Use of Ultrasonic Proximity Sensors for High-Throughput Characterization of Crop Canopies.

Ultrasonic proximity sensors use principles of sonar to measure distances with centimeter-level accuracy at working distances from 0.5 to 5 m or more. Canopy height per se is of interest, especially in relation to lodging and mechanical harvest, but height can also be used to improve interpretation of other canopy-scale measurements such as of spectral reflectance and surface temperature. The irregular surfaces of crop canopies are challenging targets for ultrasonic proximity measurement, but tests with multiple types of crops suggest that the sensors can accurately quantify canopy height and architecture. We describe results for sensors logged at 5 Hz from vehicles moving at approximately 0.5 m s^-1 through wheat and oil-seed crops. Correlations with visual estimates of height were generally over 0.9. Comparisons among four types of sensors revealed important differences related to refresh rates and response when a distance could not be estimated. Quantile analysis appears especially useful as a means of extracting information beyond a mean height. Overall, ultrasonic proximity sensors appear to have considerable utility for high-throughput monitoring of canopy growth and structure.