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Time Discrepancies in Multi-GNSS Receivers

One of the major issues in testing GNSS receivers designed for use with multiple satellite systems is that the different systems do not necessarily share the exact same time-bases. And while the differences may be tiny, time is such a critical quantity in satellite navigation that even microsecond differences can create large accuracy errors.

The problem is a legacy of the GPS system, which has used its own time-base (GPS time) since it began in the early 1980s, rather than the globally accepted co-ordinated universal time (UTC) used elsewhere. GPS time is not corrected to match the rotation of the Earth, and so it does not contain leap seconds or other corrections that are periodically added to UTC. However, periodic corrections are performed to the clocks onboard each GPS satellite to correct relativistic effects and keep them synchronized with ground clocks. And the almanac that is broadcast with each GPS navigation message includes the difference between GPS time and UTC.

In comparison, both GLONASS and Galileo are locked to UTC (or a derivative thereof), and so do not require such corrections.

Fortunately, Spirent multi-GNSS simulators give the user full control of GPS time, allowing multi-GNSS tests to use a single time-base. They also account for timeshifts between the various UTC derivatives, using a second-order Markov process and adding the shift to a state vector to be estimated in the multi-GNSS receiver's PVT.

 
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