The smart solution for road maintenance is in the sky – with SAR imagery making it possible to spot surface movement from a thousand kilometres away.
If most people were told they could identify a 10 cm vertical displacement on the surface of a road from space, they probably wouldn’t believe it. But most people don’t know about the incredible power – and potential – of Synthetic Aperture Radar (SAR) imagery.
With SAR imagery, it’s not only possible to measure 10 cm of subsidence from 1,000 km – you can even accurately identify movements of 5 cm or less.
In addition to road surfacing, the use of SAR imagery can also help resolve structural and infrastructure issues relating to:
- Development on landfill sites
- Construction of tunnels
- Underground mining
- Groundwater extraction
So, how does it work? Well, all these measurement methods generally have one thing in common: they rely on SAR imagery acquired by satellites carrying active sensors.
These sensors send signals towards the earth’s surface, and receive the signal once it bounces back. Two pieces of information can be obtained based on the echo of the signal:
- The comparative strength of the signal
- The time taken for that signal to reach the earth and return
It’s this second point – time – that’s key to measuring distance to a surface object. Deploying a sensor at two separate times will tell us if that point has moved, and by how much.
Radar engineers like to call this a ‘change in phase’. Any movement or displacement on the earth’s surface creates varying signals which can be measured. This method of calculating change is known as interferometry, and is commonly spoken of in physics.
But what benefits can this SAR data transfer provide us in practical terms? Can it be harnessed to improve our decision-making? There are several ways, but for this post I’ll just provide an example of how it can assist with transport planning.
I recently had the opportunity to analyse a particular section of road in Brisbane, to see if it had been sinking. To do this I used a stack of TerraSAR-X imagery (acquired from Airbus Defence and Space) documenting intervals between 17 October 2012 and 14 June 2014.
The results of the analysis showed deformation had occurred up to 30 mm per year over a road corridor near Brisbane Airport. The below image highlights in blue the sections where significant surface displacement has occurred.
This type of imagery provides the big picture city planners need to make smarter transport infrastructure decisions. If subsidence is increasing maintenance costs, for instance, planners may consider realigning the road to pass through more stable land.
These sort of decisions can only be made by looking at data on a regional level, as opposed to a local level. It’s also a far more economical way of working.
Trying to obtain this information by installing numerous ground-based subsidence measurement devices can prove expensive for local and state governments.
SAR technology ensures money spent on maintenance works is minimised, and value is being maximised for taxpayer-funded projects.
About the author
Dr Dipak Paudyal
Principal Consultant for Remote Sensing and Imagery
– Esri Australia, Brisbane