Radiometric and geometric characteristics of Pleiades images
Pleiades images are distributed with 50 cm ground sampling distance (GSD) even if the physical resolution for nadir images is just 70 cm. By theory this should influence the effective GSD determined by means of point spread function at image edges. Nevertheless by edge enhancement the effective GSD can be improved, but this should cause enlarged image noise. Again image noise can be reduced by image restoration. Finally even optimized image restoration cannot improve the image information from 70 cm to 50 cm without loss of details, requiring a comparison of Pleiades image details with other very high resolution space images. The image noise has been determined by analysis of the whole images for any sub-area with 5 pixels times 5 pixels. Based on the standard deviation of grey values in the small sub-areas the image noise has been determined by frequency analysis. This leads to realistic results, checked by test targets. On the other hand the visual determination of image noise based on apparently homogenous sub-areas results in too high values because the human eye is not able to identify small grey value differences – it is limited to just approximately 40 grey value steps over the available gray value range, so small difference in grey values cannot be seen, enlarging results of a manual noise determination. A tri-stereo combination of Pleiades 1A in a mountainous, but partially urban, area has been analyzed and compared with images of the same area from WorldView-1, QuickBird and IKONOS. The image restoration of the Pleiades images is very good, so the effective image resolution resulted in a factor 1.0, meaning that the effective resolution corresponds to the nominal resolution of 50 cm. This does not correspond to the physical resolution of 70 cm, but by edge enhancement the steepness of the grey value profile across the edge can be enlarged, reducing the width of the point spread function. Without additional filtering edge enhancement enlarges the image noise, but the average image noise of approximately 1.0 grey values related to 8 bit images is very small, not indicating the edge enhancement and the down sampling of the GSD from 70 cm to 50 cm. So the direct comparison with the other images has to give the answer if the image quality of Pleiades images is on similar level as corresponding to the nominal resolution. As expected with the image geometry there is no problem. This is the case for all used space images in the test area, where the point identification limits the accuracy of the scene orientation.
TELLS THE NUMBER OF PIXELS THE TRUTH? - EFFECTIVE RESOLUTION OF LARGE SIZE DIGITAL FRAME CAMERAS
The photo scale or the ground sampling distance (GSD) usually is the dominating factor for the specification of photo flights. For a comparison of cameras not only the nominal number of pixels is important, also the image quality has an influence. The effective resolution, respecting the image quality, can be determined by edge analysis. A sudden change of the brightness in the object space is causing a continuous change of the gray values in a profile across the edge. A differentiation of the gray value profile lead to the point spread function, including the information of effective resolution. DMC, UltraCamD and UltraCamX-images as well as analog aerial photos have been investigated. Of course the effective resolution is depending upon the illumination condition, expressed by the sun elevation, and the atmospheric condition. For the DMC only in one case with 20° sun elevation the effective GSD was 5% larger than the nominal value; this is different for both UltraCam. The calibration reports shows lower modulation transfer functions in the image corners. Under optimal light conditions, with 60° sun elevation, the UltraCamD in the center has a loss 4% of the effective GSD against the nominal value, but in the image corners a loss of 28% exists. With 20° and 27° sun elevation an overall loss of resolution by 16% respectively 24% has been detected. The same effect exists for the UltraCamX, showing an effective resolution 28% less than the nominal value over the whole image format. Similar investigations have been made for analog photos.
Phase characterisation of simulated high burn-up UO2 fuel
Abstract To determine the lattice constant variation with burn-up and the oxidation behaviour at high temperatures, simulated UO2-spent fuels with burn-ups up to 200 GWd tM−1 were prepared by cold pressing and sintering and characterised by X-ray diffraction analysis and thermogravimetry. The fcc-lattice parameter of the sintered samples determined at room temperature, showed a continuous decrease with the simulated burn-up. During oxidation the conversion of the simulated fuels to the (U,FP)4O9 and (U,FP)3O8-type phases was strongly delayed when the simulated burn-up exceeded 80 GWd tM−1. Also, at high burn-ups (>100 GWd tM−1), large fractions of the (U,FP)4O9-type phase remained untransformed to (U,FP)3O8 after prolonged exposure to air at 950–1000°C. This suggests a marked widening of the (U,FP)4O9-phase field when the amount of substituted uranium atoms exceeds values of about 8%.