Radio waves

75 years ago, the first American moon shot used radio waves

America’s first “moon shot” succeeded 75 years ago today. For the first time, a team of US Army engineers has bounced a radar signal off the surface of the Moon and captured its echoes here on Earth. The effort, called Project Diana, laid the foundation for radio communications with spacecraft, radar missile defense systems and surface mapping of our closest planetary neighbors.

In the aftermath of World War II, the world was counting on the chilling implications of destructive new types of warfare, especially long-range missiles like Germany’s V2 rockets and atomic bombs like those the United States dropped on Hiroshima and Nagasaki in Japan.

“During the war the Germans used the V2 rocket, which soared about 70 miles above the Earth, and the future holds the unfortunate prospect of missiles going much higher than that,” wrote Jack Mofenson, Diana Project researcher. Mofenson was right; modern intercontinental ballistic missiles explode upwards of 1,200 miles before collapsing towards their targets. As Mofenson put it, “The question of transmitting radio signals at great distances above the earth for the detection and control of such weapons becomes a problem of military significance.”

So the War Department (which was reorganized and renamed the Department of Defense in 1949) wanted to know if radar could help detect missiles plunging to Earth from space. But as far as scientists knew at the end of 1945, any ballistic missile re-entering Earth’s atmosphere would have a natural cloaking device: the outer layer of the Earth’s atmosphere, where ultraviolet radiation from the Sun strips electrons from molecules , leaving a mixture of spurious ions and electrons. surrounding the planet. This layer, called the ionosphere, begins about 50 miles from the surface and extends a few hundred miles into space.

The ionosphere is where the dancing lights of the northern Lights and aurora australis happen. All of this electromagnetic activity and solar radiation also has strange effects on radio waves, which are the basis of radar. In general, the ionosphere tends to scatter radio waves; sometimes if the transmitter is at just the right angle, it means a signal from one location can bounce off the ionosphere and reach a radio receiver on another continent.

(Note: About 30 years ago, your trusty pen pal’s father was using a handheld radio during his shift at a Houston, Texas-area stationery store one night when he accidentally ran into a deputy sheriff somewhere going to Arizona. what are you doing on this channel?”, they both understood and had a good chat.)

Since the ionosphere tends to reflect and also scatter or refract radio waves, especially short wave or high frequency waves, engineers and scientists in 1946 weren’t entirely sure that the radar could “see” through the layer to detect an incoming missile. .

In the 1920s, a team of researchers bounced a radar signal off the lower edge of the ionosphere to measure its altitude. Now the Pentagon was asking its scientists to aim even higher. The math suggested it was theoretically possible, but with the Cold War already looming on the horizon, theory wasn’t enough.

Beginning in September 1945, just a month after the war ended, Colonel John DeWitt, Jr., and his team began building the massive radio transmitter, receiver, and antenna array they would need to work. They used a mixture of purpose-built parts and modified equipment left over from World War II. Camp Evans, where DeWitt became director of the Army’s Evans Signals Laboratory, was a patch of land with a checkered history.

In 1914, a few months before the start of World War I, the Marconi Wireless Telegraph Company set up the Belmar receiving station for transatlantic messages. Marconi strung a mile of bronze antennae, supported by 400-foot-tall towers, along the Shark River near Wall Township, New Jersey. During the war, the U.S. Army acquired the land and built a Signal Corps camp there; When the war ended, Marconi regained the upper hand and sold the land to RCA – who eventually sold it to a group that turned out to be the New Jersey chapter of the racist terrorist organization, the Ku Klux. clan.

The army regained control of the land in 1942, and after the war Camp Evans became the headquarters of Project Diana.

During the fall and winter of 1945, engineers and physicists built their lunar radar station and discovered the mathematics that would make it all work. Objects in space are constantly moving relative to each other. As a radio signal reached the Moon, reflected off the lunar surface, and bounced back, the changing distance between the Moon and Camp Evans stretched the waves to a different frequency. (This is called the Doppler effect.)

DeWitt’s team had to calculate this effect each time they aimed a burst of radio waves at the Moon, because they needed to know which frequency to listen for returning echoes.

They also had to find the right time: their antennae pointed towards the horizon and could only rotate in two dimensions, so the Diana Project team had less than an hour a day, split between sunrise and sunset. moonset, to try to photograph the Moon. It still gave them a lot of chances. DeWitt and his team sent a 0.25 second pulse of radio waves to the Moon once every five seconds. Radio waves traveling at the speed of light take about 2.5 seconds to travel the roughly 239,000 miles to the Moon, then another 2.5 seconds to make the return trip.

And at 11:58 a.m. Eastern Time on January 10, 1946, a radio pulse that DeWitt’s team had transmitted five seconds earlier returned to their receivers — after echoing off the distant surface of the Moon.

Thanks to Project Diana, we now have radar installations capable of detecting an incoming ICBM (or tracking Santa’s sleigh every Christmas Eve). We can also use radios to communicate with astronauts in space or send instructions to robotic spacecraft like Voyagers 1 and 2, the Lunar Reconnaissance Orbiter and the Curiosity Rover. Project Diana also paved the way for radar astronomy, which uses reflected radar signals to study the shapes, rotation and surface properties of objects in space, ranging from near-Earth asteroids to the rings of Saturn. and on the surface of Venus.