Project Diana: Bouncing Radio Waves Off The Moon

Around noon on January 10, 1946, a powerful pulse of radio waves shot skyward from a massive radar installation at Camp Evans in Wall Township, New Jersey. Unlike the sweeping motion of traditional radar used to search for enemy aircraft, this beam was tightly focused and aimed at a pale orb just rising above the eastern horizon — the Moon.

The pulses, transmitted at a rate of one every five seconds, raced toward the luminous target at an astonishing speed of 186,000 miles per second, piercing through the ionosphere and into the unknown void beyond Earth’s atmosphere. In a tiny shack near the base of the antenna tower, a team of engineers waited breathlessly, their eyes focused on the screen of a single 9-inch oscilloscope. Presently, a tiny spike appeared — the unmistakable signal that a radar pulse had successfully bounced back from the lunar surface. This astounding feat was repeated almost every day and night for the next several months, proving beyond doubt that radio waves could penetrate the ionosphere and be reflected off the Moon.

This experiment, known as Project Diana, marked the beginning of radar astronomy. It was also a pivotal step toward space-based communication and laid the groundwork for the future of the U.S. space program.

Credit: Pierre PRESTAT

However, Project Diana didn’t begin as an astronomy or space exploration venture — it was a military initiative.

Shortly after the end of World War II, U.S. military leaders tasked the Army Signal Corps with researching the capabilities of long-range radar. Their primary concern was whether such radar could be used to detect incoming ballistic missiles. The devastation wrought on London by the German V-1 and V-2 rockets was still fresh in their minds, and they were determined to prevent similar destruction in the future.

The assignment was handed to Colonel John DeWitt and his team at the Evans Signal Laboratory in Belmar, New Jersey. DeWitt’s mission was to determine whether radar waves could penetrate the Earth’s ionosphere — a crucial requirement, since long-range missiles would pass through this atmospheric layer on their way to their targets. Without the ability to "see" through the ionosphere, early detection and tracking of incoming missiles would be impossible.

DeWitt needed a target outside the Earth’s ionosphere to bounce the signal off. With artificial satellites still more than a decade away, he chose the largest and nearest celestial body to Earth: the Moon. He named the program Project Diana, after the Roman goddess of the Moon.

In the fall of 1945, DeWitt assembled his team, which included Chief Scientist E. King Stodola, Herbert Kauffman, Jacob Mofenson, Harold Webb, and the renowned mathematician Walter McAfee. With limited resources and time, no attempt was made to design new components specifically for the experiment. Instead, the team modified existing radar equipment already available at Camp Evans, using a heavily modified SCR-271 radar set as their transmitter. The original radar transmitter, rated at 3 kilowatts, was upgraded to produce an output of 50 kilowatts. Through the use of a high-gain antenna, the effective radiated power was increased to approximately 10 megawatts.

The radar antenna used for Project Diana. Credit: www.projectdiana-eme.com

The site of the experiment. Credit: www.projectdiana-eme.com

The antenna was mounted atop a 100-foot tower and aimed toward the horizon at the Moon. It was fixed at a set elevation angle and could only be rotated horizontally, meaning the team had a narrow window of opportunity — about 40 minutes — as the Moon passed through the 15-degree-wide beam during moonrise and moonset each day.

The radars used during World War II were designed for relatively short-distance detection of enemy aircraft. These sets emitted brief bursts of microwave energy lasting only a few millionths of a second and were capable of detecting objects several hundred kilometers away. But the Moon is about 383,000 kilometers (approximately 238,000 miles) from Earth. Not only would the "echo" of a radar pulse sent to the Moon take much longer to return, but it would also be far weaker than the typical echoes detected by military radars up to that time.

To address these challenges, DeWitt and his colleagues decided to generate a much longer radar pulse — about a quarter of a second in duration — which was easier to detect than shorter ones. The interval between pulses was also increased to five seconds to allow sufficient time for the echo to return.

After a few initial setbacks, when some equipment failed to function properly, DeWitt finally saw success on January 10, 1946. That day, the Moon rose at 11:58 a.m. Around that time, the first radar pulses were transmitted, and soon the first echoes appeared on the oscilloscope. Precise timing of each pulse and its reflected echo revealed that it took 2.5 seconds for the signal to return. Since radio waves travel at a fixed speed of about 186,000 miles per second, it was not difficult to calculate the distance from the radar installation to the reflecting surface: approximately 238,000 miles. Colonel DeWitt and his radar engineers were convinced they had made contact with the Moon, because there was nothing else in space at that distance from Earth.

Oscilloscope display showing the 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. Credit: www.projectdiana-eme.com

A beaming Col. DeWitt later said: "We knew our months of thinking, planning, calculations, and design were on the right track, but to make doubly positive and sure, as our Army Laboratories must be, we aimed our radar beam at
the rising and setting satellite time and time again, so that we knew without question of a doubt that our pulses were striking the moon and echoes were rebounding back to earth."

It had been known since the early 1900s that certain radio waves, when directed at an angle, could be reflected back from the ionosphere. This technique was already in use for radio communication with locations beyond the curvature of the Earth. However, before Project Diana, it was not known whether radio waves could penetrate this electrically charged layer of the upper atmosphere.

Project Diana is widely regarded as the birth of radar astronomy and a vital precursor to the U.S. space program. It was the first successful demonstration that terrestrial radio signals could penetrate the ionosphere, paving the way for space-based communication with probes and future human explorers. The project also set a precedent for naming space missions after Roman deities — a tradition that continued with programs like Mercury and Apollo.

In addition to its scientific achievements, Project Diana proved the feasibility of using the Moon as a passive reflector to transmit radio signals from one point on Earth to another, beyond the planet’s curvature. This method, known as Earth-Moon-Earth (EME) communication or “moonbounce,” was put to practical use in 1950 when the U.S. military successfully intercepted stray Soviet radio transmissions reflected off the Moon. Although the returning signals were extremely faint, the potential was promising enough for the U.S. to begin constructing the world’s largest parabolic antenna at Sugar Grove, West Virginia, a project ultimately abandoned in 1962 due to its high cost.

A more successful application of EME technology was the U.S. Navy’s Communication Moon Relay, also known as Operation Moonbounce, which used lunar reflections to enable military communications. This method was eventually phased out in the early 1960s with the advent of communications satellites. Today, EME communication lives on as a niche pursuit among amateur radio enthusiasts.

Jack Mofenson, Harold Webb, John DeWitt, King Stodola, and Herbert Kauffman. Credit: www.projectdiana-eme.com