[1] The total zenith tropospheric delay (ZTD) is an important parameter of the atmosphere and directly or indirectly reflects the weather and climate processes and variations. In this paper the ZTD time series with a 2-hour resolution are derived from globally distributed 150 International GPS Service (IGS) stations (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006), which are used to investigate the secular trend and seasonal variation of ZTD as well as its implications in climate. The mean secular ZTD variation trend is about 1.5 ± 0.001 mm/yr at all IGS stations. The secular variations are systematically increasing in most parts of the Northern Hemisphere and decreasing in most parts of the Southern Hemisphere. Furthermore, the ZTD trends are almost symmetrically decreasing with increasing altitude, while the summation of upward and downward trends at globally distributed GPS sites is almost zero, possibly reflecting that the secular ZTD variation is in balance at a global scale. Significant annual variations of ZTD are found over all GPS stations with the amplitude from 25 to 75 mm. The annual variation amplitudes of ZTD near oceanic coasts are generally larger than in the continental inland. Larger amplitudes of annual ZTD variation are mostly found at middle latitudes (near 20°S and 40°N) and smaller amplitudes of annual ZTD variation are located at higher latitudes (e.g., Antarctic) and the equator areas. The phase of annual ZTD variation is about 60°in the Southern Hemisphere (about February, summer) and about 240°in the Northern Hemisphere (about August, summer). The mean amplitude of semiannual ZTD variations is about 10 mm, much smaller than annual variations. The semiannual amplitudes are larger in the Northern Hemisphere than in the Southern Hemisphere, indicating that the semiannual variation amplitudes of ZTD in the Southern Hemisphere are not significant. In addition, the higher-frequency variability (RMS of ZTD residuals) ranges from 15 to 65 mm of delay, depending on altitude of the station. Inland stations tend to have lower variability and sites at ocean and coasts have higher variability. These seasonal ZTD cycles are due mainly to the wet component variations (ZWD).
In this paper, we present a novel maximum likelihood (ML) decoding algorithm for space-time block codes (STBC) over fading channels. Using a lattice representation for space-time codes by transforming complex channel models into real matrix equations, we propose a new efficient ML decoding algorithm with performance identical to the conventional ML decoder. We show that the complex orthogonal space-time codes, in fact, allow a separate ML decoding for each inphase and quadrature-phase component. For rate one quasiorthogonal designs with N transmit antennas (N > 4), the proposed decoding scheme reduces the decoding complexity from O(M.'j2) to O(M.'/4) in a M,-QAM constellation. Moreover, multidimensional rotated constellations are constructed by which the quasi-orthogonal codes can achieve full diversity, while the proposed ML decoding method offers significant computational savings as compared with previous approaches.
The ionospheric slab thickness, the ratio of the total electron content (TEC) to the F2-layer peak electron density (NmF2), is closely related to the shape of the ionospheric electron density profile Ne (h) and the TEC. Therefore, the ionospheric slab thickness is a significant parameter representative of the ionosphere. In this paper, the continuous GPS observations in South Korea are firstly used to study the equivalent slab thickness (EST) and its seasonal variability. The averaged diurnal medians of December-January-February (DJF), March-April-May (MAM), June-July-August (JJA) and September-October-November (SON) in 2003 have been considered to represent the winter, spring, summer and autumn seasons, respectively. The results show that the systematic diurnal changes of TEC, NmF2 and EST significantly appeared in each season and the higher values of TEC and NmF2 are observed during the equinoxes (semiannual anomaly) as well as in the mid-daytime of each season. The EST is significantly smaller in winter than in summer, but with a consistent variation pattern. During 14-16 LT in daytime, the larger EST values are observed in spring and autumn, while the smaller ones are in summer and winter. The peaks of EST diurnal variation are around 10-18 LT which are probably caused by the action of the thermospheric wind and the plasmapheric flow into the F2-region. r
The separation of ethylenediamine (EDA) from aqueous solution is a challenging problem because its mixture forms an azeotrope. Pressure-swing distillation (PSD) as a method of separating azeotropic mixture were investingated. For a maximum-boiling azeotropic system, pressure change does not greatly affect the azeotropic composition of the system. However, the feasibility of using PSD was still analyzed through process simulation. Experimental vaporliquid equilibrium data of water-EDA system was studied to predict the suitability of thermodynamic model to be applied. This study performed an optimization of design parameters for each distillation column. Different combinations of operating pressures for the low-and high-pressure columns were used for each PSD simulation case. After the most efficient operating pressures were identified, two column configurations, low-high (LP+HP) and high-low (HP+ LP) pressure column configuration, were further compared. Heat integration was applied to PSD system to reduce low and high temperature utility consumption.
ABSTRACT:In order to determine the plausibility of GPS-derived Integrated Water Vapour (IWV) as a fog detector, the relationship between GPS IWV and meteorological observations during fog was studied in a time domain. In this research, it is assumed that fog is a simple function of water vapour and cloud water based on non-precipitation warm cloud in the bulk water-continuity model and that, during fog, the atmosphere over the fog layer is horizontally homogeneous.The case of dense fog was chosen, for which the visibility observed by the human eye ('synop') was <1 km with the regular observation at co-located GPS sites. Preliminary results showed that GPS IWV could detect the microphysical and dynamical process of fog, such as conversion of the water vapour into cloud water and inflow of water vapour into GPS observation sites. Furthermore, it could potentially be found that GPS IWV reflected unceasingly and quantitatively the state of fog after saturation.To mitigate limitations of the temporal resolution and the inaccuracy of 'synop' visibility data, a GPS receiver and a visibility meter were operated at the same site simultaneously. This experiment revealed definitively that typical radiation fog was more suitable than other types of fog for detection by GPS IWV.Based on these results, it is suggested that GPS IWV can be considered as a supplementary technique in detecting fog processes and improving fog predictability.
Computer simulations were performed to obtain highly pure tetrahydrofuran (THF) with over 99.9 mole% from the mixture of THF and water. Pressure swing distillation (PSD) was used since the azeotropic point between tetrahydrofuran and water can be varied with pressure. A commercial process simulator, PRO/II with PROVISION release 8.3, was used for the simulation studies. The Wilson liquid activity coefficient model was used to simulate the low pressure column, and the Peng-Robinson equation of state model was added to correct the vapor phase non-idealities for the modeling of the high pressure column. The most optimal reflux ratios and the most optimal feed stage locations that could minimize the total reboiler heat duties were determined.
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