Radio interferometric location finding of VLF signal transmitters

The article presents some experimental results of using a very low frequency (VLF) interferometer designed in INRTU for radio direction finding of VLF transmitters. It is evaluated that amplitude angular-position measurements provides azimuth accuracy of several degrees. Distant VLF stations (3000 - 5500 km) can be located in a 2-6 km area if the signal azimuth is known with 0.001° accuracy. In order to minimize errors associated with antenna’s angular offset the authors developed a correcting algorithm. Using both radio direction finding methods and additional geophysical data (year and daily variations of solar terminator moving, signal disturbances during solar flares) the authors ascertained that 24.1 kHz and 25.0 kHz signals are emitted from the transmitter in Mokpo (South Korea). The phase measurements give about two times lower error of 1.5-2° for the NWC station (19.8 kHz, Australia) which is located more than 8200 km away from the VLF interferometer.