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RAIN: Undergraduate Project

Rice University and the Electrical & Computer Engineering Department encourage undergraduates to participate in independent research projects. The RAIN project began as such a project and has continued over the past two academic years, supervised by Professors James Young and Rich Baraniuk. A list of the particpants is on the ProjectMembers page.

The undergraduate groups built four prototype nodes (two transmitters, two receivers) based on a simpler, single-detector rainfall rate sensor. The normalized signal variance, or rms noise, from a single horizontal line detector is proportional to the total water content in a fixed volume. This quantity can be converted to the path-averaged rainfall rate by assuming the Marshall-Palmer model of natural raindrop size distribution, and using the terminal-velocity dependence of raindrops. The prototype nodes were deployed along paths of various lengths over several months to collect rain data along with the signal from a standard >tipping-bucket rain gage. The single-detector rain sensor eliminates one detector, but it does not provide raindrop size distribution, and its rainfall rate calibration depends on the path length, detector performance and aperture, and propagation conditions.

The design of the prototype rain sensors reflects the application of the engineering principles of design for cost reduction. The laser source is a collimated diode laser module from a $6 laser pointer. With less than 5 mW output power, it is classified as a Type IIIa laser source, and meets federal eye safety regulations for unrestricted free-space propagation. The 635 nm beam is expanded by a low-power negative lens to produce a spherical beam expanding at a full angle of about 10 mr. Thus the diameter of the beam 50 m away is about 50 cm, a size that eases alignment and is tolerant of minor node motion. The detector consists of a horizontal slit, 1 mm high by 30 mm long, followed by a plastic fresnel collection lens (<$1); the light is focused onto an integrated photodiode-transimpedance amplifier (TI OPT101, <$4). The most expensive component is the glass beam expansion lens ($17) that will be replaced by an inexpensive plastic version or an adjustment of the collimating lens in the diode laser module.

Data from these sensors was digitized and recorded by a laptop computer (inside out of the rain) using National Instruments LabVIEW hardware and software. The data was processed off-line and compared to data from the tipping-bucket gauge. An example of data from a 30 m path is shown below. Note the time resolution of the laser data, and the slow response of the tipping-bucket for low rainfall rates.