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Positioning Methods
Published in Basudeb Bhatta, Global Navigation Satellite Systems, 2021
Remember that for the frequency differential approach, the compulsory requirement is to receive two signals at two frequencies (e.g., L1 and L2) issuing from the same satellite. We may recall that in Section 6.2 there was an indication of hybrid techniques, such as rapid static. This consists in using specific algorithms that allow a rapid integer ambiguity resolution once at least four satellites are tracked (a few minutes are enough). This is usually achieved in dual-frequency mode, although other modes are possible.
Performance analysis of low-cost GNSS stations for structural health monitoring of civil engineering structures
Published in Structure and Infrastructure Engineering, 2022
Nicolas Manzini, André Orcesi, Christian Thom, Marc-Antoine Brossault, Serge Botton, Miguel Ortiz, John Dumoulin
Processing was performed using RTKLib, an open-source and versatile GNSS software (Takasu, 2011; Takasu, Kubo, & Yasuda, 2007). RTKLib is also a well-documented scientific software that offers a wide range of processing methods and calculation parameters, with access to the source code. During processing, a Kalman filter Zhao, Cui, Guan, and Lu (2014) is used to obtain the final position, allowing to apply strong constraints to station’s position or speed, for example preventing it from quickly moving away from an initial or calculated position. One of the critical steps of GNSS processing, when using carrier phase positioning, is Ambiguity Resolution (AR). When computing GNSS solution, antenna position corresponds to a distance of N wavelengths cycles with each satellite. In case of a good resolution (fixed solution), N is an integer at every epochs (GPS time) for each satellite. However, due to noise and various perturbations, such solution cannot be achieved within fixed tolerance values. In those scenarios, N can be replaced with a float number, giving an approximate float solution.
A geodetic study of the Alpine Fault through South Westland: using campaign GPS data to model slip rates on the Alpine Fault
Published in New Zealand Journal of Geology and Geophysics, 2018
Chris J. Page, Paul H. Denys, Chris F. Pearson
GPS data have been processed using the Bernese software package (v. 5.2) (Dach et al. 2015) using 24 h daily position solutions. The Centre for Orbit Determination (CODE) precise satellite orbit and clock parameters, together with the I08.ATX absolute GPS receiver and satellite antenna phase centre model (Schmid et al. 2007) are used to generate daily position time series. The carrier phase ionosphere-free linear combination is used to correct the first-order ionosphere. Higher-order ionospheric effects are not considered here because Hernández-Pajares et al. (2007) and Petrie et al. (2010) showed that these effects are < 1 mm. Tropospheric effects are modelled using the Global Mapping Function, which maps the zenith troposphere delay to the elevation of each observation (Boehm et al. 2007). A 10° elevation cut-off angle is used, a compromise to constrain tropospheric effects but minimise multipath errors. Non-tidal atmospheric loading displacements are modelled according to Ray and Ponte (2003). The effects of ocean loading are corrected using the FES2004 model (Lyard et al. 2006) from the Onsala Space Observatory (holt.oso.chalmers.se/loading). Ambiguity resolution involves a recursive strategy that includes code and phase-based wide lane, QIF and direct L1/L2 fixed ambiguities, depending upon baseline length. The ITRF2014 reference frame is realised through the Helmert three-parameter transformation of the daily coordinate positions. Global IGS sites include those on the stable Pacific, Antarctic and Australian plates. Data outliers were removed using the Median Absolute Deviation (MAD) robust estimator at the 4σ level with σ = 1.4826 × MAD.