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Fall 2003 Issue |
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Aircraft Enhancement Some Insights from Bistatic Radar Theory |
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This article is an abridged
version of a paper originally presented at |
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Aircraft enhancement is widely used on the east coast of Australia for VHF and UHF contacts in the 240 to 480 mile (400 to 800 km) range. Typically, for a few minutes it produces enhanced signals that are 20 to 30 dB stronger than would be expected, based on radar reflection or tropo scatter. The key difference between aircraft enhancement and normal radar reflections is that the aircraft must be closely in line between the two stations to achieve the enhancement. Interestingly, the phenomenon that is called aircraft enhancement by Australian amateurs is a manifestation of theories put forward by the French physicist Augustin Fresnel back in 1819, and the enhancement at light wavelengths is known as the Fresnel Bright Spot. This paper draws on the literature on bistatic radar (transmitter and receiver located a large distance apart) to give some insights into aircraft enhancement. Skolnik1 gives this example: For a sphere of radius ten times the wavelength, forward scatter is enhanced by 36 dB compared to back scatter as it applies to the more normal monostatic radar (transmitter and receiver co-located). A sphere of this size—40 meters in diameter at a wavelength of 2 meters—would present a much larger area than the largest aircraft. The example does show that large enhancements can be produced. In terms of a large aircraft, such as a 747 front on, bistatic radar theory shows that while the normal radar back-scatter area is only a little more than 100 square meters, the effective forward-scatter area at 2 meters is in the order of 30,000 square meters. At 70 cm the forward-scatter area can reach 240,000 square meters. I have applied the theory to simple shapes (sphere and sections, which approximate the wings, cabin, and tail of the example aircraft) rather than the complex shape of an aircraft. Nevertheless, I believe it does give some useful insights that help explain some of the observations of amateurs who have experimented with aircraft enhancement. For example, it explains significant signal enhancements, why larger enhancements might be obtained at higher frequencies, and why large enhancements only occur when the aircraft is close to the triangular alignment of the aircraft with the opposing two stations.
Information is given on the construction of a
simple model based on a map, tracing paper, and a drawing pin that allows
the prediction of aircraft enhancement from known flight paths.
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