Continental-scale InSAR ground motion mapping

Persistent Scatterer and Small Baseline InSAR techniques convert years of Sentinel-1 radar imagery into millimetre-per-year velocity fields across entire nations, giving geological surveys and infrastructure owners a systematic picture of where the ground is rising or sinking.

Sensors

Sentinel-1 (IW mode): C-band SAR at 5.6 cm wavelength; Interferometric Wide Swath mode covers 250 km at 5 × 20 m ground range resolution. Six-day repeat at mid-latitudes with two satellites; archive from 2014. The backbone of EGMS and LiCSAR.
ALOS-2 PALSAR-2: L-band SAR at 23.6 cm wavelength. Longer wavelength penetrates vegetation and maintains coherence over forested or rural terrain where C-band decorrelates. Strip-map mode reaches 3 m resolution; 14-day repeat. Useful for extending coverage outside Europe or cross-validating C-band results.
EGMS derived products: Pan-European operational service producing geocoded PS and SBAS velocity maps at roughly 100 m posting for the Basic product and individual scatterer level for the Ortho and Calibrated products. Covers EU plus EEA countries; calibrated to GNSS reference frame. Latency is annual to multi-annual release cycles.
LiCSAR / LiCSBAS processing chain: COMET's open processing system generates interferogram networks from Sentinel-1 frames globally, then applies the LiCSBAS time-series inversion. Outputs line-of-sight velocity fields and uncertainty maps. Publicly accessible for research and near-operational use.
GNSS reference networks (e.g. EUREF, IGS): Not SAR sensors, but essential for absolute referencing. Continental InSAR products are inherently relative; tying them to continuous GNSS stations converts relative phase to mm/yr in a stable geodetic frame. Without this step, long-wavelength signals remain ambiguous.

Why a continent needs a single coherent velocity field

A national geological survey that commissions isolated InSAR studies over individual cities ends up with a patchwork: each study uses a different reference point, a different atmospheric correction, and a different time window. Comparing them is unreliable. The European Ground Motion Service addressed this directly by processing the full Sentinel-1 archive across Europe in a single, consistent pipeline, anchoring results to the EUREF GNSS network and publishing velocity fields in the ETRS89 reference frame. The result is a product where a subsidence signal in Rotterdam and an uplift signal in Scandinavia are genuinely comparable.

That consistency matters for infrastructure risk registers. A railway authority managing track across three countries cannot act on three incompatible datasets. A single velocity field, even at modest spatial density, lets asset managers rank corridors by deformation rate and prioritise ground-truth inspections accordingly.

What PS and SBAS actually measure, and where each fails

Persistent Scatterer InSAR identifies individual point targets, typically buildings, pylons, or exposed rock outcrops, whose radar backscatter remains stable across hundreds of acquisitions. Phase history at each scatterer is inverted to separate topographic error, atmospheric delay, and the deformation signal of interest. Velocity precision of 0.5 to 1 mm/yr is achievable in dense urban areas with long time series, though this figure assumes good atmospheric correction and a well-distributed scatterer network.

Small Baseline Subset methods instead form interferograms between image pairs with short spatial and temporal baselines, then invert the network to recover time-series displacement. SBAS tolerates lower scatterer density and works better over semi-vegetated or mixed terrain, but coherence still collapses over dense forest, water, and agricultural fields with rapid crop cycles. Neither technique measures vertical and horizontal motion independently: both measure displacement projected onto the radar line of sight, which is typically 30 to 45 degrees from vertical in Sentinel-1 IW geometry. Decomposing into vertical and east-west components requires combining ascending and descending pass data; north-south motion remains largely invisible to C-band polar-orbiting systems.

Atmospheric delay is the dominant noise source at short time scales. Tropospheric water vapour introduces apparent range changes of several centimetres per acquisition. Long time series average this down, but a two-year stack in a humid tropical climate will carry larger residuals than a five-year stack over an arid plateau. Users should treat velocity uncertainties as site-dependent, not as a fixed specification.

Reference frames, calibration, and the GNSS anchor problem

An InSAR velocity map without a geodetic anchor is a relative product. Every pixel is moving with respect to an arbitrarily chosen reference pixel, which may itself be moving. The EGMS Calibrated product corrects for this by subtracting a smooth long-wavelength field derived from interpolated GNSS velocities, bringing the InSAR result into the ETRS89 frame at the continental scale. The residual uncertainty in the calibrated product is on the order of 1 to 2 mm/yr, depending on GNSS station density in a given region.

Outside Europe, GNSS network density drops sharply. In parts of Africa, Central Asia, and the Pacific, continuous GNSS stations are separated by hundreds of kilometres. In these settings, absolute calibration is weaker, and long-wavelength deformation signals, such as those from glacial isostatic adjustment or broad tectonic tilting, may be absorbed into the reference field rather than resolved. Analysts must document this limitation explicitly when delivering products to clients in data-sparse regions.

Integrating velocity fields into national risk registers

A velocity map is not an engineering assessment. Converting millimetres-per-year into actionable risk requires knowing what infrastructure sits on which signal, what the tolerable deformation threshold is for each asset class, and whether the motion is linear, seasonal, or accelerating. Linear subsidence at 3 mm/yr beneath a motorway embankment may be manageable; the same rate with a seasonal amplitude of 8 mm and an accelerating trend over the last 18 months is a different problem.

