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440-L over-the-horizon forward scatter radar (OTH-F)

Over the horizon (OTH) radar, as its name implies, was conceived and developed to detect and track targets beyond the horizon. Conventional radars, by contrast, can detect targets only within line-of-sight. In 1946, scientists at the Army Air Forces' (ancestor of the USAF) Cambridge Field Station (earliest progenitor of RYH) invented a radar concept in which radar beams were reflected off the ionosphere to the earth’s surface, then back to the ionosphere, and so on.

During the late 1940s the Army Air Forces’ Watson Laboratories used such ionospheric “propagation paths” to send signals between Boston, MA and Puerto Rico, and between Boston and the White Sands, NM, Proving Grounds (The Army Signal Corps transferred Watson Laboratories to the Army Air Forces in 1945). A critical observation that enabled OTH was Doppler shifts (changes in the frequency of the radar beam caused by motion of the targets) detected in the returns of the laboratory’s radar ionospheric propagation during V-2 rocket launches at White Sands.

The observation eventually led to the effort known as Project 440L. Over-the-Horizon Forward Scatter Radar, or 440-L radar, was developed during the 1960s to detect missile launches from Chinese or Soviet territory. The 440-L was a series of high frequency radio transmitters and receivers on either side of the Sino-Soviet landmass that produced continuous signals that bounced between the ionosphere and the surface of the earth until it reached the receiving stations. Disturbances in the pattern indicated that (intercontinental) missiles were penetrating the ionosphere.

Following World War II, Americans working at the US Naval Research Laboratory experimented in HF radar with ionospheric propagation in mind. They recognised that, for such a radar to be useful, discrimination of targets against the inevitable ground return—clutter must be provided through signal processing. Target movement against the stationary background allows separation of target and ground returns on the basis of the Doppler effect.

Christian Doppler first analysed this effect in 1842. A moving source or scatterer of waves, be they acoustic or electromagnetic, causes a received signal to vary in frequency depending on the motion of the source or scatterer. The rise in the apparent note of a vehicle horn as it approaches and the fall as it recedes is the acoustic analog of the effect used for radar discrimination of moving targets.

But for the Doppler technique to be applicable to ionospheric radar, the ionosphere must be stable enough not to confuse small frequency-shifted signal returns. Thus, research into ionospheric stability was an essential step in the development of OTHR. Early experiments failed to produce better results. Certainly the average power from the pulse transmitter was not large. And undoubtedly a major factor was the relative unsophistication of the signal generation and the receiver, which relied exclusively on analog processing techniques. Advances in both areas were required before the real potential of OTHR could be realised and these advances relied on a digital approach to signal generation and processing. Advances in digital computing were a prerequisite for further progress and it was still very early days in the digital revolution.

The US Naval Research Laboratory work on OTHR isolated the computing requirement as the major problem area and had pushed ahead a dedicated processor design. This involved a magnetic drum integrator. As Headrick and Skolnik of the US Naval Research Laboratory have subsequently remarked ?The (digital) signal processor has been the key element in the success achieved with OTHR".

Through the late 1950s then classified work proceeded to demonstrate ionospheric stability and, towards the end of 1961, resulted in the first OTHR detections of air targets. For a continuous wave radar, the transmitter and receiver need to be separated. It cannot receive faint echoes at the same location from which it was transmitting a high power signal. A receiver separated from the transmitter needs some way of knowing what was transmitted so that comparisons can be made with signals returned. The problem was that, up until the early 1960s, signal sources were simply not good enough: the stability of sources was such that the ionospheric and Doppler effects being studied would be lost in the random drift and fluctuations between the signals from two sources.

Extraordinary ionospheric effects are induced by the introduction of rocket exhaust molecules at high altitudes during launch. The well-known SKYLAB effect, as investigated by Mendillo, Hawkins, and Klobuchar [1975], is a case in point. In this study Faraday rotation data for times including the SKYLAB launch were used to characterize the induced electron depletion region (i.e., ionospheric "hole") and to help identify the essential chemical mechanisms responsible for it. Booster rocket emission of such molecules as H2O and H2 in the upper atmosphere, say above 250 km where monatomic species are dominant, increases the probability of electron loss dramatically.

In the United States, the US Air Force in the 1960s fielded an operational ICBM launch detection system to give as much early warning against an attack to the nation as possible — an over-the-horizon forward scatter radar system (OTH-F) known as Project 440L. The 440-L system consisted of radio transmitter stations and receiving stations in Europe and the Pacific on either side of the Soviet-Chinese landmass. The four transmitters in established a high-frequency curtain at low grazing angles across the top of the Sino-Soviet landmass from Europe to the eastern periphery of the Pacific. Apparently reliable sources diagree as to which end had the transmitters, and which end had the receivers, and very little visual evidence of this rather rudimentary system seems to have survived.

OTH-F Receiver Sites (each with AN/FSQ-76):
R-1 Cyprus
R-2 San Vito, Italy
R-3 Aviano, Italy
R-4 Rothwesten, Germany
R-5 Feltwell, England
OTH-F Transmitter Sites (each with AN/FRT-80):
T-1 Wallace AS, P.I.
T-2 Awase, Okinawa
T-3 Tokorozawa, Japan (Hq. site for Asian operations)
T-4 Chitose, Japan

The net consisted of four high frequency AN/FRT-80 transmitters located in the Philippines, Okinawa, and Japan (2) and five AN/FSQ-76 receivers in Cypress, Italy (2), Germany, and England, with a correlation center in Aviano, Italy connected to the NORAD Combat Operation. Center in Cheyenne Mountain. The system was to be designated 440L. Using the ionosphere and the earth as a gigantic wave guide over the Soviet Union, the transmitters would generate a continuous radar signal which would be constantly monitored by the receivers in Europe.

Continuous signals from the transmitters were bounced off the ionosphere and then repeatedly back and forth between the ionosphere and the surface of the earth until they reached the receiving stations. There the receivers detected perturbations or disturbances of the transmissions caused by missiles penetrating the ionosphere under active-boost propulsion. Interruptions in the signal caused by ballistic missile launches would be detected by the receivers which would pass this information to the correlation center in Aviano, Italy, where the data would be processed to provide the time and inclination of launch. Processed data would then be flashed to NORAD.

The system provided nearly realtime (five-to-seven minutes from launch) detection of missiles launched from the USSR and China (also satellite launches and nuclear detonations), with time-of-launch and rough estimates of the launch location and type and number of missiles.

The 440L system became operational on New Year’s Eve 1965 but for a time was beset with operational difficulties which impacted upon its ability to detect launches. By 1968 these problems were not all solved and the detection rate was very low. For instance during one period 7–14 June 1968 the system was only able to detect 20 percent of the launches that actually occurred. As a result, extensive revision of the operating procedures was undertaken which resulted in a significant improvement in the number of launches detected.

In 1966 and 1967, it demonstrated a high-order capability by successfully detecting and reporting 94 percent of all Soviet ICBM test launches (198 of 210), including all 10 FOBS tested in 1967, and plans were accelerated to introduce it as a working system. It became operational in I968. The 440-L, was capable of detecting solo-launched missiles within minutes after launch. It was aimed at the threat from the Soviet fractional orbital bombardment system (FOBS), a low-altitude missile that would cut the BMEWS fan at low altitude and thus reduce the time from cutting the fan to impact to as little as five-to-seven minutes.

In March 1975 the Over-the-Horizon Forward Scatter Radar System (440L) ceased operations prior to inactivation of sites and units.

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