A Frequency-Tracking Direction Finder for Whistlers and Other Very Low Frequency Signals
STANFORD UNIV CA STANFORD ELECTRONICS LABS
Pagination or Media Count:
Whistlers and related very low frequency radio signals are guided in ducts of enhanced or reduced ionization along the geomagnetic lines of force of the earths magnetosphere. The signals convey information about the distribution of particles in the plasma through which they have propagated and about the occurrence of wave-particle interactions in the magnetosphere. Direction-finding on such signals will aid in locating the ducts and measuring their temporal drifts, thus making an important contribution to studies of magnetospheric convection. The signals, although narrowband in nature, exhibit wide frequency excursions in the 1 to 10 kilohertz range. An innovative technique is presented for tracking these frequency excursions in real time to produce a filterred, quasimonochromatic version of the signal. The voltages induced by the incident wave on two orthogonal loop antennas and a vertical monopole antenna are processed by this method. The filtered signals are then cross-multiplied in a manner analogous to a Poynting vector calculation to obtain a continuous indication of the wave direction of arrival. The design, construction, and laboratory testing of a prototype instrument are described. Field testing of the instrument was performed at Stanford, California, and Roberval, Quebec, Canada. Operating at fixed frequencies, the direction-finder produced accurate results on VLF transmissions in the 10 to 20 kilohertz range from NAA, NPG, GBR, and the Omega stations.
- Atmospheric Physics
- Direction Finding
- Radio Communications