The source declaration is not a separate behavior. Instead, it is a function that is called at the end of the Track-Out behavior. Each time that the Track-In ends, the last detection point is added to a list. That list is sorted according to distance along the direction of the flow. As long as the AUV is making progress up the plume, the first points on the list will be widely separated. When the AUV is near the source, the plume tracing maneuvers will cause several points on the list to be very near each other in the direction of the flow. When the first three points on the sorted list differ in the direction of the flow by less than 4 meters, then the most upflow point on the list is declared as the source location. An additional error component is due to the fact that the vehicle navigation system may contain accumulated errors of approximately 10 m.
Note that the chemical source is on the bottom and that the AUV drives at a nonzero altitude above the bottom (altitude of 1.5 to 2.0 m is commanded). Therefore, the chemical plume does not rise to the altitude of the AUV, which is necessary for the AUV to detect the chemical, until the chemical has traveled some distance from the source in the direction of the flow. This distance is flow-dependent and is not known. Therefore, the declared source location is expected to have an error component, relative to the true source location, that is in the direction of the flow.
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