SAR sees through clouds, day and night. It can measure ground deformation to the centimeter. This week you decode the magic.
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.
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.
SAR satellites operate in distinct microwave bands, each with different penetration and sensitivity:
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 (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.
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:
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.
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).
Download two Sentinel-1 SLC acquisitions over a known deformation event (volcanic uplift). Coregister. Form the interferogram. Identify the deformation pattern.
Test yourself. Answer key on the certificate-track page (Gold-tier feature: progress tracking and auto-grading).