SWOT (Surface Water and Ocean Topography)

The first wide-swath altimetry mission — measures water-surface elevation across a 120 km swath instead of the traditional single nadir-track point. Joint NASA-CNES mission. Maps rivers ≥100 m wide and lakes ≥250 m × 250 m globally every 21 days, plus ocean topography at 15-50 km mesoscale resolution.

What it sees

• KaRIn (Ka-band Radar Interferometer) — two 5-meter Ku/Ka-band antennas on a 10-meter mast, generating interferometric phase across a 120 km swath (split into two 50 km half-swaths separated by a 20 km nadir gap)
• ~2 m vertical accuracy on river height; ~10 cm on ocean SSH
• 250 m raster resolution for inland water; ~15 km for ocean mesoscale
• 21-day exact repeat with 1-day fast-sampling cal/val periods

Why it matters

• Pre-SWOT: river discharge measurements globally required ground gauges (sparse outside North America + Europe). SWOT generates global river discharge from space.
• Lake storage anomalies for ~1 million lakes (previously: tens of thousands).
• Mesoscale ocean dynamics at 15-50 km — eddies, fronts, mixed-layer variability that Jason-class altimetry couldn’t resolve.
• Closes a gap that affects flood prediction, drought monitoring, water-resource management, ocean climate modeling.

Where to get the data

• earthaccess Python: short names beginning SWOT_L2_* and SWOT_L2_HR_*
• PO.DAAC SWOT portal: podaac.jpl.nasa.gov/SWOT
• Hydrocron API: podaac.jpl.nasa.gov/hydrocron — REST endpoint for river-reach time series (much friendlier than raw L2)
• SWOT-galleries community notebooks: swot-community.github.io/SWOT-galleries

What it enables

• River discharge estimation in ungauged basins (combine width × slope × elevation → discharge via Manning equation)
• Lake-level + reservoir-storage time series
• Floodplain inundation extent mapping
• Mesoscale ocean eddies + frontogenesis studies
• Hurricane storm-surge mapping
• Glacier-fed lake-outburst-flood (GLOF) early warning

Gotchas

• The 20 km nadir gap is real. Each swath is two 50 km half-swaths separated by 20 km. Targets that fall in the gap are missed by the swath observation (but covered by the nadir altimeter).
• 21-day repeat is non-sun-synchronous. The same target is observed at different local solar times. Beware of diurnal-cycle bias in lake/reservoir comparisons.
• Inland water and ocean products use different processing chains. L2 HR (high-rate, inland) and L2 LR (low-rate, ocean) are distinct families.
• Calibration is still maturing. Validation papers as of 2025 show systematic bias of 5-10 cm vs ground gauges for narrow rivers. Use the QA flag.
• Use Hydrocron, not raw L2, for first analyses — it gives reach-aggregated time series directly. Raw L2 HR pixel-cloud requires significant processing.

Related missions

• Jason-3 / Sentinel-6 Michael Freilich: traditional nadir altimetry, ocean continuity (1992–present line: TOPEX/Poseidon → Jason-1/2/3 → S6MF).
• GRACE-FO (s12 →): gravity → total water storage (complementary spatial scale, ~300 km).
• ICESat-2 (s10 →): lidar altimetry, complements SWOT for narrow rivers + lakes SWOT misses.
• NISAR (s03 →): floodplain extent mapping during inundation.

Related datasets