A unit of time used in GPS navigation systems where one week equals 604,800 seconds, with weeks numbered sequentially since January 6, 1980.

GPS Week

Definition and Overview

GPS Week refers to the continuous weekly time count used by the Global Positioning System (GPS) for time measurement and synchronization. The GPS week epoch began on January 6, 1980, at 00:00:00 UTC, and weeks are numbered sequentially from this baseline. Each GPS week contains exactly 604,800 seconds, corresponding to seven 24-hour days.

Historical Context

When the GPS system became operational in the 1980s, engineers needed a precise and continuous timekeeping system that could be broadcast from satellites to receivers worldwide. The GPS week number, combined with the seconds of the week (ranging from 0 to 604,799), provides an unambiguous timestamp for any GPS observation. This system was chosen for its simplicity and ease of computation across distributed receiver networks.

Technical Structure

The GPS time system operates independently of UTC (Coordinated Universal Time), though it is kept within one second of UTC through leap second insertions. GPS time does not include leap seconds, making it a continuous linear timescale—a critical feature for precision surveying applications.

Each complete GPS week is identified by a week number, starting from Week 0 on January 6, 1980. Within each week, time is expressed as the number of seconds elapsed since the beginning of that week. This dual-component system (week number plus seconds of week) creates precise timestamps without ambiguity across months or years.

Rollover Events

GPS week numbers are stored as 10-bit values in the GPS signal, allowing for 1,024 possible week numbers (0-1023). This creates a GPS week rollover event every 19.7 years. The first rollover occurred on August 22, 1999, when week 1023 was followed by week 0 again. Subsequent rollovers occurred in August 2019 and will continue approximately every 19.7 years.

These rollovers have significant implications for surveying equipment and software that must be designed to handle the transition correctly. Modern receivers typically account for these events through firmware updates or inherent date recognition capabilities.

Applications in Surveying

In surveying and geospatial applications, GPS week time is essential for:

Precise positioning: Synchronizing observations across multiple receivers

Base station operations: Maintaining accurate time logs for differential GPS

Data post-processing: Correlating observations with known time references

Equipment synchronization: Ensuring simultaneous measurements across survey networks

Surveyors must be aware of current GPS week numbers when planning fieldwork and processing data, as timestamp errors can introduce significant positional inaccuracies.

Relationship to Other Time Systems

GPS week time differs from both Julian dates and UTC in important ways. While UTC includes leap seconds to stay aligned with Earth's rotation, GPS time remains continuous. Surveyors working with multi-sensor systems (combining GPS with other technologies) must account for these timing differences.

Modern Considerations

With the evolution of GNSS (Global Navigation Satellite Systems) beyond GPS, including GLONASS, Galileo, and BeiDou, coordination between different week numbering systems has become necessary. However, GPS week remains the primary reference for legacy systems and continues as the standard in most surveying software.

Understanding GPS week is fundamental for anyone working with GPS technology in surveying, geodesy, or precision navigation applications.