Radiometry performance of the VGOS receivers of the Onsala twin telescopes

With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver asradiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is whenobservations can be carried out at higher frequencies, closer to the water vapour emission line at 22.2 GHz.An advantage of having the radiometer in the VLBI telescope, compared to the use of a stand-alone WaterVapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing thesignal propagation delay.We have assessed this method using simulations and arrived at the following two important conclusions:(1) the receiver’s measurements of the sky brightness temperature is likely to be the main error source, rather thanthe algorithm error introduced when calculating the wet delay from the observed sky brightness temperatures;(2) the method requires an extension of the frequency range of the receiver well beyond 14 GHz in order toincrease the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHzfrom the emission line at 22.2 GHz.In spite of the fact that the present VGOS receivers observe at too low frequencies we find it meaningful toassess the radiometric stability of these receivers at the higher end of the frequency band. We have used one ofthe Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separatethe atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried outduring different atmospheric conditions and the estimated sky temperatures are compared to the observationsdone with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-freeconditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and15 GHz observations from the VGOS receiver

Актуализация принципов планирования деятельности предприятия

Рассматриваются вопросы развития некоторых принципов планирования показателей деятельности предприятий в контексте современной парадигмы менеджмента, дается характеристика принципов планирования и условий их применения

Modeling Lithospheric Thickness Along the Conjugate South Atlantic Passive Margins Implies Asymmetric Rift Initiation

The lithospheric architecture of passive margins is crucial for understanding the tectonic processes that caused the breakup of Gondwana. We highlight the evolution of the South Atlantic passive margins by a simple thermal lithosphere-asthenosphere boundary (LAB) model based on onset and cessation of rifting, crustal thickness, and stretching factors. We simulate lithospheric thinning and select the LAB as the T = 1,330°C isotherm, which is calculated by 1D advection and diffusion. Stretching factors and margin geometry are adjusted to state-of-the-art data sets, giving a thermal LAB model that is especially designed for the continental margins of the South Atlantic. Our LAB model shows distinct variations along the passive margins that are not imaged by global LAB models, indicating different rifting mechanisms. For example, we model up to 200 km deep lithosphere in the South American Santos Basin and shallow lithosphere less than 60 km in the Namibe Basin offshore Africa. These two conjugate basins reflect a strong asymmetry in LAB depth that resembles variations in margin width. In a Gondwana reconstruction, we discuss these patterns together with seismic velocity perturbations for the Central and Austral Segments of the margins. The shallow lithosphere in the Namibe Basin correlates with signatures of the Angola Dome, attributed to epeirogenic uplift in the Neogene, suggesting an additional component of post-breakup lithospheric thinning

Systematic investigation of a family of gradient-dependent functionals for solids

Eleven density functionals are compared with regard to their performance for the lattice constants of solids. We consider standard functionals, such as the local-density approximation and the Perdew-Burke-Ernzerhof (PBE) generalized-gradient approximation (GGA), as well as variations of PBE GGA, such as PBEsol and similar functionals, PBE-type functionals employing a tighter Lieb-Oxford bound, and combinations thereof. Several of these variations are proposed here for the first time. On a test set of 60 solids we perform a system-by-system analysis for selected functionals and a full statistical analysis for all of them. The impact of restoring the gradient expansion and of tightening the Lieb-Oxford bound is discussed, and confronted with previous results obtained from other codes, functionals or test sets. No functional is uniformly good for all investigated systems, but surprisingly, and pleasingly, the simplest possible modifications to PBE turn out to have the most beneficial effect on its performance. The atomization energy of molecules was also considered and on a testing set of six molecules, we found that the PBE functional is clearly the best, the others leading to strong overbinding

The Kepler Pixel Response Function

Kepler seeks to detect sequences of transits of Earth-size exoplanets orbiting Solar-like stars. Such transit signals are on the order of 100 ppm. The high photometric precision demanded by Kepler requires detailed knowledge of how the Kepler pixels respond to starlight during a nominal observation. This information is provided by the Kepler pixel response function (PRF), defined as the composite of Kepler's optical point spread function, integrated spacecraft pointing jitter during a nominal cadence and other systematic effects. To provide sub-pixel resolution, the PRF is represented as a piecewise-continuous polynomial on a sub-pixel mesh. This continuous representation allows the prediction of a star's flux value on any pixel given the star's pixel position. The advantages and difficulties of this polynomial representation are discussed, including characterization of spatial variation in the PRF and the smoothing of discontinuities between sub-pixel polynomial patches. On-orbit super-resolution measurements of the PRF across the Kepler field of view are described. Two uses of the PRF are presented: the selection of pixels for each star that maximizes the photometric signal to noise ratio for that star, and PRF-fitted centroids which provide robust and accurate stellar positions on the CCD, primarily used for attitude and plate scale tracking. Good knowledge of the PRF has been a critical component for the successful collection of high-precision photometry by Kepler.Comment: 10 pages, 5 figures, accepted by ApJ Letters. Version accepted for publication

