Answer: The GPS date is expressed as a week number and a time-of-week value. The week number is transmitted as a ten-bit field in the C/A navigation message. It can therefore signify a week within a period of 1,024 weeks or 19.69 years. GPS receivers must calculate the date based on this 1024 week period. Since going backwards in time is not a real-world situation, GPS receivers will calculate the week based on the 19.69 years looking forward from the time that they are created. For example, for a scenario date of 2007, newer GPS receivers may interpret that GPS week number to be 2007 + 19.69 years. It’s similar to the Y2K problem, where we express years as two digits, and we have to use some logic to determine when somebody says 1/1/68, does that means 1/1/1968 or 1/1/2068. This situation will happen whenever you use a scenario with an older date. Note that all of the A-GPS scenarios are for specific dates that all occurred in the past, so you will see this problem with those scenarios.

GPS weeks started with week 0 on 6 Jan 1980, and the first “GPS week rollover” that reset the week number to 0 occurred at midnight between 21 and 22 August 1999. The next GPS week rollover will occur at midnight between 05 and 06 April 2019.

GPS weeks started with week 0 on 6 Jan 1980, and the first “GPS week rollover” that reset the week number to 0 occurred at midnight between 21 and 22 August 1999. The next GPS week rollover will occur at midnight between 05 and 06 April 2019.