Accuracy assessment of the MODIS 16-day albedo product for snow: comparisons with Greenland in situ measurements

The accuracy of the Moderate Resolution Imaging Spectroradiometer (MODIS) 16-day albedo product (MOD43) is assessed using ground-based albedo observations from automatic weather stations (AWS) over spatially homogeneous snow and semihomogeneous ice-covered surfaces on the Greenland ice sheet. Data from 16 AWS locations, spanning the years 2000-2003, were used for this assessment. In situ reflected shortwave data were corrected for a systematic positive spectral sensitivity bias of between 0.01 and 0.09 on a site-by-site basis using precise optical black radiometer data. Results indicate that the MOD43 albedo product retrieves snow albedo with an average root mean square error (RMSE) of ±0.07 as compared to the station measurements, which have ±0.035 RMSE uncertainty. If we eliminate all satellite retrievals that rely on the backup algorithm and consider only the highest quality results from the primary bidirectional reflectance distribution function (BRDF) algorithm, the MODIS albedo RMSE is ±0.04, slightly larger than the in situ measurement uncertainty. There is general agreement between MODIS and in situ observations for albedo <0.7, while near the upper limit, a −0.05 MODIS albedo bias is evident from the scatter of the 16-site composite.     

Real-time server-based communication with CAN

This paper investigates the concept of share-driven scheduling of networks using servers with real-time properties. Share-driven scheduling provides fairness and bandwidth isolation between predictable as well as unpredictable streams of messages on the network. The need for this kind of scheduled real-time communication network is high in applications that have requirements on flexibility, both during development for assigning communication bandwidth to different applications, and during run-time to facilitate dynamic addition and removal of system components. We illustrate the share-driven scheduling concept by applying it to the popular controller area network (CAN). We propose a scheduling mechanism that we call simple server-scheduled CAN (S/sup 3/-CAN), for which we also present an associated timing analysis. Additionally, we present a variant of S/sup 3/-CAN called periodic server-scheduled CAN (PS/sup 2/-CAN), which for some network configurations gives lower worst-case response-times than S/sup 3/-CAN. Also for this improvement, a timing analysis is presented. Moreover, we use simulation to evaluate the timing performance of both S/sup 3/-CAN and PS/sup 2/-CAN, comparing them with other scheduling mechanisms.     

Rapid quenching and nitrogenation of Sm2Fe17 alloys

The production of hard magnetic samarium-iron nitrides by rapid quenching of Sm₂Fe₁₇ alloys followed by a nitrogenation treatment is described and analysed. Rapid quenching of Sm₂Fe₁₇ produces almost pure Sm₂Fe₁₇ phase with 40 to 100 nm grain size. At low temperature (T465°C), the nitrogenation treatment under 1 bar N₂-H₂ (5%) mainly produces the definite compound Sm₂Fe₁₇N_3– and follows reasonably well an Arrhenius law, the reaction rate being limited by diffusion through the Sm₂Fe₁₇N_3– reacted layer. At higher temperatures (T465°C), disproportionation reactions simultaneously take place, which transform the metastable nitride into free iron and tiny SmN crystallites (about 10 nm wide). It is concluded that the nitrogenation treatment should be performed at about 400°C in order to reach a sufficient reaction rate for the nitride formation while (possibly) avoiding disproportionation reactions whose by-products (Fe and SmN) are deleterious for permanent magnet applications.     

Deficits in Prenatal Serine Biosynthesis Underlie the Mitochondrial Dysfunction Associated with the Autism-Linked FMR1 Gene

Fifty-five to two hundred CGG repeats (called a premutation, or PM) in the 5′-UTR of the FMR1 gene are generally unstable, often expanding to a full mutation (>200) in one generation through maternal inheritance, leading to fragile X syndrome, a condition associated with autism and other intellectual disabilities. To uncover the early mechanisms of pathogenesis, we performed metabolomics and proteomics on amniotic fluids from PM carriers, pregnant with male fetuses, who had undergone amniocentesis for fragile X prenatal diagnosis. The prenatal metabolic footprint identified mitochondrial deficits, which were further validated by using internal and external cohorts. Deficits in the anaplerosis of the Krebs cycle were noted at the level of serine biosynthesis, which was confirmed by rescuing the mitochondrial dysfunction in the carriers’ umbilical cord fibroblasts using alpha-ketoglutarate precursors. Maternal administration of serine and its precursors has the potential to decrease the risk of developing energy shortages associated with mitochondrial dysfunction and linked comorbidities.     

