SAR: Sentinel-1, polarimetry, InSAR
SAR sees through clouds, day and night. It can measure ground deformation to the centimeter. This week you decode the magic.
When the ground shifts beneath us — Kīlauea inflating before an eruption, or the Pacific plate sliding — can satellites measure it?
Yes, to the millimeter. SAR interferometry (InSAR) does it from 700 km up using radar phase. This week you'll see the math.
Learning objectives
- Understand synthetic aperture radar fundamentals
- Distinguish single, dual, and quad polarization
- Compute interferometric phase between two SAR acquisitions (InSAR)
- Detect ground deformation at centimeter scale with InSAR
Try it: read an InSAR interferogram
InSAR phase wraps every half-wavelength of ground motion. For Sentinel-1's C-band (5.6 cm), that's 2.8 cm per fringe. Move the slider and count the fringes.
Primer
Synthetic Aperture Radar is the most underused superpower in civilian Earth observation. It sees through clouds, day or night. It measures ground deformation to the millimeter via interferometry. It distinguishes surface texture, vegetation density, and soil moisture in ways optical sensors cannot. This week is the SAR primer for space-GIS practitioners.
How SAR works
SAR is an active sensor — it transmits its own microwave illumination and measures the backscattered signal. The "synthetic aperture" trick: a single small antenna on a moving spacecraft synthesizes the effective resolution of a much larger antenna by combining returns from successive positions along the orbit. This is what enables 5–10 meter resolution from a satellite-borne radar — impossible with a "real aperture" antenna of feasible size.
Wavelengths and polarizations
SAR satellites operate in distinct microwave bands, each with different penetration and sensitivity:
- X-band (~3 cm) — TerraSAR-X, COSMO-SkyMed. High resolution, surface scattering, sensitive to roughness.
- C-band (~5.6 cm) — Sentinel-1, RADARSAT-2. The civilian workhorse. Balanced between penetration and resolution.
- L-band (~24 cm) — ALOS-2, NISAR (launching 2026). Penetrates vegetation canopy. Good for biomass and below-canopy mapping.
- P-band (~70 cm) — BIOMASS (ESA, launched 2026). Penetrates dense forest canopy entirely.
Polarization adds more information: a radar can transmit horizontally (H) or vertically (V) polarized waves and receive either. Single-pol is one combination (e.g. VV). Dual-pol is two (VV + VH). Quad-pol is all four (HH, HV, VH, VV). Quad-pol enables polarimetric decomposition, distinguishing scattering mechanisms: surface (smooth ground), volume (vegetation), and double-bounce (urban walls).
Sentinel-1: the open SAR workhorse
Sentinel-1 (ESA, two satellites A and B before B's failure in 2021; C launched 2024) provides free global C-band SAR with 5-day revisit at the equator. Three main acquisition modes: IW (Interferometric Wide, 250 km swath, 5×20 m resolution, the standard mode), EW (Extra Wide, 400 km swath, lower resolution, for sea ice), SM (Strip Map, 80 km swath, higher resolution, on request only). Data is on the ESA Copernicus Hub and on AWS Registry of Open Data.
InSAR: phase-based deformation
The most powerful trick in SAR is interferometry. The radar return at each pixel has both amplitude (signal strength) and phase (the wave's position in its cycle, modulo 2π). The phase encodes the path length from satellite to ground and back.
Take two SAR acquisitions of the same scene from very similar viewing geometries, weeks or months apart. Compute the phase difference at each pixel — the interferogram. If the ground hasn't moved between acquisitions, the phase difference is zero (modulo 2π) everywhere. If part of the ground has subsided 28 mm (half a Sentinel-1 wavelength), the phase difference is 2π × 0.5 = π — visible as a single fringe in the interferogram.
InSAR practical applications:
- Volcanic deformation (uplift before eruption, subsidence after).
- Earthquake co-seismic displacement (centimeters of horizontal slip).
- Urban subsidence from groundwater extraction (millimeters/year, slow but visible over multi-year time series).
- Landslide motion (slow creep visible over weeks).
- Building / infrastructure settlement.
Coherence
InSAR only works where the surface is stable enough between acquisitions to preserve phase. Vegetation, snow, and water are usually decoherent — the phase signal is noise. Bare rock, concrete, urban surfaces, and stable agricultural ground are coherent. Coherence (0–1) is a per-pixel quality measure. High coherence regions give reliable deformation; low coherence regions are masked out.
The lab
You'll download two Sentinel-1 SLC (Single Look Complex) acquisitions over a known volcanic deformation event from the past 5 years, use snap (ESA's Sentinel Application Platform) or a Python wrapper to coregister them, form the interferogram, unwrap the phase, and identify the deformation pattern. The output is a centimeter-scale displacement map of a real geophysical event.
SAR / InSAR is a major specialization in itself; this week is the orientation. For space GIS, SAR's relevance is increasing — both for change detection of orbital infrastructure (new pads, modified facilities) and for environmental monitoring (subsidence around launch facilities, coastal erosion at Starbase).
Connecting to Hawaiʻi: InSAR over Kīlauea
The USGS Hawaiian Volcano Observatory uses InSAR routinely to monitor Kīlauea's inflation and deflation. Before the 2018 eruption, InSAR showed millimeter-scale uplift weeks in advance. During the eruption, it tracked subsidence as magma drained. After the eruption, it continued to map ground motion as the volcano re-equilibrated. The technique works in clouds (radar sees through them), it works at night, and it works on every volcano in the world. Same technique applies to sea-level rise on coastal Hawaiʻi.
Hands-on lab: InSAR over a volcanic deformation event
Download two Sentinel-1 SLC acquisitions over a known deformation event (volcanic uplift). Coregister. Form the interferogram. Identify the deformation pattern.
Quiz — click an answer to check it
No grade, no shame. Tap any option; you'll see if it's right plus the answer if not. The point is to notice what you already know and what's still settling.
- Radio waves (active microwave)
- Visible light
- Infrared
- Gamma rays
- C-band, ~5.6 cm
- X-band, ~3 cm
- L-band, ~24 cm
- P-band, ~70 cm
- Two polarization channels transmitted and received (e.g. VV+VH)
- Two satellites
- Two passes
- Two phases
- A measure of phase stability between two SAR images, 0–1
- A radar gain
- A wavelength
- A signal-to-noise ratio in time
- Centimeter to millimeter scale (depending on processing)
- Always meter scale only
- Never below a kilometer
- Only above ground
Reflection
Take five minutes with this. Write your answer somewhere. Carry it into next week.