Landon Urquhart, Rodrigo Leandro, Dragos Catalin, Susan Heath, Jens Guenther, Sapcorda

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Abstract:

In January 2020, Sapcorda released its Premium GNSS positioning service. The service provides high precision GNSS correction data on the continental scale to enable high accuracy positioning in addition to integrity information which can be used in safety critical systems. The ionosphere is one of the primary error sources for high precision users due to the diverse nature of the ionosphere which exhibits both wide area effects such as ionosphere storms and travelling ionosphere disturbances, but also local effects caused by scintillation and small-scale gradients. For this reason, the ionosphere is one of the primary threats contributing to the target integrity risk for the correction service and which therefore must be properly handled for real-time GNSS users. This paper will describe the design considerations taken into account for characterizing the ionosphere impact on the system, the impact of ionosphere threats on correction service providers and their users as well as show examples of events observed during the first year of operation of the Sapcorda premium service. As part of the functional safety case developed for certification under functional safety standards such as ISO 26262 and IEC61508 it is necessary to properly characterize the threats which impact the system. These threats must then be allocated to the system components including the service provider and the end-user. Residual risk that cannot be properly mitigated by the service provide must then be communicated properly to the end-user so that the appropriate mitigation strategies can be implemented, and realistic protection levels can be established. This paper will describe the steps taken during concept phase including the identification of ionosphere events, from historical data, relevant literature and other case studies. Using this information the threat model will be described along with the identification of the failure modes, likelihood of occurrence and severity in order to establish the risk level. The paper will describe how the ionosphere threats were then allocated to the service provider and end user as well as the impact of this analysis on the system design, integrity concept and the development of the open-source SPARTN correction format. The SPARTN format, in addition to providing corrections for the ionosphere has been designed to properly communicate the uncertainty as well as the network design parameters which end-users can take advantage of to properly model the residual risk in their position engine. The core component of the SAPA correction service is the underlying tracking network of reference stations that are used to generate the precise corrections and integrity information. The dense reference network operates in real-time and continuously transmit their data to the Sapcorda control centers for processing. This tracking network provides an excellent opportunity for analyzing ionosphere characteristics at diverse geographic locations and at both local and regional scales. Using this data collected during the first year of test and operation, the ionosphere behavior observed by the tracking network will be categorized by region, extent and magnitude. Although the ionosphere is currently in a relatively quiet state, scintillation and other ionosphere events will be shown and analyzed and their impact on positioning and safety performance will be discussed. Finally, design considerations for end-user position engines will also be discussed showing how the SPARTN correction and integrity information can be used to mitigate the threats, what additional monitors must be implemented, and general filter design strategies which can be implemented to ensure safe operation even during these disturbed periods.