Passive Sounding of the Atmosphere

Passive remote sensing of the atmosphere from space at millimetre and sub-millimetre wavelengths is directed towards the investigation of processes linking atmospheric composition and climate and their assimilation into operational systems used to forecast weather and in the future, air quality. Further improvements of radiometer technologies in terms of sensitivity, frequency performance and resource use is crucial to the improvement of applications in the terahertz spectral region.

The work being undertaken in CEOI is designed to address this issue, so that the UK will be well positioned to exploit future millimetre and sub-millimetre radiometry in programmes already proposed for both EU/ESA GMES Sentinel and Eumetsat post-EPS satellite missions.  Specifically developments have focused on new, critical technology for two distinct, but complementary instrument developments.

1. STEAM-R is a passive microwave limb sounder, proposed by Sweden as a nationally funded contribution to the PREMIER mission. STEAM-R is dedicated to the investigation of chemical, dynamical, and radiative processes in the upper troposphere and their links with the Earth climate. It is designed to measure emission from H2O, O3, CO and other trace gases (e.g., HNO3 and N2O) in the frequency range 300-360 GHz.  STEAM-R will utilise an array of receivers, which will image different tangent heights simultaneously to providing unique information about the 2-D structure of the atmosphere.

2. Cirrus sounding instruments have been presented as possible Explorer missions (CIWSIR and GOMAS) and post-EPS will include a sounder and/or imager for which extension to sub-millimetre is under consideration. A similar instrument has been base-lined for the UK Facility for Airborne Atmospheric Measurements (FAAM). In general, these instruments consist of a number of radiometers (extending from millimetre wavelengths to near-terahertz frequencies) that can sound the atmosphere in order to discriminate cirrus components intermediate between those accessible in the IR and microwave. Although Schottky diode receiver technology is known to work at frequencies up to 1THz, the high frequency radiometer channels required by a future cirrus instrument, e.g. at frequencies around 462, 684 and 875GHz, have not yet been demonstrated with the required performance.

Key technologies chosen for development include a new sideband separating sub-millimetre, mixer, local oscillator (LO) source technology and a novel substrateless optical filter. In addition, a new optical methodology for designing microwave instruments was investigated and scientific support has been provided to the STEAM-R Phase 0 Explorer study.

SHIRM 360 GHz image separator mixer using Schottky diode technology

Excellent success has been achieved. CEOI Phase 1 highlights include the following:

- Radiative transfer and retrieval simulations have underpinned the STEAM-R instrument baseline design.

- STEAM-R is focused on the upper troposphere, in alignment with scientific objectives of PREMIER, NCEO Atmospheric Composition Theme and the wider UK community,

- Novel image separation mixer technology (SHIRM), based on UK Schottky diodes, has been demonstrated by Astrium and RAL. The UK is well positioned to supply critical technology (mixers, optical filters) and other hardware for STEAM-R.

- Astrium has developed an optical design methodology that accurately predicts antenna patterns for sub-millimetre radiometer instruments

- Novel, generic, filter technology has been developed by Queen’s University Belfast and Astrium and demonstrates state-of-the art low loss performance.

- Two studies, precursors of a possible airborne cirrus instrument have been started.The UK involvement in STEAM-R through continued work under CEOI in preparation for the PREMIER Explorer Phase 0 study will consolidate UK excellence in THz radiometry. 

This work is led by Dr Dave Matheson of STFC-RAL with Astrium and Queens University Belfast