The increasing progress in the field of satellite navigation systems (GNSS, SBAS) in the recent decades supports effort to use it for determination of train position for railway safety-related systems. Satellite-based augmentation systems (SBAS) such as WAAS in the USA, and EGNOS in Europe, are available and a new global satellite navigation system Galileo is being built by the European GNSS agency (GSA). The currently available SBAS systems were developed in order to satisfy aviation requirements. But the safety concept on railways is very different from the aviation safety concept. The railway safety concept in Europe is determined by means of the CENELEC standards (EN 50126, EN 50129, EN IEC 61508). So it is necessary to find a way how to use GNSS systems in accordance with strict railway standards. The main problem is attainment of sufficient integrity of position solution [5, 12]. Satisfaction of safety integrity level 4 (SIL4) is necessary for railways [6, 7, 8, 9]. At the beginning, it can provide low-cost controlling system for the local, regional and freight railway lines. GNSS provides a 3D position (position in horizontal and vertical plane). The value of altitude is cruical for application in aviation, in ground transportation this value is not so important. On the contrary, the value of horizontal position is cruical. For the purpose of increasing the integrity of GNSS-based position determination we propose a new method of the detection of a GNSS horizontal position error based on the relation between vertical and horizontal position error. As was mentioned for example in [4], as GPS is a three dimensional positioning system, errors between any two coordinates may be correlated, and so there can be relations between errors in individual dimensions. The general 3D GPS-based position solution can be divided into two parts:

- 2D horizontal position

- 1D vertical position

We investigated the relation between errors in the horizontal and vertical plane in real data measured by a GNSS receiver. It was static measurement and the antenna location was exactly known. The vertical position provided by GNSS is not constant. In ground transportation we can mostly make an assumption of nearly a constant value of altitude during the ride. Especially in railway transportation the changing of altitude during the ride is limited by many factors (railway standards, properties of track) So we investigate the possibility of using values of altitude to estimate a position error in the horizontal plane. As the receiver determines the values of the vertical position in real time, the detection of the horizontal position error based on the values of altitude can help detect the actual position error in horizontal plane during the train ride also in real time. The sensitivity of this method to errors in pseudoranges (error caused by multipath) was also investigated. This was done by simulation with software receiver Pegasus (Eurocontrol). The analysis was based on real data from GNSS.

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