Efficient implementation of tight response-times for tasks with offsets

Earlier approximate response time analysis (RTA) methods for tasks with offsets (transactional task model) exhibit two major deficiencies: (i) They overestimate the calculated response times resulting in an overly pessimistic result. (ii) They suffer from time complexity problems resulting in an RTA method that may not be applicable in practice. This paper shows how these two problems can be alleviated and combined in one single fast-and-tight RTA method that combines the best of worlds, high precision response times and a fast approximate RTA method. Simulation studies, on randomly generated task sets, show that the response time improvement is significant, typically about 15% tighter response times in 50% of the cases, resulting in about 12% higher admission probability for low priority tasks subjected to admission control. Simulation studies also show that speedups of more than two orders of magnitude, for realistically sized tasks sets, compared to earlier RTA analysis techniques, can be obtained. Other improvements such as Palencia Gutiérrez, González Harbour (Proceedings of the 20th IEEE real-time systems symposium (RTSS), pp. 328–339, 1999), Redell (Technical Report TRITA-MMK 2003:4, Dept. of Machine Design, KTH, 2003) are orthogonal and complementary which means that our method can easily be incorporated also in those methods. Hence, we conclude that the fast-and-tight RTA method presented is the preferred analysis technique when tight response-time estimates are needed, and that we do not need to sacrifice precision for analysis speed; both are obtained with one single method.

Surface roughness characterizations of sea ice and ice sheets: case studies with MISR data

This work is an examination of potential uses of multiangular remote sensing imagery for mapping and characterizing sea ice and ice sheet surfaces based on surface roughness properties. We use data from the Multi-angle Imaging SpectroRadiometer (MISR) to demonstrate that ice sheet and sea ice surfaces have characteristic angular signatures and that these angular signatures may be used in much the same way as spectral signatures are used in multispectral classification. Three case studies are examined: sea ice in the Beaufort Sea off the north coast of Alaska, the Jakobshavn Glacier on the western edge of the Greenland ice sheet, and a region in Antarctica south of McMurdo station containing glaciers and blue-ice areas. The MISR sea ice image appears to delineate different first-year ice types and, to some extent, the transition from first-year to multiyear ice. The MISR image shows good agreement with sea ice types that are evident in concurrent synthetic aperture radar (SAR) imagery and ice analysis charts from the National Ice Center. Over the Jakobshavn Glacier, surface roughness data from airborne laser altimeter transects correlate well with MISR-derived estimates of surface roughness. In Antarctica, ablation-related blue-ice areas, which are difficult to distinguish from bare ice exposed by crevasses, are easily detected using multiangular data.     

New methods to infer snow albedo from the MISR instrument with applications to the Greenland ice sheet

Snow-covered surfaces have a very high surface albedo, thereby allowing little energy to be absorbed by the snowpack. As the snowpack ages and/or begins to melt, the snow albedo decreases and more solar energy is absorbed by the snowpack. Therefore, accurate estimation of snow albedo is essential for monitoring the state of the cryosphere. This paper examines the retrieval of snow albedo using data from the Multi-angle Imaging SpectroRadiometer (MISR) instrument over the Greenland ice sheet. Two different methods are developed and examined to derive the snow albedo: one based on the spectral information from MISR and one utilizing the angular information from the MISR instrument. The latter method is based on a statistical relationship between in situ albedo measurements and the MISR red channel reflectance at all MISR viewing angles and is found to give good agreement with the ground-based measurements. Good agreement is also found using the spectral information, although the method is more sensitive to instrument calibration, snow bidirectional reflectance distribution function models, and narrowband-to-broadband relationships. In general, using either method retrieves snow surface albedo values that are within about 6% of that measured at the stations in Greenland.