Practical integration typically proceeds in three steps. First, the velocity raster is spatially joined to the infrastructure asset register, flagging assets that intersect pixels exceeding a defined threshold, commonly 2 to 5 mm/yr depending on asset sensitivity. Second, time-series plots are extracted for flagged assets to distinguish linear from non-linear motion. Third, a tiered alert is issued: assets showing acceleration or seasonal anomaly go to field inspection; assets with stable linear motion go to a monitoring watchlist. The EGMS documentation recommends against using the Basic product (100 m posting) for individual structure assessment; the Ortho product, which resolves individual scatterers, is more appropriate for that purpose.

Geological surveys in several EU member states have begun incorporating EGMS outputs into national geohazard inventories, though the methodological standards for doing so are still being formalised. Satellize can assist agencies working outside the EGMS footprint, including Pacific and Caribbean territories, to build equivalent processing pipelines on Sentinel-1 open-access data and calibrate them against available GNSS benchmarks.

Archive depth and the value of the 2014 baseline

Sentinel-1A launched in April 2014. A processing run started today can reach back more than a decade, giving velocity estimates that span multiple phases of groundwater abstraction, infrastructure loading, and climate variation. That archive depth is not trivial. A two-year study detects subsidence; a ten-year study can separate a drought-driven compaction signal from a long-term structural trend. For urban areas in Europe, the combination of ERS and Envisat data (1992 to 2010) with Sentinel-1 extends coherent time series to more than thirty years at some locations, though cross-sensor stitching introduces additional uncertainty.

ALOS-2 PALSAR-2 adds L-band coverage from 2014 onward for regions outside Sentinel-1's systematic acquisition plan. Its longer wavelength is particularly valuable in tropical environments, where C-band coherence degrades rapidly over vegetated terrain. The trade-off is a 14-day repeat and narrower systematic coverage outside Japan and selected international agreements.

Honest limits before you commission a product

Continental InSAR is a screening tool, not a substitute for precise levelling or GNSS campaign surveys on critical structures. It will miss deformation events shorter than the revisit interval, fail over water and dense vegetation, and underestimate motion with a strong north-south component. In areas of rapid or large-magnitude displacement, such as active landslides or post-seismic zones, phase unwrapping errors can introduce artefacts that look like real signals. Quality flags in products like EGMS encode coherence and velocity uncertainty, and ignoring them leads to false positives in risk registers.

Latency is also a genuine constraint. EGMS releases are annual at best; LiCSBAS processing on new Sentinel-1 acquisitions can be run more frequently, but operational near-real-time continental motion mapping does not yet exist as a standard product. For time-critical monitoring of individual sites, targeted PS analysis with shorter update cycles is more appropriate than a continental service.

Typical figures

SAR sensor frequency	C-band 5.405 GHz (Sentinel-1); L-band 1.2578 GHz (ALOS-2 PALSAR-2)
Ground range resolution (IW mode)	Sentinel-1: 5 × 20 m (range × azimuth); ALOS-2 strip-map: 3–10 m depending on mode
Repeat pass interval	Sentinel-1: 6 days (two satellites, mid-latitudes); ALOS-2: 14 days
Velocity detection limit	0.5–1 mm/yr in dense urban areas with long time series; 2–3 mm/yr typical in semi-rural or short-stack scenarios
EGMS product spatial posting	Basic: ~100 m grid; Ortho and Calibrated: individual scatterer level (metres to tens of metres)
Reference frame (EGMS)	ETRS89, calibrated against EUREF GNSS network; residual uncertainty ~1–2 mm/yr
Archive depth	Sentinel-1: April 2014 to present; ERS/Envisat legacy data extends to 1992 at select locations
Geographic coverage	EGMS: EU plus EEA countries; LiCSAR/LiCSBAS: global where Sentinel-1 systematic acquisition exists; ALOS-2: selected global coverage by agreement
Delivery formats	GeoTIFF velocity rasters, shapefiles of individual scatterers with time-series attributes, CSV time-series per point, GIS-ready packages
Product latency	EGMS: annual to multi-annual release cycles; custom LiCSBAS processing: weeks to months depending on stack length and compute allocation

Analytics Satellize can run

National velocity mosaic	LiCSBAS SBAS time-series inversion on Sentinel-1 IW stacks, GNSS-calibrated	GeoTIFF velocity field (vertical and east-west components where ascending and descending data overlap) with per-pixel uncertainty layer, delivered as GIS package
Infrastructure deformation report	Spatial join of PS/SBAS velocity points to client asset register; threshold filtering and trend analysis	Ranked asset list with velocity, uncertainty, and motion-type classification (linear, seasonal, accelerating); PDF summary with time-series plots for flagged assets
Acceleration detection alert	Rolling polynomial fit to PS time series; change-point detection on residual velocity	Periodic alert feed (monthly or quarterly) flagging assets where deformation rate has increased beyond a client-defined threshold since last reporting period
Seasonal signal decomposition	Harmonic regression on displacement time series to separate annual and semi-annual components from secular trend	Per-scatterer amplitude and phase of seasonal motion, useful for distinguishing groundwater-driven swelling from structural settlement; delivered as attributed shapefile
Cross-sensor velocity comparison	Co-registration and differencing of C-band (Sentinel-1) and L-band (ALOS-2) velocity fields over vegetated or mixed terrain	Discrepancy map highlighting areas where C-band coherence loss may underestimate true deformation; technical note on confidence by land-cover class
Geohazard screening layer	Cluster analysis on velocity field to delineate coherent deforming zones; overlay with geology and slope maps	Polygon layer of candidate geohazard zones with associated velocity statistics, suitable for input to national geological survey inventories

Sources

Who does the work

We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.