Abstract
Abstract. GHOST is a novel, compact shortwave-infrared grating spectrometer, designed for remote sensing of tropospheric columns of greenhouse gases (GHGs) from an airborne platform. It observes solar radiation at medium to high spectral resolution (better than 0.3 nm), which has been reflected by the Earth's surface using similar methods to those used by polar-orbiting satellites such as the JAXA GOSAT mission, NASA's OCO-2, and the Copernicus Sentinel-5 Precursor. By using an original design comprising optical fibre inputs along with a single diffraction grating and detector array, GHOST is able to observe CO2 absorption bands centred around 1.61 and 2.06 µm (the same wavelength regions used by OCO-2 and GOSAT) whilst simultaneously measuring CH4 absorption at 1.65 µm (also observed by GOSAT) and CH4 and CO at 2.30 µm (observed by Sentinel-5P). With emissions expected to become more concentrated towards city sources as the global population residing in urban areas increases, there emerges a clear requirement to bridge the spatial scale gap between small-scale urban emission sources and global-scale GHG variations. In addition to the benefits achieved in spatial coverage through being able to remotely sense GHG tropospheric columns from an aircraft, the overlapping spectral ranges and comparable spectral resolutions mean that GHOST has unique potential for providing validation opportunities for these platforms, particularly over the ocean, where ground-based validation measurements are not available. In this paper we provide an overview of the GHOST instrument, calibration, and data processing, demonstrating the instrument's performance and suitability for GHG remote sensing. We also report on the first GHG observations made by GHOST during its maiden science flights on board the NASA Global Hawk unmanned aerial vehicle, which took place over the eastern Pacific Ocean in March 2015 as part of the CAST/ATTREX joint Global Hawk flight campaign.
Highlights
The Paris Agreement (2015) describes a framework for mitigating global anthropogenic greenhouse gas (GHG) emissions
The optical design behind gas Observations of the Stratosphere and Troposphere (GHOST) was developed by the STFC (Science and Technology Facilities Council) Astronomy Technology Centre during two design studies, both funded by the UK Centre for Earth Observation Instrumentation, which demonstrated reductions in size and weight of SWIR Earth observation spectrometers by using technology originally developed for astronomy instruments
In order to avoid misalignment between the main optical ray defined by these apertures and the mechanical axis of the gimbal, we required that the end of the fibre bundle, the coupling lens, and the field stop aperture should be centred on the rotation axis of the gimbal to within 50 μm
Summary
The Paris Agreement (2015) describes a framework for mitigating global anthropogenic greenhouse gas (GHG) emissions. Data collected by commercial aircraft represent valuable knowledge of variations in the upper troposphere along air corridors and provide vertical profiles during airport ascents and descents In addition to these in situ observations, temporary networks of portable upward-looking Fourier transform spectrometers (Gisi et al, 2012) have been used to constrain GHG emissions on a local scale by making integrated column measurements of GHGs from different locations around a city (Hase et al, 2015; Viatte et al, 2017).
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