Performance assessment of onboard and scene-based methods for Airborne Prism Experiment spectral characterization

Abstract

Accurate spectral calibration of airborne and spaceborne imaging spectrometers is essential for proper preprocessing and scientific exploitation of high spectral resolution measurements of the land and atmosphere. A systematic performance assessment of onboard and scene-based methods for in-flight monitoring of instrument spectral calibration is presented for the first time in this paper. Onboard and ground imaging data were collected at several flight altitudes using the Airborne Prism Experiment (APEX) imaging spectrometer. APEX is equipped with an in-flight characterization (IFC) facility allowing the evaluation of radiometric, spectral, and geometric system properties, both in-flight and on-ground for the full field of view. Atmospheric and onboard filter spectral features present in at-sensor radiances are compared with the same features in reference transmittances convolved to varying instrument spectral configurations. A spectrum-matching algorithm, taking advantage of the high sensitivity of measurements around sharp spectral features toward spectrometer spectral performance, is used to retrieve channel center wavelength and bandwidth parameters. Results showed good agreement between spectral parameters estimated using onboard IFC and ground imaging data. The average difference be-tween estimates obtained using the O2 and H2O features and those obtained using the corresponding filter features amounted to about 0:3 nm (0.05 of a spectral pixel). A deviation from the nominal labora-tory instrument spectral calibration and an altitude-dependent performance was additionally identified. The relatively good agreement between estimates obtained by the two approaches in similar spectral windows suggests they can be used in a complementary fashion: while the method relying on atmospheric features can be applied without the need for dedicated calibration acquisitions, the IFC allows assessment at user-selectable wavelength positions by custom filters as well as for the system on-ground. Accurate spectral calibration of airborne and spaceborne imaging spectrometers is essential for proper preprocessing and scientific exploitation of high spectral resolution measurements of the land and atmosphere. A systematic performance assessment of onboard and scene-based methods for in-flight monitoring of instrument spectral calibration is presented for the first time in this paper. Onboard and ground imaging data were collected at several flight altitudes using the Airborne Prism Experiment (APEX) imaging spectrometer. APEX is equipped with an in-flight characterization (IFC) facility allowing the evaluation of radiometric, spectral, and geometric system properties, both in-flight and on-ground for the full field of view. Atmospheric and onboard filter spectral features present in at-sensor radiances are compared with the same features in reference transmittances convolved to varying instrument spectral configurations. A spectrum-matching algorithm, taking advantage of the high sensitivity of measurements around sh...

An Algorithm for In-Flight Spectral Calibration of Imaging Spectrometers

“…The spectral smile measured in the laboratory does not depend strongly on wavelength. The shapes of in-flight and laboratory measurements of the spectral smile agree well, but the in-flight values are shifted upwards, because the pressure sensitivity of the instrument affects different wavelengths differently [14]. Figure 10b shows the same, but in the along-track direction.…”

“…It measures backscattered solar irradiance in the visible and near-infrared (VNIR) and in the short-wave infrared (SWIR) with a high spatial and spectral resolution (see Table 1 for spectrometer characteristics). The instrument is calibrated regularly in ground calibration facilities [12,13], and an in-flight calibration facility can be used before and after data acquisition to improve the spectral calibration using a quartz tungsten halogen lamp and four spectral filters [14]. D'Odorico et al [14] determined APEX's spectral calibration parameters for different flight altitudes, i.e., different temperature and pressure conditions, using oxygen and water absorption lines, thus focusing on the near-and short-wave infrared.…”

Impacts of dichroic prism coatings on radiometry of the airborne imaging spectrometer APEX

“…Spectral shifts of APEX have previously been analyzed based on spectral features of both the internal in-flight calibration facility and the atmosphere during imaging runs [12]. However, these methods can only provide shift estimates at selected spectral bands where absorption features occur.…”

“…It has already been shown that APEX experiences spectral shifts during in-flight conditions, mainly brought about by changes in pressure and hence related to the operational altitude [11,12]. In the following, any constellation of environmental parameters leading to spectral misregistration will be termed a spectral shift condition.…”

APEX: Radiometry under spectral shift conditions

“…This led to the hypothesis that the artifact was created due to spectral shifts that were shown to occur during flight operations, largely explained by flight altitudes [5,6], in combination with the spectral characteristics of the dichroic coating.…”