# Autonomous Wireless Sensor Networks in
## Materials and Methods
### Gateway  
A dual-core tablet running on Windows 10 was used as a gateway prototype for the PentaSense system (Figure 3). It takes the role of managing the data-storage, to make the decisions regarding the choice for the communication medium and interfaces to external communication and localization peripherals. The tablet acts as the central sink for the WSN, and is a communication hub for the SatCom short messages, to communicate through 3 G or GPRS, and to create a precise anchor localization through GPS.  
This architecture allows us to rapidly prototype and test the envisioned gateway functionalities. It can also be very useful due to the availability of a display which can be used as a keyboard-less interface for on-field configuration of the system. Moreover, it also allows for tethering with a smartphone in case the tablet is sealed in an environmental shield or just placed and remoteconfiguration by a smartphone is necessary. The tablet also incorporates a low power SSD hard disk to achieve long term data-retention and operate in a low power-consumption mode. Data compression and data aggregation is applied before sending the data through payed connections.  
With an external solar panel and battery, the system can operate autonomously for several weeks or even months. This requires further optimization of the OS. To increase the autonomy of the gateway, it is necessary for the tablet to go regularly in sleep mode or at least to turn off the display when no user interaction is required. Putting the tablet in sleep mode might lead to specific problems regarding the power provision of the communication peripherals. To avoid this, it is necessary to enable USB power availability when in sleep mode and to set-up wake up timers.  
To monitor the system and to provide a way to remotely upgrade the gateway, we have investigated off the shelf tools for remote access support. In this study, we examined two tools--a freeware tool (The Cloudberry Remote Assistant [35]) and a licensed tool (Teamviewer [36]). This remote access tool allows for taking over all the functionalities of the Windows-based smart gateway. It can restart the gateway and send over files to perform a local upgrade of the gateway. The greatest advantage of this tool is that it can be set up for unattended access. All users with the access key can remotely access the system and no personal interaction is required. A disadvantage of the tool is that a Windows system is required to perform the remote access. It does not support tablets or smartphones. The TeamViewer software is a commercial tool which is used worldwide and provides the ability to incorporate the functionalities in your own application. This means that it is not necessary to install a separate remote access software once compiled into your own application. It is a proven tool and is used by many companies for remote assistance. The tool is very well designed and gives remote access to the smart gateway trough many devices. We have positively tested remote access through an Android smartphone. Other devices like tablets are also supported. One disadvantage of the tool is that it does not support unattended access by default. This means that once the smart gateway reboots, a local person should send over the auto-generated access-key (password) to re-establish a connection. During the tests, it happened that we lost control over the machine and a person had to be sent to retrieve the key from the smart gateway. The software, however, can provide permanent access to a remote computer (i.e., our smart gateway) through a mechanism of partner-list. This tool is not free and requires a yearly fee per user.