A VLF goniometer (direction-finding) receiver, essentially similar to that described by
Bullough and Sagredo (1973) but see also
Smith et al. (1979), is operated on a synoptic basis.
The two broadband signals from the north-south and east-west channels of the preamplifier/aerial loops
are recorded on the two channels of digital audio tape (DAT), with a bandwidth up to 22 kHz and dynamic
range of about 90 dB. Recordings are normally made for 1 minute every 15 minutes (05-06, 20-21, 35-36, and
50-51 minutes past the hour). At times of unusual activity or for special campaigns, alternative
schedules of 1 minute recording every five minutes, or of continuous recording, can be run.
A time code (IRIG-B) is also recorded on the tape. Control of the tape recorders is
through the AVDAS (Advanced VLF Data Acquisition System)
(Smith et al. 1994). The data tapes are analysed
using the interactive spectral display provided by the AVDAS. The arrival azimuth can be determined
from the relative amplitude and phase of the north-south and east-west spectra.