Abstract
With the global concern over climate change in recent years, there has been an increased interest in characterizing the sources, sinks and transport of greenhouse gases through the use of satellite data. Effective mapping of an atmospheric trace gas such as carbon dioxide (CO2) requires high precision (0.3% to 0.5%) measurements of gas concentration. This is usually achieved through identifying CO2 by its spectral absorption bands at 1.56 μm - 1.62 μm and 1.92 μm - 2.06 μm wavelength (i.e. in the Short-Wave Infrared - SWIR) and distinguishing this from other greenhouse gases e.g. water vapour by using high resolution spectrometers (e.g. 0.27 cm-1 resolution at Full Width at Half Maximum (FWHM) at a relative signal-to-noise ratio (SNR) of 300:1). These requirements impose severe design and technical challenges in terms of size, weight, power consumption and cost of instrument needed. Existing spaceborne instruments that meet required specifications are generally too large and expensive to consider flying on a microsatellite constellation, which if possible, would enable a much greater temporal resolution to be achieved. This thesis contributes to the state-of-the-art instrument design by developing a compact spatial heterodyne SWIR (COMSSWIR) Fourier Transform Spectrometer (FTS) that utilises the Spatial Heterodyne Spectrometer (SHS) technique for atmospheric CO2 monitoring. The SHS is similar to a conventional FTS but has the mirrors replaced with fixed reflection gratings. This novel compact instrument uses a standard SHS in an echelle-mode SHS configuration to record interferogram on 2-dimensions of the detector for broadband applications. It is designed to cover the wavelength range of 1599 nm to 2060 nm at an appropriate spectral resolution. The implementation makes use of two optical benches each tuned to one of the key CO2 absorption bands. The first channel covers the 1599 nm to 1611 nm band, while the second channel covers 2045 nm to 2060 nm. Both benches share a common receiver three-mirror-telescope which splits the incoming signal with the aid of a dichroic beam-splitter for the two channels. With no-moving parts, this compact, solid state and robust instrument is designed to achieve a high spectral resolution of 0.17 cm-1 and 0.13 cm-1 at FWHM in the 1.6 μm channel (Channel-1) and 2.0 μm channel (Channel-2) respectively. It further offers a high resolving power of 21,984 in the 1.6 μm and 2.0 μm channels. The spatial resolution for COMSSWIR instrument is 13 km x 13 km. The complete payload system with both instrument channels has been successfully modelled and has a total size of 0.69 m x 0.74 m x 0.2 m which meets the specification required for microsatellites. A prototype of COMSSWIR Channel-1 has been developed, calibrated and tested. It achieves a SNR of >220 and a CO2 measurement precision of ~3 ppm. With these system characteristics, high quality precision measurements of CO2 concentration becomes possible from a microsatellite constellation that gives clear sky spatial resolution coverage with a daily revisit capability if used in constellation.