A Miniature UV Imaging Spectrometer for Remote Sensing of the Atmosphere Using Micro-Satellites
- Paper number
IAC-08.B4.4.B12
- Author
Dr. Craig Underwood, Surrey Space Centre, University of Surrey, United States
- Coauthor
Mr. Juan Fernandez, United Kingdom
- Year
2008
- Abstract
In this paper, we propose a new miniature ultraviolet (UV)-band spectral imager with the objective of monitoring important atmospheric constituents: sulphur dioxide, ozone and aerosols. Ideally the instrument will operate in a micro/nano-satellite constellation in order to provide the rapid response and dynamic requirements of a very demanding application such as volcano monitoring based on one of the most important gases they emit: sulphur dioxide (SO2). Currently observations in UV region are restricted to large platforms such as NASA’s Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) and ESA’s Global Ozone Monitoring Experiment (GOME) and Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). With exception of TOMS which, was not designed to monitor SO2, all others face various trade-offs when observing the UV range between 300-315 nm. Because of the wide dynamic range observed in this region, it is common to optically split this region in two channels with different gains, thus degrading the signal to noise ratio. This occurs precisely in a region where SO2 absorption features provide easier discrimination against ozone, which also absorbs in the same region. The instrument concept proposed here is analysed in terms of the spectral resolution required at Full Width Half Maximum (FWHM). This resolution is defined in the slit function of the instrument and is sampled by a photodiode area array at the focal plane of the instrument. The imaging performance of the sampled spectra is also studied given the distortions observed from different angular fields. Radiative transfer simulations show the differential radiances under various scenarios with clean and atmospheres contaminated with SO2, taking into account the estimated slit function. An estimated Noise Equivalent Radiance (NEL) is calculated to define the system sensitivity from where existing and new algorithms can be applied. The instrument design exploits the excellent response of silicon carbide photodiodes in this region; its blindness to visible radiation (two orders of magnitude higher than silicon) provides a simple optical design based on transmission gratings to offer a high sensitivity. Other channels outside this region (331- and 360-nm) can be detected also for presence of some aerosols and determination of reflectance for algorithm purposes. The demanding requirements to consider when designing such system are discussed with analysis on: the ground sample distance aimed (7 km x 32 km) which matches OMI (50 times smaller than GOME); the optical design and its spectral imaging properties across the field-of-view and the signal-to-noise required from 20-bit electronics (consuming less than 5 W). The low-cost and small size (9 x 13 x 6 cm) of the instrument proposed makes it a suitable instrument for an atmospheric mission where the potential of small satellites can be demonstrated.
- Abstract document
- Manuscript document
IAC-08.B4.4.B12.pdf (🔒 authorized access only).
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