Improving the Performance of Galileo E1-OS by Optimizing the I/NAV Navigation Message

M. Paonni, M. Anghileri, T. Burger, L. Ries, S. Schlötzer, B.E. Schotsch, M. Ouedraogo, S. Damy, E. Chatre, M. Jeannot, J. Godet, D. Hayes

Abstract: The paper describes three new technical solutions that will be soon introduced within the Galileo E1 I/NAV message. The work leading to the design of the three solutions started with the 2012 Galileo Programme decision to re-profile the Safety-of-Life (SoL) service, thus making available a significant portion of the I/NAV message, initially marked as “reserved” in the Galileo Open Service Signal In Space Interface Control Document (OS SIS ICD, [1]). The main driver originally identified for the I/NAV message optimization work was to reduce the Time-To-First-Fix for the Galileo OS users. In particular, the design targeted on one side a shorter time to access the data necessary for the first fix (i.e. Clock and Ephemeris Data, CED) for the non-connected users and on the other a faster re-synchronization with the Galileo System Time (GST) for the connected (or assisted) users. Another key objective was to improve substantially the demodulation robustness of the Clock and Ephemeris Data (CED). At the end of a long activity including several iterations of design and performance assessment, three different solutions at data and symbol level have been selected for implementation: • Secondary Synchronisation Patterns (SSP) • Reduced Clock and Ephemeris Data (RedCED) • FEC2 Reed-Solomon encoding of the Clock and Ephemeris Data (FEC2 RS CED) The paper discusses how these new I/NAV message elements aim at the improvement of the speed and the robustness of the reception of Clock and Ephemeris Data (CED) and of Galileo System Time (GST) from each satellite. The legacy mechanism to broadcast CED and GST is maintained, and is complemented with the additional features. The Secondary Synchronisation Pattern (SSP) is introduced with the aim of enabling for the users the possibility to reconstruct the broadcast GST from each satellite without the need to wait for an actual GST broadcast, provided that the receiver is already coarsely synchronised with the GST (within +/- 3 seconds). SSPs are provided every two seconds and can be detected at symbol level, without decoding the navigation message. Successful detection of a single SSP is sufficient to resolve the GST ambiguity Reduced CED consists of a compressed set of Clock and Ephemeris Data in an optimised format that fits within one I/NAV word. Especially in environments with substantial fading and shadowing a single I/NAV word is typically faster and much easier to receive than the nominal CED consisting of four I/NAV words. In exchange and due to the compression, Reduced CED is less accurate and is expected to contribute in the order of 3m (1 sigma) to the User Equivalent Ranging Error. Reduced CED is therefore intended for a fast coarse PVT fix after start-up. The nominal I/NAV CED provided within the E1 OS signal is further protected and supported by means of the introduction of Reed Solomon based outer Forward Error Correction (FEC2). The nominal CED forms the systematic part of the code word. Four more I/NAV words of Reed Solomon parity are included into the message broadcast to the users. The Minimum Distance Separation (MDS) property of the Reed Solomon, combined with the protection of I/NAV words through the legacy Viterbi inner FEC and CRC, provides a very powerful flexibility to decode CED from all combinations of four different correctly received words (erasure correction). This yields a significant and systematic advantage in speed of reception of the nominal CED, in nearly every environment. If needed, the outer FEC2 can also be used for combined erasure and error correction, to further improve robustness of CED reception. After the three solutions are introduced and explained, an overall assessment of the anticipated performance of a Galileo receiver implementing each of them is provided, highlighting the advantages of the implementation, discussing different possible approaches and comparing the results with the typical performance of a Galileo user processing the legacy signal.
Published in: Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019)
September 16 - 20, 2019
Hyatt Regency Miami
Miami, Florida
Pages: 1134 - 1146
Cite this article: Paonni, M., Anghileri, M., Burger, T., Ries, L., Schlötzer, S., Schotsch, B.E., Ouedraogo, M., Damy, S., Chatre, E., Jeannot, M., Godet, J., Hayes, D., "Improving the Performance of Galileo E1-OS by Optimizing the I/NAV Navigation Message," Proceedings of the 32nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2019), Miami, Florida, September 2019, pp. 1134-1146.
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