Inside GNSS Media & Research

MAR-APR 2018

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Page 60 of 67 M A R C H / A P R I L 2 0 1 8 Inside GNSS 61 Land User Receiver Prototype The PIPE GNSS Receiver used to run the INLU simulations is not only able to process samples, bit-true mode in INLU terms, it has also a semi-analytic opera- tion mode that calculates correlation results based on pre-computed auto- correlation functions of GNSS signals. e semi-analytic mode runs multiple times faster than the bit-true mode and allows for simulation of long runs. e scenario input data as well as the receiv- er's implementations for items such as tracking loops and PVT computation are identical for both modes. Tracking Algorithms In addition to standard tracking meth- ods like Early-Minus-Late and Bump Jumping, the PIPE receiver offers a number of state-of-the-art tracking techniques, such as the Double Delta Correlator, Kalman filter-based meth- ods, Double Estimator, and Maximum Likelihood-based methods, such as Multipath Estimating DLL and Vision Correlator. All these methods are scru- tinized during the INLU project. For the following example, the Astrium Correlator (AC) is applied which offers robustness against locks to false side peaks while tracking binary offset car- rier (BOC) signals (F. M. Schubert et alia (2014a)). For signal tracking, the AC uses a BOC's signal subcarrier to exploit the higher accuracy that can be achieved when tracking such signals to the legacy binary phase shift keying (BPSK) sig- nals. At the same time, the AC checks if it tracks the BOC signal's central peak via the observation of the BPSK enve- lope of the signal. If a lock to a side peak is detected, the tracker is commanded to switch its tracking point toward the correct main peak. Integrity Algorithms ARAIM Tailored to the Railway Environment The main objective of INLU was the development of integrity algorithms tailored to the land user environment. e following integrity concept for rail- way users was developed as an INLU scenario. Providing integrity for train posi- tion information is a major technical challenge due to the small integrity risk that is tolerated in railway applica- tions. In the current implementation of the European Rail Traffic Management System (ERTMS), the train position is propagated using odometry, and cor- rected when a balise group is reached. A balise is a transponder that provides an absolute location reference to the on-board unit of the train, allowing the train to locate itself within a move- ment authority. GNSS positioning is being considered in the context of the ERTMS evolution for the realization of a virtual balise concept, where GNSS is used for the detection of virtual balises. e approach of virtualizing the balise transmission system aims to reduce the cost of trackside infrastructure associ- ated with the installation and mainte- nance of physical balises, while mini- mizing changes to the existing system and maximizing interoperability. Con- sequentially, requirements on the integ- rity of position information provided by the GNSS receiver are very stringent. e integrity concept within INLU can be seen as a step towards a realiza- tion of the virtual balise function. A block diagram of this concept is shown in Figure 3 . e integrity algorithm pro- cesses pseudorange measurements from a GNSS receiver, measurements from odometry, and exploits information contained in a map database. It consists of two major building blocks, a module for pseudorange measurement rejection, and the integrity module that calculates the projection level. e measurement rejection module consists of a Kalman filter that integrates the odometer measurements with the track database and the pseudoranges from the GNSS receiver. The a priori position available from this filter is used to calculate the Mahalanobis distance of each pseudorange, and upon excess of a pre-defined threshold for the Mahalano- FIGURE 2 (a) Example of common railway scenery: The red boom points from the satellite transmitter to the blue train's receiver antenna. The receiving train just rode below a railway bridge and is about to enter a cutting. Another train is approaching on the other track, also shown as a blue box. The yellow balls represent reflection points from which multipath components emanate. Trees are shown in green. (b) Channel impulse response as generated by the railway model from the situation visible in (a).

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