The Drifter Platform for Measurements in Small Rivers

Researchers at The University of Iowa have been developing a small, inexpensive floating sensor platform to enable a variety of measurements in small rivers. The platform, dubbed "drifters" consists of a PVC housing and small inflatable rubber tube, data collection electronics, and several sensors. Upon release at strategic locations and times in a river network, drifters interrogate their GPS modules for position, time, and velocity. Researchers then collect the drifters and download and analyze position and velocity data. While our primary interest is to observe river network surface water flows, drifters have the broader application of serving as instrumentation platforms for other sensors such a temperature and turbidity. The drifters are structured as follows. A temperature-compensated MEMS clock provides accurate time information. A GPS disciplines this clock and provides georeference information. A low-power microcontroller orchestrates the data collection on the drifter. The standard sensor configuration of the drifter incorporates the GPS, air- and water temperature sensors, a water turbidity sensor, and an accelerometer. The microcontroller stores the collected data on a high-capacity, non-volatile Flash memory card. Each drifter has a bar code sticker, a small RFID tag, and a unique electronic ID embedded in the electronics. These allow us to manage a fleet of drifters and the data they collect. Each drifter has contact information in case a drifter is lost, and an inexpensive short-range radio and a beeper. These allow for determining the locations of the drifters at the conclusion of an experiment as follows. The microcontroller periodically turns on the receiver and listens for the instruction to turn on the beeper. The beeper, when activated, generates a piercing sound that helps operators locate the drifter. The microcontroller also blinks a super bright LED. Two AA-size alkaline batteries typically power the system. The maximum data collection period is dependent on the number of sensors a user activates, the type of battery utilized (alkaline, lithium, NiMH, etc.), the sample rate, and ranges from 12-72 hours. We have collected several data sets in Iowa.