GALCS-Mobil

Precise GNSS-based positioning for mobile devices using Galileo Commercial Service

Aims

The central goal of the project is the development of algorithms for PPP positioning of simple mass market devices in PPP mode.

Brief Description

The GNSS-based determination of single points by means of GPS SPS (Standard Positioning Service) or Galileo OS (Open Service) allows coordinate determination in the +/- 3-5 m range in case of good satellite geometry. Higher accuracies can be achieved using commercial, mostly ground-based, correction services using the differential method (down to the cm range (RTK)), whereby the mobile device must in principle be a 2-frequency receiver in the higher price segment with RTCM functionality. It is also possible to obtain global correction data for positioning in the dm-range (for a fee) via geostationary satellites (GEOs) using SIS (Signal in Space), whereby higher-quality devices are also required and, in addition, the GEOs are not visible even in little built-up areas. An accuracy of 1-2 dm would be sufficient for a large number of today’s positioning applications, but this should be achievable with very cheap receivers, in the best case with the GNSS receivers integrated in smartphones.

As part of the Galileo services, it is planned to provide a fee-based SIS service (Commercial Service (CS)) from 2020/21 via the Galileo satellites. This comprises two core areas, namely a high-accuracy service (HA) and an authentication service (AU). The corresponding correction and authentication codes should primarily be modulated onto the Galileo carrier waves in the E6 band (E6B, E6C) or E1 band (open-AU). For this purpose, the majority of the Galileo satellites is directly and substantial permanently connected to the upload stations of the ground segment in order to be able to make the most recent orbital and clock corrections as possible available to users. Based on global satellite orbit corrections in PPP mode (Precise Point Positioning), the HA service promises position accuracy in the <1dm range for users with geodetic high-end GNSS receivers (after a coordinate convergence time of approx. 10-20 Minutes). The correction data are supplied by a so-called Commercial Service Provider (CSP) which transmits the correction data in real time to the GSC (Galileo Service Center), whence the data are uploaded to the Galileo satellites. The decoding of the E6B data signal requires a key on the user side. The CSP’s business model is based on the transmission of this key to the users.

The advantage over current SIS services for the CSP is the emission of the correction data via satellites in inclined MEO (Medium Earth Orbiter), whereby users all over the world can be reached. These services are currently limited primarily by the geostationary orbits of their satellite segment. An increase in Galileo uplink capacities (20 ULS) allows the upload of the correction data to 20 Galileo satellites in real time.

On the other hand, it is possible since the middle of 2016 to access the raw GNSS code and phase observations of mobile devices with the Android operating system (see e.g. https://developer.android.com/guide/topics/sensors/gnss.html). Before this time, only the positions calculated internally in the mobile device could be read out. Currently, the data from GPS are mainly used for the positioning of smartphones, but there is also a number of end devices on the market (Samsung, Huawei) that performs measurements on several or all GNSS systems, including Galileo. The advantage of being able to access the raw measurements is various. On the one hand, the quality of the measured variables can now be calculated realistically, and on the other hand, it is now possible to implement more complex evaluation models which also consider the phase measurement data. In addition, the user now has the possibility to model the distance errors caused by the atmosphere (troposphere, ionosphere, unlike the most used and often very simple and sometimes proprietary approaches.

Since the Galileo CS currently aims primarily at the high-end GNSS receiver market (at least 2-3 frequency receivers), the evaluation models and technical requirements within this project will be developed to help owners of simple mobile devices to improve their GPS / Galileo observation data by using the global correction data provided by the Galileo CS HA Service and the atmospheric models available in the consortium to achieve a position accuracy of a few dm (+/- 1-2dm). The position is calculated on a server installed by the operator and is to be implemented for both static and kinematic (moving) users. The positions returned to the user can also be authenticated as added value via the free Galileo CS AU service (Galileo E1). Any measurement data recorded by other GNSS (GLONASS, Beidou) will not be used in this project to reduce the complexity of the evaluation and because these systems are not supported by the Galileo CS-HA service.