Thanks to my geographic location and to the fact that i use a SDR transciever with panadapter/waterfall, it so happens that almost daily i get to identify an OTH signal somewhere in the HF spectrum, so I got a bit interested; and when i saw EI2KC's post earlier today about the incredibly strong OTH signal recieved by S57S just a few days ago, I thought I should put my thoughts in writing.
The most famous OTH radar is of course the soviet cold-war megalomanic station DUGA-3, the fully grown brother of the smaller and experimental DUGA-1 and DUGA-2. This mutant is located in the Chernobyl exclusion zone, the transmitter and reciever being about 60Km apart. The sight of such an installation, taken offline in 1989 and now deserted, brings chills down my spine.
The huge array of wideband Nadenenko dipoles and reflector plane coupled with the powerful transmitter managed to put out reported EIRP up to 40MW (!). The signal was about 40KHz wide and was pulse modulated usually at 10Hz (or rarely at 16 or 20Hz), moving across most of the HF spectrum depending on maximum usable frequency (MUF) and time of day. Two similar radars (based on backscatter, OTH-B) were built by the russians during the cold war: one at Kiev wich is also offline after the fall of the USSR, and another active one in Siberia, that watches over the North Pole/USA/Alaska. There is also another type of OTH radar (surface-wave, SW-OTH) near Vladivostok, but with just 64KW power output and about 300Km range, that watches over the sea in the Japan/N.Korea/S.Korea/China hotspot.
USA had a nice assortment of OTH radars as well, but with the end of the cold war their operations proved to be too costly and unjustified, so most of them were taken out of service and dismantled. There are still three AN/TPS-71 1MW ROTHR's (Relocatable Over-The-Horizon Radar) in service of the US Navy, located in Virginia, Texas and Puerto Rico, and used to intercept incoming drug traffic from South America.
France is also operating an experimental OTH-B radar called NOSTRADAMUS from the ex-NATO-frontline airbase of Dreux-Louvilliers, wich was developed in the 90's and started operating in 2005, but the complex design is still perfected. It uses a star-shaped antenna that allows 360deg. coverage, but the range is only about 1000Km, modest compared to the big-gun systems.
Australia uses a network of a control center, 2 transmitters, 2 recievers and 12 ionosondes called JORN (Jindalee Operational Radar Network) in order to have probably the highest performance active OTH-B radar, wich is used to monitor ship and airplane movement as well as weather conditions over a surface of more than 37000 sq Km. The official range is said to be around 3000Km, but it has successfuly detected chinese missile launches over 5500Km away. The complex sistem packed with modern technology allows it to achieve a much longer range than the old radars the americans are using with half the power (28 antennas, 20KW each), as well as detect stealth airplanes and ships.
China is one of the most mysterious places as far as this kind of equipments are concerned, international rumours are talking about 1 to 3 OTH-B radars operational starting with 1967, with one being online at the moment and another one in development near Guangzhou, using FMCW modulation (useful for Doppler effect calculations). There are also two FMCW modulated SW-OTH radars currently active in China, one of them watching over the sea towards Taiwan from the coast of Zhejiang province. Little is known about these equipments other than the fact they are heard on the air frequently and the national chinese electronics import-export company CEIEC alegedly tries to promote them for export.
One of the latest players in the OTH radar game is Iran, announcing a few years ago that they are building an OTH-B radar with a range of 3000Km; just a few months later, by the end of 2010, the commander of the Kathamolambia Air Base announced they have the capability to detect any flying object within a 3000Km radius and soon after ham operators all over the world started hearing the new Iranian eye in the sky with it's characteristic signal 60KHz wide, with 870 and 307 Hz sweep rates.
There are also a large numbers of links you can consult if you're interested in the subject, starting with the great work DK2OM is doing over at the IARU Region 1 monitoring system.