LONDON The Institute of Electronics, Communications and Information Technology (ECIT) at Queen's University Belfast has filed global patent applications for a very low loss quasi-optical frequency selective surface (FSS) that can simultaneously filter both horizontally and vertically polarised sub-millimetre wave signals.
The device will be used in a new generation of radiometers being developed to analyze polarized radiation in the Earth’s upper atmosphere in European Space Agency missions scheduled for 2018 to 2020. Previously spaceborne remote sensing instruments have only been capable of spatially separating either the vertically or the horizontally polarised components of naturally occurring thermal emissions from gasses in the Earth’s atmosphere – but not both together at the same time.
The dual polarized filter resolves this problem and will enable complex sub-millimetre wavelength imaging of clouds to be undertaken for the first time. The breakthrough is expected to yield important new insights into a range of phenomena including atmospheric composition, ozone depletion and the size of water particles in cirrus clouds.
Atmospheric scientists will use the improved data to produce more accurate global weather forecasts and to create a better understanding of climate change. One of the key challenges facing the ECIT design team is that the devices have to be strong enough to withstand the forces created during a space launch. In addition, they have to be extremely sensitive to detect falling ozone levels. ECIT engineers have overcome these challenges by developing a multilayer metalized silicon screen structure 1/100thmm thick and 30mm in diameter with 5,000 shaped slots, each etched to an accuracy of 1/1000mm.
The screens are separated by air instead of quartz which is used as the dielectric in conventional filters. This makes the new filters more robust and it increases the sensitivity of the radiometer by 50 per cent, enabling it to detect very weak molecular emissions and very low levels of ozone. ECIT has designed the devices to operate in the 250 to 360GHz range but beamsplitters operating at higher frequency ranges are also being developed. These include one designed to operate at 664GHz which will be the highest frequency dual polarisation FSS ever developed.
The work is being funded by the U.K. Centre for Earth Observation Instrumentation, the European Space Agency and EADS Astrium Ltd. ECIT is the main supplier of quasi-optical filters to the UK space industry and the European Space Agency. The institute designs and tests the devices using one of only two instruments in the U.K. capable of measuring the transmission response of filters at sub-millimetre wavelengths.
The devices are constructed by ECIT engineers and research staff at Queen’s University’s Northern Ireland Semiconductor Research Centre in Belfast. The dual polarised filter has been developed by lead engineer Raymond Dickie and reader, Robert Cahill. Dickie joined the high frequency electronics research group at Queen’s University in 2001 and completed a doctorate in Electrical and Electronic engineering specialising in Frequency Selective Surface technology.
Robert Cahill was employed by British Aerospace and Matra Marconi Space in Bristol from 1982 to 1999 when he joined the School of Electrical and Electronic Engineering at Queen’s University Belfast in 1999.