Monday, 10 July 2017

Watching Cities Grow

Great Resolution Civilian Radar Satellite

Major cities in the world have been increasing and as per the estimates of United Nations, presently half of the population of the world tends to be living in cities. Towards 2050, the figure is expected to mount to two thirds of the population of the world.

 Professor for Signal Processing in Earth Observation at TUM, Xiaoxiang Zhu has informed that this growth has given rise to high demands on building and infrastructure safety since destruction events could threaten thousands of human lives at once. Zhu together with her team had established a method of early detection of probable dangers for instance; subterranean subsidence could cause the collapse of buildings, bridges, tunnels or even dams.

The new system tends to make it possible in noticing and visualizing changes as small as one millimetre each year. Data for the latest urban images tends to come from the German TerraSAR-X satellite which is one of the great resolution civilian radar satellite in the world. Since 2007, the satellite, circulating the earth at an altitude of approximately 500 kilometres tends to send microwave pulses to the earth and collects their echoes. Zhu has explained that at first these measurements were only in a two dimensional image with a resolution of one meter.

Generate Highly Accurate Four-Dimensional City Model

The TUM professor worked in partnership with the German Aerospace Centre – DLR and was also in charge of her own working team. The DLR tends to be in control of the operation and use of the satellite for scientific purposes.

The consequence of the images is restricted by the statistic that reflections from various objects which are at an equivalent distance from the satellite, will layover with each other and this effect tends to decrease the three-dimensional world to a two-dimensional image. Zhu had not only created her own algorithm that tends to make it possible in reconstructing the third and also fourth dimension, but also set a world record at the same time.

 Four dimensional point clouds having a density of three million points for each square kilometre had been reconstructed. This rich recovered information gave rise to generate highly accurate four-dimensional city models.

Radar Measurements to Reconstruct Urban Infrastructure

The trick was that the scientists utilised images taken from slightly various viewpoints. Every eleven days, the satellite tends to fly over the region of interest but its orbit position does not always seem to be precisely the same. The researchers utilise these 250 meter orbital variations in radar tomography to localize each point in three-dimensional space.

This system utilises similar principle used by computer tomography that tends to develop a three-dimensional view of the inner area of the human body. Various radar images taken from different viewpoints have been linked in creating a three-dimensional image. Zhu states that since this system processes only poor resolution in the third dimension, additional compressive sensing system which makes it possible for improving the resolution by 15 times is applied.

Scientists could utilise the radar dimensions to restructure urban organization on the surface of the earth with great accuracy, from TerraSAR-X, for instance the 3D shape of individual buildings. This system has already been utilised in generating highly precise 3D models in Berlin, Paris, Las Vegas and Washington DC.

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