Surface Water Ocean Topography Satellite (SWOT) Mission

Table of contents

• Summary
• Resources
• SWOT Instruments
• SWOT Data Schedule
• SWOT Accuracies
• USGS Involvement with SWOT
• USGS SWOT Applications
• Contacts

Summary:

The Surface Water Ocean Topography (SWOT) satellite launched December 16, 2022, the first mission of its kind designed to globally observe water. The SWOT mission represents a collaborative effort between the United States National Aeronautics and Space Administration (NASA), Centre National d’Études Spatiales (CNES, the French Spatial Agency), the Canadian Space Agency, and the United Kingdom Space Agency. SWOT uses a novel wide-swath Ka-band radar interferometer to provide unprecedented details on the elevation and extent of global water bodies, as well as sea surface height and submesoscale ocean circulation. SWOT will monitor lakes, reservoirs, rivers, and wetlands, surveying over 90% of inland water bodies with repeat coverage about every 10 days in the US. SWOT radar sensors will measure water elevations and storage changes of over 467,000 lakes and reservoirs in the US and will also measure water elevation, slope, and discharge of rivers whose widths exceeds 100 m, which includes ~93,000 US river miles.

The U.S Geological Survey (USGS), which through the Water Mission Area and Water Science Centers operates extensive monitoring networks of US inland water bodies, has collaborated with NASA on the SWOT mission and is actively preparing to use hydrologic data produced by SWOT. We expect a successful SWOT mission to enable measurement of remote water bodies (many for the first time), vastly expand the spatial reach of hydrologic monitoring in the US, and improve understanding of national and global-scale hydrology.

Resources:

The WISP site is currently displaying and disseminating SWOT Level 2 Hydrology data in the United States on river reaches through the Hydrocron Application Programming Interface (API), developed by NASA’s Physical Oceanography Distributed Active Archive Center (PO.DAAC). SWOT data can be accessed directly through the Hydrocron API, or accessed through local or cloud resources provided by NASA PO.DAAC. In addition to Level 2 Hydrology data in a vector-based and time series format, SWOT data is also available at the raster and pixel cloud level through resources provided below. SWOT data is available at a global scale, but in this application we subset data to the United States to compare with USGS gage data.

• SWOT Mission
  • Overview of the SWOT mission, with links to data, publications, and news releases.
• SWOT Data
  • Link to NASA's repository containing SWOT data
  • Each data publication contains detailed guides under "Documentation" within the Product Description Document
• SWOT River Database (SWORD)
  • This dashboard allows users to explore river geometries that SWOT is expected to provide data for
  • This site also contains helpful links on versioning of SWOT river geometries, as well as avenues for feedback
• SWORD publication
  • The SWORD publication, led by Elizabeth Altenau, describes in detail the creation of the SWOT River Database.
• PODAAC Cookbook for SWOT
  • This "cookbook" created by NASA PODAAC provides code and background for accessing SWOT data, as well as tools for data downloading, processing, analyzing, in addition to links to workshops and other helpful resources
• Hydroweb Next
  • The Hydroweb Next Portal developed by CNES, the French Space Agency, allows users to visualize, download, and create time series of global satellite hydrology data, including SWOT data
  • Hydroweb Next also contains the official release of the SWOT Lake Database, containing lake geometries that SWOT is expected to observe
• SWOT Prior Lake Database publication
  • This preprint publication contains information on the creation and development of the SWOT Lake Database, referred to as the "Prior Lake Database"
• SWOT User Handbook
  • The SWOT User Handbook provides a general, high-level introduction to SWOT data and serves as a reference for information to facilitate the use of SWOT data.
• SWOT Level 2 RiverSP description
  • The SWOT Level 2 KaRIn high rate river single pass vector product description document provides a detailed overview of the SWOT river data products displayed on the WISP site, namely the SWOT river reach and node data. This document describes in detail the meaning of different data variables and quality flags.
• SWOT Level 2 LakeSP description
  • The SWOT Level 2 KaRIn high rate lake single pass vector product description document provides a detailed overview of the lake data products displayed on the WISP site. This document describes in detail the meaning of different data variables and quality flags.

SWOT Instruments (adapted from a NASA interview)):

• The Ka-band Radar Interferometer [KaRIn] bounces radar pulses off the surface of water and receives the return signals with two different antennas at the same time. This allows SWOT to triangulate the height of the water’s surface and “see” rivers and other small water bodies on Earth’s surface. The antennas, which stick out about 16 feet on either side of the satellite, will cover about 30 miles of Earth’s surface to the right and left of the spacecraft.
• The KaRIn is paired with an altimeter identical to Jason-3/Sentinel-6 that will look straight down and measure the height of the ocean’s surface.
• A radiometer will correct for water vapor in the atmosphere, which affects the propagation of radar pulses from the altimeter or KaRIn
• SWOT also has a few precision orbital positioning instruments – including a global position system that geolocates the satellite

SWOT Data Schedule:

• Satellite launch date: December 16, 2022
• SWOT beta pre-validated data: December 1, 2023
• SWOT Validated Version C data: March 6, 2024
• Expected availability of Version D data: January, 2025

SWOT Accuracies:

SWOT measurements of stage and discharge will be less accurate than USGS gaging station measurements, but may still support many uses by USGS and cooperators.

SWOT data product	Expected Accuracy
Water surface elevation	±10 cm for areas > 1 km2; ±25 cm for areas (250 m)2 - 1 km2
Water extent	±15% for areas > 1 km2
River slope	±1.7 cm/km over a 10 km reach

USGS Involvement with SWOT:

The USGS has collaborated with NASA since at least 2008 to plan and prepare for the SWOT mission, with a focus on science requirements and water resource applications. USGS scientists from the Water Mission Area, Water Science Centers, and other offices have:

• Collaborated with NASA in SWOT planning meetings and technical working groups
• Hosted the 2017 SWOT Science Team meeting at USGS headquarters in Reston, VA
• Served on the SWOT Science Team and Discharge Algorithm Working Group (DAWG) to develop and improve methods of discharge estimation
• Contributed a discharge algorithm, the Modified Optimized Manning Method Algorithm (MOMMA), one of several that the SWOT mission will use to operationally produce river discharge estimates
• Participated in the NASA SWOT Early Adopter (EA) program
• Worked with international collaborators to collect and collate river discharge measurements to improve SWOT discharge estimates
• Developed, cooperatively with NASA, a satellite-based water monitoring system for Alaska that is now operational and will include SWOT data when available
• Partnered with NASA's Physical Oceanography Distributed Active Archive Center (PO.DAAC) to encourage the development and provide feedback on the Hydrocron API

USGS Applications:

Potential additional USGS uses of SWOT data include:

• Improving assessments of water use, water availability, water budgets, and drought
• Monitoring water levels and discharge in unmonitored basins and extending gage records
• Previewing water levels in lakes and rivers being considered for USGS monitoring

Contacts:

• Merritt Harlan (mharlan@usgs.gov)
• Chuck Hansen (jahansen@usgs.gov)
• Rob Dudley (rwdudley@usgs.gov)
• Jack Eggleston (jegglest@usgs.gov)