GPS Week Number Rollover

The GPS Week Number count began at approximately midnight on the evening of 05 January 1980 / morning of 06 January 1980. Since that time, the count has been incremented by 1 each week, and broadcast as part of the GPS message. The GPS Week Number field is modulo 1024. This means that at the completion of week 1023, the GPS week number rolled over to 0 on midnight GPS Time of the evening of 21 August 1999 / morning of 22 August 1999. Note that this corresponded to 23:59:47 UTC on 21 August 1999.

Week beginning
at 0000 GPS Time on
GPS Week Number
broadcast by satellites
08 Aug 19991022
15 Aug 19991023
22 Aug 19990
29 Aug 19991


It is the responsibility of the user (i.e., user equipment or software) to account for the previous 1024 weeks.

Depending upon the manufacturer of your GPS receiver, you may or may not have been effected by the GPS Week Number Rollover on 22 August 1999. Some receivers may display inaccurate date information, some may also calculate incorrect navigation solutions.

Contact the manufacturer of your GPS receiver if you have been effected by the GPS week number rollover. Contacts for some GPS receiver manufacturers can be found at the GPS World website.

For more information, please refer to the following websites:
US Naval Observatory Home Page
What Time Is It? USNO Time Service Home Page
GPS Home Page at USNO