# Pido: Predictive Delay Optimization for Intertidal Wireless Sensor Networks
## 3 System Deployment and Motivation
### Intertidal WSN  
Intertidal zones have long served as a model system for investigating the impacts of environmental conditions such as temperature on the ecology of plants and animals [23]. Largely due to the rhythm of the tidal period, organisms in this habitat (barnacles, seaweeds, mussels, oysters, seastars, etc. ), which evolved in purely marine conditions, are exposed to the terrestrial environment at low tide. In an era of rapid climate change, the intertidal zone has emerged as a model for experimentally investigating the impacts of climate change over a range of scales [24]. An important approach is comparing the current and future conditions experienced by organisms in the field to their physiological limitations, measured in the lab. What makes such an approach so difficult is that environmental conditions experienced by animals are hyperlocal and have extremely high temporal and spatial variability. For example, except for mammals and birds, the body temperatures of the vast majority of plants and animals living in air is driven by exposure to solar radiation. Body temperatures in most environments are often considerably higher than air temperature, and depending on shading animals located only a few cm apart can differ in their temperatures by \(>\)15 \({}^{\circ}\)C. Body temperature and physiological stress cannot, therefore, be predicted from metrics such as air temperature [24] and the only recourse is to make direct measurements at the level of the organisms using biomimetic sensors that mimic the thermal characteristics of the animal of interest [25]. Intertidal habitats make this challenging for several reasons. First, the constant action of waves make the use of any cables or wires problematic for more than a few hours. Second, because sensors need to match the spatial scale of the organism of interest, they need to be small, despite the requirement that they be encased in a waterproof housing. This challenge has been previously solved using biomimetics based on commercial loggers that are semi-permanently attached to rock substrate and left in place for periods of up to nine months, after which time they are retrieved [3; 26]. While effective, this method has several problems. First, instrument losses are high, and especially in remote locations instrument loss is not noted until the end of the deployment. Second, when loggers are lost, so are all of the data. Finally, modern methods of climate adaptation are turning increasingly towards early warning systems, which requires real-time access to field data, and frequently from remote locations. Wireless networks have been used in terrestrial systems to circumvent similar challenges. For example, wildlife such as zebras fitted with radio collars that exchange location data when in proximity to one another are an effective means of ensuring that data can be retrieved even when animals are eaten by predators [27]. However, these solutions have not been adapted to intertidal systems. We deployed a wireless sensor network in a tidal flat in Zhoushan, China (N 29\({}^{\circ}\)56\({}^{\prime}\)43\({}^{\prime\prime}\), E 122\({}^{\circ}\)5\({}^{\prime}\)10\({}^{\prime\prime}\)), which is shown in Figure 1. Figure 1a,b reveal the low-tide and high-tide of the tidal flat, and 26 nodes (which appear as black dots) are deployed for temperature monitoring. Nodes in the system cover an area of over 45,000 m\({}^{2}\), and the average distance between nodes is 20 m. To fix the node in the tidal flat, we tie sensors on wooden sticks that are sunk into the mud. Tides in this area are semidiurnal, and nodes are submerged by water for over 12 h per day. Figure 1c reveals the hardware design of the sensor node. Nodes are driven by a micro controller STM32L151 operating at 915 MHz. A pair of electrodes are equipped to acquire the current status of each node: above water or underwater. The voltage difference between electrodes will drop to zero when nodes are submerged in water.