On April 6, navigation data from some GPS receivers will become inaccurate unless they are reset to account for the next so-called “week number” rollover event, according to a new EASA Safety Information Bulletin. If affected GPS receivers are not reset by that date, the time data used for navigation solutions will be inaccurate. “A nanosecond error in GPS time can equate to one foot of position error,” EASA said.
The GPS weekly number has a valid range of values from zero to 1,023 weeks. Unless updated by April 6, the end of the 1,024th week, the counter experiences a rollover, could reset to week zero and the GPS will think it’s the week of Jan. 6, 1980. The GPS week zero started on that date and the last week-number rollover occurred Aug. 21, 1999.
A GPS device that conforms to the Interface Specification GPS 200J and provides UTC should not be adversely affected, according to a U.S. Department of Homeland Security white paper. These devices should not be affected by the April 6 rollover date “but may experience a similar rollover event at a future date. For example, a particular GPS device might interpret the week-number parameter relative to a firmware creation date and would experience a similar rollover event 1,024 weeks after that firmware creation date.”
However, EASA said, tests of some GPS devices revealed that not all manufacturer implementations will correctly handle the April 6 rollover. The EASA bulletin is intended to help operators understand the implications of the week-number rollover and recommend steps to rectify the issue.