Operating Modes

2.  Regional mission

       Narrow swath modes
           Image mode
           Alternating polarization mode
           Wide swath

These have a high data rate and operation time limited either through data relay satellite by the instrument up to 30 minutes per orbit (including 10 minutes in eclipse), or by ground station visibility, depending on the transmission scheme selected.

ASAR has five mutually exclusive modes of operation

Global Monitoring Mode (GM)

The Global Monitoring Mode provides low resolution images (1 km) using ScanSAR technique over a  405 km swath at HH or VV polarisation. The mode has a low data 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 defined for the Wide Swath Mode are used.

Global Monitoring Mode: MPEG movie (file size 784 kb)
Video animation courtesy of Matra Marconi Space (Bristol, U.K.),
made by DENMAN Productions (U.K.)

Wave Mode (WM)

In Wave Mode, the ASAR instrument, measures changes in backscatter from the sea surface due to ocean wave action. Therefore it generates vignettes, minimum size of 5 km x 5 km, similar to ERS AMI wave mode, spaced 100 km along-track in HH or VV polarization. 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.
Examples of simulated ASAR wave mode spectra:

Simulated ASAR cross-spectra versus buoy measurements.
Comparison of real part of ERS and ASAR - Respective spectra.
Comparison of complete ERS and ASAR - Cross spectra.
Comparison of complete ERS and ASAR - Wave mode spectra
Examples courtesy of NORUT / Norway.

Wave Mode: MPEG movie (file size 827 kb)
Video animation courtesy of Matra Marconi Space (Bristol, U.K.),
made by DENMAN Productions (U.K.)

Image Mode (IM)

In Image Mode the ASAR generates high spatial resolution products (30 m) similar to the ERS SAR. It images one of the seven swaths located over a range of incidence angles spanning from 15 to 45 degrees in HH or VV polarisation.
In the table below the range of the values is due to the different orbit positions. The values are given for Level 1b products.

  Table: ASAR image swath geometry     

Image Mode: MPEG movie (file size 554 kb)
Video animation courtesy of Matra Marconi Space (Bristol, U.K.),
made by DENMAN Productions (U.K.)

Alternating Polarization Mode (AP)

Alternating Polarization Modeprovides 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 polarisations combination (HH/VV or HH/HV or VV/VH) with appoximately 30 m resolution (except IS1). Radiometric resolution is reduced compared to Image Mode.

Alternating Polarization Mode: MPEG movie (file size 426 kb)
Video animation courtesy of Matra Marconi Space (Bristol, U.K.),
made by DENMAN Productions (U.K.)

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 polarization. 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 build up for each subswath.
A ScanSAR example (200 kb) is taken from SIR-C.

Wide Swath Mode: MPEG movie (file size 354 kb)
Video animation courtesy of Matra Marconi Space (Bristol, U.K.),
made by DENMAN Productions (U.K.)

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry