Countdown to GPS Timepocalypse


There is a bug about to arrive at older GPS hardware that has echoes of Y2K. Those old enough to have experienced the transition from the 1990s to the 2000s will undoubtedly recall the dreaded "Bug of the Year 2000", which should mean the misery of civilization. Thanks to short-sighted software engineering that recorded only two digits a year, we were told that date calculations would fail massively in software that did everything from the power grid to digital clocks. Massive remediation efforts were undertaken, companies rehire programmers whose obsolete skills were suddenly back in demand and, in the end, virtually nothing really happened.

Yet another time is upon us, much less known, but potentially deeper and more insidious. On Saturday, April 6, 2019 – which is tomorrow – GPS receivers may experience software problems due to the replacement of their time counters. This could result in anything from small inconveniences to major confusions with an external chance of chaos. Some alarmists say they will not fly this weekend for fear of the consequences.

So what are the actual potential consequences, and what is the problem with GPS in the first place? Unsurprisingly, it all comes down to basic math.

History of Epoch

GPS satellites are essentially super-accurate clocks in orbit, delivering navigation messages at 50 bits per second. The navigation messages include a timestamp and information about the orbit of each satellite, which the GPS receivers below can use to determine their location. Each complete navigation message is 37.5 kilobits in length, which means that an entire page of GPS data takes 12.5 minutes to transmit.

Image Source: ESA Navipedia

The navigation message is divided into 1500-bit frames, each divided into five 300-bit subframes that take 6 seconds each to transmit. Each 300-bit sub-frame is divided into ten 30-bit words. The first 30-bit word of each subframe is a word of telemetry, encoding certain information about the health of the satellite. The telemetry word is followed by a 30-bit time-of-the-week (TOW) word, which codes the week number and time that week. Calculating GPS time is a little strange because of some gymnastics needed to encode the number of seconds in a week (604,800) into the 17 bits available in the TOW word after removing 13 bits for parity and other uses. The word TOW actually represents the number of periods of 1.5 seconds in a week, which is divided by four because there are four periods of 1.5 seconds in the six seconds that each sub frame takes to be transmitted.

Despite appearances, the complexity of time coding on the spatial side of the GPS system is not the cause of the problem, although it is related. The problem is how time data is interpreted by GPS receivers and, as in the old bug of the year 2000, they go back to the decisions made by software engineers. Of the 17 bits dedicated to the coding of the word TOW, the weekly counter uses 10 bits. This means that satellites can count up to 1024 weeks, or about 19 years and 8 months, before the meter goes to zero. Now the counter of the week is all: 1111111111. On Saturday, April 6, the counter of the week will be incremented, rolling back to 0000000000. That's where the problem lies.

It was here, done that.

Now, this is not the first time this has happened. The GPS system has been operating in various forms since the late 1970s, strictly for military use at first, and then opened to the civilian market in 1983, partly in response to the overthrow of Korean Airlines' flight 007 by the Soviet anti-aircraft defenses which claimed that the airline was a spy plane. The beginning of the GPS era was set for January 6, 1980, with the time counted from that point. This means that the first rollover occurred on August 21, 1999 – 1024 weeks after the start of the clocks.

GPS satellite image II / IIa from

The astute reader will note that the world has not come to an end the last time the GPS weeks counter rolled, so for sure, this time will not be an event either. Probably, but there are some complicating factors this time around. First, in 1999, there were very few GPS receivers in the hands of civilians. While Magellan introduced the first portable GPS receiver, the Magellan NAV 1000, in 1989, and some cell phones were already equipped with receivers since 1999, any problem with the nascent system when the date changed for the first time was not that great. to deal.

In the year following the first rollover, the US Department of Defense made the decision to transmit navigation messages with full positional accuracy enabled. For the first time, everyone would be able to get centimetric accuracy with the right equipment, and the GPS industry took off. In 2001, the Garmin and Tom Tom dashboard navigators became the GPS killer app. Cell phones would turn into smartphones soon after, and begin to incorporate GPS receivers and navigation software. By 2017, the worldwide market for GPS receivers has been estimated at almost $ 38 billion, so there are many GPS receivers out there, much more than in 1999.

Back to the future?

So, what will likely happen to your GPS devices? Probably nothing. GPS manufacturers have known this for some time, and virtually any receiver made in the last decade is already able to handle rollover. Older devices such as the old Garmin eTrex Legend, which was once the source of a lot of fun in family geocaching in 2003, but which has been in a drawer for years, may collapse on Saturday.

As the end of the second GPS era will manifest itself in specific devices it is completely dependent on how the manufacturer coded the thing. Some will interpret the scroll as a jump of 19.7 years, be it backwards or forwards. The navigation itself should not be affected, even if the weather is unstable, or only for a moment, if this happens. Non-navigational GPS receivers such as GPS time bases used to synchronize cell phone services may have more problems, but again, if the devices are old or have been fixed, there should not be a problem.

So relax and take care of your business, believing that when ten of them become ten zeros someday on Saturday, practically nothing will happen. If you're tilted, you might want to plug in your car's GPS and see if there's any update needed, but besides that, you're probably fine. And if you really want to spend some time worrying about overlaps, think about it: it's less than 19 years until we have to deal with the year 2038 problem.


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