Are Elliptical Galileo Satellites Usable for RTK ?

Abstract. Galileo satellites 5 and 6 (E18, E14), erroneously launched into elliptical orbits, were commissioned into service on Nov 30, 2020, but already on Feb 16, 2021 were again declared unusable in a wake of reports indicating repeatable daily problems with RTK, a precise phase-differential positioning technology. This decision stripped Galileo users of two apparently usable satellites even though RTK does not belong to the list of Galileo services and its availability and accuracy mostly depend upon base-rover interaction. The purpose of this paper is to analyze possible impact of deviant elliptical orbits on the performance of RTK. The statement of this paper is as follows: if the implementation of RTK algorithms is generally correct and follows Galileo ICD and good practices of GNSS theory, there is no reason to expect any anomalies due to high orbit eccentricity except for a limited range of problems caused by high age of ephemeris, which occurs few times a year due to limited uplink capacity of Galileo’s ground control segment. The impact of these extra errors on standard RTK services with base-lines of about 10 km is almost negligible except for the cases of delayed reception of differential corrections. In such cases an extra time-linear term is added to the error budget, particularly significant when time latencies reach values of 10 seconds or longer. In summary, we conclude that there is no reason to declare E14/E18 unhealthy, but it is recommended to broadcast appropriate values of the SISA index (I/NAV word 3, F/NAV p. 1) to indicate degraded accuracy of orbit predictions.