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Radar imagery - Two SAR surveys in Sweden for Sethi

In September 2010, two SAR1 imaging surveys were successfully carried out in Sweden with Onera’s Sethi flying laboratory, one for the detection of targets under vegetation cover, the other one for the evaluation of biomass.

The Sethi concept is based on the installation under the wings of a Falcon 20 (AVdef Company) of two pods housing radar and/or optronic sensors. The control and data acquisition functions are located in the cabin of the aircraft.

The first survey, Lorambis, involved the detection of targets under plant cover. The airborne experimental equipment of the Swedish FOI (Lora) was associated with Sethi in order to carry out simultaneous flights in bistatic configuration: one system transmits while the other receives. This configuration allows for the reduction of the effects of vegetable matter (trunks and foliage), so that the image reveals what is not part of the forest. The quality of the first images produced in bistatic mode attests to the good synchronization between the two systems, the fruit of major joint preparatory work in the laboratory and in flight in 2009.

One of the first "raw" bistatic images of the survey for the detection of targets under plant cover.

The second Sethi campaign in Sweden covered the measurement of the biomass in the P band². On behalf of the ESA the Onera team made an acquisition flight at the Remingstorp test site, a preparatory step for the Biomass satellite project, currently in the selection phase. After Tropisar, which focused on tropical forests, the Sethi team analyzed a northern forest. The results and images were compared with those acquired during other surveys financed by the ESA.

The radar image is a quick-look produced by the processing team during the survey in Sweden. It is an image in false colors (HH channels in red, HV in green and VV in blue).

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¹SAR: Synthetic Aperture Radar. A radar technique for imagery that uses the displacement of the radar antenna in order to obtain an angular resolution much greater than that of an static antenna. As the antenna moves, the same point is illuminated several times: we obtain a series of data for each point under the radar. By combining the variation of amplitude and phase of these returns, the aperture synthesis processing yields images of the zones observed as if we were using a wide, very high resolution antenna.

²P Band: Range of frequencies for which the wavelengths are between 30 and 100 cm (part of the microwaves of the electromagnetic spectrum). SAR radars operating in the P band have a very good capacity for penetration of plant cover and water (ice, rain, etc.).