A Sonar Phenomena: Controlling Motion
Most images from DIDSON look sharp even when objects are moving in the field-of-view or the sonar is moving while collecting data. If the movement is too rapid, the images may become distorted. This happens when there is significant motion while data are mapped to the display and there is no motion compensation in the mapping. Figure 1 shows the side of the pool when the DIDSON is panned slowly. The side is imaged correctly as a straight line. Figure 2 shows the side of the pool when DIDSON is panned too rapidly. The image of the side becomes a jagged line. The data collection time increases with increased maximum range imaged. It takes sound longer to travel to the increased range and return. To minimize the effects of motion, select a maximum range no larger than necessary to include your objects of interest. When the sonar is forward looking and mounted on a forward moving platform, the software has an optional motion compensation routine. Figure 3 was collected with the sonar mounted on the front of a REMUS ROV going 2 m/s over a small sunken boat. Figure 4 is the same data but mapped to compensate for the forward movement.
 Figure 1: A slow pan |
 Figure 2: A fast pan |
 Figure 3: DIDSON on ROV going 2 m/s. |
 Figure 4: Same data as in Figure 3, but mapped to compensate for forward motion. |