Development of a Compact Modular Radiometer for Laboratory and Field Studies

Infrared filter radiometers are used to measure accurately radiant flux across the band-pass of a filter placed in the optical path before a thermal detector. The aim of this proposal is to develop a miniature, modular radiometer with a built-in calibrator, the Compact Modular Radiometer (CMR). Miniature filter radiometers are relatively commonplace (for example, the digital ear thermometers used in hospitals) but rarely have integral, traceable calibration targets. The CMR combines technologies recently developed for miniature space instruments (miniature filters and high sensitivity un-cooled broadband detectors) with small high accuracy calibration targets, currently under development, to produce a general-purpose miniature radiometer for field and laboratory studies. This grant will be used to produce a fully functioning breadboard of the instrument, including the newly developed built-in calibration targets. The CMR offers significant improvements over previous generations of instruments in two key areas: 1. It combines robust optical design and integral calibration targets into a high performance miniature instrument (900g). Previous calibrated radiometers (e.g. Lunar Diviner, part of NASA's upcoming Lunar Reconnaissance Orbiter) have masses nearly five times as great. 2. The inclusion of an intermediate focus dramatically improves the performance of the infrared filters when compared with previous compact radiometers such as the Lunar Diviner instrument. By choosing different filter band-passes, radiometers such as the CMR can be made sensitive to specific gases in the atmosphere or different minerals on the ground due to their unique molecular fingerprints. In a properly calibrated radiometer the radiance due to these fingerprints can be accurately measured, and if multiple filters (also known as 'channels') are used then these measurements can be inverted to produce quantities such as atmospheric temperature or constituent profiles, surface temperature maps and relative mineral abundances. Instruments that perform all these tasks have been developed at Oxford for over forty years and used to make measurements from aircraft, boats, weather balloons, Earth observing satellites and missions to other planets. The CMR is a highly flexible instrument suitable for numerous terrestrial, space and commercial applications, for example: 1. The low data rate, low power consumption and onboard calibration make the instrument highly applicable for long term measurements such as remote environmental monitoring (e.g. tropospheric pollution, sulphur dioxide level measurements at volcanoes etc). The high sensitivity of the instrument allows long path length measurements, either actively or passively, increasing sensitivity to trace gases. 2. Using the scanning mirror combined with the detector array allows the instrument to act as a moderate spatial resolution imaging radiometer. With its low mass the instrument is an ideal payload for use in experiments using unmanned aerial vehicles (UAVs), atmospheric research balloons and micro-satellites. E.g. remote sensing of forest health by using filters sensitive to absorption features in the chlorophyll spectrum, sea surface temperature from a micro-satellite or ship-borne versions of the same instrument. The prototype instrument will be used to test two of these as a proof of concept: Firstly it will be used to measure low pressure mixtures of water vapour and aerosol in the lab to simulate observing the Earth's atmosphere from a micro satellite. Secondly, it will be used to measure the temperature at different heights in the atmosphere close to the ground, to show how heat in the atmosphere changes through the day in 3D.