Synthetic Aperture Radar (SAR) observations

Space-based Synthetic Aperture Radar (SAR) is a very reliable technique for monitoring changes of both the solid and aquatic surfaces of the Earth. SAR measures two independent observables, backscattered amplitude and phase, over a wide swath (10-400 km) with pixel resolution of 1-100 m depending on the satellite acquisition parameters. Backscattered amplitude, which is often presented as gray-scale images of the surface (Figure 1a), is very sensitive to the surface dielectric properties, surface inclination towards the satellite, and wave direction in oceans. Amplitude images are widely used for studying surface classification, soil moisture content, ocean waves, oil spill detection, and many other applications.

Figure 1.  (a) Location map of our study area in South Florida. (b) RADARSAT-1 ScanSAR image of Florida showing location of study area (RADARSAT data © Canadian Space Agency / Agence spatiale canadienne 2002. Processed by CSTARS and distributed by RADARSAT International). (b) Cartoon illustrating the double-bounce radar signal return in vegetated aquatic environments. The red ray bounces twice and returns to the satellite, whereas the black ray bounces once and scattered away. [Source: Wdowinski and Hong, 2015]

The second observable, backscattered phase, measures the fraction of the radar wavelength that returns to the satellite’s antenna. It is mainly sensitive to the range between the surface and the satellite, but also to atmospheric conditions and changes in the surface dielectric properties. Phase data are mainly used in interferometric calculations (InSAR) for detecting cm-level displacements of the surface (Figure 2). The method compares pixel-by-pixel SAR phase observations of the same area acquired at different times from roughly the same location in space to produce high spatial-resolution displacement maps. Such maps, termed interferograms (Figure 2b-2d), are widely used in studies of earthquake induced crustal deformation, magmatic activity (volcanoes), land subsidence due to water extraction, glacier movements, and more.

Figure 2. Interferograms showing phase changes, which were induced by water level changes in the Everglades wetlands, south Florida. (a) JERS-1 amplitude image of South Florida showing the location of the three interferograms. (b) L-band (24 cm wavelength) ALOS interferogram of 90 km wide ascending track. Each color cycle corresponds to 15 cm of water level change. (c) C-band (5.6 cm) Radarsat-1 interferogram of a 75 km wide descending swath. Each color cycle represents 4 cm of water level change. (d) X-band (3.1 cm) TSX interferogram of a 30 km wide descending swath. Some of the observed changes reflect changes in atmospheric moisture between the acquisitions. Each color cycle represents 2 cm of water level changes. L.O. – Lake Okeechobee; EAA – Everglades Agricultural Area; WCA – Water Conservation Area; ENP – Everglades National Park. [Source: Wdowinski and Hong, 2015]

 

Wetland InSAR

Wetland Interferometric Synthetic Aperture Radar (InSAR) is a unique application of the InSAR technique, which detects water level changes in aquatic environments with emergent vegetation. It provides high spatial resolution hydrological observations of wetland and floodplains that cannot be obtained by any terrestrial-based methods. Here we present wetland InSAR observations acquired over various wetland environments by various sensors (L-, C-, and X-bands) and polarizations (Figure 2). The quality of the wetland InSAR observations is evaluated by calculating interferometric coherence and the accuracy by comparing the space-based observations with ground-based stage (water level) measurements. Coherence analyses indicate that L-band data are most suitable for the wetland InSAR application, but also C- and X-band data with short time span between acquisition dates are very useful. The comparison between InSAR and stage observations indicates an accuracy level of 3-8 cm, depending on the data type. Our studies present the more advanced wetland InSAR time series techniques that provide multi-temporal high-resolution maps of absolute water levels. Useful applications of wetland InSAR observations include high spatial resolution water level monitoring, detection of flow patterns and flow discontinuities, and constraining high-resolution flow models.

Here we presents results of our recent study using Sentinel-1 observations for routine water level change measurements over the entire south Florida Everglades wetlands (Liao et al., 2020). The study utilizes 91 Sentinel-1 images acquired over a three-year period (Sep 2016 to Nov 2019) and generates routine 12-days Interferograms and correspondingly 30 m spatial resolution water level change maps over the entire Everglades. The high spatial resolution interferograms detect hydrological signals induced by both natural- and human-induced flow, including tides, gate operations, and canal overflow; all these cannot be detected by terrestrial measurements. The large number of both InSAR and ground-based gauge observations allow us to quantify the overall accuracy of the Sentinel-1 InSAR measurements, which is 3.9 cm for the entire wetland area, but better for smaller hydrological units within the Everglades. Our study reveals that the tropospheric delay for individual interferograms can be very large, as much as 30 cm (~10 fringes). When applying tropospheric corrections to all three years of Sentinel-1 InSAR observations, the overall accuracy level improved by 13% to 3.4 cm. Although our study is focused on the Everglades, its implications in term of the suitability of Sentinel-1 observations for space-based hydrological monitoring of wetlands and the derived accuracy level are applicable to other wetlands with similar vegetation types, located all over the world.

Figure 3.  A representative Sentinel-1 Interferogram (20161009–20161021) showing phase change over south Florida. (a) Most phase changes reflect surface water level change, but also tropospheric phase delay. The white solid lines mark the boundaries of the hydrologic units (WCA1, WCA2A, WCA2B, WCA3A, WCA3B, BCNP, ENP). Interesting hydrological signal are marked by red dash boxes are enlarged in figures (a1), (a2), and (a3). Each fringe cycle (from red to yellow to green to blue and back to red) in the interferogram represents a 3.6 cm vertical elevation change. All interferograms presented in the following share the same colour scale as presented here. [Source: Liao et al., 2020]