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Okay, the title of this instructable is a little bit misleading. We’re not going to listen to RADAR signals from space (which would require a giant transmitter in outer space), but RADAR signals transmitted from Earth which have bounced off of something in space. That something could be a satellite (the International Space Station, Hubble Space Telescope, etc.), a piece of space junk, a meteor as it enters the Earth’s atmosphere, an asteroid passing too close to the Earth for comfort (but at least passing safely instead of hitting the Earth), or even the moon.
The Air Force Space Surveillance System consists of three extremely high power RADAR transmitters in a East-West line across the southern United States. Formerly known as the NAVSPASUR (Naval Space Surveillance) when it was operated by the Navy, these transmitters are primarily used to track satellites in orbit around the Earth. Besides the three transmitters there are six receiving stations but there’s nothing to stop anybody else from listening in. There are over 10,000 objects tracked, basically anything larger than a 4 inch bolt. The only objects in low Earth orbit which aren’t tracked are ones which are only in space for less than a handful of orbits (so their orbits never pass over the Southern United States), a handful of extremely unusual satellites which are in orbits which hug the equator and are too far South, and objects which are in higher altitude orbits which result in RADAR pings too small to detect. The space fence was one of the most important assets for tracking the “Mystery Object” which “fell off” of the space shuttle Columbia during its final mission where it broke up during reentry.
The three transmitters are located at Jordan Lake, Alabama, Lake Kickapoo, Texas, and Gila River, Arizona. All three sites are at roughly 32 degrees North latitude. The Lake Kickapoo site is the world’s most powerful continuous wave (CW) radio transmitter, which a whopping 768 kilowatts on 216.983 MHz.
Besides the space fence the Air Force also operates a worldwide network of RADARs with tracking antennas, automated telescopes, and sensors in space. There are plans to upgrade the space fence from VHF to S-Band in order to increase the resolution and accuracy.
In 1994 NASA actually put some intentional space debris into space to help calibrate the Air Force’s worldwide tracking network. Metal spheres were used, both shiny and matte surfaced because a sphere looks the same from any angle and a couple of dipoles (simple passive radio antennas which look like long wires) to provide a sharp edge for the RADAR signals to bounce off. One of the RADARs involved was the MIT Haystack observatory, making this the world’s first case of using a haystack to search for a needle!