PRN1/SVN049 L5 Payload Drawback and PRN27 Outage on the 30th of June 2009: GNSS Receiver Reactions and Lessons Learnt

N. Suard

Abstract:	CNES was deeply involved in EGNOS operations until October 2009 by participating in the daily operations of the EGNOS Performances Assessment and Check Facility and is also involved in the EGNOS evolutions through a mixed CNES/ESA EGNOS team. In parallel, CNES is monitoring, for its own needs, GPS and SBAS performances thanks to its Navigation and Time Monitoring Facility. In this context, CNES has analysed in January 2010 more specifically the quality of the data delivered by different types of receivers spread over the world when tracking PRN1/SVN049, using its own receivers or using the IGS database. These analyses were done through different combinations of the data, in particular the bias between P1 and C1, the ionosphere delay based on the mono frequency code/phase divergence or based on the dual frequency techniques (code1/code2, phase1/phase2), the phase ambiguity and wide lane techniques. The obtained information was compared to the one obtained with PRN30 that was on the same node of the GPS constellation (B2) giving the opportunity with a few minutes of delay to discard any local effects as PRN30 was in view with a few minutes of delay in a similar situation from the receiver/user point of view. These analyses point out that observed degradation (bias as a function of the elevation that can impact some techniques to compute the ionosphere delay and residual ionosphere error) is depending on the receiver type and sometimes does not exist. From these results, we have concluded to the necessity to discard PRN1 in SBAS mask in case this satellite is declared one day “HEALTHY” and to discard any PRN1 data for the ionosphere corrections/protection computation. This information has been exchanged with EC, ESA and ESSP for EGNOS. We suppose that same kind of observations has been made on the WAAS side, because PRN1 has been excluded from the WAAS side on the 17th of September 2009. On another hand, by analysing for the sake of the EGNOS performance monitoring some local accuracy degradations not covered by the protection level before a PRN27 alert sequence reception, it has been discovered and confirmed by the GPS Navigation Centre that a short outage without NANU or description in the GPS ICD occurred on the 30th of June 2009 for that PRN. Different data sets from different receiver types were analysed showing different reactions in the tracking of this PRN. It has been also verify what the WAAS reaction was as this satellite was also in view of some WAAS stations. These observations were reported to the EUROCAE WG-62 in order to take care of them in some current analysis to try to shorten the re-tracking of satellite impacted by ionosphere scintillation. More generally, this type of short outage, not currently described in GPS applicable document could be also confusing at receiver/user level in regards to short local losses of signal due to local reasons that were only the one expected, explaining probably the different reactions observed. To confirm that, CNES is making some evolutions in its simulation means in order to go further on this hypothesis. The paper presents, for the two sets of analysis - the different types of data analysed - the different reactions observed in function of receiver types and data combination. - the SBAS (EGNOS and WAAS) status regarding PRN1/SVN049 situation and reaction regarding PRN27 short outage - the lessons learnt, recommendations and future work when relevant
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	1975 - 1983
Cite this article:	Suard, N., "PRN1/SVN049 L5 Payload Drawback and PRN27 Outage on the 30th of June 2009: GNSS Receiver Reactions and Lessons Learnt," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 1975-1983.
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