Sammendrag
Archived GPS ephemeris data for years 2000-2016 is investigated by cleansing and precision measures. The goal of this work was twofold: (1) to reveal and remove incorrect ephemeris data so that the cleansed data is as close to the real broadcasted data as possible, and (2) to analyse the quality of the cleansed data by comparing ephemeris based satellite locations to true locations. Our findings show that, besides obvious duplicates, many erroneous data epochs were also detected by monitoring the slowly changing I0 and Ω0 parameters. The cleansed data shows small deviation precision errors steadily decay in the 2000- 2016 period, as expected. Also the number of large errors show a decaying nature. Many large errors occur frequently in first operation period of new satellite vehicles, especially the new Block IIF, but these happened also to show in time epochs close to periods flagged as unhealthy. Further, it is shown that ephemeris data performance is independent on Toe parameter being modulo 100 or not.