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A new imaging riometer based on Mills Cross technique

Research output: Contribution to conferenceOther

Published

Publication date07/2003
Number of pages6
Pages26-31
Original languageEnglish

Conference

Conference7th International Symposium on Communication Theory and Applications (ISCTA 2003)
CountryUnited Kingdom
CityAmbleside
Period13/07/0318/07/03

Abstract

A new type of imaging riometer system based on a Mills Cross antenna array is currently under construction by the Ionosphere and Radio Propagation Group, Department of Communication Systems, Lancaster in collaboration with the Max-Planck-Institut für Aeronomie, Germany. The system will have an unprecedented spatial resolution in a viewing area of 300x300km. It is located at Ramfjordmoen, near Tromsø, Norway. The riometer (relative ionospheric opacity meter) determines the radio-wave absorption in the ionosphere by measuring the received cosmic-noise power. The expected variation of background noise over a sidereal day is usually referred to as the quiet-day curve (QDC). The ionospheric opacity is deduced from the difference between the QDC and the received noise power. Absorption images may be produced by utilising a number of spatially-distributed narrow beams. The Mills Cross system considered in this paper provides at least 4 times the resolution which can be achieved (with the same number of antennas) with a filled array antenna system. However, the cross correlation technique employed for producing narrow pencil beams requires information on both amplitude and phase of the signals to be cross correlated in contrast to other existing imaging riometer systems that record only signal power. This adds a considerable amount of complexity to the system which requires the use of state-ofthe- art FPGA signal processing technology. The system design and specification will be presented. While the application of correlation technique allows an increased spatial resolution—even for the same number of antennas used — it leads at the same time to an increased noise level in the measurements with adverse effect for the minimum integration time. For filled arrays the integration time can be as low as 1/8s; for a correlation system the integration time will be at least some seconds to achieve comparable uncertainties. First experimental observations have confirmed this. The measurements also indicated that antenna sidelobes introduce phase delays that result in signal reduction/increase especially in the presence of a strong noise source (radio star). To what extent this has an adverse effect on the QDC is being investigated. Adaptive beam steering and tapering techniques are being analysed to minimise the effects of the sidelobes. The results of these investigations will also be presented.