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.

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: