Soil Scouts: Description and performance of single hop wireless underground sensor nodes

Soil Scouts are palm-size wireless underground sensor nodes for monitoring of agriculture soil parameters. The system design is guided by two main criterions: (i) the data packets from underground sensor nodes must be received from up to 1 km away and (ii) the maintenance free underground sensor nodes must be capable of operating for a decade. System solutions are discussed compared to technical restraints and topology issues. Underground sensor nodes communicating with each other would require an excessive density of devices to overcome the range requirement and multi-hop routing would make the energy conservation requirement hard to achieve. This is why the single-hop approach is chosen. 11 Soil Scout prototypes are installed in a heavy clay soil, where up to 236 m distances and −110 dB calculated path losses are overcome with a directive 60° receiving antenna. A reference Soil Scout is installed in a sandy soil, where the 300 m distance is managed uninterruptedly with an omnidirectional receiving antenna. Battery voltage decline for the nodes as well as for previous Soil Scout designs are presented and observed to decline for 18 months and then obtain a sustaining annual temperature dependent fluctuation.