A timekeeping unit in the GPS system that consists of 604,800 seconds and is used for precise timing and positioning calculations.

GPS Week

Definition

GPS Week refers to a continuous count of weeks since the GPS epoch began on January 6, 1980, at 00:00:00 UTC. Each GPS week consists of exactly 604,800 seconds (7 days × 86,400 seconds per day). This standardized time reference is fundamental to the operation of the Global Positioning System and is essential for all GPS-based surveying and positioning applications.

Historical Background

The GPS system was developed by the United States Department of Defense and became fully operational in 1995. The choice of January 6, 1980, as the GPS epoch was arbitrary but has remained constant throughout the system's history. GPS time is maintained independently from UTC (Coordinated Universal Time) and does not account for leap seconds, making it a continuous linear timescale.

GPS Week Number

The GPS week number is a counter that increments with each passing week. Starting from week 0 on January 6, 1980, the system counts upward consecutively. This numbering system is transmitted by GPS satellites to receivers and is crucial for determining precise time and location. The GPS week number was designed to cycle through 1,024 weeks (approximately 19.6 years) before returning to zero—a rollover event that occurred on August 21, 1999, and will occur again in 2137.

Application in Surveying

In surveying, GPS week numbers are critical for several reasons:

Positioning Accuracy: Surveyors use GPS week information combined with the time-of-week (TOW) to calculate the precise positions of satellites at any given moment. This enables accurate determination of ground control points and site positioning.

Data Processing: Survey-grade GPS receivers record GPS week and time-of-week data for post-processing. This allows surveyors to download raw observation data and process it using specialized software to achieve centimeter-level or better accuracy.

Base Station Networks: Real-time kinematic (RTK) surveying systems rely on GPS week synchronization between base stations and rover units. Precise time alignment ensures accurate differential GPS corrections.

GPS Week Rollover Events

GPS week numbers roll over approximately every 1,024 weeks. The first rollover occurred on August 21, 1999, and the second on April 6, 2019. These events required careful management and updates to surveying equipment and software to maintain accuracy. Future rollovers will require similar attention from the surveying industry.

GPS Week vs. Calendar Time

GPS week differs from standard calendar time and should not be confused with ISO week numbering or other calendar systems. GPS week provides a continuous, unambiguous time reference that is independent of time zones, daylight saving time, and leap seconds. This makes it ideal for precise technical applications like surveying.

Modern Relevance

With the modernization of GPS and the addition of new signals (L2C, L5), GPS week remains central to the system's operation. Other Global Navigation Satellite Systems (GNSS) such as GLONASS, Galileo, and BeiDou have their own time systems, but GPS week continues to be the dominant standard in surveying practice worldwide.

Understanding GPS week is essential for surveyors working with GPS technology, particularly when dealing with raw data processing, quality control, and integration of historical survey data.