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Despite the extremely small radar signature of the plane a great amount of time goes into routing the flight path of each stealth fighter to minimize its exposure to known threats such as surface-to-air missile batteries and radar. At certain points in a mission however the stealth fighter can become quite visible to enemy radar. Turning suddenly at low altitudes can reflect radar waves right back to the radars that sent them. The most visible and risky moments are when the bomb bay door is open - briefly making the plane un-stealthy. Military officials suspect that Serbian spotters, perhaps starting with agents in Italy watching the F-117's take off, were able to determine a time table of how long it took the planes took to cross the Adriatic and from there on to Belgrade. With this information Serbian radar operators had a better sense of when and where to watch for the plane. Once the aircraft's radar reflection crossed their screens all the radar operators would be alerted down the line. When the F-117 dropped one of its two laser guided bombs on a target near Belgrade this probably provided the final clues to fix its position and fire several missiles.
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Radar is simply an application of radio, and in today’s wireless age, radio waves surround us. In particular, in industrialized nations, cell phone towers can be found every few miles, sometimes every hundred feet. The Roke Manor researchers have envisioned the use of cell phone towers as a extremely dense network of radar transmitters and receivers, interconnected via communications links. The sheer number of cell phone towers makes detection much easier than with solitary radar sites. If you have bistatic radars, in particular a very large number of sources, so that you excite the target from a wide range of angles, and you have a multiplicity of receivers in many locations, you essentially will get around the stealth target’s redirection capabilities. It is highly likely that an incident wave from a cell tower will be redirected towards one or more receivers.
Having gotten around the stealth aircraft’s redirection capabilities, the system then puts together all the data from the cell phone towers. Until recently, this was not possible. However, increased computational power and advanced signal processing techniques have made it possible to sort through all the signals and form a coherent radar picture. Ironically, the further development of the same computing technology that originally made stealth possible has now made it possible to detect stealth aircraft.
“If you can get a radar return, you can get all kinds of information from the return signal if you can process it sufficiently,” Hansman said. “For example, if you an look at the Doppler shift of the returned signal, you can get aircraft velocity. If you are sensitive enough, you can see frequency effects, such as engine rotation or structural vibration. If you have several receivers or different imaging angles, you can begin to reconstruct an image of the target.”
With velocity and shape information, as well as software specifically designed to detect the inconsistencies that give away a stealth airplane, it becomes considerably easier to separate planes from birds in the sky.