  City-scale sensing holds the promise of enabling a deeper understanding of
our urban environments. However, a city-scale deployment requires physical
installation, power management, and communications---all challenging tasks
standing between a good idea and a realized one. This indicates the need for a
platform that enables easy deployment and experimentation for applications
operating at city scale. To address these challenges, we present Signpost, a
modular, energy-harvesting platform for city-scale sensing. Signpost simplifies
deployment by eliminating the need for connection to wired infrastructure and
instead harvesting energy from an integrated solar panel. The platform
furnishes the key resources necessary to support multiple, pluggable sensor
modules while providing fair, safe, and reliable sharing in the face of dynamic
energy constraints. We deploy Signpost with several sensor modules, showing the
viability of an energy-harvesting, multi-tenant, sensing system, and evaluate
its ability to support sensing applications. We believe Signpost reduces the
difficulty inherent in city-scale deployments, enables new experimentation, and
provides improved insights into urban health.
