A comparison of two ranging approaches to an integrated active, optical, reflectance and ranging sensor

Active optical sensors that contain their own modulated light sources are becoming popular for 'sensing' photosynthetically-active biomass in crops. Primarily confined to on-ground deployment, these sensors rely on detecting optical reflectance in two or more wavebands (for example red and near infrared). The derived spectral vegetation indices, such as the widely-used normalised difference vegetation index (NDVI) are subsequently calibrated to a measure of biomass, tiller number, leaf area index or the like. However research has demonstrated the accuracy of the conversion process can often be improved by including both spectral index and a corresponding measure of plant geometry such as height. This paper describes an active, optical sensor that integrates the 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 (ISQ) of reflected radiation from a target and another based on position-sensitive detector (PSD). Both ranging methods proved capable of reliably delineating target distances out to 4.0m from the source. Over this range, the PSD detector exhibited a distance-invariant uncertainty of ±2.6cm whilst the ISQ method exhibited an almost linear increase in error of ±25% of the measured distance.