Owners of old global positioning system devices take note: your trusty navigation buddy may think it's back in the 1990s after Saturday unless it's been updated.

And it's down to a matter of time — or, rather, timing.

To accurately calculate location, signals sent by the constellation of GPS satellites whizzing around Earth include a time stamp.

It includes a "week number" transmitted as a string of 10 binary digits or bits — 1s and 0s — and of which there are only 1,024 possible combinations.

This means each "GPS epoch" equals 1,024 weeks or 19.7 years.

How GPS works Around 30 satellites in the GPS constellation circle the planet 20,000km overhead, each completing two orbits of the Earth in a day

Around 30 satellites in the GPS constellation circle the planet 20,000km overhead, each completing two orbits of the Earth in a day Each satellite broadcasts microwave signals containing information about its position and time towards Earth

Each satellite broadcasts microwave signals containing information about its position and time towards Earth GPS receivers combine signals from several satellites in a process called trilateration to calculate its position on Earth

GPS receivers combine signals from several satellites in a process called trilateration to calculate its position on Earth Local global navigation satellite systems use geosynchronous satellites, which are in a fixed position above a specific location

Local global navigation satellite systems use geosynchronous satellites, which are in a fixed position above a specific location These local systems can provide accurate position information, even among tall buildings

Because GPS time began on January 6, 1980, the first rollover transpired on August 21, 1999.

The second rollover happens this Saturday, April 6 at 23:59:42 UTC (which is Sunday morning in Australia).

If you think this whole situation sounds a bit like the Millennium or Y2K bug, you're not wrong — it too was a "rollover to zero" issue.

In that case, early software measured years using two digits (90) instead of four (1990). And after midnight on December 31, 1999, those years reset to zero.

That sounds bad. Should I run for the hills (with a map)?

Relax. Just as the risks of Y2K were (mostly) mitigated by companies in the years leading up to it, GPS hardware manufacturers have had loads of time to test their gear and update firmware and software.

If your GPS-enabled device is less than around nine years old, generally you should be fine, RMIT University geospatial scientist Ken Harima said.

It's the pre-2010 models, or those that haven't been updated, that might run into rollover problems.

After the weekend, some of these devices could suddenly be out by 19.7 years, reverting their date to the beginning of the previous GPS epoch, August 21, 1999.

And because accurate GPS positioning relies on extremely precise synchronisation between the time on a receiver and that sent in a satellite signal, an error of 19.7 years renders a navigation device useless — but only on the remote chance the receiver is badly designed.

Come the weekend, older models of GPS receiver might start malfunctioning unless they've been updated. ( Flickr: Paul Downey (CC by 2.0) )

While most of us use the GPS to find our way around, it also has another crucial job. Those signals from the satellites provide timing information to better than a millionth of a second.

And this is crucial for industries such as banking and financial services, which perform billions of high-speed and automated transactions every day, Dr Harima said.

"For them, reliable time stamps are very important."

So the rollover will most affect those services that use the GPS for precise timekeeping.

Still, if you want to check if your navigation device might be affected, the official US GPS recommendation is to contact your device manufacturer.

But for newer devices, Saturday (and Sunday) should be like any other day.

Does it affect all navigation networks?

While the GPS might be the best known global navigation satellite system, it's not alone.

And these other networks, too, reset to zero — albeit with different frequencies.

Europe's Galileo and China's BeiDou use 12- and 13-bit week numbers respectively, so we have quite a bit of time before they reset to zero: February 2078 for Galileo and January 2163 for BeiDou.

Then there's Russia's GLONASS, which eschewed the week number and used a "day number" instead.

It resets every four years, with the next due on New Year's Day, 2020.

"It happens so often that any manufacturer worth existing will have this in mind," Dr Harima said.