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Session B2: Trends in GNSS Augmentation Systems

PPP-RTK with SSR-Based Corrections via Digital Audio Broadcasting in Germany
Thomas Krawinkel, Steffen Schön, Leibniz University Hannover, Institute of Geodesy; Jens Schroeder, RFmondial GmbH; Christopher Perschke, Geo++ GmbH; Christoph Wallat, Markus Vennebusch, Landesamt für Geoinformation und Landesvermessung Niedersachsen; Christoph Knöfel, Bundesamt für Kartographie und Geodäsie
Date/Time: Wednesday, Sep. 18, 5:08 p.m.

Real-time kinematic (RTK) positioning enables challenging applications such as autonomous driving and precision farming. To that end, applying corrections to the received GNSS signals is essential. Typically, those corrections are derived from a network of continuously operating reference stations (CORS) such as the German satellite positioning service SAPOS. The corrections account for satellite orbit and clock errors, atmospheric refraction (troposphere, ionosphere), and signal delays in both satellite and receiver antennas. Since they are usually modeled in the so-called error state space, this concept is referred to as observation space representation (OSR). This approach requires bidirectional communication between the service provider and the user. The reason for this is the fact that the user has to send his position to the service provider so that the latter can compute and transmit appropriate OSR corrections back. Transmission of those data takes place via mobile phone networks. However, due to uneven network coverage and/or topographical conditions, so-called dead spots, in some regions or network congestion in crowded areas, communication between provider and user can be interrupted. Furthermore, the increasing demand for precise real-time applications leads to an increasing computational and communication burden for the service provider.
All of this requires establishing a new service. For the dissemination of correction data, the Digital Audio Broadcasting (DAB, DAB+) standard can be used, which is widely available in most European countries. Because of the unidirectional broadcasting nature of DAB, a new way of providing the actual correction data to the user needs to be introduced. The state space representation (SSR) model, where the user receives corrections directly for the error states instead of precomputed observation-specific corrections, meets those requirements. Such a service that combines the advantages of globally available PPP with the advantages of regional network RTK, referred to as PPP-RTK, enables the dissemination of GNSS correction data to a virtually unlimited number of users over a wide area like the whole of Germany. Therefore, practical studies like the one described in our contribution are very important to evaluate the quality of the service already in its early stages.
Starting in 2021, the Federal Office for Cartography and Geodesy (BKG) is testing transmission of SSR correction data via DAB+ in Germany. The feed for this broadcast is operated by the company RFmondial GmbH. We will use these data to investigate the differences between two concepts. The classical approach using OSR corrections (RTCMv3 format) from the SAPOS high precision real-time positioning service transmitted via mobile internet, and the new approach with SSR corrections (SSRZ format) broadcast over DAB+. For this purpose, we carry out dedicated static and kinematic GNSS experiments. Such an experiment is based on a zero-baseline measurement configuration, which in this case consists of six receivers, namely two Septentrio PolaRx5e, two Septentrio Mosaic-X5, and two JAVAD OMEGA, all connected to the same NovAtel VEXXIS GNSS-850 antenna. In each of these three receiver pairs, one receiver works with OSR corrections and the other works with SSR corrections. Unfortunately, SSR corrections are not yet natively supported by any commercially available GNSS receiver. Therefore, we convert those corrections from SSR to OSR with the Geo++ software tool SSR2OBS. To be specific, the SSR data received by a DAB+ software-defined radio are transferred via TCP to a laptop running SSR2OBS, and after conversion to OSR corrections forwarded to the GNSS receivers in RTCMv3 format. On the one hand, we will investigate the new SSR-based approach regarding latency and integrity of the DAB+ signal. On the other hand, both classical and new approach will be evaluated and compared by means of typical geodetic performance parameters such as precision, accuracy and reliability of the position solution computed by each receiver.
In conclusion, we will give a short overview of our collaborative project, its goals and current status. The main focus of our contribution will be on the presentation and discussion of the most recent findings regarding the use of SSR corrections transmitted via DAB+ in typical high-precision GNSS applications.

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