In the GPS L1 C/A navigation message, the week number is contained in a 10-bit binary number. This limits the maximum week number to a total of 1024, effectively making 1023 the maximum week after it loops back to 0. Hence, the event named “week rollover” occurs every 19.7 years. Modernized signals solve, or reduce significantly the occurrence of week rollover by using more bits, but many receivers rely solely on the original L1C/A signal to determine the current time.
The first ever GPS week started on January 6, 1980. The weeks counter already rolled over on August 21, 1999, and is scheduled to reach its limit again on April 6, 2019, roughly 10 months from now.
A week rollover is a great example of a difficult, if not impossible, event to test without a GNSS simulator. Testing with the live sky, in this case, is definitely not applicable. Waiting for the event to occur with fingers crossed is not a great idea either.
So, what are the implications?
Time is at the heart of GNSS. Without accurate timing, positioning is imprecise. Systems using GPS L1 C/A signals for navigation are at risk of exhibiting errors, potential malfunction or even causing incidents.
Moreover, many applications make use of the precise timing of GPS in a different context than positioning and navigation. For example, both banking transactions and power grids’ electrical current phases synchronization rely on GPS time by integrating receivers in their sub-systems. These critical applications must be resilient to any event that may disrupt their acquisition of proper time coming from GPS satellites’ signals, or consequences could be dire. Less critical applications may also suffer from improper handling of the situation by crashing/stopping, or reporting erroneous time, causing financial woes.
Receivers have various means of establishing a reference date to validate the current week number. If your receiver is a recent model or had its firmware updated in the past few years, it should be all right. However, if you are using older equipment (10 years old or more), or are unsure of the receiver’s date or maintenance, an investigation should be initiated. One starting point is to contact the manufacturer’s support to inquire about the handling of the rollover week. If the receiver is part of a larger integrated equipment installation, an inspection should be planned.
That being said, there is a small, but non-negligible risk that the GPS week rollover will cause important disruptions. However, since this event occurs once every 20 years, are you willing to go without testing? We’d rather not.
Our friends at Talen-X have published a guide on how to effectively test your system and interpret the results of this event.