A continuous count of weeks since January 6, 1980, used as the primary time reference system in Global Positioning System measurements and surveying applications.

GPS Week

Definition

GPS Week refers to the number of complete weeks that have elapsed since the GPS epoch, which began at 00:00:00 UTC on January 6, 1980. This continuous week counter forms the foundation of the GPS time system and is essential for all positioning, navigation, and timing applications derived from satellite signals.

Historical Background

The GPS system was designed with a specific reference epoch that predates the satellite constellation's operational deployment. The January 6, 1980 date was selected as an arbitrary starting point for the GPS timekeeping system. Since its inception, GPS Week has been continuously incremented, providing an unambiguous reference for temporal measurements throughout the global surveying and navigation community.

Week Rollover Events

One of the most significant technical challenges associated with GPS Week is the rollover phenomenon. The original GPS Week counter was designed as a 10-bit number, allowing it to count from 0 to 1,023 weeks before resetting to zero. This cycle repeats approximately every 19.7 years. The first GPS Week rollover occurred on August 22, 1999, when the counter reset from week 1023 to week 0. Subsequent rollovers have occurred at predictable intervals, with major events in 2019 and will occur again in 2038 and beyond.

These rollover events present challenges for surveying equipment and software that may not be adequately updated to handle the discontinuity in the week counter, potentially causing significant positioning errors if systems misinterpret the week number.

Relationship to GPS Time

GPS Week works in conjunction with the GPS Time of Week (TOW) counter, which measures seconds within a specific GPS week. Together, these two parameters establish an absolute time reference. GPS Time differs from UTC by an integer number of leap seconds, which must be accounted for in precise surveying applications. As of recent years, GPS Time leads UTC by approximately 18 seconds due to accumulated leap second insertions.

Surveying Applications

In professional surveying, GPS Week is critical for:

Data Logging: Recording precise timestamps for all survey measurements

Post-Processing: Calculating accurate positions through differential GPS techniques

Baseline Calculations: Establishing precise distances between survey control points

Network Adjustments: Synchronizing multiple receiver measurements across extended projects

Modern Considerations

Contemporary GPS receivers and surveying software now incorporate extended week numbers or secondary time references to mitigate rollover issues. The addition of extended GPS Week numbers in newer signal formats provides backward compatibility while eliminating ambiguity for decades into the future.

Surveyors working with legacy equipment must ensure their systems can properly handle GPS Week rollovers. Many professional surveying organizations maintain databases that cross-reference GPS Week numbers with calendar dates to prevent temporal misalignment in their measurement records.

Conclusion

GPS Week remains fundamental to all satellite-based positioning work despite the technical complexities surrounding its implementation. Understanding GPS Week numbering and rollover characteristics is essential for surveyors working with GNSS technology, ensuring accurate temporal referencing of all measurements and maintaining data integrity across long-term projects.