5:15 PM - 7:15 PM
[ACG41-P25] Ray-trace-based performance analysis of imaging Fourier transform spectrometer for atmospheric observation
Keywords:infrared sounder, Fourier Transform Spectrometer, imaging spectroscopy, ray tracing
Hyperspectral observation is a valuable tool for identifying materials exposed on the Earth’s surface and measuring the spatial and temporal distribution of atmospheric temperature, water vapor, and trace gases. Fourier Transform Spectrometer (FTS) is one of the most promising spectroscopic instruments for providing both high spectral resolution and signal-to-noise ratios over a broad wavelength band. FTS is usually employed in infrared sounders for meteorological and greenhouse gas (GHG) observation (e.g., IASI/MetOp, TANSO-FTS/GOSAT) partly because dispersive spectrometer systems for the long-wavelength infrared band are physically large. To further improve the temporal resolution of infrared sounding, it would be desirable to use imaging FTS (iFTS), which is capable of spectral imaging based on the same principle as FTS, on a geostationary satellite. For meteorological observations, China’s FY-4A recently employed iFTS for a geostationary infrared sounder in 2016. MTG, to be launched in 2025, is also scheduled to employ iFTS [1-2]. Although there have been no cases in the field of GHG observation, the necessity of such geostationary observations has been proposed for a long time, as several missions were previously planned [3-4].
FTS is a spectrometer that measures the light intensity modulation (an interferogram) caused by changes in the optical path difference (OPD) and obtains a spectrum by applying Fourier transformation to the interferogram. FTS has a high modulation efficiency when observed in the direction of the optical axis. However, with iFTS, the OPD increases as the light incident angle shifts from the optical axis, and hence, constructive and destructive interferences can occur within the same off-axis pixel. Consequently, the modulation efficiency obtained at the edge of the field of view (FOV) is lower than at the center (in addition, the calculated spectrum is shifted toward the long wavelengths), affecting spectral performance. To evaluate the utility of iFTS for future Earth observation, it is necessary to accurately calculate the impact of these effects on spectral performance for a variety of optical designs and observation conditions (e.g., FOV, wavelength, and maximum OPD).
This study constructed an optical model to accurately analyze the spectral performance of basic Michelson-interferometer iFTS, which can compute the optical flux incident on an arbitrary pixel involving the interference effects. Although ray tracing typically has a high computational cost, we reduced the amount of calculation by sparse-sampling the interferogram and by signal model fitting, thus obtaining efficient interferogram analysis that includes modulation efficiency and wavelength shift. This technique enables us to evaluate spectra at the edge of the FOV under a variety of observation conditions. To validate the ray-tracing method, we also constructed a prototype iFTS and compared its measured interference fringes and prediction results with its optical model. We plan to utilize this evaluation platform to link observation performance requirements with optical system specifications necessary to realize them for future Earth observation missions.
<references>
[1] J. Yang et al., 2017, BAMS, Vol. 98, pp. 1637--1658
[2] K. Holmlund et al., 2021, BAMS, Vol. 102, p. E990--E1015
[3] W. L. Smith et al., 2006, Proc. SPIE, Vol. 6405, p. 64050E
[4] B. Moore III et al., 2018, Front. Environ. Sci., Vol. 6, pp. 1--13
FTS is a spectrometer that measures the light intensity modulation (an interferogram) caused by changes in the optical path difference (OPD) and obtains a spectrum by applying Fourier transformation to the interferogram. FTS has a high modulation efficiency when observed in the direction of the optical axis. However, with iFTS, the OPD increases as the light incident angle shifts from the optical axis, and hence, constructive and destructive interferences can occur within the same off-axis pixel. Consequently, the modulation efficiency obtained at the edge of the field of view (FOV) is lower than at the center (in addition, the calculated spectrum is shifted toward the long wavelengths), affecting spectral performance. To evaluate the utility of iFTS for future Earth observation, it is necessary to accurately calculate the impact of these effects on spectral performance for a variety of optical designs and observation conditions (e.g., FOV, wavelength, and maximum OPD).
This study constructed an optical model to accurately analyze the spectral performance of basic Michelson-interferometer iFTS, which can compute the optical flux incident on an arbitrary pixel involving the interference effects. Although ray tracing typically has a high computational cost, we reduced the amount of calculation by sparse-sampling the interferogram and by signal model fitting, thus obtaining efficient interferogram analysis that includes modulation efficiency and wavelength shift. This technique enables us to evaluate spectra at the edge of the FOV under a variety of observation conditions. To validate the ray-tracing method, we also constructed a prototype iFTS and compared its measured interference fringes and prediction results with its optical model. We plan to utilize this evaluation platform to link observation performance requirements with optical system specifications necessary to realize them for future Earth observation missions.
<references>
[1] J. Yang et al., 2017, BAMS, Vol. 98, pp. 1637--1658
[2] K. Holmlund et al., 2021, BAMS, Vol. 102, p. E990--E1015
[3] W. L. Smith et al., 2006, Proc. SPIE, Vol. 6405, p. 64050E
[4] B. Moore III et al., 2018, Front. Environ. Sci., Vol. 6, pp. 1--13