The town in 3D, with radar

3D rendering of a part of the city of Toulouse from radar images acquired in polarimetric and interferometric (PolInSAR) modes, with Onera airborne experimental equipment. The texture in shades of grey is the total intensity image.
The colored information that appears hereafter is obtained with polarimetric data: the red channel corresponds to the intensity measured for the HH+VV polarization, the green channel corresponds to the intensity for the HH-VV polarization and the blue channel corresponds to HV polarization.

During the PolInSAR workshop organized by the ESA in Frascati at the end of January 2011, Onera presented a remarkable video of 3D urban reconstruction obtained with a combination of polarimetry and interferometry, in open synthesis (PolInSAR technique). This is the first time that such a demonstration of its feasibility on a large scale has been achieved.

The polarimetric SAR technique means radar images can be obtained for several different emission and reception polarizations. The PolInSAR technique combines interferometry and polarimetry. It uses two polarimetric images, acquired with two slightly different viewing angles, in order to be able to create an elevation map of the scene.

The recent satellite measuring campaigns have already shown the potential of this technique, in particular for hydrological resources or the estimation of biomass, which is indispensable for the quantification of carbon flows (see the TropiSAR campaign with the airborne SAR of Onera’s Sethi).

Radar systems can be used whatever the meteorological conditions, and even at night. They are therefore an attractive and complementary alternative to optical systems. Radar contour imaging could be applied, for example, to the surveillance and management of the growth of the urban fabric.

• Polarimetry
  Polarimetry is the science of measuring the polarization of electromagnetic waves
• Polarimetric interferometry
  By studying the interferometric properties (related to interferences) of polarimetric data, information can be found regarding the diffusion source, since the polarimetric signatures can be distinguished by analyzing the data.
  In the 3D reconstruction algorithm developed by Onera, the polarimetric information is used on two scales. Firstly, it is used in an algorithm that partitions the image into several homogeneous zones (segmentation). Then, height estimations are improved using interferometry, in a “coherence optimization” algorithm.
• SAR - Synthetic Aperture Radar
  Radar technique that uses antenna displacement to obtain an angular resolution far superior to that of a static antenna. The angular resolution of an antenna is inversely proportional to its size, which is necessarily reduced when mounted on an aircraft.
• The PolInSAR workshop
  The growing interest in radar techniques that produce three dimensional earth data for forest and urban areas was consolidated at a “PolInSAR” workshop organized by the Esa in Frascati, Italy, at the end of January 2011. Devoted to the science of polarimetry and to polarimetric interferometry and its applications, PolInSAR has brought together more than 200 researchers from 24 countries, all keen to know more about the latest advances in the field.
• Sethi
  New generation on-board radar imaging system designed at Onera for scientific campaigns. Developed and certified in 2007, it produces ground radar images, day and night, whatever the meteorological conditions. “There It has many possible applications in both the civil sector (remote detection, archeology, environmental studies) and the military sector”, explains Olivier Ruault du Plessis of the Electromagnetism and Radar Department. The airborne campaign for this study was carried out with an airborne system that predates Sethi (Ramses), but the processes using the PolInSAR technique are very recent.

The following people took part in this study: Nicolas Trouvé –doctorand, Elise Koeniguer, Hubert Cantalloube, research engineers in the Electromagnetism and Radar Department.