Project Diana and the birth of radar astronomy

Project Diana radar antenna at Fort Monmouth, New Jersey, USA.
Credit: InfoAge Science History Learning Center and Museum

On January 10, 1946, a team of military and civilian personnel at Camp Evans, Fort Monmouth, New Jersey, USA, reflected the first radar signals off the Moon using a specially modified SCR-270/1 radar. The signals took 2.5 seconds to travel to the Moon and back to the Earth. This achievement, Project Diana, marked the beginning of radar astronomy and space communications. The effort resolved doubts about whether electromagnetic waves suitable for long-range communication and radar could penetrate the Earth’s ionosphere. It was the first documented experiment in radar astronomy and in actively probing another celestial body, and was the dawn of the space age.

   Following the end of World War II, Col. John H. DeWitt Jr. (1906–1999), Director of the Evans Signal Laboratory at Camp Evans (part of Fort Monmouth), in Wall Township, New Jersey, was directed by the Pentagon to determine whether the ionosphere could be penetrated by radar, in order to detect and track enemy ballistic missiles that might enter the ionosphere. He decided to address this charge by attempting to bounce radar waves off the Moon. For this task he assembled a team of engineers that included Chief Scientist E. King Stodola (1914–1992), Herbert Kauffman (1914–1980), Jacob Mofenson (1914–1969), and Harold Webb (1909–1989). Input from other Camp Evans units was sought on various issues, including most notably the mathematician Walter McAfee (1914–1995), who made the required mathematical calculations. 

Project Diana staff from left: Jacob Mofenson, Harold D. Webb, John H. DeWitt, Jr., E. King Stodola, and Herbert P. Kauffman.
Credit: US Army Communications Electronics Museum

   The reflective array antenna consisted of 64 half-wavelength dipoles in an 8x8 array in front of a flat reflective screen, and had a gain of 24 dB and a main lobe beamwidth of about 15°. It was driven by a 50 kW modified SCR-271 radar set that produced quarter-second pulses. The whole assembly was mounted atop a 100-foot reinforced tower. The signals had a frequency of 115.5 MHz, the peak power was 15 kW. The echoes took about 2.5 seconds to return. The receiver had to compensate for the Doppler shift in frequency of the reflected signal due to the Moon's orbital motion relative to the Earth's surface, which was different each day, so this motion had to be carefully calculated for each trial. The antenna could be rotated in azimuth only so the experiment could only be done at moonrise and moonset as the moon passed through the antenna's horizontal beam.

   The first successful echo detection came on January 10, 1946 at 11:58 am local time by Harold Webb and Herbert Kauffman. The experiment was concluded at 12:09 pm, when the Moon moved out of the radar's range. The experiment was repeated daily over the next three days and on eight additional occasions during the month.

    Although the possibility of reflecting radar signals off the Moon had been discussed in the scientific literature, there are no prior documented similar achievements. Zoltán Bay (1900–1992) and a Hungarian team achieved a similar result on February 6, 1946. Because their receiver did not have the sensitivity required, and their antenna did not have the gain needed to directly detect the reflected signal, they used an accumulating coulometer to acquire a 30 fold increase in the signal to noise ratio, producing a signal, post processing, 4% above the noise floor.

   Demonstrating that signals could travel from the Earth to the Moon and back was proof of concept for the idea of what is known as Earth-Moon-Earth (EME), or “moonbounce” communication. Following the success of the project, the US Navy set out to explore the implications and applications of this form of communication - the idea of a reliable, secure EME scheme. The system in its completed state began seeing use in 1960 and was expanded to accommodate ship-to-shore transmissions. In the later 1960s it became obsolete due to the advent of artificial satellites in orbit to serve the same purpose.

Oscilloscope display showing the Project Diana radar signal. The large pulse on the left is the transmitted signal, the small pulse on the right is the return signal from the Moon. The horizontal axis is time, but is calibrated in miles. It can be seen that the measured range is 238,000 mi (383,000 km), the distance from the Earth to the Moon. Credit: Radio News Magazine

Radar astronomy is concerned with the investigation of solar system bodies by radar methods. The technique involves transmitting a beam of microwave radiation from an Earth-based radio telescope or an orbiting spacecraft towards a target and analyzing the faint ‘echo’ that returns from the target’s surface. Radar techniques can yield information on the precise distance between the Earth and the target body, the rate at which the target body is rotating, the altitude of a spacecraft above a body’s surface, the vertical relief and topography on the surface of a planet or satellite, and the nature and roughness of that surface. The ability to control and measure the source of the transmission allowed scientists to extract information that was difficult to obtain before, such as composition and relativistic data. Since 1946, this technique has been used to gather a wealth of data about the geological and dynamic properties of many of the planets, moons, and asteroids that orbit our sun. Additionally, it has been used to determine the length of the astronomical unit (au) to a much higher precision than had previously been possible, and to determine the scale of the solar system itself.

   The first unambiguous detection of radar echoes from Venus was made by the Jet Propulsion Laboratory on March 10, 1961. JPL established contact with the planet Venus using a planetary radar system from March 10 to May 10, 1961. Using both velocity and range data, a new value of 149,598,500 ± 500 km was determined for the astronomical unit. The spread of frequencies in the returning signal provides information on the rotation rate of the target body. This technique enabled the retrograde rotation period of cloud-covered Venus to be determined in 1962, and the 59-day rotation period of Mercury to be determined in 1965.

Jet Propulsion Laboratory Goldstone 26-meter HA-DEC antenna that was used with the 26-meter AZ-EL antenna to detect radar echoes from Venus in 1961. Credit: NASA/JPL-Caltech

   Detailed mapping of the surface topography of Venus has been carried out using ground-based radar and radar instrumentation carried on a succession of spacecraft that have been placed in orbit round that planet. The radar instrument aboard the Cassini spacecraft has penetrated the thick veil of smog and aerosols on Saturn's moon Titan to determine the physical state, composition, and topography of its surface. It has confirmed the existence of large hydrocarbon seas and lakes on Titan.

Asteroid 2015 TB145 is depicted in eight individual radar images collected on Oct. 31, 2015 between 5:55 a.m. PDT and 6:08 a.m. PDT. The 70-meter DSS-14 antenna at Goldstone, California, was used to transmit high-power microwaves toward the asteroid. The radar echoes were received by the National Radio Astronomy Observatory's 100-meter Green Bank Telescope in West Virginia. The radar images achieve a spatial resolution of 4 meters per pixel. Credit: NASA/JPL-Caltech/GSSR/NRAO/AUI/NSF

Images from the Radar instrument aboard NASA's Cassini spacecraft show the evolution of a transient feature in the large hydrocarbon sea named Ligeia Mare on Saturn's moon Titan. Credit: NASA/JPL-Caltech/ASI/Cornell

References:

Website and blog about Project Diana: Project Diana: Radar Reaches the Moon
The National WWII Museum New Orleans: Project Diana: To The Moon And Back
ETHW: Milestones: Detection of Radar Signals Reflected from the Moon, 1946
Butrica, Andrew J.: To See the Unseen: A History of Planetary Radar Astronomy. NASA SP-4218 (1996)

© 2026, Andrew Mirecki