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The Permanent Platform of Atomium Culture brings together some of the most authoritative universities, newspapers and businesses in Europe to increase the movement of knowledge: across borders, across sectors and to the public at large.
La plataforma permanente Atomium Culture reúne a las universidades, periódicos y empresas más prestigiosos de Europa para promover el flujo del conocimiento más allá de fronteras, entre sectores y hacia el público en general.

Observing the Earth from the Stars

Por: | 17 de enero de 2013

Atomium_Culture_Earth from Stars


By Andrea Nascetti of the University of Rome, La Sapienza 

Four hundred years after Galileo built and used a telescope to observe the night sky, "new telescopes" are taken to the stars in order to better observe the earth.  


"All truths are easy to understand once they are discovered . . . the point is to discover them." These wise words were spoken by Galileo Galilei, the first person that built and used a telescope to observe the night sky. Thanks to his observations, Galileo was able to find remarkable evidences of the heliocentric theory.

Four hundred years later, we have taken "new telescopes" to the stars and turned them towards us here on Earth; and now, for the first time in human history, we have the opportunity to observe the Earth from the stars in detail. The first images of the Earth were captured in the 1960s.

The relevance of this imagery for the understanding of the planet we inhabit, was not immediately clear; The impact of this information on our everyday lives is only a very recent phenomenon.

Thanks to the technological development and the larger and larger availability of satellite imagery, services such as Google Earth, Microsoft Bing or the new Apple iOS 6 Maps have been made possible. In a couple of seconds we can access the internet by our mobile phones and see pictures of our houses acquired by some satellite in orbit around the earth.

Is that all? Hundreds of millions of dollars spent to see my house? Are there other applications of satellite imagery, especially of the ones with the highest resolution? What is the real potential of this data?

No, that's not all; the even more recent advent of high-resolution radar (Synthetic Aperture Radar, or SAR) sensors, with the launches of TerraSAR-X (German), RADARSAT-2 (Canadian) and COSMO-SkyMed (Italian) satellites started in 2007, has greatly extended the potentialities of remote sensing.

Now the shape of the earth and its movements can be continuously monitored through the satellite data. Not counting the artificial satellites that have been launched for military purposes, a kind of competition started (since 1999, with the launch of IKONOS) among different companies and agencies to launch optical and radar sensors with higher and higher resolution for civilian applications.

Meanwhile, many research groups around the world started to develop methodological and practical investigations about the large number of applications of this new satellite imagery, such as land monitoring, archeological and environmental studies, cartography and mapping. It was clearly understood that the high-resolution sensors permit an easier acquisition of data of the same area at regular intervals, which is useful to monitor natural or anthropic phenomena evolving in time (e.g., urban growth, natural disasters); moreover, the high-resolution remote sensors allow us to obtain images of areas where it may be difficult to carry out traditional surveys for logistic reasons (e.g., developing countries).

One of the most important applications of SAR remote sensing is the generation of Digital Surface Models (DSMs), that is, three-dimensional models of the Earth’s surface; these 3D models have a large relevance in many engineering, environmental, civil protection, safety and security applications. Starting with the SAR data, two different methods may be used to generate DSMs: the well-known interferometric and the radargrammetric one, and the importance of the latter approach is rapidly growing because of new high-resolution imagery (up to 1 m).

Radargrammetry in principle requires just a couple of images to reconstruct the three-dimensional shape of the Earth, like human stereo vision or classical photogrammetry applied to optical imagery; on the contrary, radargrammetry does not need the good coherence among two (or a series of) images, a strict requirement for the success of the interferometric approach which can be guaranteed only with an extremely short (tens of seconds) revisit time.

I decided to follow the radargrammetric approach in order to investigate the present potentialities of the high-resolution SAR sensors for DSMs production with the minimal requirement of a couple of images.

My research was focused on the crucial step for DSMs generation, which is the image matching. The matching process is the automatic identification of pixels representing the same object in the two (or even more) images. If corresponding pixels are recognized, than a simple geometric intersection is needed to compute the position of the object in space. The development of a fully automatic, precise and reliable image matching method that adapts to different images and scene contents is a challenging problem. Dissimilarities between SAR images due to occlusion, illumination differences, radiometric differences and speckle noise must be take in account and this is the reason why many image matching approaches have been developed in recent years.

In the last year an original matching procedure, able to extract DSMs from a couple of high-resolution SAR images, was developed by the research group of the Geodesy and Geomatics area of the University of Rome "La Sapienza".

The effectiveness of the novel procedure was assessed with some TerraSAR-x and COSMO-SkyMed SpotLight imagery acquired in northern Italy (over the Merano and Trento areas) reaching an absolute and relative DSM accuracy of 3.5 m and 2.5 m, respectively. These results are encouraging, even if further tests and researches are needed, especially to improve the performances in urban areas or in zones with more complex morphologies.

In conclusion, these advanced "telescopes" facing the Earth and the images collecting information are great opportunities for mankind; the enormous quantity of data available can be useful for a large number of research fields and may become even more a part of our daily routines and activities.


Andrea Nascetti
University of Rome, La Sapienza

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