Estimation of extinction in L band and X band using PolInSAR and the RVoG model

Chapter 1: Introduction 1.1 Brief background of Radar RADAR is an acronym of RAdio Detection And Ranging. (It was originally developed for military purposes during the Second World War). The original radar systems measured range to a target, via time delay, and direction of a target via antenna directivity. Afterwards, the Doppler shifts were used to measure target speed, and then to obtain fine resolution in a direction perpendicular to the range or beam direction. Radar operates in the microwave portion of the electromagnetic spectrum beyond the visible and thermal infrared regions. Imaging radars are generally considered to include wavelengths from 1 mm to 1 meter. Longer wavelengths (lower frequencies) are generally areas devoted to communication and navigation purposes among other uses. Operating in the microwave region improves signal penetration (decreases attenuation) especially trough atmosphere. Imaging Radars are not affected by clouds cover or hazes, as optical sensor are, and operate usually independent from weather conditions. Water clouds have a significant effect only on Radar operating at wavelength below 2 cm. The effect of rain is relatively inconsequential at wavelengths above 4 cm. The Radar is an active sensor, transmitting an electromagnetic signal that illuminates the terrain and recording or measuring the response returned from the target or the surface. Thus the term “active microwave” is an often synonymous used with Radar. As an active sensor, Radar is independent of the sun and sun conditions and can operate day and night. A Radar system records the signal from a target at a single specific wavelength. Therefore the microwave part of the electromagnetic spectrum is divided into several frequency bands (see table 1.1). The name originates from military, where they were used for security reasons, but they are now well established also for civil applications. Important frequency bands for remote sensing are X,C,L and P band, whereas P band until now, is only used by airborne systems. Band Wavelength λ [cm] Frequency ν=c 0* λ -1 [MHz] P 30-100 1000-300 L 15-30 2000-1000 S 7,5-15 2000-4000 C 3,75-7,5 4000-8000 X 2,4-3,75 8000-12,500 K u 1,67-2,4 18,000-12,500 K 1,1-1,67 26,500-18,000 K a 0,75-1,1 40,000-26,500 Table 1.1: Radar Bands designation 1