Message posted by Ron Milione on February 16, 2010 at 9:37:42 PST:

RADAR has a long history starting with the early experiments by Hulsmeyer in 1903 of scattering of radio waves off of ships followed by his patent in 1904. However, the German military did not support the developments due to the immaturity of radio wave components at that time. As Skolnik points out, RADAR did not generate much scientific interest until the 1920s and 1930s. The experiments of Marconi and developments at the Naval Research Lab demonstrated the ability to detect ships on the surface and aircraft in the air. Both long wave and microwave RADAR were being developed in Europe and the US. By the start of the Second World War, the technology was sufficient to detect aircraft and ships at long ranges from ground installations. Soon aircraft were being provided RADARs for all weather detection of air and ground targets. However, the ground clutter was a significant problem to early airborne RADARs, and no real attempts were made to image or penetrate this obscuration for detecting objects.

Battlefield surveillance RADAR was started in early 1960s with the development of the OV-1 APS-94 Side Looking Array Radar (SLAR) by the US Army, for detecting military encampments and large groups of artillery and mechanized vehicles on the battlefield. In the early 1970s the Army determined that there was also a need for detecting large numbers of moving vehicles, at a significant range from the forward edge of the battle area (FEBA). The first ground moving target indication (GMTI) system for battlefield surveillance was developed as the Stand-Off Target Acquisition System (SOTAS). It was constructed using the APS-94 radar with a moving target mode, and operated from an UH-1 helicopter. The helicopter was necessary to minimize the platform motion and provide sufficiently low minimum discernable velocity detections over a wide area. The SOTAS prototype was tested in the United States and in Germany under the Reforger Exercises, as shown in Figure 1 2, and was accepted by military leaders as a capable operational capability. Because of the demonstrated importance of detecting slow moving troops at long distance, the Army started to develop an operational system to be installed on a Black Hawk UH-60 helicopter. The Blackhawk had a bigger payload and longer endurance than the UH-1, but the system development was stopped in 1978 by the Secretary of Defense due to the need for longer endurance and more survivability of a manned platform.

The battlefield surveillance capabilities of SLAR and SOTAS soon led to the development of the Joint Surveillance and Target Acquisition system (JSTARS) for use by the Army and Air Force. The JSTARS standoff battlefield surveillance capabilities could be integrated on a high-altitude, multi-engine aircraft for longer endurance and significantly longer standoff for survivability. The benefits of JSTARS combining SAR and GMTI on the battlefield are extensively documented, and have been reproduced and fielded on many international platforms. All of these early battlefield surveillance RADAR systems were developed in the microwave frequency band. Microwave frequencies were important to provide all-weather, long-range, high probability of detection of vehicles and structures, and be small enough that they could be carried on tactical aircraft.

However, there was one important operational issue -- the adversaries understood X-band RADAR’s limitations to see through forest cover. Tactics were being developed to deny these radars the ability to image the movement and location of ground forces. Hiding in tree lines and using other forms of camouflage and concealment quickly countered microwave RADAR. This countering tactic was becoming as effective as demonstrated against early optical surveillance. There was a need to develop the ability to detect fixed and moving targets under foliage as a complement to the very capable microwave battlefield surveillance RADAR systems.

The first application for foliage penetration (FOPEN) RADAR was during the Vietnam conflict; where early systems were developed to detect and recognize ground moving targets. Specifically, there was a compelling need to detect and locate insurgent soldiers walking through the dense tropical forests. Two innovations were needed: coherent waveforms and signal processing, and RADAR installations on tall hills and masts. These two features reduced both the foliage loss and clutter motion that masked the small returns from personnel and vehicles. However it did not provide the ability to detect manmade objects that were stationary. A parallel development of FOPEN Synthetic Aperture Radar (SAR) was needed to detect manmade objects under the trees. The required innovation for foliage penetration SAR was wideband image processing and coherent discrimination of man made objects from the background clutter.

More to come....

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