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    24-Jul-2014
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Measurement Data Set containing spectra. 1 MDSR per spectra.
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ASA_XCH_AX: ASAR External characterization data
ASA_XCA_AX: ASAR External calibration data
ASA_INS_AX: ASAR Instrument characterization
ASA_CON_AX: ASAR Processor Configuration
Browse Products
ASA_WS__BP: ASAR Wide Swath Browse Image
ASA_IM__BP: ASAR Image Mode Browse Image
ASA_GM__BP: ASAR Global Monitoring Mode Browse Image
ASA_AP__BP: ASAR Alternating Polarization Browse Image
Level 0 Products
ASA_WV__0P: ASAR Wave Mode Level 0
ASA_WS__0P: ASAR Wide Swath Mode Level 0
ASA_MS__0P: ASAR Level 0 Module Stepping Mode
ASA_IM__0P: ASAR Image Mode Level 0
ASA_GM__0P: ASAR Global Monitoring Mode Level 0
ASA_EC__0P: ASAR Level 0 External Characterization
ASA_APV_0P: ASAR Alternating Polarization Level 0 (Cross polar V)
ASA_APH_0P: ASAR Alternating Polarization Level 0 (Cross polar H)
ASA_APC_0P: ASAR Alternating Polarization Level 0 (Copolar)
Level 1 Products
ASA_IMS_1P: ASAR Image Mode Single Look Complex
ASA_IMP_1P: ASAR Image Mode Precision Image
ASA_IMM_1P: ASAR Image Mode Medium Resolution Image
ASA_IMG_1P: ASAR Image Mode Ellipsoid Geocoded Image
ASA_GM1_1P: ASAR Global Monitoring Mode Image
ASA_APS_1P: ASAR Alternating Polarization Mode Single Look Complex
ASA_APP_1P: ASAR Alternating Polarization Mode Precision Image
ASA_APM_1P: ASAR Alternating Polarization Medium Resolution Image product
ASA_WSS_1P: Wide Swath Mode SLC Image
ASA_WVS_1P: ASAR Wave Mode Imagette Cross Spectra
ASA_WSM_1P: ASAR Wide Swath Medium Resolution Image
ASA_APG_1P: ASAR Alternating Polarization Ellipsoid Geocoded Image
Level 2 Products
ASA_WVW_2P: ASAR Wave Mode Wave Spectra
ASAR Glossary Terms
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ASA_WVI_1P: ASAR Wave Mode SLC Imagette and Imagette Cross Spectra
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3.1.2 ASAR Instrument Functionality

ASAR will provide continuity of the ERS SAR Image and Wave Modes, but with the opportunity for better temporal frequency of coverage. The nominal 30 m spatial resolution and swath coverage of the ASAR Image Mode (100 km) and Wave Mode (5 km) are the same as the ERS Image Mode, and ASAR will also be on a 35 day repeat orbit.

ASAR has five mutually exclusive modes of operation and offers, by exploiting the combinations of polarisations and incidence angles, 37 different and mutually exclusive operating modes in high, medium (Wide Swath Mode), and reduced (Global Monitoring Mode) resolution. These modes are operated mainly in response to user requests. Wave mode is mutually exclusive with respect to all the other modes. It is a low-rate mode operated systematically over oceans as part of the Global Mission.( See figure3.3 in the section entitled "Instrument Description" ).

Global Monitoring and Wave Modes are recorded systematically when operated. ASAR high- and medium-resolution imaging modes are either transmitted on a real-time link (direct X-band or via Artemis Ka-band link) or recorded on the on-board solid state recorder for ground data recovery. The high- and medium-resolution data is acquired only when required to satisfy either a background mission scenario and/or user requests. The operation modes are divided into two categories:

3.1.2.1 Global Mission

These modes have a low data rate (data generation of 0.9 Mbps) with sytematic on-board recording and operational capability of up to 100% of the orbit. The two modes, GM and WV described below, are systematically recorded to an on-board tape recorder that is dumped every orbit under visibility of an ESA Station, such as the Kiruna Station in Sweden or the ESRIN Station, in Italy.

  • Global Monitoring Mode (GM)

The Global Monitoring Mode provides low-resolution images (1 km) using the ScanSAR technique, over a 405 km swath, at HH or VV polarisation. As explained above, this mode has a low data rate due to a slightly reduced along-track duty ratio and the use of digital filtering for reduction in the across-track direction. The same subswaths as those defined for the Wide Swath (WS) Mode are used.

