Delta-X: AirSWOT L2 Geocoded Water Surface Elevation, MRD, Louisiana, 2021

This dataset contains Level 2 (L2) AirSWOT geocoded products. Data were collected during the Delta-X Spring 2021 deployment over the Atchafalaya and Terrebonne basins of the Mississippi River Delta, Louisiana, USA from 2021-03-26 to 2021-04-18. AirSWOT is an airborne calibration and validation instrument for the Surface Water Topography Mission (SWOT) satellite. AirSWOT is capable of producing high resolution digital elevation models over land and water bodies using near-nadir wide-swath Ka-band radar interferometry to measure water-surface elevation and produce continuous gridded elevation data. AirSWOT elevation data is useful for calibrating elevation and slopes along the main channels, as well as tying observations to open ocean tidal conditions. The AirSWOT Level 2 data contains a set of GeoTIFF rasters in UTM map coordinates for each AirSWOT flight line.

This dataset contains 1554 Cloud Optimized GeoTIFF (*.tif) files, seven for each of the 222 flight lines. In addition to the raster files, 222 plain text annotation files (*.ann) are also included with basic metadata such as the flight line ID, start and end UTC times of data acquisition, processor version number, and the date and time of different processing stages.

This dataset contains Level 2 (L2) AirSWOT geocoded products. Data were collected during the Delta-X Spring 2021 deployment over the Atchafalaya and Terrebonne basins of the Mississippi River Delta, Louisiana, USA from 2021-03-26 to 2021-04-18. AirSWOT is an airborne calibration and validation instrument for the Surface Water Topography Mission (SWOT) satellite. AirSWOT is capable of producing high resolution digital elevation models over land and water bodies using near-nadir wide-swath Ka-band radar interferometry to measure water-surface elevation and produce continuous gridded elevation data. AirSWOT elevation data is useful for calibrating elevation and slopes along the main channels, as well as tying observations to open ocean tidal conditions. The AirSWOT Level 2 data contains a set of GeoTIFF rasters in UTM map coordinates for each AirSWOT flight line.

Project: Delta-X

The Delta-X mission is a 5-year NASA Earth Venture Suborbital-3 mission to study the Mississippi River Delta in the United States, which is growing and sinking in different areas. River deltas and their wetlands are drowning as a result of sea level rise and reduced sediment inputs. The Delta-X mission will determine which parts will survive and continue to grow, and which parts will be lost. Delta-X begins with airborne and in situ data acquisition and carries through data analysis, model integration, and validation to predict the extent and spatial patterns of future deltaic land loss or gain.

Related Datasets

Denbina, M.W., M. Simard, E. Rodriguez, X. Wu, and C. Michailovsky. 2021. Pre-Delta-X: L2 AirSWOT Water Surface Elevations, Atchafalaya Basin, LA, USA, 2016. ORNL DAAC, Oak Ridge, Tennessee, USA. https://doi.org/10.3334/ORNLDAAC/1818.

• Contains L2 AirSWOT products from the Pre-Delta-X Spring 2015 deployment.

Denbina, M.W., M. Simard, and E. Rodriguez. 2021. Delta-X: AirSWOT Level 1b Interferogram Products across MRD, LA, USA, 2021. ORNL DAAC, Oak Ridge, Tennessee, USA. https://doi.org/10.3334/ORNLDAAC/1996.

• Contains L1b AirSWOT products from the Delta-X 2021 deployment

Acknowledgments

This work was supported by NASA Earth Venture Suborbital-3 (EVS-3) program (grant NNH17ZDA001N-EVS3).

Spatial Coverage: Atchafalaya and Terrebonne Basins, LA, USA

Spatial Resolution: 3.6 m

Temporal Coverage: 2021-03-26 to 2021-04-18

Temporal Resolution: Measurements taken at irregular intervals

Site Boundaries: Latitude and longitude are given in decimal degrees.

