Introduction

We demonstrate the first deployment of a large-scale, cloud-based, fully buried Wireless Underground Sensor Network (WUSN) with a very long, continuous operation time on an university campus in an urban area – the Thoreau WUSN at the University of Chicago, built using the Sigfox wireless network. Thoreau consists of an above-ground Sigfox base station with the receiving antenna mounted on the roof of the William Eckhardt Research Center and a number of fully buried underground sensor nodes, each with 4 sensors, a radio and antenna. All the buried sensor nodes communicate directly with the receiving antenna through a single hop. Thoreau covers an area of about one square mile on the university campus, and the sensors nodes are buried 6 to 14 inches underground in different soil conditions. We estimate that the Sigfox low-power IoT solution will allow Thoreau nodes to operate on eight AA batteries for about five years. 

Sensors

In the Thoreau sensor network, each node is equipped with high-precision sensors that measure four important soil parameters: soil temperature (T); electric conductivity (EC), which is an indication of soil salinity and accordingly nutrient content; volumetric water content (VWC), which is an indication of soil moisture; and water potential (WP), which indicates the tendency of water to move in soil. All these parameters are related to soil water, which is vital for plant growth because not only does it maintain the aqueous condition required by biochemical reactions for life, but also dissolves salts that supply nutrients to plants. Real time sensing of these parameters allows monitoring of the health condition of the plants, scheduling irrigation, planning for flood management, etc. On the other hand, as part of the natural water cycle, soil water content influences the weather by affecting the water and heat energy exchange between the land and the atmosphere by means of evaporation and plant growth activities. Therefore, continuous monitoring of these parameters in Thoreau provides important information for soil and weather studies.

We use high-performance Decagon sensors to monitor the above properties. Decagon GS3 measures soil temperature, electric conductivity and volumetric water content, while Decagon MPS6 measures water potential and soil temperature. In GS3, volumetric water content is measured by sensing the dielectric permittivity of the surrounding soil using electromagnetic fields. This is done by sending a radio wave to the sensor prongs and measuring the stored charge at the prongs, which is determined by the dielectric permittivity of the surrounding medium and water content, and then through a calibration relation the measured dielectric permittivity can be converted to the volumetric water content. Electric conductivity is directly measured by sending radio waves to two electrodes and monitoring the resistance between them, which is inversely proportional to the electric conductivity. In MPS6, water potential is measured by introducing a porous ceramic disc as a matrix and measuring its dielectric permittivity when it comes into hydraulic equilibrium with the surrounding soil and therefore has the same water potential as in the surrounding soil. Then through the moisture characteristic curve (relationship between water potential and moisture content) which is known for this ceramic material, the measured dielectric permittivity can be converted to water potential. In both sensors, temperature is measured using a small thermistor which is a type of resistor whose resistance changes as a function of temperature. All these sensors provide general calibration curves that are insensitive to specific soil types, so they can be directly applied in Thoreau where soil types and conditions various from site to site.

Wireless

Thoreau uses the Sigfox network in the unlicensed 902 MHz band using a frequency-hopped ultra narrow band (UNB) transmission scheme with BPSK. Packets are a maximum of 12 bytes long. The bandwidth for each packet is only around 100 Hz, with a transmitted power of 22 dBm. Each packet is transmitted three times sequentially on three randomly chosen frequencies, thus providing both temporal and frequency diversity. Furthermore, packets received by up to three BSs in the reception area can be combined in the backend, thus providing spatial diversity as well. The maximum number of messages each node can transmit per day is 140, and the maximum payload size per packet is 12 bytes. The medium access protocol is extremely light: devices transmit on the uplink asynchronously, without any collision avoidance mechanism, and hence do not need to stay awake to receive synchronization signals from the base station. Messages are not acknowledged or retransmitted, which reduces power but may lead to missed packets. In our application, this is an acceptable trade-off, since soil parameters are fairly stable and a few missed measurements are tolerable. All these measures help to reduce power consumption, increase the reception range, reduce the interference from other users in the unlicensed band, and allow a single base station to handle millions of messages per day. Therefore, the Thoreau WUSN can be conveniently scaled up to a very high node density. While it is well known that frequencies lower than 902 MHz are preferred for underground transmission, we nevertheless chose this frequency for this proof-of-concept deployment since commercial products like Sigfox are readily available and the required antenna size is more manageable in the sensor node.

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