2.5 ASAR Level 0 Products

2.5.1 Introduction

The general product structure, used in all ENVISAT products, is given in "Definitions and Conventions." 2.3.2.1.

2.5.1.1 ASAR Instrument Source Packet

The Level 0 product contains Annotated Instrument Source Packets (AISPs). These are Instrument Source Packets as received from the instrument, with a small header attached by the Front End Processor (FEP).

In addition to the AISP, each Measurement Data Set Record (MDSR) of the Level 0 product contains a time stamp in MJD 2000 format which gives the on-board sensing time of the ISP which it contains (one MDSR per AISP).

The MDSRs within the Level 0 product are:

• demultiplexed per instrument (and per mode for some instruments);
• in ascending chronological order;
• without overlap between the real-time and onboard recorder;
• without obsolete data from the onboard recorded data;
• stored on a computer-compatible format, media and interface.

2.5.1.2 Level 0 Products

ASAR products produced from the Level 0 data can be classified as high-rate or low-rate imaging modes or calibration modes (see also the product summary tables presented in the section "Organisation of Products").

There are nine Level 0 products for ASAR. The first seven correspond to the five main measurement modes, with the AP Level 0 divided into 3 possible products. The final two Level 0 products are the External Characterisation Level 0 and Module Stepping Mode Level 0 products.

The Level 0 products may be Unconsolidated or Consolidated, as described in "Organisation of Products."

The complete list of Level 0 ASAR products to which the information in this section applies is summarised in table 2.7 below.

2.5.1.2.1 Processing Performed

The determination of the satellite position and conversion of Satellite Binary Time (SBT) to Universal Time Co-ordinated (UTC) is accomplished using ESA software. These are the only algorithms applied when forming the Level 0 products.

2.5.1.2.2 High Data Rate Level 0 Products

The high data rate Level 0 products, as depicted in Figure2.10 "ASAR High Data Rate Tree" found in Section "Organisation of Products," are the Image Mode (IM), Alternating Polarisation Mode (AP), and the Wide Swath Modes (WS). There is also a Level 0 External Characterisation product, used for calibration purposes. All of these are discussed below.

2.5.1.2.2.1 Image Mode (ASA_IM_0P)

The Image Mode Level 0 product consists of time-ordered Annotated Instrument Source Packets (AISPs) collected while the instrument is in Image Mode. The echo samples contained in the AISPs are compressed to 4 bits/sample using Flexible Block Adaptive Quantisation (FBAQ). This is a high-rate, narrow swath mode so data is only acquired for partial orbit segments and may be from one of seven possible image swaths.

Details of extracting level 0 data are explained below and in the Level 0 Packets section.

The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL (fully consolidated) version is available from the PAC after 2 weeks.

Max size: 22,500 Mbytes
Coverage: 100 km * 12,000 km (30 min)

2.5.1.2.2.2 Alternating Polarisation (AP) Mode

There are three AP Mode Level 0 products:

These products contain time-ordered AISPs corresponding to one of the three possible polarisation combinations: HH & HV, VV & VH and HH & VV, respectively). The echo samples in the AISPs have been compressed to 4 bits/sample using FBAQ. This is a high-rate, narrow swath mode, so data is only acquired for partial orbit segments and may be from one of seven different image swaths.

The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL, fully consolidated, version is available from the PAC after 2 weeks.

Max size: 22,500 Mbytes
Coverage: 100 km * 12,000 km (30 min)

2.5.1.2.2.3 Wide Swath (WS) Mode (ASA_WS_0P)

The WS Mode Level 0 product consists of time-ordered AISPs collected while the instrument is in WS Mode. The echo samples in the AISPs have been compressed to 4 bits/sample using FBAQ. This is a high-rate, wide swath (ScanSAR) mode so data is only acquired for partial orbit segments and is composed of data from five image swaths (SS1 to SS5). The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL (fully consolidated) version is available from the PAC after 2 weeks.

Max size: 22,500 Mbytes
Coverage: 400 km * 12,000 km (30 min)

2.5.1.2.2.4 Level 0 External Characterisation (ASA_EC_0P)

The External Characterisation Level 0 product consists of time-ordered AISPs collected while the instrument is in External Characterisation Mode. All samples in the AISPs are represented by 8 bits/sample. This is a calibration mode, which allows characterisation in orbit; departures from ground measured characteristics of the instrument calibration loop and the passive part of the antenna during the overflight of a ground receiver. The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL (fully consolidated) version is available from the PAC after 2 weeks.

