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This image and the ones below demonstrate the increased capabilities of the Advanced Synthetic Aperture Radar (ASAR) onboard the Envisat spacecraft as compared with the SAR sensors on the earlier ERS generation of satellites, while offering a continuity of service to users.

Envisat’s ASAR instrument is the first permanent spaceborne radar to incorporate dual-polarisation capabilities – the instrument can transmit and receive signals in either horizontal or vertical polarisation.
This Alternating Polarisation (AP) mode can improve the capability of a SAR instrument to classify different types of terrain. Because the reflective properties of a surface are dependant on the polarisation of the incoming radar signal, the use of more than one type of polarisation provides valuable extra information.

Different types of terrain might look very similar using only one polarisation mode. However they can be much more clearly discriminated using a combination of several modes.

The ASAR AP mode provides two simultaneous images (or channels) of the same scene taken with different radar polarisation options. Since the radar can transmit and receive in both the horizontal (H) and vertical (V) polarisations, researchers can create image pairs, with possible combinations, HH &VV (the images shown here), HH&HV and VV&VH.

A cross-polarisation option employs a channel in which the polarisation signal transmitted by the spacecraft is received on a different polarisation, i.e. transmitted using a horizontal polarisation and received on the vertical for an HV combination, or the reverse for a VH combination.

Some of the different applications of the ASAR’s unique AP mode include:

Land classification
The cross-polarisation combinations offer better discrimination between terrain types, such as between vegetation and bare soil, or between forests and deforested, clear-cut regions. Cross polarisation channels become much stronger when terrain such as vegetation causes multiple scattering of the radar signal before it returns to the receiver, compared with making simple single bounces from surfaces such as bare soil.

Sea ice
The AP mode can help better identify the boundaries between sea ice and open water. This is an important distinction for mariners operating in ice-laden waters.

Envisat’s ASAR can highlight different oceanographic features with different AP schemes. For example, the HH polarisation channel can better identify ocean wave phenomena that could include internal waves or effects caused by the proximity of the ocean floor. In contrast, a VV polarisation channel can be more effective at recording ocean features caused by localised differences in atmospheric conditions.

The images are colour composites generated using ASAR’s AP mode and acquired on 8 April 2002. The red channel contains the VV polarised image, the green channel contains the HH polarised image and the blue channel contains the magnitude of the difference between the amplitudes in VV and HH.

Dzerzhinsk and the Volga River
This image shows an area, 100-km wide, around the Russian city of Dzerzhinsk, 300 km east of Moscow. Visible in the north is the lower part of the Gorkovskoye Reservoir, formed by a dam across the Volga River, Europe’s longest. The river in the south is a Volga tributary, the Oka River.

Urban areas are generally visible in bright yellow tones, indicating equally high backscatter in VV and HH polarisation. The green-yellow colour that dominates the image results from the extensive forests in this region (medium backscatter in VV and HH). Agricultural land in the river plains to the east and south appears in magenta and purple shades (VV backscatter dominates). A number of small river systems, probably frozen, are highlighted in cyan (dominant HH backscatter).

This shows an area, 100-km wide, around the city of Arzamas in Russia, 300 km east of Moscow.

The city appears bright yellow because backscatter is equally high in VV and HH polarisation. To the north and south are agricultural regions, where individual fields are visible in shades of magenta and purple, because of the strong VV backscatter. The green – yellow colour that dominates the central portion of the image results from the forests in this region (medium backscatter in VV and HH). The Tesha River skirts the northern edge of the lower region. In the centre of the image the banks of a large river system, probably frozen, are highlighted in cyan (HH backscatter dominates).

Archangel and the White Sea
The image displays an ice-laden segment of the White Sea on Russia’s northern coast.

Flowing into it from the southeast is the Severnaya Dvina, with the city of Archangel located on its eastern bank. To the north is the peninsula of Kolskiy Poluostrov, Russia’s northwestern extremity. Since the 1600s, Archangel has been considered the cradle of Russian shipbuilding and is of major strategic importance roje port, from where lumber, cut from the surrounding forests, is exported throughout the world.

This image illustrates that during early April the sea that surrounds the port is still heavily infested with ice floes. It is also interesting to note the offshore barrier along the eastern coastline that has presumably been constructed to provide an ice-free passage for this important shipping route.

For further information, please contact:

Frederic Le Gall
European Space Agency
00 39 06 94180757