VLF/ELF Multichannel Receiver
A new multichannel receiver - VELOX (VLF/ELF Logger Experiment) - deployed at Halley in
January 1992, superseded both the multichannel system described by Smith and Yearby (1987),
which formed part of the Mk-1 AVDAS, and the VLF/ELF channels of the Real-time Antarctic
Logger Facility (RALF) system (Gough et al., 1987). The frequency bands covered by the
VELOX are shown in the Table. The selections have been designed to be as similar as
possible to the multichannel receivers previously operated at Halley, and with certain
other receivers which operate or are planned to operate in Antarctica. The latter include
the Stanford University ULF/ELF/VLF Radiometer (Fraser-Smith and Helliwell, 1985) and the
US AGO (`PENGUIN') VLF receiver (Inan, private communication, 1989).
For channels 1-9, each of the broadband preamplifier outputs is anti-alias filtered, and
digitised to 12 bits at a 512 kHz rate. Filtering into the selected bands is achieved
digitally. For channel 10, a separate tunable receiver is provided (not implemented at
the time of writing). Automatic gain control is used to achieve an overall dynamic range
of 90 dB.
Scalar power (P) and vector power (Px, Py, Pz), averaged over each of the frequency
bands, are computed at intervals of 10 ms, using the complex Fourier transforms of the
signals received by the loop aerials (proportional to the east-west and north-south
components respectively, of the horizontal wave magnetic field).
In each recording interval (programmable in the range 1-60 s, but normally 1 s), the
parameters defined below are computed.
- Mean log power, i.e. the average of log(P) over the interval. This
parameter is preferable to mean power for ground data, since it discriminates against
intense but impulsive terrestrial atmospheric noise from thunderstorms, thus giving more
weight to magnetospheric sources.
- Mean vector power, i.e. the average of (Px, Py, Pz) over the interval.
Data contaminated by large spherics are excluded from the averages. Polarisation and
arrival azimuth are computed from the direction of the vector. For perfectly coherent
signals only, the magnitude of the vector is proportional to the mean power.
- Peak value of P in the interval (10 ms time constant).
- Minimum value of P in the interval (100 ms time constant).
- Impulse counts: Counts of the number of times in the interval that P exceeds
3 thresholds (3 kHz and 9.3 kHz channels only). (Not implemented at the time of writing.)
- Digital data are recorded on an optical disk. Further processing yields the small
set of GGS key parameters (see Table) for this instrument.
More information and data.
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