Receiver Inter System Bias Impact on SBAS Dual Constellation Positioning and Integrity

C. Boulanger, N. Suard, F. Mercier, C. Rodriguez, D. Lapeyre

Abstract:	In the current context of MEO constellation development and modernization, interoperability aspects are more and more important and already several publications are related to this topic with a focus on inter system biases. This particular point will have to be carefully addressed for SBAS as well, like for EGNOS V3 for which the GPS and GALILEO simultaneous augmentation is foreseen. In the beginning of 2013 over Europe, the SDCM system - the SBAS currently being developed by the Russian Federation as a component of GLONASS - offered the first opportunity to assess in real conditions what can be the inter system bias impact on positioning and to get elements to be discussed about integrity issues that could be induced. Indeed even in test mode, as SDCM performs integrity monitoring of GPS and GLONASS satellites, the corrections broadcasted by the Luch 5B satellite were applicable on both constellations. This paper will aim at describing the assessments carried out by CNES with these real data and a specific focus will be made on the system time differences and on the MT#12 format and content, analysed according to the current EUROCAE SBAS L1/L5 Interface Control Document [1]. It has to be noted that the GPS time to GLONASS time difference can be deduced from the monthly BIPM Circular T that gives a daily offset of each system time to UTC, enabling to compute the difference between both system times. The RINEX data used for these analyses were recorded with several types of receivers available in CNES or collected on GNSS IGS-MGEX (Multi-GNSS Experiment) networks in January and February 2013. In particular, 3 COTS receivers, from different manufacturers, Septentrio, Javad and Trimble, were connected to the same antenna in Toulouse (France) to check the receiver behaviour in front of the same signals in the same environment. This location is in the footprint of the Luch 5B geostationary satellite from which the messages were directly received. Toulouse is not in the SDCM precise service area due to the absence of enough ionosphere corrections and monitored Ionosphere Grid Point but remains in the SDCM service area as defined in its MT#27 so enabling the En-Route/NPA mode processing. However it was enough to observe the system time management and to exhibit each receiver contribution to the inter system bias. Other receiver data from the GNSS IGS-MGEX networks have been computed, in the SDCM service area (Trimble in KZN2) or not (Septentrio in BRUX) for example, in order to compare their internal inter system bias to the receivers located in Toulouse and used as reference. Different periods in January and February 2013 have been processed. If there are opportunities to complete the available dataset - in case of other Luch emissions before summer 2013 – additional analysis will be performed to give information on the studied bias stability. These data have been processed with ADN (Analysis and Display of Navigation data) tool, a software tool developed by Thales Services SAS in the frame of a CNES contract. This software is able to use all the signals and messages emitted by the MEO constellations and augmentation systems according to current standards or proposed new ones to compute the GNSS position and associated information (protection levels, residuals, biases…) with a full parameterization of the algorithms. Therefore it is possible to use directly the GLONASS time to SDCM time Offset broadcast in the SDCM MT#12 as defined in the EUROCAE SBAS L1/L5 ICD [1]. In that case we observe a degraded performance in the position domain depending on the receiver technology mainly when a change occurred in the number of satellites in view. It is also possible to add a fifth unknown in the position equations and to compute a Delta GGTO in order to cancel the receiver internal inter system bias. In this configuration, a feared event occurrence has to be analysed in detail. We can compare also the different information obtained in SDCM MT#12 and by Delta GGTO computation to the GGTO issued from a direct GPS/GLONASS solution. All these cases will be discussed in the paper. By specification the SDCM time shall be closed to the GPS time, assessment of that being done, the GLONASS time to SDCM time offset can be compared to information on GPS time and GLONASS time extracted from BIPM circular T: results of such comparisons will be also presented. In addition, the GLONASS FDMA signals being very specific, some complementary assessments on the GGTO with GPS and GALILEO using real data will be carried out (without SBAS for the moment) to draw our conclusions and relevant lessons learnt. [1] EUROCAE SBAS L1/L5 Interface Control Document (ED 134 May 2008)
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	854 - 864
Cite this article:	Boulanger, C., Suard, N., Mercier, F., Rodriguez, C., Lapeyre, D., "Receiver Inter System Bias Impact on SBAS Dual Constellation Positioning and Integrity," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 854-864.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In