Bounce radio signals off the moon? Unthinkable! Well, it certainly was in 1946, when scientists at the US Army Signal Corps at Fort Monmouth (Wall Twp., NJ) resumed trying (after World War II ended) to pierce the Earth's ionosphere with electromagnetic energy. Hardly anyone knew much about this program called “Project Diana” (named for the Greek goddess of the moon), and many still don’t today (Figure 1). This is a shame, as the project and its participants removed one of the greatest obstacles to space travel.
Figure 1: Project Diana has been noted as the birth of the US space program and radar astronomy.
In 1946 it was believed that RF energy could not penetrate the ionosphere, thus making communication between Earth and space impossible. Without the ability to communicate between Earth and manned or unmanned spacecraft, space exploration was also off the table. In 1946, one way to know for sure that this was true was to transmit a high-energy signal directly at the moon and measure whatever (if any) reflections were detected.
But there was a fundamental obstacle: No one had determined the velocity of a point on the moon relative to a point on Earth. Enter mathematician and physicist Dr. Walter S. McAfee, who worked at Ft. Monmouth’s Electronics Research Command (Figure 2). After hundreds of hours of calculations, he ultimately determined the answer, a feat that eliminated the barrier to conducting the experiment.
Figure 2: Mathematician and physicist Dr. Walter S. McAfee contributed calculations that were crucial to Project Diana's success.
Lt. Col. John DeWitt and chief scientist E. King Stodola, accompanied by a few radar researchers, modified a ‘bedspring’ radar antenna to deliver 24dB of forward gain, a transmitter to produce 3kW at 111.5MHz in 250-ms pulses, and a receiver that compensated for Doppler modulation of the reflected signal (Figure 3). Since the antenna could be rotated only in azimuth and not in elevation, attempts could only be made as the moon passed through the 15°-wide beam at moonrise and moonset. This meant that there were only 40 minutes of observation on each pass as the moon transited the various lobes of the antenna pattern.
Figure 3: The image shows the 111.5MHz reflective array antenna the US Army Signal Corps used to bounce a radar signal off the Moon.
On the morning of January 10, 1946, the team aimed the antenna at the rising moon and transmitted a series of radar signals. To their delight, the first successful echo detection appeared at 11:58 am EST, and the echo was received in exactly 2.5 seconds (Figure 4).—exactly as McAfee had calculated! Ten years later, the era of spaceflight began.
Figure 4: Oscilloscope display of the radar signals: transmitted signal on the left and radar return on the right. The arrow at right shows the distance in miles to the Moon.
Barry Manz is president of Manz Communications, Inc., a technical media relations agency he founded in 1987. He has since worked with more than 100 companies in the RF and microwave, defense, test and measurement, semiconductor, embedded systems, lightwave, and other markets. Barry writes articles for print and online trade publications, as well as white papers, application notes, symposium papers, technical references guides, and Web content. He is also a contributing editor for the Journal of Electronic Defense, editor of Military Microwave Digest, co-founder of MilCOTS Digest magazine, and was editor in chief of Microwaves & RF magazine.