The effect of snow accumulation on imaging riometer performance

In January 1998 an imaging riometer system was deployed at Halley, Antarctica (76°S, 27°W), involving the construction of an array of 64 crossed-dipole antennas and a ground plane. Weather conditions at Halley mean that such an array will rapidly bury beneath the snow, so the system was tuned to operate efficiently when buried. Theoretical calculations indicate that because the distance between the ground plane and the array was scaled to be 1/4λ in the snow, as snow fills the gap the signal will increase by 0.6–2.5 dB. Similarly, the short antennas are resonant when operated in snow, not in air. Theoretical calculations show that the largest effect of this is the mismatch of their feed point impedance to the receiver network. As the signal for each riometer beam is composed of a contribution from all 64 antennas, for each antenna that buries the signal level will increase by 1/64 of ∼9 dB. The measured response of the system to burial showed significant changes as snow accumulated in and over the array during 1998. The changes are consistent with the magnitude of the effects predicted by the theoretical calculations. The Halley imaging riometer system, having now been buried completely, is operating more efficiently than if a standard air-tuned configuration had been deployed. The results are of considerable relevance to the ever-increasing community of imaging riometer users regarding both deployment and the subsequent interpretation of scientific data. Some systems will experience similar permanent burial, while others will be subject to significant annual variability as a result of becoming snow-covered during winter and clear during summer.

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