(Photo: ESA)
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The ENVISAT-1 Satellite is composed of the payload complement embarked on the Polar Platform.

The Payload comprises a set of seven ESA Developed Instruments (EDI’s), complemented by three Announcement of Opportunity Instruments (AOI’s). The Polar Platform is constituted of two major assemblies:

• the Service Module (SM) which accommodates most of the satellite support subsystems such as
  • power generation, storage and distribution,
  • Attitude and Orbit Control System (AOCS),
  • communication on S-band,
  • support structure and launcher interface,
• the Payload Module (PLM) carrying the instruments and the payload dedicated support subsystems
  • instrument control and data handling,
  • communication on X- and Ka-band,
  • power distribution,
  • support structure.

Orbit

To fulfill its mission objectives, the orbit selected for the ENVISAT-1 satellite is sun synchronous, with a mean altitude of 800 km. This is a similar orbit as that of RADARSAT-1 and later RADARSAT-2. It allows for timely and repeated coverage of various regions of the globe. The satellite over-passes are always at the same local time. This is particularly important in obtaining multi-temporal data sets, minimizing the influence of effects due to diurnal variations.

This orbit will be maintained such to ensure that the deviation of the actual ground track is kept within 1 km of the reference orbit track and the mean local solar time is maintained within 1 minute.

Synthetic Radar Aperture (SAR) vs.
Advanced Synthetic Radar Aperture (ASAR)

Products calibration

Data products

As part of the ground segment concept, ESA has approved the following type of products:

• Level 0 products: time-ordered instrument source packets formatted as PDS products
• Level 1 products: geolocated and calibrated engineering parameters
• Level 2 products: geolocated geophysical quantities

In-flight calibration

The in-flight calibration requires processing of data acquired via:

• use of instrument-internal calibration loops, the instrument carrying its own stimuli
• use of instrument-external well characterized stimuli, natural targets (e.g. stars, rain forests, deserts or ice sheets) or specific artificial targets (ASAR transponders). One such series of target, located in The Netherlands, was built MPB Technologies.

The corresponding results will be processed off-line in the Instrument Engineering Calibration Facility (IECF), specifically equipped for handling these data and for generating the calibration tables to be delivered to the FDS for the corresponding level in the processing chains.

Level 2 Algorithm Validation

The Level 2 processing algorithm use numerous models to concept engineering quantities into geophysical parameters (absorption lines into atmospheric species, brightness temperature into surface temperatures, radar cross-section return echo into wind or wave). Wherever possible, these models are being validated using the experience acquired before ENVISAT launch, using in particular data acquired in-flight (when available) or airborne campaign data.

Preparation for in-orbit commissioning

As shown above, the provision of Level 1-B and Level 2 products to users requires the setting up of a comprehensive list of complementary archives in parallel with the satellite development phase. These archives aim at preparing the in-orbit Commissioning Phase; all processing chains have to be operable with corresponding mechanisms for updating the tunable parameters. Throughout the 6-month Commissioning Phase, the calibration activities will allow delivery of level 1-B products.

The validation of Level 2 products occurs at the end of the Commissioning Phase. Products will be delivered to the users. However, for the entirely new instruments, it is expected that the products will start to be released with warnings concerning error bars. The validation of these products will be completed after the end of the Commissioning Phase.

Calibration / Validation throughout the Operation Phase

At the end of the Commissioning Phase, the mission will enter in its Operation Phase, which is planned for four and a half years. The monitoring of the products and the corresponding calibration and validation will be maintained throughout the Operation Phase.

Main difference between RADARSAT-1, ENVISAT and RADARSAT-2 data products

Signal polarization

RADARSAT-1 provided single polarization data. The signal was sent and received in a horizontal (HH) polarization state antenna. This provided HH products.

The ASAR sensor on ENVISAT has multi-polarization capability. It will provide:

• 2 choices of selective single polarization (HH or VV); or
• 3 choices of dual-polarized data products (HH/VV or HH/HV or VV/VH). The dual-polarized products are delivered as an image with 2 layers of information.

RADARSAT-2 will have complete polarimetric capability. It will provide the choice of:

• 2 choices of selective single polarization (HH or VV);
• 2 choices of dual-polarized data (HH/HV or VV/VH). The dual-polarized products are delivered as an image with 2 layers of information; or
• Complete set of quad-polarized data (HH/VV/HV/VH). The polarimetric data are measured coherently, this means the quad-polarized data are not only a four-layer image. This product is delivered in a matrix format that gives full information of the received signal (amplitude and relative phase). This opens up the possibility of extracting unique information in the image in a way that is not possible with other space borne civilian SAR sensors.