TCarta: Exploring SAR for Marine/Hydrospatial Environments
Exploring "Collaborative and Complementary" Remote Sensing techniques in the Hydrospatial domain
INTRODUCTION: "GOLDEN AGE OF SAR/RADAR REMOTE SENSING IN EARTH OBSERVATION"
Microwave/RADAR/SAR Remote Sensing has significantly emerged in the last decade and continues to makes its mark in the future of Earth observation. (“AN EYE IN THE SKY!!”)
SAR Remote Sensing: Concept, Advantages, Applications
Remote sensing of the earth in optical region of electromagnetic spectrum has provided useful scientific operational data for various earth resources applications. Optical sensor essentially records the reflectance of sunlight from the earth’s surface. However, under cloud cover conditions, operational capabilities of optical sensors become limited.
In contrast cloud cover is considerably more transparent at microwave wavelengths, hence microwave sensors plays important role in data collection, which is independent of weather and sun light conditions. The microwave region of the electromagnetic spectrum extends from wavelengths of about 1cm to about 1m.
Microwave remote sensing techniques can be classified into two categories viz., active microwave remote sensing and passive microwave remote sensing. Active microwave sensors used in remote sensing are Side Looking Airborne Radar (SLAR), Synthetic Aperture Radar (SAR), Scatterometer and Altimeter. Passive microwave sensors are mainly radiometers.
EM spectrum and various frequency/wavelength bands used by SAR sensors in Microwave Remote Sensing
Example of an Optical image with clouds present and a clear SAR image (Capella SAR) over a part of Teller, Alaska
How does a Synthetic Aperture Radar (SAR) capture an Image??
Basic SAR Image formation process
SAR is an active microwave imaging system with the capability to penetrate cloud cover and image during night. It transmits pulses of microwave radiation to a target and receives the back scattered radiation from the target in the form of complex samples. The SAR image represents the Radar reflectivity (backscatter) of the target as a function of position. The time delay of the return signal gives information about the position of the target.
Depiction of synthetically increasing aperture for a given sensor and using concepts from camera optics to analogize the processes
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Applications of SAR Remote Sensing in Coastal/Marine Studies:
Due to its ability to penetrate through darkness, clouds, and rain, SAR can be practically used for most earth observation tasks, monitoring and surveillance activities in various thematic realms (Agriculture, Forestry, Geology, LULC, Mapping, Hydrology, Sea-Ice study, Oceans and Coastal monitoring). Some of the noteworthy marine applications are as follows......
- Coastal feature Identification and Classification
- Coastal infrastructure Monitoring & Surveillance; maritime traffic checks
- Shoreline extraction and Coastal Bathymetry
- Coastal Inundation
- Ocean currents & wave parameters, wave pattern identification
- Oil Spill Detection
- Vessel/Ship Detection
- Sea-Ice (Cryosphere) changes, and many more.....
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TCarta Initiatives : SAR for Hydro-spatial Applications
As a part of phase-2 of the National Oceanic and Atmospheric Administration Small Business Innovation Research (NOAA SBIR) grant, that focuses on multi-sensor integration, custom sensor tasking parameterization for marine imaging, and adaptation of Satellite Derived Bathymetry techniques to Alaska and Arctic waters, attempts are being made to evaluate the collaborative and complimentary use of “new age, high temporal and spatial resolution” Microwave/SAR Remote Sensing datasets, for new as well as existing space based hydrospatial derived products.
The following SAR based studies were conducted in synchronization with tidal information obtained from NOAA tides and currents ( https://tidesandcurrents.noaa.gov/tide_predictions.html )
The raw SAR data was pre-processed and converted to a backscatter (dB: sigma naught) image, to allow visual identification (greyscale and colored) and classification of features based on their backscatter values.
2.Yakutat, Alaska: Temporal evaluation of Coastal features and Shoreline changes/extraction during High Tide (HT) and Low Tide (LT) events
For this study- high resolution, X-band Capella SAR data (in stripmap mode) was tasked in synchronization with the date & a close time overlap of predicted Low Tide (0.2 ft) and High Tide (6.6ft) events on 15 and 24 September 2022 (from NOAA), respectively.
The SAR swipe visual clearly shows the visual variations as well as differences in backscatter values of various features over Yakutat during a Low tide and High tide window.
In case of the high tide, one can clearly see the impact of surface roughness (wind and waves) on the water, submerged sandy shoreline and sandbars/land area.
Temporal changes in pixel values /bacskcatter values of Shoreline, sandbar, and waterbody (calm and with waves)
Description for “Tidal Mean backscatter plot”
- Shoreline pixels: mean backscatter during LT is high from shoreline due to exposed sand pixels, and low during HT due to water existing in those areas due to high tide.
- Sandbar pixels: same reasoning as above 1). Sandbar pixels are completely covered with water during HT, giving very low backscatter values during HT.
- Open water pixels: Calm water in the sea exhibits lowest backscatter values during LT, while high values of water are seen due to wave and corresponding surface roughness during HT.
A fair attempt of extracting the shorelines (High Tide Line-HTL and Low Tide Line-LTL) from the high resolution SAR images was made (visual on the right).
While the work is still in progress, the SAR extracted shorelines were validated with HTL and LTLs extracted from optical images (during similar tidal events), as well as Alaska-NOAA CUSP shorelines.
SAR Hight tide and Low tide Image stack for Yakutat- showing clear differences in the shoreline and sandbar boundaries
3. Teller, Alaska: SAR based observations, Shoreline (HTL & LTL) extraction and evaluation of changes- Pre and Post Merbok typhoon
"Collaborative and Complimentary is the way to go for Earth Observation"
The visual shows a high resolution SAR and Optical Image overlay over Teller region, clearly adding more textural element (from SAR) to the multi-spectral image, for enhanced information extraction and analysis.
The swipe visual shows the mean High Tide Line (in red), and the mean Low Tide Line (in green), extracted and overlaid on respective mean tidal (HT and LT) images.
Evaluating changes in Teller SAR images- Pre and Post Typhoon Merbok
The visual on the left: Dark brown to yellow colors depict land areas and surface roughness (high backscatter), while tones of green represent water, increase moisture laden areas or low backscatter regions).
The shoreline and the settlement areas around Teller clearly show low backscatter (dark brown-yellow to green) after the impact of Typhoon Merbok.
Teller: Changes in SAR extracted Low tide Shoreline pre (23 Aug 2022) and post (30 Sept 2022) typhoon Merbok
