A comparison of two ranging approaches in an active, optical plant canopy sensor

Active optical sensors that contain their own modulated light sources are becoming popular for “sensing” photosynthetically-active biomass in crops and pastures. These sensors detect optical reflectance to derive spectral vegetation indices, such as the normalised difference vegetation index (NDVI), and are subsequently calibrated to measure plant parameters e.g. biomass. However, research has demonstrated the accuracy of the derived measurements can often be improved by including both a spectral index and a corresponding measure of plant height. This paper describes an active, optical sensor that integrates modulated reflectance sensing with the ability to measure (range) the distance between the source and a target surface. Two ranging techniques are evaluated; one based on the inverse square law (ISL) of reflected radiation and another based on a position-sensitive detector (PSD). Both ranging methods proved capable of reliably delineating target distances out to 4.0 m from the source. Over this range, the PSD detector exhibited a distance-invariant RMSE of ± 2.6 cm whilst the ISL method exhibited an almost linear increase in error of ± 25 % of the measured distance to a spectralon target. Application to a vegetative target (Kikuyu grass), demonstrated the ISL ranging method to yield an average RMSE of ± 3.0 cm in the range of 0.60-1.40 m, while the average RMSE of the PSD over a range of 0.50-1.10 m was observed to be ± 10.0 cm. Despite superior accuracy, target reflectance variations may prove problematic in the use of a PSD ranging sensor and requires further investigation.