VisualGPS, LLC

GPS Time Armageddon – Should you worry?

Monte Variakojis 11/19/2018

Remember the Year 2000 (Y2K) problem back in 1999? This is where most of the financial software used a 2-digit year to represent the date and when midnight 2000 occurs, the date goes to zero. This caused problems and there was a huge effort (and panic) to fix software that handled the date using a two-digit year. For the most part, midnight 2000 came and went without a huge disaster. Did you know that the Global Positioning System (GPS) also had a time rollover problem? It occurs every 19.7 years and the fist occurred just before Y2K on midnight August 22, 1999? This is called GPS week rollover.

Back when GPS was designed computers were expensive, not very powerful compared to todays standards, and memory was a limited resource. The transmission of data from the satellite to the receiver is slow, very low power, and the designers needed to keep it a minimum. The data rate is about 50 bit/second. The designers needed to make use of every bit of information. A GPS receiver holds the state of the entire GPS system and that data is known as the almanac. The satellite health, the age of the data, and the week number is stored in the ephemeris. This is the data transmitted from the satellite to the receiver. A GPS receiver uses the week number (Week Number - ICD-GPS-200 subframe 1 bit 61-70) to determine the current date (month, day, and year) and the week number is only a 10-bit number. The key to our conundrum.

GPS time started when the GPS atomic clocks were synchronized on January 6th, 1980 and this included the GPS week number where it was set to zero. GPS week is a number that represents the number of weeks since this GPS time epoch, January 6th, 1980. The GPS week is incremented every seven days (a week) and the GPS receiver can derive the date based on the GPS week number. To derive the time, the time is the number of seconds into the week for current week number.

So why is 10-bits a big deal for the GPS week number? A 10-bit number has (2^10) - 1 combinations or a number from 0 to 1023 giving us 1024 combinations. This translates to about 19.7 years of week numbers. August 21st, 1999 is week 1023 and on midnight August 22nd, 1999 the GPS week rolls over to zero. Remember, GPS week of zero is January 6, 1980. A GPS receiver will use the date and time to calculate which specific satellites to search for based on information in the almanac, so it can calculate time and position on earth. With the time being off 19.7 years will confuse this search algorithm and may cause the GPS receiver to report a wrong position.

On April 6th, 2019 – about 19.7 yeas since August 21st, 1999 there’ll be another GPS week rollover. Today, we rely on GPS for our wireless telecommunications, metrology, aircraft navigation, and finding the nearest coffee shop.

How is this fixed? Software update. This is a common method and the software in your GPS receiver knows when it was built. That is if the software was build in 2005, the GPS week is 1024 + the current GPS week number reported by the satellite. This holds true for GPS week rollover 2019. At best, the GPS receiver will work for less than 39.4 years. Other methods would be help from the user or some sort of assisted method where something will tell the receiver what year it is. Assisted GPS (AGPS) in your cell phone could tell the GPS the year.

GPS navigation and GPS timing manufactures will use GPS simulators to test for GPS rollover and update their software to deal with the anomaly.

Rare, but the first rollover still causes problems. Recently I’ve received an email from a GPS user claiming that his GPS timing software on his PC was not functioning and faulty. It turned out that the GPS receiver was reporting 1998 for the year even though the year was 2018, the GPS indicated it was locked, and navigating. A software update (if available) for this GPS would fix the problem.

Should you worry about the GPS week rollover in 2019? No – for the most part, the GPS used in our cell phones, guiding our aircraft, and getting us to the nearest coffee shop will continue to work. If you have an important application that uses GPS, you may also want to contact the manufacture and ask them if their GPS will handle GPS week rollover.