Wednesday, January 27, 2016

Over The Horizon

The video above is Iran's over the horizon radar system. It's a wideband signal heard on 10 meters with a varying bandwidth of 60 KHz to about 90 KHz. It was hear later the same year at 28000 kHz with a 300kHz bandwidth. These are messy and were received all the way to Chicago, IL. The Russian OTH radar in Gorodezh (near Nizhny Novgorod) also causes strong QRM splattering from 10125 up to 14 MHz. The signals are received so far away it's often mistaken for jamming. But it's not. Last year Matt Waid at ARRL wrote a good column on the IARUMS reported increase in OTH radar signals (among other Eastern military radio traffic) [HERE] he wrote:
"At least some of these intruders were likely to be audible in other parts of the world. Monitors in Europe reported a Russian over-the-horizon (OTH) radar in Gorodezh on 14.108 MHz, causing strong interference daily and often exhibiting splatter. In addition the Russian Navy was reported active frequently on 14.192.0 MHz using FM CW.  ...some of the worst offenders are OTH facilities in Russia and Iran. The signals can result in broad swaths of noise in the 20 meter band"

So let's discuss what OTH is all about. OTH systems are a type of radar system with the ability to detect targets at very long ranges. This can be over 500 miles away. while this is not over the horizon as you might think of it at sunset, this is beyond the radar horizon, (the distance limit for normal radar.) There are several OTH radar systems most of which were developed during the peak of the cold war over the 1950s and 1960. 

The technology exploits a long-understood artifact of AM radio propagation, skip. With OTH, the broadcaster generates a powerful shortwave signal from a large transmitting antenna which reaches a receiving antenna beyond the horizon by bouncing off the ionosphere. A return signal (echo) is sent back from the receiving station by the same route. All other systems use low-frequency radio waves that due to diffraction follow the curvature of the Earth to reach beyond the horizon. The ionospheric refraction is the more common application.

But this time of refraction has two big limitations. Firstly the angle is limited to about 2 to 4 degrees off the horizon. Second they require enormous antenna arrays to transmit at this angle so the stations are expensive and immobile. The first development on this was done in the Soviet Union Engineers starting in 1946 under the Veyer research Project. The word is a surname, a city and the word for 'fan' so it's significance is unknown since the project was ultimately cancelled in 1949. A working model was developed in the U.S. under Dr. William J. Thaler at the Naval Research Lab. Their first experimental system, MUSIC (Multiple Storage, Integration, and Correlation) went online in 1955 at Cape Canaveral.

Iran didn't get into the OTH game until 2013 with their Sepehr project. It is purported to have a range of over 1500 miles. Sepehr  is the Iranian word for "sky" which at least makes more sense than 'fan'.