The International Terrestrial Reference Frame is the most important measuring system you’ve never heard of. It’s what we use when we measure the position of an object on Earth or in space, and quite simply, we’d be lost without it.
Accurate positioning and measurements are increasingly important for global and national economic stability and defence. Uses vary between industries, from measuring sea level rises and identifying valuable ore deposits in mines, to precision agriculture and driverless vehicles.
“When you’re doing any sort of navigational positioning, when you’re trying to do things like measuring small changes in sea level rise, you have some sort of coordinate frame that you're defining it in,” says Professor Simon Ellingsen, Head of Physics at the University of Tasmania.
“Effectively, it’s your ruler. When it comes down to it, if you don’t have an accurate ruler, you can’t measure small things.”
The science of measuring Earth’s shape, orientation in space, and gravity field is known as geodesy. It uses a range of techniques to create an accurate reference frame, the best-known of which are the Global Navigation Satellite Systems (GNSS) – a generic term used to describe constellations that provide geospatial positioning information, such as the US Global Positioning System (GPS).
To achieve consistently accurate measurements, scientists use techniques like GPS, which measures signals from different satellites to an Earth-bound receiver, and Satellite Laser Ranging (SLR), where a laser is reflected off a satellite, but both have significant limitations.
“We always have a problem that, at some point, we cannot distinguish whether the satellites are moving, or if Earth is rotating faster or slower,” explains Dr Lucia McCallum, a geodesist at the University of Tasmania.
The solution is to look further out into the Universe to distant astronomical objects called quasars, which are a highly luminous source of radio waves.
In a technique called Very Long Baseline Interferometry (VLBI), the arrival time of a wavefront from a quasar is measured at different Earth-based telescopes located very far apart from each other. The difference in arrival times at these telescopes allows scientists to map out reference frames on Earth with great precision – currently to an impressive four millimetres.
North vs South
Until 2014, the accuracy of the International Terrestrial Reference Frame varied greatly, with the Northern Hemisphere achieving far greater accuracy than the Southern Hemisphere.
There are two reasons for this, says Professor Ellingsen.
“There’s more land mass in the Northern Hemisphere, but more importantly, there are more wealthy countries there, too. So they tend to have more of the radio telescopes that are required for these sorts of observations.”
While Australia and New Zealand have the infrastructure in the Southern Hemisphere, they can’t do it all on their own.
“You need telescopes across the whole globe, so we use telescopes in Africa and South America,” says Professor Ellingsen.
The VLBI technique has been around for a generation, long before GPS, and it was the first technique to provide a measurement of plate tectonics on Earth.
“Now we know that Tasmania and the rest of Australia is moving to the northeast at about 6 centimetres per year,” says Professor Ellingsen.
The University operates the geodetic VLBI facilities for the whole of Australia using three telescopes: one in Tasmania; another in Katherine in the Northern Territory; and one near the West Australian town of Yarragadee, inland from Geraldton.
It’s the only university in the world to operate a continent-wide array of radio telescopes such as this. It’s known as the AuScope VLBI array, and is operated in partnership with Geoscience Australia.
Since being commissioned in 2011, data from the AuScope VLBI array has improved the accuracy of the terrestrial reference frame of the Southern Hemisphere by 30 percent. In 2014, it had enabled the Southern Hemisphere to finally catch up to the Northern Hemisphere in terms of accuracy.
But there’s still plenty of work to be done to make the system even better.
“We hope to improve the fine grain of it,” says Dr McCallum. “If you look at geological scales or earthquake scales, we really need to see to the millimetre in order to see what's actually going on inside and on the surface of Earth.”
“Sea level rise is, at the moment, at a level of about three millimetres a year. Our systems are possibly at the sector of five or 10 millimetres above this, but in order to work out whether the first island [in the Solomon Islands] will be flooded in 10 years or two, we need to be able to see the millimetre changes.”
Economics may hurry that progress along. ACIL Allen Consulting forecasts that the economic benefit of precise positioning systems on Earth and in space is between $7.8 and 13.7 billion from 2012 to 2020. Some $2.4 to $3.1 billion of that is in the mining sector alone.
“And it’s not just more profitable,” says Professor Ellingsen. “It means safer mines and ones where, essentially, you’re extracting only the bits of the ore body that you really want. You can be much more selective.”
The United Nations recently recognised the vital need for precise positioning, making the International Terrestrial Reference Frame one of its highest priorities.
“But no country can do it on its own,” says Professor Ellingsen. “It really is a global endeavour to do this.”
“We want to share the data, especially to help the small countries that will be the first affected by climate change.”
Australia plays a leading role in this effort. The Federal Government committed $200 million over the next 20 years to ensure that Australia maintains and improves the accuracy of the global reference system.
“The big challenge – not just for us, but for everybody – is to move this reference frame away from being just a research endeavour to actually a product that is seen like global infrastructure, like building highways and roads," says Dr McCallum.
Professor Ellingsen agrees.
“It really is an underlying part of Australia's critical infrastructure for the well-being of the country and the world. But it’s one of those things where the message is not necessarily out there in the public.”
About the researchers
Dr Lucia McCallum is a Geodesist at the School of Physical Science, working in the group of radio astronomy. Her field of research is the Very Long Baseline Interferometry (VLBI) technique, using signals from far distant radio galaxies to measure the Earth. After finishing her PhD at the Technische Universität in Vienna (Austria), she joined the University as a post-doc in 2014.
Professor Ellingsen is a Professor in Physics and Radio astronomy and Head of Discipline for Physics. His major areas of research are in the formation of massive stars, in particular through the study of methanol masers and other molecular maser species. He is also involved in research of active galactic nuclei through very long baseline interferometry, flux density monitoring and studies of megamaser emission.
- University of Tasmania-operated facilities comprise almost half of the routinely observing VLBI antennas in the Southern Hemisphere.
- Data from the AuScope VLBI array improved the accuracy of the terrestrial reference frame in the Southern Hemisphere by 30%.
- Thanks to the AuScope VLBI array, the Southern Hemisphere matched the Northern Hemisphere in terrestrial reference frame accuracy in 2014.
- In 2016, the University of Tasmania received the largest individual philanthropic gift in its 125-year history and created Australia’s only endowed chair in Astrophysics, currently held by renowned French researcher Dr Jean-Philippe Beaulieu.
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