Two orthogonal horizontal components ( north-south and east-west) of the VLF/ELF wave magnetic field are received by two vertical single-turn 56 m2 loop antennae, and amplified by twin low-noise preamplifiers. The frequency range is 0.1-25 kHz. The aerials and preamplifiers are located sufficiently distant (1.8 km) from Halley station, that locally generated electromagnetic noise is negligible over the whole frequency range. Sensitivity (typically 10 -31 T2 Hz -1 at 5 kHz, equivalent to a wave power flux of 2.4 × 10 -17 W m -2 Hz -1) is normally limited by global thunderstorm noise (radio atmospherics) rather than receiver system noise. Calibration is achieved by simulating a signal of known intensity, frequency, and arrival direction, through control of the amplitude and phase of a.c. currents flowing in two small orthogonal coils mounted at the centre of the loop aerials. Short (1 s) 5- frequency calibration tones are injected once a minute, with longer tones on the 10 minutes past the hour ( 3 s) and on the hour (10 s). The broadband signals from the preamplifiers are inputs to the specialised receivers described below:
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