GPS Galileo Time Offset: How It Affects Positioning Accuracy and How to Cope with It

摘要

Looking forward for the advent of the future European navigation system Galileo, we anticipate a big part of its potential users to utilize combined GPS/Galileo receivers. It will allow such users to get benefits of observing two satellite constellations (= more satellites in view), and utilizing services of two independent navigation systems (= more reliability). The combined use of GPS and Galileo requires the systems to be interoperable. Already on the early stages of the Galileo program, interoperability with GPS was set as a goal for Galileo design and implementation. Leaving aside such issues like signal design, frequency allocation, signal processing etc, we concentrate here on the timing aspects of the interoperability. The time offset between Galileo System Time (GST) and GPS Time will cause a slowly changing bias between GPS and Galileo measurements in a combined navigation solution. After discussing the GPS Galileo time offset (GGTO) magnitude, we consider two approaches how user can cope with its effect: utilisation of GGTO correction which is planned to be broadcast in the navigation messages of GPS and Galileo, or introduction of an additional unknown into the positioning solution. For both approaches, we assessed the potential accuracy of user positioning with the help of large-scale simulations. Also, we present a method to simulate user applications in an urban environment. Finally, we discuss two options for determination of the broadcast GGTO correction: a) using a two-way time transfer link between Galileo Precise Timing Facility (PTF) and the US Naval Observatory (USNO), and b) using a GPS time receiver connected to the physical realisation of GST as generated at the Galileo PTF.