Radar
Radar (Radio detection and ranging) works similar to LiDAR in transmitting electromagnetic signals and detecting the retunes. However, radars use short wavelengths of radio region from 0.1 mm to 1 m, known as microwave region (see figure 3). Due to the long wavelength, radar signals can penetrate through clouds and other particles in the sky, forming an all-weather sensor. Moreover, each wavelength (in radars, usually frequency is used rather than the wavelength) can penetrate through objects to a different extent; thus, it can reveal specific information. For example, frequency of 8-4 GHz, known as C-band, contains lots of information on vegetation dynamics and agricultural data, while higher frequencies 18-12.5 GHz, known as Ku Band, are suitable for crop separability [1]. In addition to frequency, polarization and incident angles are also important parameters in radar studies.
Diverging nature of radar signals requires large antennas to collect the back-scattered signals. This limitation is improved by Synthetic Aperture Radar (SAR), in which a longer antenna is electronically “synthesized”. This radar technology has been mounted on satellites and aircrafts platforms such as UAVSAR, used for vegetation mapping, soil moisture, and crop separability on a large scale[2]. Although most of the reported remote sensing studies using radar are satellite-based or airborne, recently, some prototypes of lightweight radars mounted on sUAS are used for high-resolution (1600 points/m2) radar mapping[3]. However, radars have a long way to reach the popularity of optic sensors mainly because the world is less known in radio frequencies.
References
[1] S. C. Steele-Dunne, H. McNairn, A. Monsivais-Huertero, J. Judge, P.-W. Liu, and K. Papathanassiou, “Radar remote sensing of agricultural canopies: A review,” IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 10, no. 5, pp. 2249–2273, 2017.
[2] UAVSAR, “What is UAVSAR? - UAVSAR,” 2014. https://uavsar.jpl.nasa.gov/education/what-is-uavsar.html (accessed Feb. 02, 2020).
[3] C. Noviello, G. Esposito, G. Fasano, A. Renga, F. Soldovieri, and I. Catapano, “Small-UAV Radar Imaging System Performance with GPS and CDGPS Based Motion Compensation,” Remote Sensing, vol. 12, no. 20, p. 3463, 2020.