Calibration Challenges for Future Radio Telescopes

Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and 2-antenna interferometers, current instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to provide meaningful output (images), accurate calibration is of critical importance. Calibration must solve for the unknown antenna gains and phases, as well as the unknown atmospheric and ionospheric disturbances. Future telescopes will have a large number of elements and a large field of view. In this case the parameters are strongly direction dependent, resulting in a large number of unknown parameters even if appropriately constrained physical or phenomenological descriptions are used. This makes calibration a daunting parameter estimation task, that is reviewed from a signal processing perspective in this article.Comment: 12 pages, 7 figures, 20 subfigures The title quoted in the meta-data is the title after release / final editing

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Antibioticagebruik in de veehouderij wordt door de buitenwereld kritisch gevolgd

Retrieval of canopy component temperatures through Bayesian inversion of directional thermal measurements

Evapotranspiration is usually estimated in remote sensing from single temperature value representing both soil and vegetation. This surface temperature is an aggregate over multiple canopy components. The temperature of the individual components can differ significantly, introducing errors in the evapotranspiration estimations. The temperature aggregate has a high level of directionality. An inversion method is presented in this paper to retrieve four canopy component temperatures from directional brightness temperatures. The Bayesian method uses both a priori information and sensor characteristics to solve the ill-posed inversion problem. The method is tested using two case studies: 1) a sensitivity analysis, using a large forward simulated dataset, and 2) in a reality study, using two datasets of two field campaigns. The results of the sensitivity analysis show that the Bayesian approach is able to retrieve the four component temperatures from directional brightness temperatures with good success rates using multi-directional sensors (Srspectra˜0.3, Srgonio˜0.3, and SrAATSR˜0.5), and no improvement using mono-angular sensors (Sr˜1). The results of the experimental study show that the approach gives good results for high LAI values (RMSEgrass=0.50 K, RMSEwheat=0.29 K, RMSEsugar beet=0.75 K, RMSEbarley=0.67 K); but for low LAI values the results were unsatisfactory (RMSEyoung maize=2.85 K). This discrepancy was found to originate from the presence of the metallic construction of the setup. As these disturbances, were only present for two crops and were not present in the sensitivity analysis, which had a low LAI, it is concluded that using masked thermal images will eliminate this discrepanc

Set-up effects of piles in sand tested in the centrifuge

The bearing capacity of piles increases over time. Research has shown that this is caused by an increase in shaft friction combined with a constant or only slightly increasing base capacity. Although there are some ideas on the mechanisms that play a role there is no quantitative model to describe this mechanism. From the literature the shaft friction seems to increase linearly with the logarithm of time. For piles in the field this is proven by load tests performed between 1 until approximately 1000 days after installation. Literature indicates that set-up as a function of time is also present minutes and hours after installation. This allows investigating the set-up mechanisms under controlled conditions in a centrifuge. Therefore two test series have been performed to investigate the set-up for a single pile and a pile group. This paper presents the relevant literature and describes the position of the tests in the on-going research program on piles in The Netherlands. Furthermore, the results will be described and discussed. Time dependency in bearing capacity in sand can be observed in the centrifuge tests, although it is not certain whether some of the increase has not been caused by other mechanisms. It appears that the testing conditions as well as the effects of installation of neighboring piles are of great importance on the time effects