In GNSS and geodetic applications, an epoch refers to a specific instant in time used as a reference point for measurements, coordinates, or data collection. This fundamental concept serves multiple important purposes in satellite navigation, including timestamping observations, defining when reference frame coordinates are valid, and organizing data collection in receivers and processing software.
Within GNSS receivers and observation files, epoch typically refers to the specific moment when a set of measurements was recorded. Receivers collect pseudorange, carrier phase, and Doppler measurements from all visible satellites at discrete time intervals, commonly every second (1 Hz) or faster for high-rate applications (up to 100 Hz). Each measurement set is tagged with its epoch timestamp, enabling precise temporal correlation during processing. The quality and density of epoch data directly impacts positioning performance, particularly for kinematic applications tracking moving receivers.
In geodetic reference frame contexts, epoch has a broader meaning related to coordinate validity. Because the Earth’s surface is not static, tectonic plates move, land subsides or uplifts, and earthquakes cause sudden displacements, coordinates in precise reference frames are only accurate at a specific moment. The International Terrestrial Reference Frame (ITRF) publishes station coordinates and velocities at a reference epoch (e.g., ITRF2020 at epoch 2015.0), and users must apply velocity models to obtain current coordinates. Failing to account for epoch differences when combining historical and current survey data can introduce centimeter to meter-level errors.
Different GNSS constellations maintain their own time systems with defined epochs: GPS Time started at midnight January 5-6, 1980; GLONASS Time aligns with UTC+3 hours; Galileo System Time began August 22, 1999; and BeiDou Time started January 1, 2006. Understanding these time system epochs and their relationships to UTC and each other is essential for multi-constellation GNSS processing.