  • Wave Mode (WV)

In Wave Mode, the ASAR instrument measures the changes in backscatter from the sea surface due to ocean wave action. Therefore, it will generate vignettes with a minimum size of 5 km x 5 km, similar to the ERS AMI Wave Mode, spaced 100 km along-track in HH or VV polarisation. The position of the wave vignette across-track being selected as either constant or alternating between two across-track positions over the full swath range.

3.1.2.2 Regional Mission

These modes are high data rate (downlink rate of 100 Mbps) modes for narrow swath, like the Image Mode (IM) and Alternating Polarisation (AP) modes, or for the Wide Swath (WS) mode, with operation time up to 30 minutes per orbit (including 10' in eclipse). The high-rate mode data is recovered according to one of the following schemes:

  • real transmission via an X-band link to ESA
  • another station
  • real-time transmission via tire Ka-band link using the Artemis Data Relay Satellite to the ESA ESRIN Station in Italy
  • recorded on board the Solid State Recorder and dump in X- or Ka-band link in visibility of an ESA station
  • Image Mode (IM)

In Image Mode, the ASAR generates high spatial resolution products (30 m) similar to the ERS SAR. It will image one of the seven swaths located over a range of incidence angles spanning from 15 to 45 degrees in HH or VV polarisation.

Alternating Polarisation Mode (AP)

Alternating Polarisation Mode provides high-resolution products in any swath, as in Image Mode, but with polarisation changing from subaperture to subaperture within the synthetic aperture. Effectively, a ScanSAR technique is used but without varying the subswath. The results are in two images of the same scene in different polarisation combinations (HH/VV or HH/HV or VV/VH) with appoximately 30 m resolution (except IS1). Radiometric resolution is reduced compared to Image Mode.

Wide Swath Mode (WS)

In the Wide Swath Mode, the ScanSAR technique is used, providing images of a wider strip (405 km) with medium-resolution (150 m) in HH or VV polarisation. The total swath consists of five subswaths and the ASAR transmits bursts of pulses to each of the subswaths in turn in such a way that a continuous along-track image is built up for each subswath.

The ERS high-resolution products PRI, SLC, and GEC will be continued for Image Mode, and generated for Alternating Polarisation Mode on user request. The Wave Mode products are continued, and their quality improved, thanks to cross-spectra algorithms.

Within each mode, several different image swaths may be used. The swath layout is depicted in figure3.19 below.

ASAR Swath Designations
Figure 3.19 ASAR swath designations

In addition, ASAR supports 2 Auxiliary Modes, known as Test Mode and Module Stepping Mode, and one Calibration Mode, known as External Characterisation Mode. As their names imply, these are used for testing, calibration and instrument monitoring. Finally, one of five possible on-board data quantisation methods may be used for each mode, though each has a default choice which will be used in most cases.

The nominal characteristics of the various ASAR measurement modes are summarised in table 3.1 below:

Table 3.1 Nominal ASAR characteristics
Image Mode
(IM)
VV or HH polarisation images from any of 7 selectable swaths. Swath width between approximately 56 km (swath 7) and 100 km (swath 1) across-track. Spatial resolution of approximately 30 m (for precision product).
Alternating Polarisation Mode (AP)
Two co-registered images per acquisition, from any of 7 selectable swaths. HH/VV, HH/HV, or VV/VH polarisation pairs possible. Spatial resolution of approximately 30 m (for precision product).
Wide Swath Mode (WS)
400 km by 400 km wide swath image. Spatial resolution of approximately 150 m by 150 m for nominal product. VV or HH polarisation.
Global Monitoring Mode (GM)
Spatial resolution of approximately 1000 m in azimuth by 1000 m in range for nominal product. Up to a full orbit of coverage. HH or VV polarisation.
Wave Mode (WV)
A small imagette (dimensions range between 10 km by 5 km to 5km by 5km) is acquired at regular intervals of 100 km along-track. The imagette can be positioned anywhere in an Image Mode swath. Up to two positions in a single swath or in different swaths may be specified, with acquisitions alternating between one and the other (successive imagettes will hence have a separation of 200 km between acquisitions at a given position). HH or VV polarisation may be chosen. Imagettes are converted to wave spectra for ocean monitoring.