The effect of snow accumulation on imaging riometer performance

In January 1908 an imaging riometer system was deployed at Halley, Antarcti ca (76 degrees S, 27 degrees 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 indi cate that because the distance between the ground plane and the array was s caled to be 1/4 lambda in the snow, as snow fills the gap the signal will i ncrease by 0.6 2.5 dB. Similarly, the short antennas are resonant when oper ated in snow, not in air. Theoretical calculations sinew that the largest e ffect of this is the mismatch of their feed point impedance to the receiver network, ns the signal for each riometer beam is composed of a contributio n from all 64 antennas, for each antenna that buries the signal level will increase by 1/64 of similar to 9 dB. The measured response of the system to burial showed significant changes as snow accumulated in and over the arra y during 1998. The changes are consistent with the magnitude of the effects predicted by the theoretical calculations. The Halley imaging riometer sys tem, 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 riomete r users regarding both deployment and the subsequent interpretation of scie ntific data. Some systems will experience similar permanent burial, while o thers will be subject to significant annual variability as a result of beco ming snow-covered during winter and clear during summer.