Adaptable Beam-width Ultrasound Array for Remotely Sensing Vegetation Properties

We describe an active ultrasonic sensor for remotely sensing vegetation biomass. The sensor comprises co-located log-spiral arrays of speakers and microphones, and normal operation is about 0.8 m above pasture, mounted on a farm quad bike moving at up to 20 km/h. Since axial sensing is used, individual sensors are not phased. But the sensors are arranged in a number of rings for both transmission and reception, allowing considerable scope for examining the effect of changing the ‘footprint’ diameter. Linear FM chirp pulses are transmitted and, with matched filtering, very high axial spatial resolution is obtained. Echoes are obtained from blades of grass in the pasture canopy and from the ground. However, because of the strongly reflecting ground surface, secondary reflections also occur (reflection from ground then pasture, or from pasture then ground). This is the situation which also causes challenges for satellite sensing of vegetation using Synthetic Aperture Radar, which is called ‘bounce’ in that context. The result is that the ground position is hard to identify, with possible difficulty in defining the extent of the vegetative layer. We describe how our multiple footprint diameter sensor addresses this scattering problem.