Technical Note: Fledermaus Geocoder Toolbox Improved AVG modes

What are the changes?

For the Fledermaus Geocoder Toolbox (FMGT) to process backscatter data: all the Angle Varying Gain (AVG) modes (Trend, Adaptive and Flat) have been improved to better handle sonar data from multibeam systems running with narrow angular swaths (less than +/-45 degrees).

The AVG algorithm used by FMGT computes the average seafloor response level within a band of 30-60 degrees (grazing angles) to normalize the data. These are depicted by the red squares in the upper plot of Figure 1. For most MBES, this is not an issue, however for MBES that run very narrow angular swaths, the AVG algorithm would produce substandard results that would translate into a striping effect in the resulting mosaic (See left image of Figure 2).

The changes are that the AVG will now consider a larger subset of angles than stated above. See dotted lines in the lower plot in Figure 2, which shows the increased angular range of data that FMGT will consider in the tabulation of the mean seafloor response level for the purposes of normalization. The increase in the examined angular range is only done for cases where less than 50% of the nominal angular range provides data, i.e. the angular sector is less than +/-45 degrees. In this case when there is not enough data in the original angular range, the angular range is expanded to include data up to 70 degree grazing angle (70 is closer to nadir than 60). The result is now that depicted in the right image of Figure 2.

It is important to note that normalizing the angular response with such a small angular sector will likely lead to some biases since the normalization level will be slightly noisier than it would be with the usual 30-60 degree window.

Figure 1. Plot found in the ARA tab from FMGT used to depict, with the ARA expected seafloor response model (line in blue) and some example ARA results. These graphs are just for reference to depict the changes done for the AVG normalization algorithm.

Who does this impact?

This improvement affects MBES with less than +/-45 degree coverage. Data from systems running with a larger angular coverage will not be affected, and FMGT will use the original established AVG algorithm which is suitable in those cases.

Example:

Figure 2. Before and after resulting mosaic screengrabs, using filter AVG normalization algorithm in flat mode for processing.