# AUV Adaptive Sampling Methods: A Review
## 4 Mission Objectives of Sampling
### Source Localisation
#### 4.1.2 Insect-Inspired Behaviours  
The components of olfactory-based algorithms adopted by insects are Surge and Cast [82]. When the signal is present, the insect flies upstream (Surge). When the signal is absent, the insect flies back and forth across the wind until it makes contact with the plume (Cast). Therefore, Surge is to make progress upstream to reduce the distance between the agent and the source, while Cast is a local search action to reacquire the plume through local search after the signal is lost. [PERSON] and [PERSON] [83] highlighted the importance in adjusting counterturns (a turn followed by another turn), as opposed to having consistent counterturns, in a plume-following algorithm. They presented a tuned-counterturns strategy consisting of course behaviours in accordance with the presence of a plume, as shown below in Table 10.  
[PERSON] et al. [84] presented the Spiral Surge (SS) strategy based on binary information sensing (See Table 11). This approach starts with an initial outward spiral search (SprialGap1), which allows a thorough coverage. When the odour patch is encountered, the vehicle performs a surge behaviour, sampling the wind direction and moving upwind for a set distance (StepSize). After reaching the surge distance, the vehicle performs a tighter spiral casting behaviour, looking for another plume (SpiralGap2). If the plume is re-encountered, the surging behaviour will be repeated, if it is not encountered for a set time (CastTime), the plume is assumed to be lost and the algorithm returns to the initial searching behaviour. [PERSON] et al. [62] presented a behaviour-based planning (BBP) method to trace a plume and identify the location of its source. Their strategy was inspired by male moth \begin{table}
\begin{tabular}{c c} \hline \hline
**Preceding Condition** & **Behaviour** \\ \hline None & Make turns across the wind line at regular intervals \\ \hline If plume is detected & Make upwind progress between turns \\ \hline If plume is not detected & 1. Fly at right angles to the wind line \\  & 2. Increase the time gradually between turns \\ \hline \hline \end{tabular}
\end{table} 
Table 10: A tuned-counterturns strategy proposed by [PERSON] and [PERSON] [83]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Label** & **Definition of Manoeuvre** \\ \hline SpiralGap1 & Initial spiral gap width \\ \hline SpiralGap2 & Plume reacquisition spiral gap width \\ \hline StepSize & Surge distance post odour hit \\ \hline CastTime & Length of time before reverting from reacquisition to initial search spiral \\ \hline SrcDecThresh & Significance threshold between consecutive separate odour hits \\ \hline SrcDecCount & Number of significant differences before source declaration \\ \hline \hline \end{tabular}
\end{table} 
Table 11: List of individual parameters of the Spiral Surge (SS) algorithm, developed from information in [84]. a pheromone plume to seek a female moth. The original chemical plume tracking (CPT) method introduced by [PERSON] [62] constituted five sets of subtasks: Go-to, Find, Track-in, Track-out and Reacquire. Then, two more modules (Source declare and Post-declare Manoeuvre) were added later [74]. A clover-leaf-shaped trajectory generated by the Reacquire manoeuvre is used to reacquire the plume signal after contact is lost. The Bowtie manoeuvre allowed the vehicle a smooth transition to Track-In by performing a 15 degree turn relative to the up-flow direction. When detection is not made by the time the Bowtie completes, the final leg would be followed by a clockwise turn with a 10 m radius toward the down-flow direction. This will result in a minimum of a 23 m search on each side of the centreline of the flow. Two manoeuvres (FLYBY and STAMP) were designed in their method to acquire additional useful data after the source is declared. While the AUV was commanded to perform the FLYBY manoeuvre, following the lines with a 2 m gap between, an underwater video camera captured the location of the chemical source. This is needed to verify whether it is the true source.