GPS skew-whiff

For more than a decade, major earthquakes around the world have been interfering with our Earth-bound global positioning system (GPS) sites. A 2013 study by Australian, Belgian and US scientists shows that all but three (central Australia, western Europe and the eastern tip of Canada) of our GPS network of ground stations have undergone small but measurable shifts. These shifts have introduced errors into our network of satellites and ground stations.

17 great earthquakes

The study, published 6 May 2013 in the Journal of Geophysical Research: Solid Earth, says the 21st century has seen 17 ‘great earthquakes’ (Mw >8), including some of the largest earthquakes ever recorded.

“Numerical modelling of the earthquakes shows that nearly half of the Earth's surface has undergone horizontal coseismic deformation >1 mm, with the 2004 Sumatra-Andaman earthquake dominating the global deformation field.”

How GPS works

GPS, developed and operated by the US Government, is an open-access navigational system currently made up of:

• 31 satellites in medium-Earth orbit
• 3–4 decommissioned satellites, called ‘residuals’, that can be reactivated
• an Earth-based master control station
• an alternate ground master control station
• 12 ground command and control antennas
• 16 ground monitoring sites
• millions of receivers.

A person or object with a GPS receiver can calculate their position by precisely timing the signals sent by at least four GPS satellites. However, any given position or direction or speed of movement is always relative to something else’s position, direction or speed. For the satellite to broadcast your position accurately, it relies on the accuracy of coordinates of ground stations or what is commonly referred to as the ‘terrestrial reference frame’.

GPS was originally developed for military applications, such as missile guidance and troop navigation. However, today GPS is used by just about everyone for a huge range of purposes including in-car navigation devices, cell phone tracking, tracking the movements of children, teenagers and elderly relatives, tracking stolen cars or workplace vehicle fleets, ship and plane navigation, and possibly the most important civilian use – surveying.

Growing distortions

The researchers have only calculated horizontal deformations of the ground stations in the order of a few tenths of a millimetre, which doesn’t have an impact on the GPS receivers in phones and cars. They say that, as the GPS network is used for calculating satellite orbits (that require very precise measurements), sea level rise and earthquake hazards, the slow but growing distortions can not be ignored. Indeed, some scientists involved in calculating satellite orbits already have to run additional calculations to allow for tectonic plate and earthquake-driven movements.

Scientists also use reference frames, usually one for each tectonic plate, to compare movements between plates. For example, to look at movements of the Pacific plate, it might be compared to a reference frame on the North American plate and/or one on the Indian-Australian plate. In this study, the scientists write that “the accumulated global deformation field shows that two regions, Australia and the north Atlantic/Arctic Ocean, have been largely undeformed by these great earthquakes, with accumulated deformations generally <0.5 mm. Using GPS estimates of surface deformation, we show that the majority of the Australian continent is deforming at <0.2 mm/yr, the northern part of New Zealand is rotating clockwise relative to the Australian Plate with relative horizontal velocities of ~2 mm/yr, while the southeastern coast of Australia is undergoing post-seismic relaxation caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake.”

Reference for GPS needs updating

The authors say that this on-going ‘post-seismic relaxation’ is occurring thousands of kilometres from plate margins, which “violates the secular/linear assumption made in current terrestrial reference frame definitions”.

The authors urge that the next update of the International Terrestrial Reference Frame System (that happens every few years), the internationally agreed upon reference for GPS, needs to account for the effects of big earthquakes.