EUREF Analysis Centres Workshop

How good are the broadcast ephemeris of GPS, Glonass, Galileo and Beidou? An evaluation based on recent data

26 Oct 2017, 12:15

15m

Meridian Room (Royal Observatory of Belgium)

Open session

Speaker

Alessandro Caporali (University of Padova, Department of Geosciences)

Description

We assess the quality of a sample of broadcast orbits of GPS, Glonass, Galileo and Beidou taking precise orbits provided by CODE as reference. First we analyze the alignment of the reference systems adopted by broadcast orbits respect to that provided by precise orbits. In the second part we analyze the coordinate differences in the RSW (radial, along-track, out-of-plane) reference system. The study focuses on GPS week 1950, from 21/05/2017 to 27/05/2017. For each day we have obtained four (one per each constellation) set of seven Helmert parameters, which define the transformation from precise to broadcast reference systems, and consist of 3 translations (Tx, Ty, Tz), 3 rotations (Rx, Ry, Rz) and one scaling factor (k). We find that for GPS and Galileo the translation of the origin is less than 0.10 m and rotations are less than 1 milli-second of arc. For Glonass and Beidou we have higher differences: the translation of the origin is about 0.35 m and the rotations are comprised between 1 and 4 milli-seconds of arc. The scale factor is about 0.02 mm/km for all the systems except for Glonass, which shows a value 4 times higher. We now address coordinate differences in the RSW frame: GPS satellites show radial and along-track differences with mean values less than 1.0 m, in absolute value. Standard deviation is about few decimeters up to about one meter, respectively. Out-of-plane differences have a nearly zero mean values, with a standard deviation of about few decimeters, and show an oscillating trend, characterized by a period of about 12 hours and an amplitude up to 0.8 m. Clock differences have a nearly zero mean value with a standard deviation of maximum 2 ns. Glonass coordinate and clock differences show an oscillating trend with a period of about 12 hours. The amplitudes are about 1 m, 2 m and 2.5 m for radial, along-track and out-of-plane components, respectively, and 8 ns. Mean values of radial and out-of-plane differences are maximum few decimeters, in absolute value, whereas mean values of along-track differences are up to 3 m in absolute value. Mean values of clock differences are different from a satellite to another, varying from -20 ns to 20 ns, with a standard deviation up to 10 ns. Also Beidou coordinate and clock differences show an oscillating trend. The period is about 12 hours for Medium Earth Orbit (MEO) satellites (C11, C12, C14) and about a whole day for the other satellites (Inclined Geosynchronous Satellite Orbit - IGSO). The amplitudes are generally lower than 1.0 m, but in some days we observe slightly higher amplitudes for MEO satellites in the along-track component. Clock differences are continuous and with constant values. Mean values are generally comprised between 90 and 120 ns, except for C13 for which we observe a clock difference of 56 ns. The standard deviation of clock differences is 1-2 ns, except for C11 for which we observe two large discontinuities: one on 21th and the other on 23th. For Galileo, it is important to remember that three types of ephemeris are available: I/NAV E1-B, F/NAV E5a and I/NAV E5b. We have chosen to use I/NAV E1-B ephemeris. In addition, referring to paragraph 5.1.9.3 of Galileo OS SIS ICD, we have rejected I/NAV blocks with Health Status equal to 1 (Signal out of service) and accepted the others. For 11 of the 17 satellites we have found always the SV health value set to 0, which means Data Validity Status = “Navigation Data Valid” and Health status = “Signal OK”. For the other 6 satellites we have found the value 455, which means Data Validity Status = “Working without guarantee” and Health status = “Signal Component currently in Test”. These satellite are the last 4 launched on 17/11/2016 (E03, E04, E05, E07) and the two launched on 22/08/2014 (E14, E18) into non nominal elliptical orbit. For Galileo we have found irregular spacing between ephemeris blocks. Often between two consecutive blocks there are 10 minutes and sometimes up to several hours. Since the time validity of each ephemeris block is +/- 3600 s, it is questionable to compute coordinates and clock using the broadcast data, outside the validity time of an ephemeris block. Coordinate differences are smaller compared to those of other systems, except for E14 and E18. We report mean values lower than 0.2 m, in absolute value, with a standard deviation of about 0.1-0.2 m for radial and out-of-plane components and 0.2-0.5 m for along-track component. Clock differences, like GPS, have a nearly zero mean value (except for E08 which shows an offset of about 8 ns) with a standard deviation of maximum 2 ns. E14 and E18 show scattered coordinate differences with very large values, so about 75% of coordinates are rejected by Bernese and not considered in the estimation of Helmert parameters. We speculate that the modified Keplerian model may not be suitable for eccentric orbits. Clock differences show an offset of about 4-8 ns, with the same standard deviation as for the other satellites.