Mitigation of Ionospheric Effects on GNSS Positioning at Low Latitudes

Jihye Park, Sreeja Veettil, Marcio Aquino, Lei Yang, Claudio Cesaroni

Peer Reviewed

Abstract:	Ionospheric conditions at low latitudes are extremely harsh mainly due to the frequent occurrence of scintillation and the presence of strong TEC gradients. As the ionospheric effect is one of the crucial factors in GNSS RTK positioning, it is very important to mitigate this effect in order to obtain a highly accurate GNSS positioning solution. For this study, the São Paulo state region in Brazil is chosen as a test area where scintillation frequently occurs during post sunset hours and where there is a strong demand for high accuracy GNSS positioning to support applications such as precision agriculture. The GNSS reference stations in the São Paulo State RTK network are very sparsely distributed, the average inter-distance between stations is around 125km, presenting yet another challenge for high accuracy GNSS RTK positioning. This study presents a strategy to mitigate the ionospheric impact on RTK positioning in the São Paulo State region with an experimental result. The proposed strategy explores two approaches that can be applied simultaneously: a) to mitigate the scintillation effect on the GNSS signals by refining the stochastic model of the corresponding observations; b) to precisely estimate the residual double difference ionospheric delay by exploiting an accurate external TEC map. These approaches were developed in the context of the FP7/GSA funded CALIBRA (Countering GNSS high Accuracy applications Limitations due to Ionospheric disturbances in BRAzil) project (http://www.calibra-ionosphere.net/). The strategy was tested on a long baseline (88km), where kinematic processing was performed under strong scintillation conditions (DOY21 in 2014). Two different techniques were used to improve the stochastic model, the first based on the tracking jitter variances estimated per satellite/receiver link from the post correlation In-phase (I) and Quadra-phase (Q) samples, namely IQ based approach, and the second based on the tracking jitter variance, also estimated per satellite/receiver link, but from a modified version of the scintillation sensitive tracking model introduced by Conker et al. (2003), which we refer to as the Conker’ approach. The innovation with respect to the original Conker model lies on a new method to estimate the amplitude scintillation index (S4) that overcomes the limitation of the original Conker model, which cannot handle strong scintillation, i.e. when S4>0.707. For the ionospheric delay estimation, a regional TEC map, referred to as the CALIBRA TEC Map (CTM), was used in order to mitigate the strong TEC gradients observed in the region. The use of this mitigation strategy was compared with the use of conventional state of the art (called herein ‘non-mitigated’) approaches, whereby the stochastic model is computed based on the carrier to noise ratio (c/n0) of each observation and the ionospheric delay is estimated by Global Ionosphere Map (GIM). In the case study presented in the paper, the 3D positioning RMS error by applying the non-mitigated solution together with the GIM was 1.814m during the post sunset strong scintillation hours (0:00-6:00 UT), whereas by applying the IQ based technique simultaneously with the CTM, the result significantly improved, with a 3D RMS of 0.756m. When the Conker’ based technique was used simultaneously with the CTM further improvement was obtained, with a 3D RMS of 0.412m. These results represent improvements over the ‘non-mitigated’ strategy of 58.3% and 77.3%, respectively.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	3550 - 3556
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In