Integrity Monitoring (IM) for the Singapore Satellite Positioning Reference Network (SiReNT) infrastructure

Abstract

The topic of this thesis is Integrity Monitoring (IM), with special focus on the Singapore Satellite Positioning Reference Network (SiReNT) infrastructure. This network infrastructure supports and improves Global Navigation Satellite System (GNSS) applications, related to positioning, navigation, tracking and (deformation) monitoring. This project focuses on what IM is with regards to the SiReNT infrastructure, and how the current methodology can be improved so that its output is easier to interpret/understand for its users. This research consists of two parts, a theory study, and a data study. The theory study is aimed at developing a fundamental understanding of GNSS and integrity, and the need for an IM component, as well as what comprises IM around the globe. The data study, which includes IM data processing, is performed to develop an understanding of the stability and performance of the SiReNT infrastructure itself, and its sensitivity to several degrading factors. For this the IM displacement data of a period of up to 5 months is analysed, in combination with the atmospheric information, and the number and geometry of tracked satellites. The Trimble method, with its default thresholds, has been applied to this IM displacement data, in order to determine what percentage was supposedly reliable, and inside these thresholds. It was also investigated whether these thresholds are performing adequately, if applied, and whether they have any impact on simultaneous recorded Rover data. The latter was assumed; the IM displacement data is thought to be usable to control the Rover data. When it comes to positioning solutions, the quality parameters include, integrity, accuracy, precision, availability, reliability, continuity, capacity and redundancy. These quality parameters are affected by several degrading factors. At the moment the Trimble Pivot Platform (TPP) is used to perform the network processing, as well as the integrity monitoring of the Continuous Operating Reference Stations (CORS) of the SiReNT infrastructure (and with that the impact of several of these degrading factors). This method is an example of the so-called external monitoring method. ‘External’ as the monitoring does not take place within the receiver itself, such as in the other method available; Receiver Autonomous Integrity Monitoring (RAIM). The external monitoring method and RAIM can either be used individually and/or independently, or in some sort of combination, complementing each other. In several other countries, namely the Netherlands, Australia (Victoria), Hong Kong, and Malaysia, variants of the external monitoring method are applied in performing IM of their GNSS infrastructures. Usually this includes baseline and coordinate processing/comparison, both in real-time and through post-processing (often using the Bernese GNSS software). All of them have backup systems in place. When it comes to IM visualization, even though there are similarities, the approaches differ per country. Some only visualise as much as the status of their reference stations, others visualise the real-time positioning solution of their IM stations, or have a complete dashboard visualising a number of parameters. From this research can be concluded that the integrity (in terms of CORS’ stability and positioning solution quality) of the SiReNT infrastructure is actually quite good. Of the IM displacement data which was supposedly reliable (following from the applied method), over 99% was within the set thresholds, for all CORS. The default thresholds (therefore) are iv

considered to be quite a good trade-off; they filter out most of the main disturbances (especially the ones which are assumed to be related to sudden severe fluctuations in the Ionospheric activity), while minimizing the amount of data loss. There is room for improvements however; as a start, the (overall) redundancy should be increased, making the SiReNT infrastructure more robust and increase its reliability. The method in which the IM is currently performed could be improved in several ways as well. This could for example be done by tracking more constellations/signals, constructing additional CORS and backup systems, and using multiple different hardware brands for the equipment that is used in the SiReNT infrastructure. Also, the SiReNT infrastructure should be monitored with respect to surrounding IGS stations, rather than using a CORS that is part of the SiReNT infrastructure itself as a reference. An important addition to the IM component could be the feedback from the Rover data and performance (the initialization time, and positioning solution quality). Currently made assumptions related to that do turn out to be erroneous, however. Any threshold applied to the IM displacement data cannot be used to control the Rover data. The way in which the output resulting from this IM is visualised, should be more intuitive/easy to interpret. For this a dashboard is to be developed (in design following the example of the dashboard of the Dutch Cadastre), which homepage should be visualizing several parameters, all in real-time. This would enable the (end-)user to see what is the current status with regards to all these parameters, in one view. One would however also be able to access further information related to these parameters, by selecting the one of interest. These parameters are to be the number of tracked satellites, the number of connected users, an indication of the Ionospheric delay, the predicted geometric error, and the Rover performance. The content of the information presented to the public should be kept to a minimum (only the necessary), where the content presented to the system administrators can be more elaborate. The conclusions of this research point towards the need for further research into how the applied methods/algorithms and thresholds (as well as the other available options) can be utilized in order to improve the IM procedure in Singapore. On top of that, the impact of interference should be identified and monitored. Being able to do this, will improve the (integrity of the) infrastructure and the information provided by it.