1.1.1 Geophysical Measurements

The ENVISAT mission has both global and regional objectives, with the corresponding need to provide data to scientific and application users on various time scales.

Following on from the very successful ERS-1/2 SARs, ASAR is an all-weather, day-and-night, high-resolution imaging instrument that will provide radar backscatter measurements indicative of terrain structure, surface roughness, and dielectric constant. Important new capabilities of ASAR include beam steering for acquiring images with different incidence angles, dual polarisation, and wide swath coverage.

Important contributions of ASAR to the global mission include:

• measuring sea state conditions at various scales
• mapping ice sheet characteristics and dynamics
• mapping sea ice distribution and dynamics
• detecting large scale vegetation changes
• monitoring natural and man-made pollution over the ocean

ASAR will make a major contribution to the regional mission by providing continuous and reliable data sets for applications such as:

• offshore operations in sea ice
• snow and ice mapping
• coastal protection and pollution monitoring
• ship traffic monitoring
• agriculture and forest monitoring
• soil moisture monitoring
• geological exploration
• topographic mapping
• predicting, tracking and responding to natural hazards
• surface deformation

Some of the global and regional objectives (sea state condition, sea ice applications, marine pollution, maritime traffic, hazard monitoring, etc.) require near real-time (NRT) data products (within a few hours from sensing). Some of these products are generated systematically since they are assimilated by meteorological offices (e.g. wave and wind information from wave mode acquisitions) while others are generated according to user requests. Other applications (e.g., agriculture, soil moisture, etc.) require fast turnaround data services (within a few days).

As well as using ASAR to satisfy specific operational and commercial requirements, major systematic data collection programmes will be undertaken, to build up archives for scientific research purposes.

ASAR has dual polarisation capabilities and a special Alternating Polarisation Mode has been implemented that permits half of the looks from a scene to be acquired with horizontal and half with vertical polarisation, thereby considerably increasing the target classification capability (especially if used in conjunction with multi-temporal imaging).

Wide area coverage will be achieved by switching between different swaths using the ScanSAR technique. This will enable 405 km coverage at resolutions of either 150 m or 1 km. At the 1 km resolution the data rate is low enough for tape recording on board the spacecraft and the recording capacity will be sufficient for downloading low-resolution global coverage through a single receiving station.

In contrast to the ERS SARs, which had a fixed swath position (23° mid-swath incidence angle), ASAR Image Mode will provide data acquisition in seven different swath positions (i.e., IS1 to IS7), giving incidence angles ranging from 15° to 45°. This subject is discussed in the section "Selectable Incidence Angles" 1.1.5.2.

Imaging radars can transmit horizontal or vertical electric field vectors, and receive either horizontal or vertical return signals, or both. The basic physical processes responsible for the like-polarised return are quasi-specular surface reflection and surface or volume scattering. The cross-polarised return is usually weaker, and often associated with multiple scattering due to surface roughness or multiple volume scattering. Scattering mechanisms and the returns from different surfaces may also vary markedly with incidence angle. For a further discussion of this topic see the section entitled "Dual Polarisation." 1.1.5.1.

ASAR medium and low-resolution images, provided by the Wide Swath and Global Monitoring Modes, open up new possibilities for applications requiring large area coverage and/or more frequent revisit. Both modes will provide 405 km swath coverage with a spatial resolution of 150 m for the Wide Swath and 1000 m for the Global Monitoring. For applications where higher resolution is necessary, better than a 5-day repeat interval is possible for approximately 100 km x 100 km areas using Image Mode. For a further discussion on this topic, see the section entitled "Wide Area Coverage and Frequency of Coverage." 1.1.5.3.

ASAR offers continuity with 35-day repeat images for SAR interferometry, although interferometric pairs will need to be taken with the same incidence angles. The availability of ASAR interferometric pairs acquired at higher incidence angles will be useful for improving the visibility of steeper slopes, and there will also be new possibilities for low-resolution interferometry. For a further discussion on this topic, see the section entitled "Interferometry." 1.1.5.4.

ASAR Wave Mode will provide wave spectra derived from imagettes of minimum size (5 km x 5 km), similar to the ERS AMI-Wave Mode, spaced 100 km along-track in VV polarisation. For a further discussion on this topic, see the section entitled "Wave Spectra." 1.1.5.5.