The frequency bands covered by the VELOX are shown in the Table; the columns are channel number, centre frequency, and bandwidth, respectively.

nf0 (kHz)BW (kHz)

The selections have been designed to be as similar as possible to the multichannel receivers previously operated at Halley, and others which operate or have operated in Antarctica, e.g. the US AGO (`PENGUIN') VLF receiver.

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. Automatic gain control is used to achieve an overall dynamic range of 90 dB.

The following quantities, averaged over each of the frequency bands, are computed at intervals of 10 ms, where Hx and Hy are 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):

  • Scalar Power :- P = (|Hx|2 + |Hy|2)/2
  • Vector Power :-
    • Px = (|Hx|2 - |Hy|2)/2;
    • Py = Re(HxHy*);
    • Pz = Im(HxHy*)

In each 1 s recording interval, the parameters defined below are computed and digitally recorded.

  • Mean log power, i.e. the average of log(P) over the interval, which is proportional to the horizontal Poynting flux. 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, approximately 10 dB, 20 dB and 30 dB above the receiver noise level (3 kHz and 9.3 kHz channels only).

Channels 1-8 were implemented in 1992; the remaining channels and the impulse counters will be installed in 1997, as Phase 2 of the VELOX deployment.

* The polarisation is described by an ellipticity parameter, (-Pz/P), which takes values in the range +1 to -1. For a plane wave it is a function of the horizontal projection of the polarisation ellipse: zero for linear polarisation and ±1 for left-handed and right-handed polarisation respectively (in the southern hemisphere). The arrival azimuth parameter, ½tan-1 (-Py/Px), is in the range 0-180°; for a horizontally propagating plane wave it is the bearing (eastward of north) which would be measured by a goniometer, and has a 180° ambiguity.

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