Abstract
Serendipitous observations of our Moon or planets of the inner solar system provide a unique means to the characterization of the (quasi-)optical properties of instruments on meteorological research satellites in flight, because their diameters are much smaller than the size of a pixel or the field of view. We investigated how beamwidth, channel co-registration, and pointing accuracy can be determined with such observations. For this we identified and analysed intrusions of the Moon in the deep space views of AMSU-B (Advanced Microwave Sounding Unit - B) and MHS (Microwave Humidity Sounder) on various satellites in polar orbits and as well images obtained with SEVIRI (Spinning Enhanced Visible Infra-Red Imager) on Meteosat-10 and -11, which had Mercury or Venus standing close to the Earth in the rectangular field of view.
When the Moon moves by chance through the field of view of a microwave sounder, it is possible to calculate the beamwidth and pointing direction of each channel. We found significant discrepancies between the results of these measurements in flight and those obtained during ground tests with both AMSU-B and MHS on certain satellites. In several of these cases we detected non-compliance with requirements that had not been recorded in test reports.
Studies of the channel alignment and image quality are presented as well for SEVIRI. As the angular resolution of this instrument is about a hundred times higher than with microwave sounders, we replaced here the Moon with Mercury and Venus. We characterized the typical smearing of the planet’s radiance into neighbouring pixels with infrared channels and occasionally we detected large shifts of the scan lines in the raw data that tear a planet’s image apart by dozens of pixels. Once these misalignments between scan lines are corrected, however, the band-to-band registration is accurate within a small fraction of a pixel, even for detectors on different focal planes.
Characterization for IR and Microwave Instruments with Solar System Objects
Serendipitous observations of our Moon or planets of the inner solar system provide a unique means to the characterization of the (quasi-)optical properties of instruments on meteorological research satellites in flight, because their diameters are much smaller than the size of a pixel or the field of view. We investigated how beamwidth, channel co-registration, and pointing accuracy can be determined with such observations. For this we identified and analysed intrusions of the Moon in the deep space views of AMSU-B (Advanced Microwave Sounding Unit - B) and MHS (Microwave Humidity Sounder) on various satellites in polar orbits and as well images obtained with SEVIRI (Spinning Enhanced Visible Infra-Red Imager) on Meteosat-10 and -11, which had Mercury or Venus standing close to the Earth in the rectangular field of view.
When the Moon moves by chance through the field of view of a microwave sounder, it is possible to calculate the beamwidth and pointing direction of each channel. We found significant discrepancies between the results of these measurements in flight and those obtained during ground tests with both AMSU-B and MHS on certain satellites. In several of these cases we detected non-compliance with requirements that had not been recorded in test reports.
Studies of the channel alignment and image quality are presented as well for SEVIRI. As the angular resolution of this instrument is about a hundred times higher than with microwave sounders, we replaced here the Moon with Mercury and Venus. We characterized the typical smearing of the planet’s radiance into neighbouring pixels with infrared channels and occasionally we detected large shifts of the scan lines in the raw data that tear a planet’s image apart by dozens of pixels. Once these misalignments between scan lines are corrected, however, the band-to-band registration is accurate within a small fraction of a pixel, even for detectors on different focal planes.