# Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia
## 2 SAR Sensing of Hazards  
In recent decades, SAR has become one of the fastest growing fields of EO and remote sensing (e.g., [11]). SAR is an active remote-sensing technology that operates by illuminating the ground with microwave radiation and recording the reflected response received back at the sensor. Unlike optical EO systems, this means SAR imagery can be acquired in all weathers, through smoke, and at night. A full description of SAR systems, including the effects of imaging geometry and other sources of bias, is beyond the scope of this paper but can be found in [12]. Table S1 in the Supplementary Material also lists key resources for non-specialist readers. Pixels within SAR images each contain a phase and amplitude component, both of which have applications for detecting, monitoring, and mapping the hazards described in the following sections (Figure 2B). Like optical EO systems, the pixel amplitude, A (often converted to intensity, I, where I = A\({}^{2}\)), provides information about features and changes on the Earth's surface. However, the information derived from SAR is complementary to that of optical systems, as A is a function of surface roughness (or structure) and dielectric constant (indicative of moisture and salinity content) that is dependent upon the SAR wavelength. These factors govern the strength and pattern of the backscatter. For example, a flat surface, such as a water body in calm conditions, results in scattering of the incident SAR microwave away from the sensor, thus surface water can be identified by low intensity/amplitude pixel values. For this reason, SAR backscatter intensity thresholding can be used to map surface water coverage dynamics (lakes, rivers, extreme or seasonal floods: Section 3.1). Areas burned during bushfires have also been mapped using temporal variations in backscatter intensity (Section 4). Backscatter can be interpreted using scattering models to discriminate the most direct wave bounce mechanisms (direct or double-bounce scattering) from complex interactions (volume or surface roughness scattering). Volumetric and other indirect scattering mechanisms can be most efficiently retrieved by using multi-polarized SAR datasets that contain information about the signal phase shifts (e.g., [13]). The pixel phase is dependent on the summed phase response of all the individual scatterers contained within the resolved area (pixel footprint) on the ground and is essentially random. For phase to be a useful measurement in hazard monitoring, relative phase differences are measured over time, which cancels out the absolute random component. These phase differences are dependent upon the path length between the satellite and Earth's surface. Variations in pixel phase between multiple SAR images that are acquired over the same region, but at different times (expressed hereafter in yyyymmdd format), can therefore be used to measure mm-to-cm scale displacements of the Earth's surface, a unique capability of SAR systems compared to other types of EO. This technique, known as Interferometric Synthetic Aperture Radar, or InSAR [14], produces maps of ground displacements called interferograms in the 1D oblique satellite Line-of-Sight (LoS). With only a single LoS geometry, it is not possible to determine, unambiguously, the magnitude of vertical versus horizontal ground motion in the measured InSAR signal unless ground-based geodetic data are available (e.g., [15]) (note that, due to the near-polar orbit of SAR satellite, InSAR measurements are less sensitive to the north-south component of surface displacements). However, if two or more LoS geometries are available from both an ascending (approximately south-to-north) and descending (approximately north-to-south) orbit, then the decomposition of the LoS signals into vertical and horizontal components is possible [16].