2.2 Organisation of Products

Raw Data is data as it is received from the satellite (serial data stream, not demultiplexed). Raw data is recorded from the X and Ka band demodulator output interfaces and stored on High-Density Data Tapes (HDDTs). The Raw Data is not considered a product, and its structure is not defined in this document.

Level 0 is reformatted, time-ordered satellite data (no overlap), in computer-compatible format. The Level 0 product is the lowest level product in the ENVISAT PDS. Table 2.7 in the section "Level 0 Products" shows a full list of ASAR Level 0 products.

Level 1B products are geolocated engineering foundation products in which data has been converted to engineering units, auxiliary data has been separated from measurements, and selected calibrations have been applied to the data. These products are the foundation from which higher level products are derived.

The Level 0 product is transformed into a Level 1B Product by application of algorithms and calibration data to form a baseline engineering product. The Level 1B product may be based on the whole of the source Level 0 product, or only a selected set of instrument packets from within the Level 0 product, based on a time range or other selection criteria.

Level 1B products may be produced in the following ways:

• Unconsolidated: generated in Near Real Time (NRT); an intermediate step in near real time processing, used for instrument performance monitoring. It is produced in NRT using available NRT auxiliary data (i.e., may not contain the most precise orbit vectors or calibration information).
• Fully Consolidated: generated off-line, time ordered, no overlap, no data gaps (except where the instrument has been turned off), fully validated; the basis for any further off-line processing. It is produced using the most precise auxiliary information available. It is compiled to respect the defined product boundaries (e.g., ascending node to ascending node at the equator for full orbit products).
• Partially Consolidated: a product that has been consolidated in all ways except that it does not use the most precise auxiliary data possible to perform processing. Since several different levels of accuracy exist for auxiliary data (e.g., 5 possible orbit state vector sources), varying levels of consolidation may exist for a product.

Level 1B products make up the majority of ASAR products, and are outlined in the section "Level 1B Products" 2.6. . Table 2.39 in that section provides a summary of these products.

Level 2 consists of geolocated geophysical products:

• Near Real Time: generated in near real time from unconsolidated Level 1B products
• Off-line: generated from consolidated Level 1B products

In general, the Level 1B product is transformed into one or more Level 2 products through higher-level processing to convert engineering units into geophysical quantities and to form a more directly interpretable and useful measurement data set. The Level 1B products used are created at the time of Level 2 processing.

The general rule for product structure is that NRT and off-line versions of Level 2 products share the same structure, the only difference being that the off-line product is of better quality due to the fact that more accurate (fully consolidated) data is used during production than for the NRT product; and that the start and stop times of the data may be different to reflect consolidation of the data to standardised portions of the orbit. Level 2 products may be processed systematically or upon user request.

At present, there is only one ASAR Level 2 product, which is the "Wave Mode Ocean Wave Spectra" product (ASA_WVW_2P). This is discussed more fully in the section "Level 2 Products and Algorithms." 2.7.

Browse

Browse products are decimated images which can be ordered from the ENVISAT inventory. These products are very small to support electronic transmission while querying the catalogues. They may be further reduced in size through the use of standardised data compression algorithms.

Browse products will be generated systematically for ASAR (except wave mode) and will contain image lines derived from a data segment (up to a full orbit of data).

The user will view the browse product to assist in ordering the required products by displaying scenes starting anywhere in the browse product. A comprehensive description of the Browse products is provided in the section entitled "Browse Products." 2.6.2.1.3.

Figure2.9 below shows the generalised product tree:

Figure 2.9 The Generalised Product Tree

As discussed in the Introduction 2.1. , ASAR products can be classified according to the mode in which the SAR instrument is operating. These are:

• Image (IM) Mode - these products are high-resolution, narrow swath products based on data acquired at one of seven subswaths.

  The Precision Image product is a multi-look, ground range, digital image (in either Horizontal Polarisation Transmitted/Horizontal Polarisation Received (HH) or Vertical Polarisation Transmitted/ Vertical Polarisation Received (VV) polarisation) suitable for most applications. It is intended for multi-temporal analysis and for deriving backscatter coefficients. Engineering corrections and relative calibration are applied to compensate for well understood sources of system variability. Absolute calibration parameters are provided in the product annotations.

  The Ellipsoid Geocoded Image product is similar to a Precision Image, but with rectification to a map projection. It is geocoded using a map projection selectable by the user, such as UTM, or Polar Stereographic.Absolute calibration parameters are provided in the product annotations.

  Single-Look Complex (SLC) image data is intended for SAR image quality assessment, calibration and interferometric or wind/wave applications. A small number of corrections and interpolations are performed on the data in order to allow freedom in the derivation of higher-level products. Absolute calibration parameters are provided in the product annotations.

  A Medium-Resolution Image product will be available at 150 m resolution, which is specifically aimed at sea ice and oceanography applications. Absolute calibration parameters are provided in the product annotations.

• Alternating Polarisation (AP) Mode - these products are similar to Image Mode products but include a second image acquired using a second polarisation combination. The raw data is acquired in bursts of alternating polarisation.

  The polarisation combinations are:

  • the co-polarisation submode (one image HH and one image VV)
  • the cross-H polarisation submode (one image HH and one image HV)
  • the cross-V polarisation submode (one image VV and one image VH)

  Products contain two images corresponding to one of the three polarisation combination submodes, at reduced spatial resolution in comparison to Image Mode.

• Wide Swath (WS) Mode - these products provide greater swath width at reduced spatial resolution through the use of the ScanSAR technique.

  The standard product available for Wide Swath Mode will be a 150 m resolution image with the full 405 km swath width.

• Wave (WV) Mode - these products are similar to those in Image Mode but are much smaller, covering an area of approximately 5 km by 5 km. Wave mode products may also contain secondary information like cross spectra.

  ASAR Wave Mode products are based on small high-resolution, complex images of ocean scenes also called imagettes. These are processed to derive spectra of the ocean backscatter and consequently the wavelength and direction of ocean waves.

  An SLC Imagette and Imagette Cross Spectrum is the basic Level 1B product from the ASAR Wave Mode. A minimum number of corrections and interpolations are performed on the SLC data in order to allow freedom in the derivation of higher-level products. Absolute calibration parameters are provided in the product annotations.

  ASAR Ocean Wave Mode Spectra are available as a Level 2 product. A Level 2 product is an ocean wave wavelength and direction measurement.

• Global Monitoring (GM) Mode - is similar to WS mode, but has a lower data rate since the data is subsampled on board the satellite.

  The Global-Monitoring Mode product is a 1 km resolution image with a 405 km swath width, suitable for dissemination by electronic links in near real time.

The nominal characteristics of the various ASAR measurement modes are summarised in the following table:

Table 2.5 below gives a summary of the ASAR products, categorised by product level, and Table 2.5 below gives a product summary categorised by mode. As for all ENVISAT instruments, the interface for browsing and ordering these products is via the PDS User Service Facility (USF).

To view the summary table that provides the standard specifications for ASAR data products, see "Products and Algorithms Introduction." 2.1.

The Image Mode (IM), Alternating Polarisation mode (AP), and the Wide Swath modes (WS) are all high data rate products. Figure2.10 below gives the generalised product tree for high data rate products.

Figure 2.10 ASAR high data rate product tree

The Global Monitoring (GM) and Wave Modes (WV) are low data rate products. The generalised trees for these products is given below in Figure2.11 .

Figure 2.11 ASAR low data rate product tree