# Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia
## 7 Geomorphological Changes
### Synergies between SAR and other EO Data Types for Hazard Monitoring  
In addition to increases in the volume of available SAR data, the growing volume and variety of available EO datasets lends itself to identifying new ways to exploit synergies between multiple sources of EO data to improve the spatial resolution of measurements, extend observation time-series, increase signal to noise ratios, and improve the interpretation and/or usability of final data products. For SAR-based hazard monitoring in Australia, three data synergies are identified to be of particular promise, including the integration of SAR with Gravity Recovery and Climate Experiment (GRACE) missions measurements of time-variable gravity, multi-spectral EO imagery, plus the integration of SAR data acquired across multiple frequency bands.  
GRACE and the follow-on GRACE-FO monitor temporal changes of Earth's gravity field that, on the time-scale of the observations (2002-2017 and 2018-present, respectively), are largely dominated by the effects of water storage change. Given the water management challenges that face many countries globally, GRACE and GRACE-FO data provide a valuable source of remote large-scale observations, but at a resolution (250-400 km) that is too low to be directly applicable to many water management activities [34]. Downscaling strategies often utilise auxiliary higher-resolution observations of mass change (e.g., [34]), which are relatively easy to obtain for surficial mass changes (e.g., glacial mass losses: [84]) but challenging to obtain for sub-surface masses such as groundwater. In this instance, large-scale InSAR-derived ground displacement maps (Section 3.2.1) can be used as a proxy for groundwater mass change distribution, offering an unprecedented opportunity to fill this gap [34; 35]. It is important to note, however, that any non-groundwater mass changes, such as soil moisture storage change, surface water storage change (e.g., [34; 84]), or mine extraction (as shown in the Northwest of Australia [85]), have to be subtracted to GRACE total mass change data to extract the groundwater storage change map used in the downscaling procedure.  
Floods, erosion, landslides, fires, and groundwater depletion can be exacerbated by anthropogenic perturbations of the land cover such as logging, grazing and vegetation clearance (e.g., [86]). Multi-spectral EO imagery is used for land-use/cover classification (LUCC), but SAR provides a complementary dataset for this purpose, providing information about the physical structure of features on the ground (angle, orientation, roughness, and texture) to reduce ambiguities in these statistical classifications (e.g., [87; 88]). While numerous other studies have explored such synergies for LUCC, no published studies from Australia are available to date.  
Similar to how SAR and multi-spectral imaging provide information about different physical properties of features on the ground, SAR sensors operating at different frequencies (e.g., Table S2) provide different information due to the role of the SAR wavelength in influencing interactions of the microwave with scatterers of different sizes, the scattering mechanism (direct, double-bounce, volumetric: Section 2) and the penetration of the radar signal into the volume of ground targets. Different SAR frequency bands can be integrated for improving, e.g., LUCC classification accuracy (e.g., [24; 89]), and there is currently scope to test a larger SAR frequency range than ever before with imagery now available at C-band (Sentinel-1), X-band (commercial providers), L-band (commercial providers but with future open-access from the NISAR mission due for launch in 2021, plus other missions such as ESA's ROSE-L in due course) and S-band (NovaSAR-1).