Site	Westernmost Longitude	Easternmost Longitude	Northernmost Latitude	Southernmost Latitude
Atchafalaya and Terrebonne Basins, LA, USA	-91.60	-90.21	29.86	28.98

Data File Information

This dataset includes 1554 datafiles in GeoTIFF (*.tif) format encompassing 222 flight lines with seven types of files per flightline (Table 1). Generally, files contain high resolution elevation data and related variables. In addition to the raster files, 222 plain text annotation files (*.ann) are also included with basic metadata such as the flight line ID, start and end UTC times of data acquisition, processor version number, and the date and time of different processing stages. Each .ann file is for one flight line

Data File Details

• CRS: "WGS 84 / UTM zone 15N" (EPSG: 32615)
• Bands: 1
• Missing data value: -10000

File Naming Convention

Files are named utm_m0m_flightline.variable.tif (e.g., utm_m0m_20210401144714.dhdphi.tif), where:

• utm = represents the Universal Transverse Mercator map units  .
• m0m = represents ‘mode 0 with motion compenasation’, the AirSWOT output product
• flightline = identifies the specific AirSWOT flightline in YYYYMMDDHHMMSS format in UTC time indicating the start of the flight line. Note: The flight line start time and the data start time, used to create the data products (available in the annotation companion files), are not identical.
• variable = indicates the type of data product (i.e. the variable represented in the data file), see Table 1.

Annotation files are named: o5release_flightline.ann (see File Naming Convention above for a description of flightline.

Table 1. Data dictionary for utm_m0m_flightline.variable.tif files.

Variable	Units of measurement	Description
corr		Interferometric correlation (values between 0 and 1).
dhdphi	m radian-1	Height sensitivity expressed as the conversion factor between interferometric phase and vertical height.
err	m	Estimated height error (1-sigma standard deviation).
hgt	m, referenced to WGS-84	Estimated water surface elevation in respect to the WGS84 ellipsoid surface.
inc	radians	Incidence angle estimated from the reference DEM.
mag	m	Backscatter magnitude product.  Equal to the square root of the geometric mean of the reference and secondary channel sigma-nought.  The file contains values in linear units of magnitude, but can be converted to sigma-nought in dB using the equation: sigma0_in_db = 20*log10(.mag)
mag_ns	m	Backscatter magnitude product after the estimated noise-equivalent sigma-nought has been subtracted from the data.  Equal to the square root of the geometric mean of the reference and secondary channel sigma-nought after noise subtraction.  The file contains values in linear units of magnitude, but can be converted to sigma-nought in dB using the equation: sigma0_in_db = 20*log10(.mag).

AirSWOT uses airborne radar interferometry to measure open water surface elevation. It was developed in collaboration with NASA’s Surface Water and Ocean Topography (SWOT) project and has supported multiple ocean and hydrology campaigns including Tanana River, Yukon Flats Basin, and Willamette River. AirSWOT’s elevation measurements can be used to calibrate and validate hydrodynamic models and estimate river discharge.

Data quality of the AirSWOT water level products was assessed by comparison with data from in situ water level gauges at locations throughout the study area. The calibration and global adjustment analysis estimated phase drift rate to make global calibration adjustments based on cross-over flight lines.

In addition, an estimated height uncertainty product is included in this L2 product (utm_m0m_flightline_err.tif).  This product provides an estimated 1-sigma standard deviation of the AirSWOT height for each image pixel.

AirSWOT produces continuous gridded elevation data. On a pixel basis, elevation error was dominated by random noise, and contiguous pixels along a river channel were averaged to reduce measurement noise. Assuming a total averaging area of 1 km², the height noise was less than 10 cm for the Pre-Delta-X L2 products after final phase calibration.

AirSWOT used near-nadir wide-swath radar interferometry (Rodriguez et al., 2017; Altenau et al., 2017) to measure water-surface elevation. The instrument was operated at Ka-band (35.75 GHz) and included six antennas that formed multiple baseline pairs for along-track and across-track interferometry. These antenna combinations provided contiguous coverage from about 500 m to 4 km from nadir, which removed water-motion-induced geolocation and height errors. The instrument was coupled with a high-precision Applanix GPS/IMU system to compensate for the effects of aircraft motion and attitude. AirSWOT was flown on a Langley Research Center (LaRC) King Air aircraftt. There are three components to the overall instrument suite: (1) radar mounted in the nadir forward port of the aircraft, (2) Avcon electronics rack in the main cabin, and (3) Welch power distribution rack in the rear of the aircraft. Radiometric calibration data was acquired prior to deployment by imaging a corner reflector array in Rosamond, CA.

