Satellites provide data for research on water and ice

10 August 2020

It's hard to think that satellites do more than just beam signals back to Earth for telecommunications. But it turns out there are satellites that are specifically tasked with monitoring the Earth's ice sheets, oceans and water resources from space.

This work is so important that after scientists finish collecting data in the current mission, NASA already has another mission scheduled for 2026 that will continue to collect data on these areas.

Professor Paul Tregoning, a geodesist from the ANU Research School of Earth Sciences and the Institute for Water Futures, has previously studied sea level variations, terrestrial water resource availability and the changes in the ice volumes in Antarctica.

Professor Tregoning, who is also a mission specialist with the ANU Institute for Space (InSpace), will be closely monitoring the successor to the Gravity Recovery and Climate Experiment (or GRACE) missions after the decommissioning of the second mission, GRACE Follow-On currently in orbit, which is expected to last to at least 2023.

The GRACE and GRACE Follow-On missions have made detailed measurements of the Earth's gravity field which have helped inform scientists about changes in the Earth's water resources and ice sheets for the purposes of climate and environmental science.

"The way this mission works is there are two satellites flying at about 450-490 kilometres above the surface of the Earth and they're separated by about 200 kilometres. We measure the change in distance between the satellites, accurate to about a tenth of the thickness of a human hair," he says.

"Believe it or not the changes of mass on the surface of the Earth, such as a flood, or high tide or melting of a polar ice sheet, cause small changes in the gravity field of the Earth which makes the satellites fly differently around the Earth. That changes the distance between the two satellites and that's what we can measure.

"We can quantify changes in groundwater, changes in soil moisture and increased ocean mass because of polar melting regions. We can weigh the continents - for example measuring the change in mass of Antarctica - and that tells us how it's melting and contributing to global sea level rise."

Professor Tregoning and his team at the ANU Research School of Earth Sciences have developed their own software to analyse the data taken from the satellites.

"We are currently working on contracts for Geoscience Australia to assess whether remote sensing can be used to quantify changes in groundwater in the Great Artesian Basin," Professor Tregoning says.

"The project is funded by the Department of the Environment and we're providing the expertise in using the space gravity data for that project.

"We're one of about 8 or 10 centres in the world who have that capability and we're the only ones in Australia that can do this analysis."

Geodesists use satellites for a range of other projects as well.

"We use satellites to measure crustal deformation of the Earth related to plate tectonics and earthquake ruptures. We can also use satellites to measure changes in sea level both globally and regionally," he says.

"We can also use space gravity to measure changes in the mass distribution on the surface of the Earth. At the moment, I devote most of my research time to this field."

Aside from the GRACE missions, there are also satellite altimetry missions that use satellites to 'ping' the Earth's surface and measure the time that the signal takes to travel from the satellite, bounce off the Earth and return to the satellite.

"We know, accurately, the orbit of the satellite -that is, how high the satellite is above the centre of the Earth - and we know the distance from the satellite to the surface of the Earth - be it the ice-sheet surface or the surface of the ocean. Therefore, we can calculate the height of the ocean or the ice sheet relative to the centre of the Earth. If these measurements are repeated over a number of years or decades it is possible to measure the change in the ocean height or ice sheet."

Back in the millennial drought, Professor Tregoning was involved in a study using the GRACE data to quantify the changes in water in the Murray Darling Basin.

Dr Tregoning says the change equated to around 200 cubic kilometres of water which had been lost, or around 20 centimetres across the whole of the Murray Darling Basin.

"It was predominantly a loss of groundwater as much of the soil moisture dried up in the first couple of years of the drought and then it was just a depletion of groundwater."

NASA has committed $300 million to a successor to the GRACE Follow On mission, named the Mass Change Mission, due for launch in 2026.

"In addition, in September 2021, there should be the launch of the SWOT mission (Surface Water Ocean Topography), which will provide very high resolution changes in ocean height including currents and waves," Professor Tregoning says.

"Over land it will be able to measure heights of water bodies which are more than a hundred metres wide. Depending on the repeat time of the satellites, we will be able to determine the heights of not only rivers and lakes but also dam storage and reservoirs.

"This will be able to contribute to water policing in terms of water allocation and extraction from rivers and that's going to be really important."