941 research outputs found

    Journal Staff

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    This master’s thesis paper is an exploratory study along with conceptual design of launch and recovery systems(LARS) for unmanned vehicles and RHIB:s, which has been conducted for ThyssenKrupp Marine System AB inKarlskrona, Sweden. Two concepts have been developed, one for aerial vehicles (UAV:s) and one for surfaceand underwater vehicles (USV, RHIB and UUV). The goal when designing the two LARS has been to meet thegrowing demand within the world navies for greater off-board capabilities. The two concepts have been designedto be an integrated solutions on a 90 m long naval ship and based on technology that will be proven in year2015-2020. To meet the goal of using technology that will be proven in year 2015-2020, existing and futurepossible solutions has been evaluated. From the evaluations one technique for each concept was chosen forfurther development.In the development of a LARS for aerial vehicles only fixed wing UAV:s have been considered. The conceptwas made for a reference UAV based on the UAV Shadow 200B, which has a weight of 170 kg. The conceptthat was developed is a parasail lifter that can both launch and recover the reference UAV effectively. In thedevelopment of a system for surface and underwater vehicles only vehicle lengths in the span 1-12 m have beenconsidered. The concept that has been developed is a stern ramp that uses a sled to launch and recover all threevehicle types. The two concepts that has been developed are in an early design state and the papers results shouldtherefore be seen as an estimation of what each system are capable of performing

    Three Dimensional Radiative Effects in Passive Millimeter/Sub-Millimeter All-sky Observations

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    This study was conducted to quantify the errors prompted by neglecting three-dimensional (3D) effects, i.e., beam-filling and horizontal photon transport effects, at millimeter/sub-millimeter wavelengths. This paper gives an overview of the 3D effects that impact ice cloud retrievals of both current and proposed (Ice Cloud Imager) satellite instruments operating at frequencies of approximate to 186.3 and approximate to 668 GHz. The 3D synthetic scenes were generated from two-dimensional (2D) CloudSat (Cloud Satellite) observations over the tropics and mid-latitudes using a stochastic approach. By means of the Atmospheric Radiative Transfer Simulator (ARTS), three radiative transfer simulations were carried out: one 3D, one independent beam approximation (IBA), and a one-dimensional (1D). The comparison between the 3D and IBA simulations revealed a small horizontal photon transport effect, with IBA simulations introducing mostly random errors and a slight overestimation (below 1 K). However, performing 1D radiative transfer simulations results in a significant beam-filling effect that increases primarily with frequency, and secondly, with footprint size. For a sensor footprint size of 15 km, the errors induced by neglecting domain heterogeneities yield root mean square errors of up to approximate to 4 K and approximate to 13 K at 186.3 GHz and 668 GHz, respectively. However, an instrument operating at the same frequencies, but with a much smaller footprint size, i.e., 6 km, is subject to smaller uncertainties, with a root mean square error of approximate to 2 K at 186.3 GHz and approximate to 7.1 K at 668 GHz. When designing future satellite instruments, this effect of footprint size on modeling uncertainties should be considered in the overall error budget. The smallest possible footprint size should be a priority for future sub-millimeter observations in light of these results

    Backus-Gilbert footprint matching methodology applied on MWI and ICI observations

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    Final report of the EUMETSAT study with contract number EUM/CO/18/4600002075/CJA

    A contrarian investment strategy for equity fund selection

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    The following thesis examines the existence of contrarian profits in the Norwegian equity fund market. Three different pricing models are used to determine if a contrarian strategy is able to create abnormal returns in the Norwegian equity fund market from 1995-20141 . Firstly, we apply a similar approach as De Bondt and Thaler (1985) by using a single-index CAPM model. Our results initially support the work of De Bondt and Thaler, as we find significant reversals in fund returns with a two-year ranking and two-year holding strategy. Secondly, we expand the CAPM model by adding size- and value risk factors as suggested by Fama and French (1993), which results in a statistically insignificant alpha, suggesting that the strategy significantly loads on size and value. Finally, we extend the model even further, by adding Carhart's momentum factor, which also yields an insignificant alpha. Our research suggests that the single-index CAPM model, initially tested by De Bondt and Thaler, is an inferior model compared to the three-factor model introduced by Fama and French. The results indicate that contrarian investors do not obtain abnormal returns as they are simply compensated for the inherent risk of their portfolios, mainly suggested by the size effect literature by Banz (1981).nhhma