AirSWOT produces data products in multiple modes and employs cross-track and along-track interferometry to both estimate the height of the water surface and to compensate for the motion of water scatterers. The delivered products were from mode “m0m (mode 0 with motion compensation) and represent the standard output product for the AirSWOT outer swath mode. Data from this mode has also been used in the pre-Delta-X campaign (Denbina et al., 2019),  as well as other previous studies to measure water surface elevation and slope (e.g., Altenau et al., 2017; Altenau et al., 2019; Pitcher et al., 2019; Tuozzolo et al., 2019).

AirSWOT performance in the floodplain was limited by the presence of vegetation and by the very shallow slopes characteristic of floodplain discharge. Therefore, the bulk of the AirSWOT data collections were targeted at larger channels. Channel discharge supplies water for potential overflow to islands and floodplains. In situ data were incorporated in the phase calibration procedure because the standard AirSWOT phase calibration could be adversely affected by wetland vegetation, which covers a significant portion of this study area (Section 5 of Denbina, 2018). The AirSWOT instrument suffered from small phase drifts creating potential slope artifacts in the cross-track direction.  To correct the phase-drift artifacts, the flight pattern was designed to include crossovers that were used to make global adjustments and estimate phase drifts when producing the L2 products.

In addition to the raster files, a plain text annotation file is included with basic metadata, including flight line ID, start and end UTC times of data acquisition, processor version number, and the date and time of different processing stages.

Altenau, E.H., T.M. Pavelsky, D. Moller, C. Lion, L.H. Pitcher, G.H. Allen, P.D. Bates, S. Calmant, M. Durand, and L.C. Smith. 2017. AirSWOT measurements of river water surface elevation and slope: Tanana River, AK. Geophysical Research Letters 44:181–189. https://doi.org/10.1002/2016GL071577

Altenau, E.H., T.M. Pavelsky, D. Moller., L.H. Pitcher, P.D. Bates, M.T. Durand, and L.C. Smith. 2019. Temporal variations in river water surface elevation and slope captured by AirSWOT. Remote Sensing of Environment 224:304- 316. https://doi.org/10.1016/j.rse.2019.02.002

Denbina, M.W., M. Simard, and E. Rodriguez. 2022. Delta-X: AirSWOT Level 1B Interferogram Products in Radar Coordinates, Spring 2021. ORNL DAAC, Oak Ridge, Tennessee, USA. https://doi.org/10.3334/ORNLDAAC/1996

Denbina, M.W., M. Simard, E. Rodriguez, X. Wu, and C. Michailovsky. 2021. Pre-Delta-X: L2 AirSWOT Water Surface Elevations, Atchafalaya Basin, LA, USA, 2016. ORNL DAAC, Oak Ridge, Tennessee, USA. https://doi.org/10.3334/ORNLDAAC/1818

Denbina, M. 2018. AirSWOT processing considerations for flight line planning. Pasadena, CA: Jet Propulsion Laboratory, California Institute of Technology.

Denbina, M., M. Simard, E. Rodriguez, X. Wu, A. Chen, and T. Pavelsky. 2019. Mapping water surface elevation and slope in the Mississippi River Delta Using the AirSWOT Ka-band interferometric synthetic aperture radar. Remote Sensing 11:2739. https://doi.org/10.3390/rs11232739

Pitcher, L.H., T.M. Pavelsky, L.C. Smith., D.K. Moller, E.H. Altenau, G.H. Allen, C. Lion, D. Butman, S.W. Cooley, J.V. Fayne, and M. Bertram. 2019. AirSWOT InSAR mapping of surface water elevations and hydraulic gradients across the Yukon Flats Basin, Alaska. Water Resources Research 55:937–953. https://doi.org/10.1029/2018WR023274

Rodriguez, E., D. E. Fernandez, E. Peral, C. W. Chen, J. De Bleser, and B. Williams. 2017. Wide-swath altimetry. Pp. 71-112 in D. Stammer and A. Cazenave (Eds.), Satellite Altimetry over Oceans and Land Surfaces. Taylor & Francis Group, London. https://doi.org/10.1201/9781315151779-2

Tuozzolo, S., G. Lind, B. Overstreet, J. Mangano, M. Fonstad, M. Hagemann, R.P.M. Frasson, K. Larnier, P.A. Garambois, J. Monnier, and M. Durand. 2019. Estimating river discharge with swath altimetry: a proof of concept using AirSWOT observations. Geophysical Research Letters 46:1459–1466. https://doi.org/10.1029/2018GL080771