrument commissioning still had to continue.

Ionospheric research also created a training path. In 1965, H. C. Carlson completed the first doctoral dissertation using Arecibo's incoherent-scatter radar, followed by other graduate students. Improving the feed became a long-term task; a new circularly polarized 430-megahertz feed installed in 1972 finally produced a marked performance improvement. A major upgrade in 1974 renewed the dish surface and expanded capabilities.

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Planetary radar quickly became an important part of the observatory's work. Gordon Pettengill and Rolf Dyce studied Mercury echoes in 1965. Pettengill's report that year described transmission of short pulses lasting about 0.5 milliseconds, separating surface regions by echo delay and analyzing frequency changes to estimate rotation.

They obtained a sidereal rotation period of approximately 59 ± 5 days, inconsistent with the then-accepted 88-day synchronous-rotation interpretation. The report retained the uncertainty range and proposed repeating measurements at another position in Mercury's orbit. Other radar researchers had not all confirmed the result at the time, so explaining the method and conducting subsequent measurements mattered as much as announcing the new value.

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Arecibo operated through cross-institutional collaboration. Cornell and the University of Sydney signed a faculty and student exchange arrangement in 1964. Pettengill's previous work at MIT's Lincoln Laboratory brought software, processing methods and planetary-ephemeris research to the new observatory. Graduate students from Cornell and MIT used its data for dissertations.

Ordinary planetary ephemerides lacked the accuracy needed for radar, so Arecibo supplied observations to Lincoln Laboratory to help improve predictions of planetary positions. NASA-funded lunar radar work beginning in 1965 also contributed to studies of Apollo landing areas. On October 1, 1969, the National Science Foundation took over funding and management oversight, and the observatory continued its development from an initially military-supported construction project into a shared multidisciplinary facility.

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