The uses for this mode are discussed in the section entitled "ASAR Characterisation and Calibration" 2.11. and in the Chapter 3 section entitled "In-flight Performance Verification." 3.2.2.

2.5.1.2.3 Low Data Rate Level 0 Products

The low data rate Level 0 products, as depicted in Figure2.11 "ASAR Low Data Rate Tree" found in the section "Organisation of Products," are the Wave Mode (WV) and the Global Monitoring Mode (GM). There is also a Level 0 Module Stepping product 2.5.1.2.3.3. , used for calibration purposes. Each of these are discussed below.

2.5.1.2.3.1 Global Monitoring (GM) Mode (ASA_GM_0P)

The GM Mode Level 0 product consists of time-ordered AISPs collected while the instrument is in GM Mode. The low data rate is achieved with on-board resampling and filtering. The echo samples in the AISPs have been compressed to 4 bits/sample using FBAQ. This is a low-rate, wide swath (ScanSAR) mode in which data may be acquired over the entire orbit and is composed of data from five image swaths (SS1 to SS5). The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL (fully consolidated) version is available from the PAC after 2 weeks.

Level 0 products include the instrument source packet plus essential input data required for processing.

Max size: 675 Mbytes
Coverage: 400 km * 40,000 km (1 orbit)

2.5.1.2.3.2 Wave Mode (WV) Level 0 Product (ASA_WV_0P)

The WV Mode Level 0 product consists of time-ordered AISPs collected while the instrument is in WV Mode. The echo samples in the AISPs have been compressed to 2 bits/sample using FBAQ. This is a low-rate mode where data is acquired in small imagettes (nominally 10 km x 5 km to 5 km x 5 km in size, high-resolution complex images of ocean scenes) allowing data acquisition to occur periodically (every 100 km) around a full orbit. The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL (fully consolidated) version is available from the PAC after 2 weeks. These are processed to derive spectra of the surface backscatter, and consequently the wavelength and the direction of ocean waves. The vignette can be positioned anywhere in an Image Mode swath. Up to two positions in a single swath, or in different swaths, may be defined with acquisitions alternating between one and the other, which means the vignette will have a separation of 200 km between successive acquisitions at a given position.

The data from this mode is recorded on board and can be obtained anywhere around the orbit when the instrument is not operating in any other mode.

(For a further discussion on Wave Mode products see Level 1B Wave Products 2.6.2.1.2. and Level 2 Products 2.7.2. .)

Level 0 products include the instrument source packet plus essential input data required for processing.

Max size: 608 Mbytes
Coverage: 400 * 5 km * 5 km (1 orbit)

2.5.1.2.3.3 Level 0 Module Stepping Mode (ASA_MS_0P)

ASAR has a dedicated Module Stepping Mode, which is used to gather data from all of the instrument's 320 Transmit/Receive Modules (TRM) automatically. The entire procedure takes less than one second. The data is downloaded to the ground for processing. After processing, the results are compared with the reference data from on-ground tests in order to determine any TRM gain or phase drifts, temperature behaviour, and any eventual module failures. Using this information, it is possible to implement any necessary correction to the TRM coefficients and eventually re-synthesise the antenna beam patterns if required. See the Chapter 3 subsection entitled the "ASA Subsystem" for a discussion of the TRM.

The Module Stepping Mode Level 0 product consists of time-ordered AISPs collected while the instrument is in Module Stepping Mode. All samples contained within the AISPs are represented by 8 bits/sample. This mode provides an internal health checking facility on an individual module basis. The purpose of the mode is to identify malfunctioning modules which may need to be switched off, and to identify modules to which calibration offsets are to be applied.

The Level 0 product is produced systematically for all data acquired within this mode. The NRT version of the product is available from the PDHS 1 day after data acquisition, while the OFL, fully consolidated, version is available from the PAC after 2 weeks.

The uses for this mode are discussed in the section entitled "ASAR Characterisation and Calibration" 2.11. and in the Chapter 3 section entitled "In-flight Performance Verification." 3.2.2.

2.5.1.3 Level 0 Formats

A summary of the general structure of the Level 0 product is shown in Table 2.13 "Level 0 Product Structure" below. Details of the Measurement Data sets are given in the following section Level0 Source Packet Description. 2.5.2.

For the description of the Level 0 SPH, including the Data Set Descriptor (DSD) formats, as well as the Level 0 Measurement Data Set (MDS) Format, including a description of the Front End Processor (FEP) Annotations, refer to the Data Formats(Chapter 6. ) section.

For Level 0, the only auxiliary data required is the orbit state vectors and the SBT to UTC time conversion parameters.