The inherent limitations of 'passive' plant canopy reflectance sensors requiring 'standardised' illumination conditions, and calibration standards have resulted in 'active 'optical sensors that contain their own illumination sources and utilise synchronous detection. A variety of such devices are now available. However, owing largely to a combination of limited target illuminance and detector sensitivity, these sensors have, to date, been exclusively deployed in handheld or vehicle-mounted configurations close to the target canopies. But on-ground deployment is limited to situations where there is minimal crop damage incurred (young crop, controlled traffic etc) and when the soil conditions support vehicular or foot access.

Two commercially-available, active sensors have now been tested on low-level airborne platforms.  A Crop Circle^TM sensor proved capable of producing normalized difference vegetation index (NDVI) values from a sorghum crop when flown in an aircraft up to altitudes of 6 m above the crop canopy. Simple ratio (SR=NIR/R) maps, derived from 20 m transects flown at 4 m altitude (above ground level -AGL) showed close agreement with SR images acquired from a meter-resolution airborne imaging system (1800 m AGL) on the same day. A more powerful Raptor ^TM sensor, tested over wheat along 30 m transects generated NDVI maps at altitudes ranging from 15 to 45 m AGL. Raptor-derived NDVI values proved highly correlated to those of an on-ground Crop Circle sensor with close to unity slope and zero offset. The subsequent NDVI maps proved to be invariant to sensor height over the 15-45 m altitude range meaning this type of sensor can be deployed over undulating crop and pasture fields.  In addition to discussing the performance of the two airborne sensor configurations, the potential for using these sensors as part of a real-time ‘sense and apply’ aerial fertilizer application methodology is discussed.