Ultra-wideband feed systems for the EVN and SKA - evaluated for VGOS

The design of the Square Kilometre Array (SKA) project for radio astronomy is now materializing at a rapid speed; the EU Horizon 2020 RadioNet project BRoad-bAND (BRAND) has the ambition to deliver a decade bandwidth receiver for EVN. The ultra-wideband quad-ridge flared horn (QRFH) feed systems developed for these projects show good performance within the geodetic VLBI Global Observing System (VGOS) frame due to the overlapping frequency bands and reflector geometries. We estimate, through simulation, system equivalent flux density (SEFD) of the two feed systems in the VGOS reflector and compare the it to the existing system installed on one of the 13.2 m diameter reflectors of the Onsala twin telescope (OTT). The two frequency bands analyzed cover 1.5−15.5 GHz and 4.6−24 GHz. Both systems show SEFD better than 1000 Jy over large parts of resp. frequency band - comparable to the 3−18 GHz feed systems. For the SKA QRFH over 4.6−24 GHz, the water vapor absorption line at 22 GHz is within the operational band, therefore we study the application of water-vapor radiometry in line-of-sight of the telescope

On the information content in linear horizontal delay gradients estimated from space geodesy observations

We have studied linear horizontal gradients in theatmospheric propagation delay above ground-based stationsreceiving signals from the Global Positioning System (GPS).Gradients were estimated from 11 years of observations fromfive sites in Sweden. Comparing these gradients with thecorresponding ones from the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses shows thatGPS gradients detect effects over different timescales causedby the hydrostatic and the wet components. The two stationsequipped with microwave-absorbing material below the antenna,in general, show higher correlation coefficients withthe ECMWF gradients compared to the other three stations.We also estimated gradients using 4 years of GPS data fromtwo co-located antenna installations at the Onsala Space Observatory.Correlation coefficients for the east and the northwet gradients, estimated with a temporal resolution of 15 minfrom GPS data, can reach up to 0.8 for specific months whencompared to simultaneously estimated wet gradients frommicrowave radiometry. The best agreement is obtained whenan elevation cut-off angle of 3\ub0 is applied in the GPS dataprocessing, in spite of the fact that the radiometer does notobserve below 20\ub0. We also note a strong seasonal dependencein the correlation coefficients, from 0.3 during monthswith smaller gradients to 0.8 during months with larger gradients,typically during the warmer and more humid part of theyear. Finally, a case study using a 15 d long continuous verylong-baseline interferometry (VLBI) campaign was carriedout. The comparison of the gradients estimated from VLBIand GPS data indicates that a homogeneous and frequentsampling of the sky is a critical parameter

Conservation of grassland birds in North America: understanding ecological processes in different regions

Many species of birds that depend on grassland or savanna habitats have shown substantial overall population declines in North America. To understand the causes of these declines, we examined the habitat requirements of birds in six types of grassland in different regions of the continent. Open habitats were originally maintained by ecological drivers (continual and pervasive ecological processes) such as drought, grazing, and fire in tallgrass prairie, mixed-grass prairie, shortgrass prairie, desert grassland, and longleaf pine savanna. By contrast, grasslands were created by occasional disturbances (e.g., fires or beaver [Castor canadensis] activity) in much of northeastern North America. The relative importance of particular drivers or disturbances differed among regions. Keystone mammal species grazers such as prairie-dogs (Cynomys spp.) and bison (Bison bison) in western prairies, and dam-building beavers in eastern regions of the continent. Deciduous forests played a crucial, and frequently unappreciated, role in maintaining many grassland systems. Although fire was important in preventing invasion of woody plants in the tallgrass and moist mixed prairies, grazing played a more important role in maintaining the typical grassland vegetation of shortgrass prairies and desert grasslands. Heavy grazing by prairiedogs or bison created a low \u27grazing lawn\u27 that is the preferred habitat for many grassland bird species that are restricted to the shortgrass prairie and desert grasslands. Ultimately, many species of grassland birds are vulnerable because people destroyed their breeding, migratory, and wintering habitat, either directly by converting it to farmland and building lots, or indirectly by modifying grazing patterns, suppressing fires, or interfering with other ecological processes that originally sustained open grassland. Understanding the ecological processes that originally maintained grassland systems is critically important for efforts to improve, restore, or create habitat for grassland birds and other grassland organisms. Consequently, preservation of large areas of natural or seminatural grassland, where these processes can be studied and core populations of grassland birds can flourish, should be a high priority. However, some grassland birds now primarily depend on artificial habitats that are managed to maximize production of livestock, timber, or other products. With a sound understanding of the habitat requirements of grassland birds and the processes that originally shaped their habitats, it should be possible to manage populations sustainably on \u27working land\u27 such as cattle ranches, farms, and pine plantations. Proper management of private land will be critical for preserving adequate breeding, migratory, and winter habitat for grassland and savanna species