The VLF at Dunedin, New Zealand, and Halley (the receivers now at Halley were formerly at
Faraday station, now the Ukrainian Vernadsky station) receive ducted whistler mode signals from three
US Navy transmitters: NAA, 24.0 kHz at Cutler, Maine; NSS, 21.4 kHz at Annapolis, Maryland; and NLK, 24.8 kHz
at Seattle, Washington. The narrow-band receivers of the type described by
Thomson (1981) are able to separate the whistler mode signals
from the stronger sub-ionospheric signal, and measure the group delays (tg), Doppler shifts
and arrival bearings of the whistler mode component. Results from these experiments have been published by
Smith et al. (1987),
Saxton and Smith (1989), and Clilverd et al. (1992).
The method of determining the group delays, L-shells and propagation paths of the whistler mode signals is discussed in detail by Smith and Clilverd (1991) and Clilverd et al. (1991). The arrival bearings of the whistler mode signals are determined from the relative strengths of the signals in two orthogonal vertical loops at each site together with phase comparisons with the signal from a vertical electric dipole antenna (Thomson, 1985; Strangeways and Thomson, 1986).
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