    Time series inversion of spectra from ground-based radiometers

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    Retrieving time series of atmospheric constituents from ground-based spectrometers often requires different temporal averaging depending on the altitude region in focus. This can lead to several datasets existing for one instrument, which complicates validation and comparisons between instruments. This paper puts forth a possible solution by incorporating the temporal domain into the maximum a posteriori (MAP) retrieval algorithm. The state vector is increased to include measurements spanning a time period, and the temporal correlations between the true atmospheric states are explicitly specified in the a priori uncertainty matrix. This allows the MAP method to effectively select the best temporal smoothing for each altitude, removing the need for several datasets to cover different altitudes. The method is compared to traditional averaging of spectra using a simulated retrieval of water vapour in the mesosphere. The simulations show that the method offers a significant advantage compared to the traditional method, extending the sensitivity an additional 10 km upwards without reducing the temporal resolution at lower altitudes. The method is also tested on the Onsala Space Observatory (OSO) water vapour microwave radiometer confirming the advantages found in the simulation. Additionally, it is shown how the method can interpolate data in time and provide diagnostic values to evaluate the interpolated data

    Impact of ice aggregate parameters on microwave and sub-millimetre scattering properties

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    Microwave scattering properties for 1101 aggregates were calculated using DDA (Discrete Dipole Approximation), at three typical radar bands (13.4, 35.6, 94.1 GHz) and three passive microwave frequencies (183.31, 325.15 and 664 GHz). The aggregates were generated in a semi-physical stochastic fashion and are composed of hexagonal crystals of varying axis ratio, ranging from 1/15 (plates) to 15 (columns). Horizontally aligned particles were assumed and scattering properties were assessed for zenith/nadir observations. Crystal axis ratio, number of crystals, effective density and aerodynamic area, were found to correlate with extinction and back-scattering efficiencies. However, the dependency between these variables and scattering properties vary between the frequencies. Interestingly, bulk extinction was found to have a relatively low sensitivity to particle shape at 664 GHz. Furthermore, extinction was found to be less shape sensitive than back-scattering. These results are promising for the sake of the upcoming Ice Cloud Imager (ICI) mission. In addition, for the considered set of aggregates, it is shown that both bulk extinction and back-scattering are more directly related to snow fall than ice water content. Triple frequency signatures were also calculated, which demonstrated clear dependence on constituent crystal axis ratio and conversely on aggregate effective density, in agreement with the literature

    The 22 GHz radio-aeronomy receiver at Onsala Space Observatory

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    We present a radiometer system for regular long-term measurements of water vapour in the middle atmosphere. To be able to do continuous and long-term measurements a simple, robust, reliable and automatic system is needed. Our system therefore is based on a stable, uncooled, HEMT amplifier frontend and on a digital spectrometer backend. In order to minimise reflections in the frontend transmission line, which distort the signal due to standing waves, we have designed a corrugated receiver horn, which combines good characteristics (low return loss and sidelobes) and narrow beamwidth to simplify the receiver optics. In order to make the radiometer system as simple as possible, we use the sky as the calibration cold load. This is possible since we use the observed brightness temperatures of an already existing broadband dual-channel 21.0/31.4-GHz radiometer, at the observation site, to estimate the brightness temperature of the sky at . However, we have developed a calibration method, which makes it possible to estimate the sky brightness temperature even if we cannot use the dual-channel radiometer. Despite new measurements, which became available in recent years, the determination of middle atmospheric water vapour distribution still remains a challenge due to the fact that there is a large dispersion among the different measurement methods and data sets, which are obtained on a sparse and sporadic basis. This is the reason why several instruments similar to ours currently are developed in Europe

    Study to support the definition of Arctic Weather Satellite (AWS) high frequency channels

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    This study compares the options of having a channel at 229 GHz or having some around\ua0325 GHz from a single perspective, cloud filtering/correction of 183 GHz data.Final report of EUMETSAT study under contract :\ua0 EUM/CO/20/4600002417/CJ

    An experimental 2D-Var retrieval using AMSR2

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    A two-dimensional variational retrieval (2D-Var) is presented for a passive microwave imager. The overlapping antenna patterns of all frequencies from the Advanced Microwave Scanning Radiometer 2 (AMSR2) are explicitly simulated to attempt retrieval of near-surface wind speed and surface skin temperature at finer spatial scales than individual antenna beams. This is achieved, with the effective spatial resolution of retrieved parameters judged by analysis of 2D-Var averaging kernels. Sea surface temperature retrievals achieve about 30 km resolution, with wind speed retrievals at about 10 km resolution. It is argued that multi-dimensional optimal estimation permits greater use of total information content from microwave sensors than other methods, with no compromises on target resolution needed; instead, various targets are retrieved at the highest possible spatial resolution, driven by the channels\u27 sensitivities. All AMSR2 channels can be simulated within near their published noise characteristics for observed clear-sky scenes, though calibration and emissivity model errors are key challenges. This experimental retrieval shows the feasibility of 2D-Var for cloud-free retrievals and opens the possibility of stand-alone 3D-Var retrievals of water vapour and hydrometeor fields from microwave imagers in the future. The results have implications for future satellite missions and sensor design, as spatial oversampling can somewhat mitigate the need for larger antennas in the push for higher spatial resolution
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