Acoustic (Sonar)
Besides electromagnetic signals, sound waves are also an option for remote sensing purposes and can be used as an active sensor to detect objects' range or other acoustic properties. Acoustic waves in frequencies ranging from 10−2 to 107 Hz and distances from cm level to km level can be exploited for a wide variety of remote sensing activities in air and water [1]. However, due to air compressibility and pressure change that alters the sound speed, using acoustic waves in the air over long distances is complex and limited. Hence, short distances are preferred. For example, ultrasonic sensors, which use the same principle as radars and measure the roundtrip time for a pulse of sound wave between the sensor and a target, can be installed on a robot navigating within an orchard to approximately locate the trees, as they were used within a pistachio orchard for sensing the canopies and precision spraying[2]. As another example, acoustic sensors can be used to detect the changes on a surface texture. This method was used in a study to remotely (~1 m) measure the running surface water on soil and estimate the drainage speed[3]. All in all, acoustic sensors are widely used for quality assessment of agricultural products in a non-destructive, remote, or proximal way. However, in longer distances, due to the lack of good spatial resolution, complexity, and uncertainties, the use of these sensors is not well developed for remote sensing.
References
[1] D. R. Dowling and K. G. Sabra, “Acoustic remote sensing,” Annual Review of Fluid Mechanics, vol. 47, pp. 221–243, 2015.
[2] H. Maghsoudi, S. Minaei, B. Ghobadian, and H. Masoudi, “Ultrasonic sensing of pistachio canopy for low-volume precision spraying,” Computers and Electronics in Agriculture, vol. 112, pp. 149–160, 2015.
[3] A. Radionova, C. Ghimire, L. Grundy, S. Laurenson, S. Bradley, and V. Snow, Acoustic Remote Sensing for Irrigation Systems Control in Agriculture. Universitätsbibliothek der RWTH Aachen, 2019.