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What is a GPS epoch? I encounter this often in technical discussions and it does not have a clear meaning to me. Lately it was used in this post and it seems to mean the receiver update rate (the frequency at which is the position updated).

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GPS Epoch is a continuous time system for all satellites and observation systems. It is represented in seconds since Jan 5, 1980 (presumably when GNSS went online). It is measured as the average of the atomic clocks for the observations and satellites - the accuracy of positioning depends on the precise timing accurate to better than two nanoseconds, given the speeds the satellites are traveling.

The time is represented as a week number (starting Saturday night at midnight), and seconds in week. Thus the seconds count can reach 604,800 then roll to zero. Because the number is 10bit data, when the weeks hit 1023 (210 - 1) they then roll back to zero (this happened in 1999 for the first time).

GPS epoch doesn’t adjust for leap seconds, so is 16 seconds ahead of current UTC time (and growing over time).

Source is Satellite Geodesy by Günter Seeber: http://www.geokniga.org/bookfiles/geokniga-seeber-g-satellite-geodesy-2003.pdf

Edit: linked to digital copy of source, thanks @Mapperz

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In terms of data received by GPS receivers, epoch usually means the time interval between consecutive points. Here's a source for that: Glossary of GPS Terminology

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