Visit to Comoros archipelago - Grande Comores

Jean-Paul RUDANT, emeritus professor at University Paris-Est and speaker during ESA radar training commented : "The 2360m-high summit of Mount Karthala lies in the southern half of Grande Comore Island. It is approximately halfway between the eastern and western shoreline, each being 10km away. The Sentinel-1 image has been acquired during a descending orbit, its antenna looking West, North-West.”

"The resulting view is different with this image acquired during an ascending orbit looking East, South-East. ”

The coral reef and nuances in lava flow colour show using Sentinel-2 natural colour. The degree of weathering of lava flows is indicative of their age.

The traditional slash and burn agricultural methods cause erosion of the steep sloped soils. This environmental issue also lessens the yield of the crops in the same way it does in its seriously deforested bigger neighbour, Madagascar. It also competes with the Ramsar status acquired by a 130km² area around Mount Karthala. The increase of the demographic pressure and the change in eating habits have made the country becomes more dependant on food importation.

Professor Jean-Paul Rudant added: "Being several kilometres large, the central crater of Mount Karthala shows clearly. Radar shadows appear wherever abrupt relief drops hide part of the ground surface from the the radar beam. Depending on the radar view direction, the 2360m-high crater appears with a 4-km across track offset either toward East (descending orbit) or West (ascending orbit).”

"This animation of images using nearly opposite view vectors bring a new perception. On the west-looking image, the eastern side of the mount is compressed and overbright while on the east-looking image the overbright areas are on the western side. This overbrightness is stronger on the eastern side, which shows its slope is steeper, up to 30°, while it only reaches 15° on the western side.

The overbrightness is caused by the compression of the image where the slope is tilted toward the radar beam. A larger ground surface is located at the same range of the sensor and thus mapped on the same pixel. As a result, the energy backscattered toward the radar sensor is significantly higher for these areas. The overbrightness effect increases as the difference between the beam incidence and the slop decreases. In the case of Karthala, the radar beam has an incidence close to 33°.”

"The volcano regularly erupts (four times between 2005 & 2007) which can be deadly. In particular since the capital of Comores, Moroni, lies on previous lava flows located in the south-west of Grand Comores Island. The energy backscattered by urban areas toward the antenna is in general higher than for the surrounding landscape. Urban areas thus appear brighter on the radar images.”