Physics – Optics
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.474e..12d&link_type=abstract
Proceedings of the SPECTRA Workshop - The concept of a space-borne Earth Observation Mission addressing the terrestrial componen
Physics
Optics
Scientific paper
The SPECTRA system will allow to achieve the scientific objectives by observing defined land targets in various spectral bands of the optical range of the electromagnetic spectrum from the Visible into the Thermal Infrared and with variable directions of observations. These observation capabilities are intended to provide a sampling of the Bi-directional Reflectance Distribution Function (BRDF) of the vegetation objects in various spectral bands and also in the thermal infrared region. In order to monitor the dynamic of the variations in time of the vegetation properties, the observations must be performed with a frequency of 3 days, assuming cloud-free conditions. The system supports two observation modes: a normal mode, which is the default mode in daylight condition, and a night mode, outside of daylight conditions. Only the two spectral bands in the Thermal Infra Red (TIR) spectral region are used in night mode, and only nadir or near-nadir scenes are acquired in this mode. For observations in normal mode, the system provides spectral images at the near-nadir direction (i.e. at null along-track angle) and a minimum of six along-track angles placed around the near-nadir direction. The along-track co-elevation angles (referred as BRDF angles) may approximately be: ± 30 ° or ± 45 ° and ± 60 ° and ± 70 °, the deviation from this values being not larger than 5°(TBC). The required revisit time of 3 days with a sun-zenith angle lower than 70° together with the local time definition allows to select the a sun synchronous orbit of about 680 km altitude, a 14 + 9/14 cycle,11h LTDN. Revisit time is met by implementing a 35° off-track roll pointing capability. The SPECTRA payload provides spectral images in two regions : Region 1, corresponding to the VIS/VNIR/SWIR spectral domain, and Region 2, corresponding to the TIR domain. Spectral region 1 includes bands specifically in the following wavelength intervals: 0.45 to 1.11 μm, 1.18 to 1.32 μm, 1.49 to 1.76 μm, 2.06 to 2.35 μm. The sampling interval is lower than 10 nm for all wavelength. The spectral width is lower than 1.2 spectral sampling interval, with a spectral misregistration of less than 0.15 spectral sampling interval. Spectral region 2 includes the two following spectral bands in the TIR spectral range: Band B1: 10.3 to 11.3 mm. Band B2: 11.3 to 12.3 mm. The two spectral bands in the TIR and a minimum of 60 elementary spectral bands in region 1 are acquired in any direction specified direction. These spectral bands are predefined for each site at least two weeks (TBC) prior to the observations. However, the system supports the acquisition, storage, handling and transmission to ground of observations of a given site obtained using all the spectral bands in region 1 and region 2. These observations may occur at a maximum rate of once per orbit, and the proposed design is compatible with this need in terms of storage and transmission, although not required. Images are provided in the form of a two-dimensional array of image elements with an equal number of rows and columns. The image swath and the image length is approximately equal to 50 km at nadir. The spatial sampling interval shall be not larger than 50 m for any along track angle, while the along track spatial width shall be not larger than 50 m at nadir and not larger than 150 m for 70° BRDF angle and assuming no across track depointing. In terms of spatial misregistration, the most stringent requirement with respect to the proposed design is the one applicable to pairs of spectral images within spectral region 2, where it shall be smaller than 0.2 (TBC) spatial sampling interval. Images in region 1 are coregistered on board by design, while coregistration between region 1 and region 2 images is performed by instruments alignment. The implication of the spatial requirements on the instrument design are dictating the main constraints for the spatial width in the TIR channels, which imposes optics diameter of about 200 mm. The radiometric resolution achieved by the design meets the specifications over the whole spectral range, except in two critical regions: the blue range [0.4 μm; 0.55 μm], where the very low signal level makes the high SNR requirement quite impossible to meet. This status is even worsen by the high photonic noise inherent to short wavelengths, the drop of the silicon detector response below 0.5 μm and the high dispersion characteristics and low transmission of standard glass materials, and the extreme SWIR range [2.0 μm; 2.4 μm] where a very low reference signal level is specified. This is made more difficult by instruments features such as the instrument background signal, increasing the photon noise level, and the detector darkness noise. The radiometric performances in region 2 easily meet the requirements of NedT lower than 0.1 K for a scene temperature of 300 K.
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