GGTO – an interesting correction

It turns out, as reported by Inside GNSS (Sept/Oct 2007), that of course the GPS and Galileo time-scales will not be the same. GGTO is the GPS-Galileo Time Offset The navigation solution calculated by receivers using signals from both navigation systems will incur in an error if the GGTO is not accounted for. The Galileo and GPS navigation messages will include the GGTO which will allow the measurements from each system to be properly combined to a common time. the pseduoranges determined with Galileo will be referenced to the Galileo System Time, and the GPS pseudoranges to the GPS System Time. But since the system times are not the same the offset between the times will be given as the GGTO. As reported by Inside GNSS (Sept/Oct 2007) position calculations are not too sensitive to errors in the GGTO reported in the Navigation message, we will see!

We do have some experience with this as we currently have many dual-system receivers tracking GPS and GLONASS. Using the raw data from these receivers requires that at least we estimate a common time offset between the GLONASS pseudoranges and the GPS pseudoranges. The time offset in this case are hardware dependent since they depend greatly on the type of receiver for the values, so it has been speculated that decorrelating and processing the GLONASS signal takes longer in the dual-system receivers even if the receivers timestamps all the pseudoranges as occuring at the same time, cleary they are not at the same time!!

Recently it has been shown that actually each receiver-GLONASS satellite link requires an individual time offset calculation for the estimation of all the GLONASS quantities to be correct. IF an individual correction is not calculated in each receiver-GLONASS satellite link the error goes into the estimated satellite clock bias and it can be as large as 5 or 6 ns.

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