Gauging systems currently in use have an end-of-life prediction accuracy of typically a few months up to a year for geostationary satellites. Replacement of a scientific or commercial satellite after 10-15 years in service is very expensive, putting pressure on improvement of end-of-life gauging accuracy for life-time extension. For comparison of gauging methods in use, a system analysis is performed on typical mono- and bi-propellant systems to evaluate the factors influencing gauging over life related to the propellant load from ground filling towards end-of-life operations in orbit. In general the gauging accuracy reduces over time. Three common gauging methods have been investigated: PVT (gas law), Bookkeeping (propellant flow integration) and (T)PGS ("Thermal Knocking"). The accuracy analysis shows that the propagation of uncertainties is related the propellant load, system design, ground filling, orbital operations and applied sensors. Using state-of-the-art data it was found that Bookkeeping currently provides the highest gauging accuracy between ±3 months for 10 years up to ±12 months for 15 years. A significant improvement of system analysis, ground operations and in-flight sensor technology is recommended to improve the gauging accuracy up to ±1 months for new satellites.
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