ScienceWise - Spring 2012

Following GRACE

Article Illustration
Dr Roland Fleddermann, Danielle Wuchenich and Professor Daniel Shaddock in the Gravity Wave Lab preparing to optically test the GRACE Follow-on retroreflector
Article Illustration
The twin GRACE Follow-on satellites will orbit the globe 200km apart. The ranging laser beam has to be reflected back with incredible accuracy if the signal is to be received back at satellite one. That’s the job of the retroreflector optical arm seen at the top here

Australian space technology to monitor groundwater

Agriculture constitutes a very significant part of Australia’s economy and with the rising world population, food production looks set to become an ever more important part of what we do. However in spite of Australia’s good weather and abundant land there is one constant threat to farming and that’s the variability of our water supply.

In the good years there’s plenty to go round but all too frequently rainfall is low and we rely increasingly on groundwater for irrigation. The trouble is no one’s really sure just how much groundwater there is and at what rate it’s depleted and refreshed.

It’s not an easy thing to measure. Sure, you can drill a hole and see if there’s water there. But is it ancient water or relatively newly fallen rain? Is it flowing from one area to another? And perhaps most importantly are we using it too quickly or could we safely use more and boost production?

NASA, the German Space Agency and the Australian Space Research Program are currently looking to answer just those questions and many others using a pair of satellites that map local variations in gravity. The mission is known as the Gravity Recovery and Climate Experiment or GRACE and has been gathering data since 2002.

Associate Professor Daniel Shaddock from the Australian National University leads a team that are developing instruments for the next mission, GRACE Follow-on. “GRACE has given scientists a vast amount of data which has provided a real insight into the changes in groundwater and ice across the globe” Professor Shaddock says, “The original satellites are now coming to the end of their service lives so we’re working on the development of their replacements.”

Measuring gravity is a bit more complicated than it might at first appear. On the ground all you need is a known mass and an accurate balance. It’s possible to make a gravity map by walking your balance across the landscape but that’s not really a very practical method to map an entire continent.

For a spacecraft crossing a continent is just a few minutes work. But a mass and balance system won’t work on a satellite because an orbiting spacecraft is effectively in free fall, so anything inside it is weightless.

The characteristics of the craft’s orbit are however influenced by the gravity of the Earth below. Imagine if the Earth were to suddenly get twice as massive. All the satellites currently in orbit would be pulled into lower orbits. If the Earth’s mass halved they’d swing further out into space. Local variations in gravity are of course, not even remotely on that scale, but the effect is the same. Fly over something heavy on the Earth’s surface like a mountain and the satellites will wobble very slightly.

“It would in principle, be possible to simply have a single satellite orbiting the Earth and monitor its position from the ground.” Professor Shaddock says, “But the tracking equipment would have to slew rapidly across the sky and you’d need multiple identical stations round the globe. The atmosphere also introduces problems with the ranging signals. However, if you have two satellites in space, you get round all those problems because one acts as the ranging station for the other.”

The two GRACE satellites, aptly nicknamed Tom and Jerry, chase each other across the sky about 200km apart and about 500 km above sea level. Because the lead satellite flies over gravitational anomalies first, its distance to the trailing satellite changes slightly as its orbit is perturbed.

“On the existing mission this distance is measured using microwaves.” Professor Shaddock  explains, “But we can achieve far greater accuracy with optical wavelengths.”

GRACE Follow-on will carry identical microwave instruments to the original, partly because the technology is proven and partly because it will help scientists get consistency between the old and new data sets. But in addition GRACE Follow-on will have a laser interferometer range sensing system that will be able to measure changes in the distance between the satellites of less than a ten thousandth of a millimetre.

One of the difficulties with this technique is that spacecraft tilt and rotate about their centre of mass slightly in flight. If you measure from an arbitrary point on the outside casing, this will distort the data because as it turns, the spacecraft corners get closer or farther away even though the centre of mass hasn’t moved. “Ideally we’d like to have a laser and detector at the centre of mass of one satellite and bounce that beam off a reflector fixed at the centre of mass of the other, “Professor Shaddock says, “But that’s not possible due to other design considerations.”

To get around this the team have designed a retroreflector system that bends the light around the spacecraft in such a way as to simulate a corner cube reflector placed at the centre of mass.

The prototype system was built at CSIRO’s Australian Centre for Precision Optics at Lindfield in NSW, with help from German collaborators. “We’re currently in the process of testing the prototype to ensure that it meets all the optical specifications,” Professor Shaddock says, “Then the next step is to literally shake it up. We have to ensure that this and any other instruments that will hitch a ride into space on top of a rocket can withstand the considerable violence of the launch.”

Local Canberra aerospace company EOS Space Systems will perform these so-called “shake and bake” tests before delivering the finished prototype to Germany at the end of the year.

What the scientists learn from this prototype will be used to construct the actual mission instrument which is scheduled to fly in 2017.

“We’re really excited to be part of this mission, “Professor Shaddock says, “The new instruments should be able to give us data with unprecedented precision.” However it’s not just a matter of mapping the gravity variations round the Earth. Interpreting that data is a daunting task in itself.

Changing groundwater and ice thickness affect the local gravity a little. But other factors such as the height of the terrain and the Sun and Moon’s gravity affect it hugely more.

“One of the key tasks in this mission is processing the data to eliminate other gravitational effects,” Professor Shaddock says, “Getting better measurements will be good but then it’s up to the guys working on the data processing to figure out ways to sufficiently eliminate all the other gravity effects in order to really make best use of that extra precision. And that’s a really difficult task.”

GRACE has already provided excellent insights into the behaviour of ground water in Australia and across the globe. GRACE Follow-on, will build on that data set so that we will have an even better idea of what’s happening to Australia and how best to manage our resources to ensure that our agricultural economic momentum isn’t lost.

For those who wonder why a country built on primary production should spend money on fundamental physics research – this is one example of exactly why we should!   

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