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

GPS Week

Definition

GPS Week refers to the continuous numbering system that tracks time in the Global Positioning System (GPS). It represents the number of complete weeks that have elapsed since the GPS epoch, established as January 6, 1980, at 00:00:00 UTC (Coordinated Universal Time). This system serves as the backbone of GPS timekeeping and is critical to all positioning, navigation, and timing applications.

Historical Background

The GPS week numbering system was established when the first GPS satellites were deployed in the early 1980s. January 6, 1980, was selected as the zero point—GPS Week 0—for the entire constellation's timekeeping reference. This date marked the beginning of what would become the world's most widely used satellite positioning system.

Technical Specifications

Each GPS week contains exactly 604,800 seconds and runs from Saturday through Friday in the GPS time frame. Within each week, time is further subdivided into GPS seconds, with Week Number combined with seconds-of-week providing precise temporal references. The GPS week counter increments automatically at the start of each week cycle.

Week Number Rollover

A significant technical consideration is the GPS week rollover, which occurs every 1,024 weeks (approximately 19.6 years). The first major rollover happened on August 21, 1999, and the second on April 6, 2019. During these events, the week counter resets to zero, potentially causing errors in systems not programmed to handle the transition. Future rollovers will occur approximately every 19.6 years.

Application in Surveying

Surveyors rely heavily on GPS Week for accurate data collection and positioning. When conducting surveys with GPS receivers, timestamps are recorded using the GPS week and seconds-of-week format. This provides a universal, continuous time standard that remains independent of time zones and daylight saving time considerations.

Precise GPS week information is essential for:

Data Synchronization: Ensuring multiple GPS receivers collecting data simultaneously maintain temporal coherence

Post-Processing: Correlating observations from different sessions and instruments

Quality Control: Verifying data integrity and detecting timing anomalies

Baseline Calculations: Computing accurate positions in geodetic surveys

GPS Time vs. UTC

GPS Week operates within GPS Time (GPST), which differs from Coordinated Universal Time (UTC). GPS Time runs continuously without leap seconds, while UTC includes periodic leap second adjustments. The difference between GPS Time and UTC has grown to 18 seconds as of 2024, a factor surveyors must account for when correlating GPS data with other time-referenced information.

Modern Considerations

With the maturation of GPS and the development of augmentation systems like WAAS, DGPS, and Real-Time Kinematic (RTK) positioning, GPS Week remains fundamental to all these applications. Modern surveying equipment automatically handles GPS week calculations, but professionals should understand this underlying framework for troubleshooting and quality assurance.

The transition to multi-constellation positioning systems (including GLONASS, Galileo, and BeiDou) has not diminished GPS Week's importance, as many integrated systems still reference GPS Time as a common standard for inter-system coordination.

Conclusion

GPS Week represents one of the most important timekeeping references in modern surveying and geospatial work. Understanding its structure, rollover characteristics, and relationship to UTC ensures accurate data collection and processing in contemporary surveying operations.