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Department of Cartography, Sao Paulo State University, Roberto Simonsen – 305, Presidente Prudente, SP, 19060-900, Brazil
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- PublicationRestrictedStochastic modelling considering ionospheric scintillation effects on GNSS relative and point positioning(2010-05-03)
; ; ; ; ; ; ; ;Alves da Silva, H.; Department of Cartography, Sao Paulo State University, Roberto Simonsen – 305, Presidente Prudente, SP, 19060-900, Brazil ;Camargo, P.; Department of Cartography, Sao Paulo State University, Roberto Simonsen – 305, Presidente Prudente, SP, 19060-900, Brazil ;Galera Monico, J. F.; Department of Cartography, Sao Paulo State University, Roberto Simonsen – 305, Presidente Prudente, SP, 19060-900, Brazil ;Aquino, M.; Institute of Engineering Surveying and Space Geodesy (IESSG), University of Nottingham, University Park, Nottingham NG7 2RD, UK ;Marques, H. A.; Department of Cartography, Sao Paulo State University, Roberto Simonsen – 305, Presidente Prudente, SP, 19060-900, Brazil ;De Franceschi, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Dodson, A.; Institute of Engineering Surveying and Space Geodesy (IESSG), University of Nottingham, University Park, Nottingham NG7 2RD, UK; ; ; ; ; ; Global Navigation Satellite Systems (GNSS), in particular the Global Positioning System (GPS), have been widely used for high accuracy geodetic positioning. The Least Squares functional models related to the GNSS observables have been more extensively studied than the corresponding stochastic models, given that the development of the latter is significantly more complex. As a result, a simplified stochastic model is often used in GNSS positioning, which assumes that all the GNSS observables are statistically independent and of the same quality, i.e. a similar variance is assigned indiscriminately to all of the measurements. However, the definition of the stochastic model may be approached from a more detailed perspective, considering specific effects affecting each observable individually, as for example the effects of ionospheric scintillation. These effects relate to phase and amplitude fluctuations in the satellites signals that occur due to diffraction on electron density irregularities in the ionosphere and are particularly relevant at equatorial and high latitude regions, especially during periods of high solar activity. As a consequence, degraded measurement quality and poorer positioning accuracy may result. This paper takes advantage of the availability of specially designed GNSS receivers that provide parameters indicating the level of phase and amplitude scintillation on the signals, which therefore can be used to mitigate these effects through suitable improvements in the least squares stochastic model. The stochastic model considering ionospheric scintillation effects has been implemented following the approach described in Aquino et al. (2009), which is based on the computation of weights derived from the scintillation sensitive receiver tacking models of Conker et al. (2003). The methodology and algorithms to account for these effects in the stochastic model are described and results of experiments where GPS data were processed in both a relative and a point positioning mode are presented and discussed. Two programs have been developed to enable the analyses: GPSeq (currently under development at the FCT/UNESP Sao Paulo State University – Brazil) and PP_Sc (developed in a collaborative project between FCT/UNESP and Nottingham University – UK). The point positioning approach is based on an epoch by epoch solution, whereas the relative positioning on an accumulated solution using a Kalman Filter and the LAMBDA method to solve the Double Differences ambiguities. Additionally to the use of an improved stochastic model, all data processing in this paper were performed using an option implemented in both programs, to estimate, for each observable, an individual ionospheric parameter modelled as a stochastic process, using either the white noise or the random walk correlation models. Data from a network of GPS Ionospheric Scintillation and TEC Monitor (GISTM) receivers set up in Northern Europe as part of the ISACCO project (De Franceschi et al., 2006) were used in the experiments. The point positioning results have shown improvements of the order of 45% in height accuracy when the proposed stochastic model is applied. In the static relative positioning, improvements of the order of 50%, also in height accuracy, have been reached under moderate to strong scintillation conditions. These and further results are discussed in this paper.373 37 - PublicationRestrictedImproving the GNSS positioning stochastic model in the presence of ionospheric scintillation(2009-03)
; ; ; ; ; ; ; ; ;Aquino, M.; Institute of Engineering Surveying and Space Geodesy (IESSG), University of Nottingham, Nottingham, UK ;Monico, J. F. G.; Department of Cartography, Sao Paulo State University, Pres. Prudente, São Paulo, SP, Brazil ;Dodson, A. H.; Institute of Engineering Surveying and Space Geodesy (IESSG), University of Nottingham, Nottingham, UK ;Marques, H.; Department of Cartography, Sao Paulo State University, Pres. Prudente, São Paulo, SP, Brazil ;De Franceschi, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Alfonsi, Lu.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Romano, V.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Andreotti, M.; Geospatial Research Center Ltd., Christchurch, New Zealand; ; ; ; ; ; ; Ionospheric scintillations are caused by timevarying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and бφ represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were computed, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.390 44