All users of Global Positioning System (GPS) receivers should be aware that on 6 April 2019, the GPS Week Number will rollover into the third epoch. This will be only the second time the GPS week number has rolled over; as such, this may affect Coordinated Universal Time (UTC) derived from GPS. Tests have shown that some older GPS receivers that have not been kept up to date may not handle the rollover correctly, giving incorrect UTC following the rollover. Navigation solution should not be affected, but associated time tags could be incorrect thus still corrupting navigation data.
It is recommended that all GPS users should:
1. investigate and understand their possible dependencies on GPS for obtaining UTC
2. contact their GPS manufacturers to understand if their equipment will be affected
3. ensure that the firmware of such devices is up-to-date.
The week number in a GPS signal is encoded into the data stream by a 10-bit field. A binary 10-bit word can represent a maximum of 1,024 weeks, which is approximately 19.7 years. Each 19.7 year period is known in GPS terms as an "epoch". The last week number rollover was August 21, 1999. GPS time is currently in the second Epoch and the next week number rollover is 6 April 2019.
Further information can be obtained here:
Memorandum for U.S. owners and operators who use GPS to obtain UTC Time
UK Maritime Coastguard Agency Safety Bulletin 13 - GPS Week Number Rollover.