Advanced Biomass Sensing Using Active Optical Sensors
The sensing and measurement of above-ground crop and pasture biomass is of considerable interest to commercial agriculture, as well as for crop and pasture agronomic research purposes. Biomass sensing is an important tool for crop management; to measure spatial and temporal variations in ‗vigour‘ throughout a field, to predict yield, to ascertain damage from pests or diseases, and other various agronomic responses, for example seeding rate, soil and plant fertility, soil moisture, and also to inform variable rate applications of fertilisers across a field or property.
Optical sensors have been utilised for estimating the biomass content of a crop or pasture from spectral reflectance of the plant canopy. These tend to saturate at high biomass levels. Other, primarily non-optical, ranging type sensors have attempted to quantify biomass through sensing of plant canopy height. The vertical resolution of these sensors are often limited by the often large sensor-target distances used.
This thesis aims to combine both approaches into a single optical sensor. The two approaches have been developed around utilising the reflection of radiation from a plant canopy. In effect, the combination can be classed as a ‗reflectance-based sensor‘, although for clarity ‗reflectance‘ and ‗ranging‘ are terms allocated to each of the two basic sensor modes. Hence, a combined spectral reflectance and ranging prototype sensor has been constructed for the use over agricultural crops and pastures. The active optical sensor utilises two laser diodes (Red and NIR wavelengths) and four different optical detectors. This study developed and tested a sensor that not only measures spectral reflectance in the Red and NIR wavebands, but it also incorporates two different optical arrangements to measure the height of the vegetation canopy. Two laterally displaced detectors are employed to utilise the inverse square law (ISL) of reflected radiation to measure the sensor-target (canopy) distance while a second distance sensing component comprises a single one dimensional position sensitive detector (PSD).
The design, construction and extensive laboratory testing of the individual sensing modalities is presented as well as their integration into a single sensor configuration. The sensor was subsequently field-tested on a four wheel drive vehicle; the reflection, ISL and PSD components tested in combination over a field of Tall fescue (Festuca arundinacea) pasture.
Deployed under these field conditions the combined sensor verified that the target height (canopy height) was more significant than the NDVI in responding to biomass changes. The sensor accurately measured the NDVI of the field and compared well with a commercial spectral reflectance sensor (CropCircleTM ACS-210) with an RMSE of deviation between the two sensors of ± 0.02 across ten trial transects.
The performance of the ranging components of the sensor were compared to the measured average height of each of the transects. It was found that the dual-detector ISL method overestimated the pasture height for large sensor target distances and displayed an overall RMSE of deviation from the actual height of 0.22 m, this was greater than the total average crop canopy height of the field. In comparison, the PSD ranging component performed more favourably, displaying random fluctuations in the measurements and an overall RMSE of only 0.05 m for the ten Tall fescue transects.
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