Case study on the reliability of unattended outdoor wireless sensor systems

Designed to support in-situ soil moisture measurements for the National Aeronautics and Space Administration (NASA) SMAP Mission, the SoilSCAPE project started more than 6 years ago and currently it has more than 141 sensor nodes and 500 soil probes spread in 7 sites, each one with a Wireless Sensor Network (WSN) connected to a central data server by means of cellular text (SMS) and 3G links. These unattended WSNs are among the largest existing outdoor networks (in terms of node sparsity) and they are operating continuously for more than 2 years. All regular sensor nodes employ non-rechargeable batteries and the failure rate due to software and electronic issues is steadily zero during the years. A very high reliability of the system is paramount in order to reduce the total cost of ownership (TCO) of WSN solutions deployed in unattended and harsh scenarios because in many cases the maintenance costs may be multiple times the value of the device to be fixed. Therefore, we designed a Watchdog Timer (WDT) solution which is supported by a special software engineering methodology involving software and hardware modules that are WDT-aware. The main focus of this paper is to describe this novel WDT approach for embedded systems and to analyze its effectiveness for unattended systems, in particular outdoor WSNs.