Avoiding Future GPS - Week Rollover Concerns - Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design

Avoiding Future GPS – Week Rollover Concerns


In the past few months, there have been articles in the press providing information and misinformation in advance of the GPS week rollover that occurred on April 6. Readers were warned to retire obsolete receivers, update software, check with manufacturers, and even perform testing of critical devices. Articles have explained that the GPS Coarse/Acquisition (C/A) signal used by billions of receivers worldwide is modulated by a data message that, among other items, provides time to receivers. Part of this time information describes the number of weeks since January 6, 1980 by a 10 bit number. Every 210 = 1024 weeks, or slightly more than 19 and a half years, this number resets. This week number reset can cause errors in the calendar date output by receivers or their host systems, unless software properly handles the rollover. If there is a problem, it might become evident either before or after the actual rollover, depending upon the software design.

Some articles have claimed that the rollover can cause errors in position outputs of GPS receivers, but the test results we have seen have not shown evidence of such errors. Further, based on understanding how GPS works, we would not expect such position errors, since the week number should not be used by receivers in computing position, but only in converting time measurements to calendar dates. However, there have been reports that some systems using GPS time were temporarily degraded or even disabled because they did not properly handle the rollover.

Having now survived the second GPS week rollover (the first occurred in 1999), people are thinking about how to avoid the next one early in 2039. Recommendations include improved specification and design of receiver software, extended testing, and other steps toward greater discipline in receiver design, development, and sustainment.

But these recommendations omit the most obvious one—receivers should be using the modernized GPS signals long before 2039. 

Modernized GPS signals offer many potential advantages over the C/A signal. The GPS L1C and L5 signals are transmitted at higher power, improving a receiver’s ability to operate indoors or under foliage, as well as to overcome interference. Their spreading modulations also enable receivers to better resist interference, and to provide greater accuracy under challenging multipath conditions. All modernized GPS signals are equipped with features that allow receivers to better correct noise-induced errors in received data messages, with improved detection of uncorrected errors. Other modernized signal design features enable receivers to maintain signal tracking under more challenging conditions than with the C/A signal. Satellite positions and satellite clock time are represented in modernized data messages with more precision for greater positioning and timing accuracy.

Modernized GPS signals’ data messages also represent the GPS week with 13 bits, rather than 10 bits, extending the week rollover period by a factor of 23 = 8. Thus, the first GPS week rollover for modernized signals will occur into the 22nd Century, long after the likely operating lifetime of any receiver built in the next decade, and likely beyond the lifetime of any current InsideGNSS reader. By then, people may well be obtaining position, velocity, and time using completely different technologies, with satellite-based navigation and timing (satnav) artifacts relegated to history books and museums. 

Of course, GPS still needs to launch four more satellites to provide a full 24-satellite constellation broadcasting the modernized L2C signal, 11 more satellites to provide a full 24-constellation broadcasting the modernized L5 signal, and 23 more satellites to provide a full 24-satellite constellation broadcasting the new L1C signal. And the ground system to fully control these signals is a few years away from operational status.

But these steps should be completed long before 2039, and receivers can begin using these signals even before they are broadcast by a full constellation.

So, while the C/A signal has been an outstanding success and the foundation for the world’s embrace of satnav, it’s time to look beyond the C/A signal and get ready to embrace the next generation of GPS signals—the modernized GPS signals. Doing so will provide many benefits to users, while avoiding some of the issues with which C/A signal receivers are encumbered. Then, the GPS week rollover will be also relegated to history books. 

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