Our second demonstration of the deployment of a large-scale, cloud-based, fully buried Wireless Underground Sensor Network (WUSN) with long, continuous operation time takes place in an agricultural site - specifically, a farm field - near Fermi National Laboratory. The Thoreau WUSN at the agricultural site consists of an above-ground Sigfox base station in the center of the field alongside 25 buried sensor nodes. Each sensor node collects soil data which is then relayed to the receiving antenna in a single hop. We estimate that this low-power IoT solution will operate on four AA batteries for just over four years.
In this Thoreau sensor network deployment, each node is buried between 14 and 15 inches below the surface, and broadcasts wirelessly from that position every thirty minutes. Each sensor contains a high-precision soil sensor, the Teros 12, which measures soil temperature (T), electrical conductivity (EC), and a raw sensor value that may be converted to dielectric permittivity (DP) or volumetric water content (VWC). These parameters, all related to soil water, will allow us to track and understand agricultural development in the field over the course of months and years in real time.
As in the previous campaign on the University of Chicago Hyde Park campus, this Thoreau deployment uses the Sigfox network in the unlicensed 902 MHz band using an ultra-narrow band transmission scheme. Packets are a maximum of 12 bytes long, and is allowed a bandwidth of 100 Hz with a transmission power of 22 dBm. To minimize power use, we select that each packet is transmitted precisely once per cycle (as opposed to three times, which is default) on a randomly chosen frequency within the 902 MHz band. Furthermore, the packets are sent asynchronously, and are not acknowledged or retransmitted. The data packets are received by the central base station planted in the field, then transmitted to the Sigfox cloud and our internal server for processing.
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