Description of the code ANVIL (ANisotropic Vhf impulse location)

We have developed a general, three-dimensional method to locate sources of earth-directed radiation that takes into account the flight paths of reflected signals. While time-of-arrival algorithms exist for locating radio sources using line-of-sight propagation paths, radio sources originating above the surface of the Earth will not necessarily emit strong power along direct paths to satellites. A combination of direct and ground-reflected pulses or only ground-reflected signals from such sources could be received by satellite-borne sensors. The work presented here applies to satellite detection of subionospheric radio sources in a vacuum environment with ideal reflection off the Earth's surface. Because satellites are not static receivers, their configuration with respect to a radio source is not always optimal. Therefore, a statistical study is performed using 1,000 randomized configurations of 24 satellites in middle Earth orbit. For each configuration, the radio source latitude and longitude is fixed, and its altitude is varied from 1 to 97 km. An analytic direct-path algorithm using the five satellites nearest the radio source provides an initial guess for the radio source latitude and longitude. We find that, using this approximation, the mean error in the calculated nadir location has a maximum value of 134 m for the 97-km source altitude. The coordinates of the initial guess are used to define the origin of a grid within which an all-points search refinement is performed. Using this procedure, the overall maximum mean error in radio source position is found to be on the order of the computational grid size.