ScienceWise - Spring 2013

Drifting off

Article Illustration
The GPS navigation system works by timing the signals from four or more of a constellation of satellites

Are GPS coordinates shifting beneath our feet?

The GPS navigation system in your car calculates your location by timing the arrival of signals from four or more of a constellation of satellites in orbit around the Earth. It’s a tricky feat to pull off because the signals travel at the speed of light so if your electronics is a hundred millionth of a second off, the position is out by three meters – easily enough to put you in the wrong lane!

In spite of the technical difficulty, the GPS system works superbly well for most purposes, providing navigation with an accuracy of a metre or so. But what if you’re a scientist wanting to study the tiny creep in the movement of tectonic plates, or measure the rate of sea level rise? In those cases, plus or minus a metre simply isn’t good enough.

Dr Paul Tregoning from the ANU Research School of Earth Sciences was a member of a scientific team using GPS to study the movement of tectonic plates in the Papa New Guinea region. “When you’re looking for movements smaller than a few millimetres per year, standard GPS isn’t good enough.” Dr Tregoning says, “So we have to throw away the usual coded time signals and instead make use of the phase of the microwave beam that they’re carried on.”

Any wave has peaks and troughs so, in principle, you can work out how far along a microwave beam you are by counting those. Obviously it isn’t possible to count every peak right back to the satellite because the distance is too great and the wave is hurtling along at the speed of light. But if you stand in one place and count the peaks going past, you can work out how fast the satellite is moving relative to you. You can then estimate the satellite’s orbit, how many peaks and troughs there were between you and the satellite when you started counting and your own position. From 24 hours of continuous measuring, the position can be worked out to better than 5 mm.

“It’s quite amazing that we’re able to do this since the satellites are travelling at thousands of kilometres per hour and tectonic plates are crawling along at less than 70 millimetres per year, but by using several satellites at the same time and keeping careful electronic count of the trillions of wave peaks we can do exactly that.”

But, as is often the case in science, when the team studied the PNG plate movements they discovered a lot more than they bargained for. “We had expected to see a steady linear drift as the plates slowly moved around,” Dr Tregoning explains, “But the data were all over the place and it was a real puzzle to work out what was happening.”

It turns out that the scientists could see a number of significant shifts in the land caused by 17 massive earthquakes that have occurred this century. At the moment of the fault slip during an earthquake there’s a sudden violent shift in the crust at that point, but the subsequent deformation effects can extend right across the surrounding continents. And whilst the initial quake is very rapid, the deformation and relaxation of the entire region can take many years to complete.

One of the complicating factors in identifying tiny land movements is that there are other forces involved which are far larger. The combined gravitational pull of the Sun and the Moon causes the Earth’s crust to rise and fall by as much as 40cm – a phenomena called the Earth tide. Although the majority of this movement is vertical, there is also a daily horizontal cycle of up to 50mm that would swamp any tectonic plate movement if not corrected for.

Then there’s the effect of ocean tides. Because eastern Australia has a long continental shelf there’s a colossal mass of additional seawater pressing down on it at high tide, which moves most of the continent. Incredibly, when the tide is in at Batemans Bay, Canberra 250km inland, moves down by about 8mm!

Even the weather changes the position of the land. A high-pressure system contains millions of tonnes of extra air, which pushes down the Earth’s surface by as much as 15 mm.

“One of the most difficult parts of this work are the many corrections that have to be applied to the measurements before we can tease out the tiny signals corresponding to tectonic plate movement or local shifts dues to earthquake processes.” Dr Tregoning says, “The more you understand about these measurements the more you realize how difficult it is to identify a fixed frame of reference from which to measure.”

The GPS satellites all orbit about the centre of gravity of the Earth but even that shifts with the Earth tide and changing distributions of ocean and air. “The best we can do is to try to identify the most stable parts of the Earth on which to place reference stations and that’s where the earthquake work becomes so important.”

All parts of the Earth move with tectonic plate drift. But that’s generally smooth, linear and easy to compensate for. However the unexpected horizontal shifts caused by massive earthquakes greatly reduce the value of any reference station located in such an area. “By creating global maps of earthquake shifts we’ve been able to identify which regions are prone to such motion and which simply follow smooth continental drift. From this we’ve selected eight measuring stations that can create a good base line for all such studies.”

A few millimetres here or there in the GPS system won’t cause you much hassle driving your car down the highway, but to science that can be very significant. For example when we talk about rising sea levels what do we measure that relative to? A tide gauge fixed to the pier is fine but how do you know that the pier isn’t moving up or down over the years?

The GPS system contributes to monitoring sea level changes, but again, you have to be able to calibrate that system accurately if you want to see changes of the order of millimetres.

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