Describing the various options available for processing navigation when creating a side scan mosaic.
The above image shows a typical problem that can occur with the processing of navigation from a towed side scan sonar, especially when using USBL (acoustic) positioning .
The tools available in Perspective Mosaic have been designed to cope with these problems, because simply applying a "Boxcar" filter to data like this will not give good results.
Note that the navigation processing is performed only on the cache file, the original XTF file remains unchanged.
In the process of creating a mosaic using the Wizard the following dialog will appear:
Source of heading:
In order to orientate the side scan swaths correctly the program needs to know the towfish heading, there are a number of ways to obtain that information:
Sensor - the heading sensor field from the .XTF files (usually the heading sensor mounted in the towfish).
Note: in some cases the Sensor heading field in the .XTF file may be filled with the course made good computed by the GPS receiver read from the NMEA $VTG sentence.
Ship - the value from the Ship's Gyro (Do NOT use this for a towed fish).
Line Median (straight lines only) - the Median value of all the computed Course Made Good heading values for the whole line. If the survey line is nominally straight this can give a useful result especially in cases where the towfish position is erratic (as above).
Check the box Smooth Navigation and click Settings.
Whether the dialog is invoked during the Wizard or done interactively (see below) the main navigation dialog looks like this:
Window - The weight of smoothing to be applied, larger values give more smoothing.
Source - Refers to the source of the navigation to be input into the filtering routines:
In a conventional XTF file there are two places the navigation can be stored, referred to as:
Sensor - sensor position is also known as “Towfish position” in Triton Isis.
Ship - ship's position.
Smoothed – in Perspective all navigation processing is done in the cache (.xtf_idx file), the navigation is processed and stored in the cache – this becomes the smoothed navigation. Choosing the smoothed option causes Perspective to read the already processed navigation from the cache.
The implication being that if processing (boxcar, filtering etc) has already be enabled in the Wizard and the processing run once then choosing this option will cause navigation smoothing to be run on already smoothed data.
Further, if Smoothed is chosen as the source, when instead of using the wizard you jump out (hit skip) after the tracks are initially displayed, (See Interactive Mode below) it is possible to repeatedly apply the same process over and over, this is the equivalent operation to repeatedly clicking the "Smooth Now" button in the earlier Triton mosaicing products.
Speed
Filter Speed(kts) - Used to remove large jumps in the navigation between consecutive points, any point for which the speed computation falls either above or below the limits is ignored.
Change cut-off - Used to remove points that exceed a change in computed speed set by the input value.
Compute
Never – do not compute a speed (use the speed in the XTF file).
Always – always compute a speed.
When Zero – only compute a speed if the value in the data is zero.
Heading - choose the source of the heading information for orientation of the side scan swaths in the mosaic.
Compute CMG
Compute the Course Made Good from the navigation and use a filtered version of that information for the orientation of the side scan swaths.
Change Cutoff (*) - a point may jump “sideways” across the track - but only by a distance that does not cause the point to be rejected by the speed filter (above); however if the computed heading has changed by more than the set value the point will be rejected.
Mag Heading Deviation - Applies if the chosen source of the heading is Sensor. Almost all towfish have a fluxgate (magnetic) compass which is used as the source of the heading information, the values entered are the corrections to be applied for the four cardinal headings – (intermediate values are interpolated). This assumes that the user has performed some kind of calibration of the heading sensor and knows what these errors are.
In practice we have found all fluxgate heading sensors are just about useless for accurate positioning of the sss swaths and we recommend that course made good is used in almost all situations.
See also Line Median option (above)
Assuming that the filter has been set up correctly then a number of points in the initial image above would hopefully have been rejected, (red in the image below)
A best-fit line is drawn through the remaining points (blue)
And the data from the rejected points is distributed along that line, the time tag from each rejected point is used to position the points on the line (green)
The screenshots are taken from some real data that used a towed fish and acoustic (USBL) tracking, it is very typical of the problems associated with this type of navigation.
After the XTF files have been loaded for the first time the normal procedure is to continue using the Wizard to configure the remainder of the processing steps. However, it is also possible to process the navigation interactively and view the results of the processing on the displayed track in the map window, to invoke this option, answer "Skip" to the import Wizard, the display will show the navigation tracks in green. Right Click on the Navigation>Sidescan node in the tree view, and choose Process Navigation from the drop-down menu:
Now click on Filter Setup to open the Boxcar Settings dialog as described above, make any adjustments then hit Start to see the results.
Note:
If the results are not what you expect Right Click on the Navigation>Sidescan node in the tree view, and choose Reset Navigation from the drop-down menu to reload the raw navigation again. To display the orientation of the Beam Lines click Show Beamlines, useful for determining if the heading is being processed properly...
As noted above if Source is set to Smoothed just hitting Start
will re-process again using same settings.