The GPS week number rollover occurs every 1,024 weeks, risking software failures and time miscalculations in systems reliant on GPS timing.
Key Facts
- Rollover cycle
- Every 1,024 weeks (~19.6 years)
- Week counter bit size (legacy)
- 10 binary digits, range 0–1,023
- New CNAV counter bit size
- 13 binary digits, range 0–8,191
- New CNAV rollover period
- Approximately every 157 years
- Previous rollover
- 1999 (first GPS rollover)
By the Numbers
Cause → Event → Consequence
The legacy GPS NAV message format stores the week number in only ten binary digits, limiting its range to 0–1,023. Once the counter reaches 1,023, an integer overflow causes it to reset to zero. This architectural constraint was built into the original GPS signal design and recurs approximately every 19.6 years.
On April 7, 2019, the GPS week number counter rolled over from 1,023 back to zero for the second time in the system's history. Systems using the legacy L1 NAV signal format that lacked rollover-handling code were at risk of interpreting the date incorrectly, effectively being shifted back in time by a multiple of roughly 20 years.
Systems dependent on GPS for precise timing—including payment processors, mobile network operators, and broadcasters—faced potential synchronization errors if their software had not been updated to handle the rollover. The newer CNAV signal format, introduced in the mid-2010s with a 13-bit counter, mitigates this issue by extending the rollover period to approximately 157 years.