Frequency Selective Surface Filters
Queen’s University Belfast (QUB) and Astrium Ltd
Pathfinder Project (typically 6-12 months duration)
The project has developed a prototype sub-mm wave Frequency Selective Surface (FSS) which is required for a space borne radiometer instrument with a mission launch date in 2018. The maximum operating frequency of 664 GHz is more than 50% higher than other resonant FSS structures previously constructed for EO missions. The objective of this project is to demonstrate the UK’s capability in FSS design, fabrication, and spectral measurement techniques up to 700 GHz. This work builds on a previous CEOI funded study which was undertaken by Queen’s University Belfast (QUB) and Astrium Ltd which successfully developed FSS technology up to 456.7 GHz.
Spaceborne radiometers enable the retrieval of a wide range of geophysical parameters on a global scale. They operate by detecting thermal emission from the Earth’s surface and atmosphere. The wavelength of the thermal emission retrieved provides information on the observed geophysical phenomena. Detection of emissions at millimetre and sub-millimetre wavelengths enables the discrimination of cirrus cloud components intermediate between those accessible in the IR and microwave ranges. However current limitations in FSS and receiver component technology prevent full exploitation of this part of the electromagnetic spectrum. Consequently, realistic estimations of cloud parameters which are fundamental to the prediction of weather conditions are currently not well assimilated into Numerical Weather Prediction (NWP) models.
The next generation European platform for operational meteorology, Eumetsat’s Post-EPS mission, aims to address the deficiency in cloud measurements thereby improving the accuracy of global and regional NWP models, and contribute to fulfilling requirements common to Nowcasting and Very Short-Range Forecasting. Due to enter service in 2018 the mission incorporates two passive millimetre wave instruments for atmospheric sounding and imaging. The Microwave Imager (MWI) will provide size and shape information of ice particles in cirrus clouds through detection of orthogonal polarisations of radiation in spectral bands between 173 GHz and 671 GHz. Complementary missions, the Cloud-Ice Water Sub-millimetre Imaging Radiometer (CIWSIR) and the Geostationary Observatory for Microwave Atmospheric Sounding (GOMAS) have been proposed within ESA’s Earth Explorer programme. These missions are the subject of a common airborne demonstrator feasibility study with the aim of further defining the scientific and technical requirements prior to spaceborne missions.
FSS demultiplexing elements are a key enabling technology for these advanced instruments and are used in the quasi-optical receiver to spectrally separate the radiation that is collected by the antenna. This hardware topology enables many spectral channels to be incorporated in a compact instrument footprint. In order to satisfy the requirements for these missions, the following critical drivers for the FSS development have been identified:
(1) To significantly increase the frequency of operation and spectral measurement capability which is currently limited to 457 GHz for ultra low loss FSS.
(2) Provide additional FSS functionality to enable simultaneous filtering of orthogonally vertical and horizontally polarised signals at higher operating frequencies.
The main aim of the programme of work was to create for the first time a 664 GHz high pass FSS filter which is sensitive to both the vertical and horizontal polarised components of the incident signal. The filter specification satisfies the requirement for the MWI and CIWSIR/GOMAS spaceborne missions to separate the 664 GHz channel from four frequency bands in the range 183 GHz – 448 GHz.