Until now incandescent lamp, sun and moon calibrations have been successfully applied for in-flight calibration of spaceborne Earth observation imaging sensors. The performance development of LEDs in the past decade guided to higher luminous efficiencies, broader spectral coverage, lower degradation of light output over time and lower power consumption. These advantages make LEDs to a candidate for radiometric and spectral calibration of spaceborne spectrometers. For analysing LEDs for space in-flight calibration a set of LEDs has been characterised and a simulation of space radiation quantities (i.e. proton and electron radiation for a poiar low-Earth orbit) has been carried out. Additional vacuum tests (outgassing behaviour) demonstrated a possible application of LEDs with epoxy housing for the future space environment. Further on, a concept for long-term temperature stabilisation has been developed for solving the main problem of LED in-flight calibration, i.e. the temperature dependency of the irradiance.Consequently, this study demonstrates that (1) a degradation of LEDs due to space environment is not expected, that (2) long-term temperature stability of LEDs can be ensured, and that (3) the higher blue part of 'white' LEDs would best suit ocean-colour scientists needs.
The imaging pushbroom scanner MOS measures the spectral radiance of the backscattered radiation ofthe earth surface in the VIS/NIR spectral region from orbit altitudes. The two main problems, the investigation of ocean/land and atmospheric properties, require a modular optomechanical and -electronical spectrometric device: MOS-A especially for data acquisition in the absorption band ofthe atmospheric oxygen in 4 spectral channels with 1,4 nm bandwidth and MOS-B with 13 channels of 10 nm bandwidth between 0,4 tm and 1,0 rim.The spectral and radiometric properties of MOS are chosen with respect to the spectral characteristic of the upwelling radiation, which is influenced by the object spectral reflection function. They are a compromise between the dominant measuring quantities like radiometric, geometric and time resolution requirements. This problems in connection with the design and calibration concepts will be discussed.
Since March 1996 the Modular Optoelectronic Scanner (MOS) provides remote data from a 820 km sun synchroneous polar orbit. It measures the spectral radiance of the atmosphere-surface system in 18 spectral channels and up to 420 pixels in a 200 km swath. MOS consists oftwo irna.ging spectrometers A and B with gratings and a camera C with an interference filter. MOS-A has 4 channels with a spectral halfwidth A? 1.4 urn in the absorption band ofatmospheric oxygen near 760 mM, MOS-B hIS 13 channels between 400 and 1010 urn with AX 10 nrn and the MOS-C channel is at 1.6 jim with iX2 100 iun. Beside the on ground laboratory calibration as the basis ofcalculating the spectral radiance of the earth objects, the long time mission requires a periodic recalibration or at least a stability check of instrument properties in orbit to support the reliability of the remote data. Internal lamps and the extraterrestric sun radiation provide actual data sets to derive corrections on remote data if any changes in the peformance data arises.
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