The Chinese BeiDou Navigation Satellite System has shown potential for precise positioning with a comparable accuracy to that of the Global Positioning System (GPS) at the centimeter level for the horizontal component and the subcentimeter level for the vertical component. However, a longer convergence time limits the performance of BeiDou precise point positioning (PPP) compared to the GPS PPP solutions. In this study, we applied the tropospheric delay information, derived from the European Centre of Medium-Range Weather Forecasts (ECMWF) analysis and prediction data, into the simulated real-time BeiDou-only and BeiDou/GPS PPP to augment the solutions. Observations from stations in Southeast Asia, which are capable of tracking the BeiDou constellation from the International GNSS Service (IGS) Multi-GNSS Experiment and Pilot Project (MGEX) network, are processed with different strategies: the standard PPP and the introduced ECMWF-augmented PPP with analysis and prediction data, respectively. The positioning results demonstrate that the ECMWF-augmented BeiDou-only and BeiDou/GPS PPP methods using prediction data perform as well as those using analysis data. In the case of BeiDou-only PPP scenarios, remarkable advancements of 80.6% for the convergence time are achieved by two ECMWF-augmented PPP solutions with respect to the standard PPP method. For the positioning accuracy, the two proposed augmented PPP methods attain 6.6 cm in three-dimensional (3D) accuracy when the standard PPP solution get converged (10 cm), representing a remarkable improvement of 34%. As for the north/east/up component, improvements of 14.7 and 8% for positioning accuracy are obtained for the north and east components, respectively, while a remarkable improvement of 37.3% is achieved for the vertical component. In terms of the BeiDou/GPS PPP solutions, the ECMWF-augmented PPP scenarios attain over 10% improvements in 3D accuracy in all processing session lengths. These improvements totally come from the vertical component, whereas almost no enhancements are obtained in two horizontal components.
Abstract:The circulation pump in an organic Rankine cycle (ORC) increases the pressure of the liquid working fluid from low condensing pressure to high evaporating pressure, and the expander utilizes the pressure difference to generate work. A portion of the expander output power is used to offset the consumed pumping work, and the rest of the expander power is exactly the net work produced by the ORC system. Because of the relatively great theoretical pumping work and very low efficiency of the circulation pump reported in previous papers, the characteristics of the expander power used for offsetting the pumping work need serious consideration. In particular, the present work examines those characteristics. It is found that the characteristics of the expander power used for offsetting the pumping work are satisfactory only under the condition that the working fluid absorbs sufficient heat in the evaporator and its specific volume at the evaporator outlet is larger than or equal to a threshold value. OPEN ACCESSEnergies 2014, 7 4958
Different from researches in two-terminal nanoscale graphene structures, the negative differential resistance (NDR) phenomenon in graphene nanoribbon superlattice (GNSL) field-effect transistors (FETs) is studied in this reported work. Numerical analyses of two types of GNSL FETs with different gate voltages reveal that NDR occurs in some 'Z'-type GNSL FETs under some gate voltages, which develops NDR research compared with the traditional two-terminal nanoscale structures. Based on these results, two trends are observed: the 3m + 2 series GNSL FETs easily exhibit NDR, whereas it is more difficult to achieve this phenomenon with narrow FETs. This phenomenon is explained by the transmission coefficient as well as ab-initio calculations of the energy levels, where the entire channel of the FET is considered as a supercell. Through this analysis, the effect of gate control on energy-level localisation is uncovered, and a heterojunction-like explanation is proposed. This new explanation bridges the gap between a novel structure's physical analysis and the general semiconductor device concept, which can also provide inspiration for improving our understanding of novel nanostructure devices.
Ionosphere products that are relatively precise are available thanks to the efforts of the International GNSS Service (IGS), and it might be possible to obtain a high success rate for the fixed integer ambiguities for medium-or longerbaseline ambiguity resolution (AR) using the ionosphere products as a priori information constraints. In this study, we used the IGS precise ionosphere products as a priori information before forming double-difference (DD) measurement equations using only original observables in a mid-range relative positioning and estimated the ionosphere residuals explicitly after DD. Furthermore, we proposed a sequential and partial ambiguity resolution (SPAR) strategy under the integer least square condition to realize fast and reliable AR. To demonstrate our proposed strategy, we randomly selected seven baselines ranging from 30 to 111 km and undertook positioning in a post-processing mode using real GPS dual-frequency data. According to the results, the SPAR strategy has a faster convergence process compared with batch AR. For instance, the convergence time with >90 % cumulative frequency percentage (probability) for 30, 40, 56, 66, 80, 95, and 111 km baselines was advanced by 55, 50, >75, 85, >110, 65, and >35 epochs, respectively, with a 30-s sample interval. By considering ionospheric correction before DD, we found further improvement in the initialization performance with the use of the SPAR strategy.
The modernization of Global Navigation Satellite System (GNSS) including the transmission of signals on multiple frequencies has greatly promoted the development of the popular PPP (Precise Point Positioning) technique. A key issue of multi-frequency PPP is the handling of the observable-specific signal biases in order to allow for carrier-phase ambiguity resolution (AR). As a result, PPP modeling at a user side in the multi-frequency case varies depending on the definition of the applied phase bias products. In this study, we investigate the positioning performance of GPS L1/L2/L5 and Galileo E1/E5a/E5b/E6 undifferenced ionosphere-float model in the conventional PPP mode and the single-epoch mode using the uncombined code and phase bias products generated at the French CNES (Centre National D’Etudes Spatiales). A series of widelane ambiguities are configured in our multi-frequency PPP functional model instead of forming the classical Melbourne–Wübbena (MW) combination. The best integer equivariant (BIE) estimator is used for the ambiguity resolution in a conventional cascading scheme according to the wavelength of the combined ambiguities for each constellation. Real data collected at IGS stations with a 30-s sampling interval is applied to evaluate the above models. For the conventional kinematic PPP configuration, a significant accuracy improvement of 63% on the east component of the fixed solution is obtained with respect to the ambiguity-float solution. The PPP convergence is accelerated by 17% after the AR. Regarding the single-epoch positioning, an accuracy of 32 and 31 cm for north and east components can be achieved, respectively, (68th percentile) with the instantaneous widelane-ambiguity resolution, which is improved by 13% and 16% compared to multi-frequency code-based or float solution.
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