2.7.2 Level 2 Product

Level 2 product (ASA_WVW_2P)

The Wave Mode Ocean Wave Spectra product is created by inverting the cross-spectra computed from inter-look processing of the SLC imagettes to derive the directional ocean product ocean wave spectra. Auxiliary ADSs included with the product remains the same as for the Cross-Spectra product. ( see Level 1B Wave Products 2.6.2.1.2. )

This is the basic Level 2 product from ASAR Wave Mode. A new Level 2 product is foreseen, taking into account SAR ocean wave inversion using the cross-spectra methodology.

Below is an example of an ASA_WVW_2P product spectral view ( see the section on the EnviView tool ):

Figure 2.63 ASA_WVW_2P spectral view ( as seen in EnviView )

The output product follows the format of the ENVISAT ASAR Level 1B Wave Mode Imagette Cross-Spectra (ASA_WVS_1P) product ( see Level 1B Wave Products 2.6.2.1.2. ). This is done in order to be compatible with the ground segment products of ENVISAT ASAR.

The products are written in the general form shown in figure2.64 , below.

Figure 2.64 Wave Mode Products Structure

The MPH will be the same as described in section 2.2. . The ASAR wave mode SPH provides only the information which applies to all wave cells within a product file. The format will be identical for all ASAR wave products, except for the DSD sections.

Data Sets

The data sets which make up the ASAR Wave product structure are defined in the following sections. Data Sets are of mixed-binary format. ASCII strings may be included, but are not contained within quotes like for the MPH and SPH.

Wave Mode SQ ADS

There is one Wave Mode SQ ADSR per wave cell. The ADSR contains information pertaining to both the imagette and the spectrum. This consists of the ASAR image SQ information previously defined in INSERTLINKHERE, plus new information pertaining to the cross spectra.

Wave Mode Geolocation Grid ADS (or LADS)

Due to the reduced size of the imagettes compared to normal ASAR images, each Wave Mode Geolocation ADSR contains only the geodetic latitude and longitude of the center point of the imagette. One ADSR is produced for each wave cell.

The center point given in the Geolocation ADSR corresponds to the ground range center point of the wave cell. This will not correspond exactly to the latitude and longitude of the center sample of the SLC imagette (in slant range) but provides a more accurate positioning of the location of the cross spectra center point.

Note that the imagettes do not cover a contiguous region geographically. Thus, Wave Mode Geolocation ADS entries may differ substantially for different imagettes.

Wave Mode Processing Parameters ADS

The Wave Mode Processing parameters ADS details all the parameters used to create the imagette and other parameters specific to Wave Mode Imagette processing. These consist of those found in the Main Processing Parameters ADS of the Image Products format, plus Doppler Centroid parameters, Chirp Parameters, Antenna Elevation Patterns, Slant Range to Ground Range (SR/GR) parameters, Geolocation Grid Tie points, and parameters specific to Wave Mode. Note that the imagette is an SLC image. Therefore the SR/GR parameters and Antenna Elevation parameters pertain to the cross-spectra creation process, not the imagette itself.

There is one ADSR per wave cell.

Cross-Spectrum MDS

This MDS contains the cross spectrum of the imagette. There is one MDSR per wave cell.

The cross spectrum is given in the MDS in complex form on a polar grid. The size of the cross spectrum grid is given in the SPH. Typically this will be 24 by 36 bins in wavelength and direction respectively. As the real part of the polar spectrum is symmetric, and the imaginary part is anti-symmetric, only 0° to 180° need to be given in order to reconstruct the entire spectrum (i.e., only 18 directional bins).

The cross spectrum values shall be stored in an unsigned integer format, each part linearly scaled to the maximum and minimum values of the representation (e.g., 0 to 255). The original maximum and minimum values for each part is reported.

The cross spectrum statistics are derived from a high resolution polar spectrum, prior to encoding for output to the product, or at other intermediate stages of the processing.

If the processor is unable to produce the cross spectra for a given wave cell, the MDSR is time stamped with the time that the cross spectra would have corresponded to, the Quality Flag is set to -1, and the MDSR data fields are all set to zero.

The concept of the Wave Mode cross spectrum product is shown below. The product has 18 sectors with 24 cells each. Because of symmetry the whole circle is not necessary to be supplied. The remaining half is computed according to even- or odd-symmetry of the spectra, as shown below.

Figure 2.65 Wave Mode Cross Spectra Format

Figure 2.66 Method to Reconstruct Full Cross Spectra

Ocean Wave-Spectrum MDS

This MDS contains the ocean wave spectrum of the imagette. There is one MDSR per wave cell. The format of the wave-spectrum MDS closely matches that of the cross spectrum MDS where some fields in the cross-spectrum MDS are substituted by new values pertaining to the ocean wave spectrum product. Each ocean wave spectrum MDSR is described below:

The ocean wave spectrum is given in the MDS as real samples distributed on a polar grid. The size of the ocean wave spectrum grid is given in the SPH. Typically this will be 24 by 36 bins in wavelength and direction respectively.

The ocean wave spectrum values shall be stored in an unsigned integer format, each part linearly scaled to the maximum and minimum values of the representation (e.g., 0 to 255). The original maximum and minimum values are reported.

If the processor is unable to produce the ocean wave spectra for a given wave cell, the MDSR is time stamped with the time that the ocean wave spectra would have corresponded to, the Quality Flag is set to -1, and the MDSR data fields are all set to zero.

MDS Containing Imagettes

The structure of the MDS containing an imagette will be identical to that specified in Section See Measurement Data Set. There is one MDS per imagette.There are up to 400 MDSs per product. The MDSs are not included for WVS and WVW products.

If PF-ASAR is unable to produce an imagette, the MDS will consist of only one MDSR with the time stamp corresponding to what the first line of the imagette would have been, a range line number of 1, and the quality flag set to -1.