The current developments in international politics, mainly the advent of rogue states attacking sovereign countries from air, causes a necessity of proliferation of cheap air defense solutions. Key part of air defense is the awareness, usually maintained by a network of ground radar stations. In the end of 50's, Czech Republic developed a passive radar system called PRP-1/Kopac (Korelacni Patrac, Correlation Seeker), which was later replaced by more advanced system Ramona and even more advanced Tamara. Then the Revolution came, bringing the inevitable international pressures that led to the bankrupcy of the Tamara developer company, following false indictments of its top managements which lead to revocation of the company's arms sale licence. Shortly after this, articles in the world press appeared about "groundbreaking" passive radar system being developed by - guess who? Lockheed. (After "15 years of research", good part of which consisted from reverse-engineering of seized shipment of I am not sure if Ramonas or Tamaras.) See also http://www.techtydenik.cz/tt1998/tt10/panoram5.htm The system allows locating and identification of aerial, ground, and (when installed on the shore) sea-based EM sources. The passive radar system consists from four main parts. Three are wideband receivers, listening for any characteristical transmitting activity. They talk to the fourth one, where a correlator is located - an electronic system calculating the position of the signal sources from the differences of times when the listening posts received their signals. The civilian sector electronics is developing fast; component prices fall down, computing power goes up, anybody can buy a machine that just few decades ago would make everyone in Pentagon salivating. Naturally, this opens interesting possibilities. The threat rogue states with overwhelming air force pose to other countries makes it a necessity to develop a cheap, open passive radar system, effectively bringing a key part of air defense down to easy affordability on a municipiality level. Let's call it GNU/Radar. We need the four stations: three listening ones, and the correlating one. The correlating station (CS) may be built as a MOSIX or Beowulf cluster. Its job is to handle signals from LPs, identifying the targets, and tracking their position. The listening posts (LPs) need a receiver - a suitably wideband one, a digitizer (a fast ADC card), optionally a DSP board to take some calculations off the shoulders of the CPU, a source of precise timebase for synchronizations (may be a GPS, which also provides information about the location of the listening post which is what the CS needs to know, or may be a receiver of a time synchronization signal broadcasted from somewhere if we want a backup for case of GPS being shut down. The receiver may be possibly adapted from the GNU/Radio project. The timing pulses can be also delivered optically, eg. by a modification in the Ronja unit mentioned later. The LPs crunch the received signals, isolate the interesting-looking ones, mark the precise moments of their reception, and send their arrival times and key characteristics to CS. The transmission channel may be anything with sufficient bandwidth - from an Internet leased line to Ronja-based 10Mbps optical links in case of direct visibility between LPs and CS. As an active twist, we can also use a separate unit, Illuminating Transceiver (IT), periodically broadcasting a pulse of known characteristics, easy to recognize by the LPs when it bounces from an aerial target. This unit has to be cheap and expendable - it's easy to locate and to destroy by a HARM missile. As a bonus, forcing the adversary to waste a $250,000+ AGM-88 missile on a sub-$100 transmitter may be quite demoralizing. There can be a whole hierarchy of ITs; when one of them transmits, the other ones sleep - when the transmitting one is destroyed, one of the sleeping units wakes up and continues in illuminating the airspace. This is within reach of capabilities of a simple microcontroller. Even other sources can serve as involuntary ITs. The landscape is littered with cellular base stations and civilian TV and radio transmitters. Just pick the suitable frequency and listen on. Remember that Kopac was built about 50 years ago, on vacuum tubes. It should be far from impossible to replicate it with contemporary COTS electronics. Using lower frequencies than the gigahertz band usual for modern military radars reduces accuracy, but also dramatically reduces the effectivity of aircraft stealth features. There are already prototype results in this field: http://www.wired.com/news/print/0,1294,16762,00.html Some other sources: http://ronja.twibright.com/ http://slashdot.org/articles/01/06/11/1617239.shtml Opinions, comments, ideas?