# AUV Adaptive Sampling Methods: A Review
## 4 Mission Objectives of Sampling
### Source Localisation
#### 4.1.3 Crustacean-Inspired Behaviours  
Lobsters in nature have a pair of antennules consisting of chemoreceptor cells on their head to trace chemical odour plumes for foraging and mating. They make use of combined information obtained from sensor cells located on both antennules and legs [75]. Two commonly used biological search methods inspired by crustaceans: chemotaxis and odour gated Rheotaxis (OGR) are discussed in this section.  
[PERSON] et al. [87] implemented a simple chemotaxis algorithm for their robotic lobster, Robolobster. It was of a real lobster size, possessing a pair of conductivity sensors to track a salt plume in a water tank. The chemotaxis approach only makes use of the chemical concentration as the cue to locate the source. Therefore, two simple rules were embedded in their algorithm (Table 12).  
A moving forward manoeuvre, when both sensors detect a similar concentration reading, allows the agent to approach closer towards the source. On the other hand, a moving backward manoeuvre leads the agent to re-entre the plume, minimising the possibility of losing contact with the plume. This method works on a very simple mechanism; hence, it has low power and data storage requirements. This is the same as other chemotaxis approaches. However, it only works effectively in the vicinity of the source.  
[PERSON] and [PERSON] [88] developed a more advanced method for biological plume tracing and source localisation for their Biologically Inspired Chemical Sensing Aquatic Autonomous Robot (BICSAAR), named Wilbur. Highlighting the limits of pure chemotaxis utilising a single chemical sensor, they integrated the direction information of the mean flow with the chemical reading to guide the agent toward the source. They conducted a two-dimensional chemical-plume-tracing experiment in the test tank with fluorescein dye. The mean flow was a salient environmental cue for intuitive sensing. The robot's movement in the upstream direction of the mean flow, while in contact with the plume, reduced the distance between the source and the agent because it is certain that the flow carries the chemical from the source to the agent. Wilbur was set to a frame of reference in the mean flow. Wilbur was oriented parallel to the mean flow using a gyroscope at the beginning of each bout. Manoeuvres of OGR are simply composed of surge and cast motions, yet there is an additional layer of logic to judge whether it is inside or outside of the plume. If the agent arrives in a hole of the plume, it has to select either a left or right direction to perform the cast motion so as to re-enter the plume. However, the possibility of choosing each side is equal. Therefore, the agent utilises a rudimentary memory that stores how many stimulations of the plume it has received by each (left and right) sensor. Surge and cast motions are continuously performed until a third bottom sensor detects the odour, indicating it has arrived at the source. This stochastic approach follows a simple logic and utilises the environmental information readily available without having to have many payload sensors.  
The key notion of these approaches is to trigger a series of behaviours to ensure the localisation of the target. Local maxima do not always correlate with proximity to the source [60]. When the source is dynamic such as with moving sources, a higher level of command and layered structure with a distributed sensor network is required to counteract the four-dimensional dynamics of the source.