Higher order ionospheric effects in GNSS positioning in the European region

Abstract

Abstract. After removal of the Selective Availability in 2000, the ionosphere became the dominant error source for Global Navigation Satellite Systems (GNSS), especially for the high-accuracy (cm-mm) demanding applications like the Precise Point Positioning (PPP) and Real Time Kinematic (RTK) positioning.The common practice of eliminating the ionospheric error, e.g. by the ionosphere free (IF) observable, which is a linear combination of observables on two frequencies such as GPS L1 and L2, accounts for about 99 % of the total ionospheric effect, known as the first order ionospheric effect (Ion1). The remaining 1 % residual range errors (RREs) in the IF observable are due to the higher -second and third, order ionospheric effects, Ion2 and Ion3, respectively. Both terms are related with the electron content along the signal path; moreover Ion2 term is associated with the influence of the geomagnetic field on the ionospheric refractive index and Ion3 with the ray bending effect of the ionosphere, which can cause significant deviation in the ray trajectory (due to strong electron density gradients in the ionosphere) such that the error contribution of Ion3 can exceed that of Ion2 (Kim and Tinin, 2007).The higher order error terms do not cancel out in the (first order) ionospherically corrected observable and as such, when not accounted for, they can degrade the accuracy of GNSS positioning, depending on the level of the solar activity and geomagnetic and ionospheric conditions (Hoque and Jakowski, 2007). Simulation results from early 1990s show that Ion2 and Ion3 would contribute to the ionospheric error budget by less than 1 % of the Ion1 term at GPS frequencies (Datta-Barua et al., 2008). Although the IF observable may provide sufficient accuracy for most GNSS applications, Correspondence to: Z. G. Elmas (isxzge1@nottingham.ac.uk) Ion2 and Ion3 need to be considered for higher accuracy demanding applications especially at times of higher solar activity.This paper investigates the higher order ionospheric effects (Ion2 and Ion3, however excluding the ray bending effects associated with Ion3) in the European region in the GNSS positioning considering the precise point positioning (PPP) method. For this purpose observations from four European stations were considered. These observations were taken in four time intervals corresponding to various geophysical con- (Marques et al., 2011) was used to calculate the magnitudes of Ion2 and Ion3 on the range measurements as well as the total electron content (TEC) observed on each receiver-satellite link. The program also corrects the GPS observation files for Ion2 and Ion3; thereafter it is possible to perform PPP with both the original and corrected GPS observation files to analyze the impact of the higher order ionospheric error terms excluding the ray bending effect which may become significant especially at low elevation angles (Ioannides and Strangeways, 2002) on the estimated station coordinates.

Statistics of GNSS amplitude scintillation occurrences over Dakar, Senegal, at varying elevation angles during the maximum phase of solar cycle 24

“…Ionospheric effects are significant components of GNSS error budgets, particularly during disturbed periods [ Akala et al ., ; Elmas et al ., ]. These effects manifest in two ways: group delay and scintillation.…”

A Study of GPS Carrier-Phase Time Transfer Noise Based on NIST GPS Receivers

“…However, the second-order ionospheric noise cannot be cancelled since we do not use a triple-frequency GPS receiver. [27] shows that the second-order ionospheric noise can play some role in GPS positioning (up to ~ 2 cm positioning error), especially when an extreme space weather event occurs. We currently do not know the impact of the second-order ionospheric noise on GPS timing (theoretically, the GPS timing should be correlated with the GPS positioning).…”

Five-frequency Galileo long-baseline ambiguity resolution with multipath mitigation

“…Forming the ionospheric-free linear combination removes the first-order term of the ionospheric delays, which accounts for about 99% of the total ionospheric delays (Elmas et al 2011). However, the ionospheric-free linear combination has the drawback that it has less flexibility to further strengthen the model, e.g., to constrain the temporal or spatial ionospheric behaviors (Mervart et al 2013;Teunissen and Khodabandeh 2015).…”