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What puts the “Advanced” in Envisat’s Advanced Synthetic Aperture Radar? 
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:
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).
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 as a 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.
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