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InSAR Ground Motion Monitoring for NZ Infrastructure

A satellite the size of a small car can spot the ground beneath it shift by less than the width of a fingernail.


Interferometric Synthetic Aperture Radar (InSAR) does this by comparing the phase of microwave echoes recorded on repeat satellite passes; any change in path length shows up as coloured fringes that can be converted into millimetres of motion.


Because radar works through cloud and in darkness, the technique delivers truly all-weather ground-deformation maps, turning the whole of Aotearoa into a passive sensor network for geotechnical, civil-engineering and environmental applications.


A three-dimensional geospatial visualization of InSAR radar data over the Three Sisters volcanic region, rendered as two stacked panels against a white background: Top panel (Interferogram on shaded relief): A tilted rectangular block shows a gray-scale shaded-relief digital elevation model of the central Oregon Cascades, over which a rainbow-colored interferometric fringes map is draped. Colors range from deep blue through green and yellow to bright red, indicating line-of-sight radar range change from 0 to 28.3 mm (0 to 2π radians). Concentric rainbow rings centered on a summit mark localized uplift or subsidence. Thin yellow lines trace U.S. highways (20, 126, 242, 97) and the Pacific Crest Trail. White dashed outlines denote the Three Sisters Wilderness boundary. Text labels identify the peaks “Sisters” at upper left and “Three Sisters Wilderness Boundary” at upper right. A small inset map of Oregon at lower right shows a red square marking the study area. Bottom panel (Point-cloud deformation map): Directly below, a second tilted block presents the same region as a colorized 3D point cloud. Here, cooler purples and blues transition to warmer greens and reds on the summit, corresponding to radar range change values from –130 mm to +30 mm. The town of McKenzie Bridge is labeled at left. Both panels share latitude tick marks at 44.5° and 44° N and longitude tick marks at 122° and 12° W. The overall composition conveys spatial context (topography, roads, wilderness boundary) alongside high-precision surface deformation measured by InSAR.
InSAR diagram, showing the 3D data layer which this technology provides geotechs.

InSAR technology explained


What is Interferometric Synthetic Aperture?


A SAR instrument is an active sensor: it transmits a microwave pulse and times the echo’s return. By moving along its orbit and stitching thousands of echoes together, the satellite synthesises a very long antenna, achieving metre-class resolution from hundreds of kilometres away. Unlike optical sensors, microwaves are largely unaffected by cloud, rain, ash or darkness, giving year-round coverage.

Each image pixel stores two pieces of information:

  • Amplitude – how bright the target is at the chosen wavelength.

  • Phase – the position along the microwave’s cycle when the echo was received.


Phase is the secret sauce: if the satellite revisits the same spot from almost the same geometry, any change in the line-of-sight distance- even a few millimetres- will shift the phase.


From raw images to an interferogram

Processing starts by aligning (“co-registering”) two images to sub-pixel accuracy and subtracting their phases. The result is a wrapped interferogram, a psychedelic pattern where each colour cycle represents half the radar wavelength of motion. Unwrapping removes the 2π ambiguity, turning colour fringes into continuous centimetre maps.


Single interferograms are noisy. Atmospheric water vapour, orbital inexactness and changes in surface roughness all distort the signal. That is why modern InSAR chains analyse dozens to hundreds of scenes, extracting only the consistent phase shifts.


A high‐angle aerial view of a new residential subdivision under construction, overlaid with white InSAR-derived contour lines showing subtle ground movements. In the upper third of the frame, finished single-story homes with dark tiled roofs sit on gently sloping lawns behind a newly paved road. Parked along the road are two SUVs—one white, one dark—flanking a yellow hydraulic excavator whose arm rests across the surfacing. Below the road, the middle of the image reveals broad swaths of exposed earth and stockpiled soil, crisscrossed by fine white lines that trace lateral shifts in the terrain. In the lower third, a deep rectangular excavation pit plunges sharply into the subsoil; the steep walls inside the pit are marked by stepped InSAR contour lines that fan inward toward the bottom, highlighting zones of greatest deformation. Sunlit mounds of spoil border the pit’s edge, and the overlay of regular, polygonal line segments across the entire scene conveys a precise, centimetre-scale measurement of land movement occurring as the subdivision is cut into the hillside.
New InSAR-derived contour lines reveal subtle ground movements around a residential subdivision excavation as heavy machinery and newly built homes sit above.

From SAR to land & surface displacement


Raw images pass through four main stages:


  1. Precise orbit correction – GPS and star-tracker data refine satellite position to centimetres.

  2. Co-registration & interferogram formation – images are aligned to sub-pixel accuracy; phase differences produce “wrapped” interferograms.

  3. Noise reduction – multi-temporal filters isolate coherent pixels and suppress atmospheric artefacts

  4. Time-series inversion – Persistent Scatterer (PS) and Small Baseline Subset (SBAS) algorithms convert stacks of interferograms into velocities and displacement histories


Advanced processors blend PS and SBAS approaches so that stable point targets (e.g., building corners) and distributed scatterers (e.g., bare soil) are both retained, delivering nation-wide deformation grids at 10–20 m spacing.



Application domains and typical examples of InSAR

Sector

Conventional Monitoring

InSAR Advantage

Transport corridors (highways, rail, runways)

Total-station prisms every 500 m

6-day updates over the full corridor; mm yr⁻¹ precision flags cut-slope or embankment creep

Utilities & pipelines

Burst history & spot surveys

Detects sub-cm ground strain; improves pipe-failure risk models and renewal planning

Hydro & tailings dams

Extensometers, levelling

Wall-to-wall coverage identifies uneven settlement or uplift at early stage

Coastal land-level change

Tide gauges, GNSS

Separates tectonic/subsidence signals from eustatic sea-level, informing adaptation design

Ground-water & subsidence basins

Borehole levelling

Maps clay-aquifer compaction; supports sustainable abstraction limits


A wide-angle view across a high-rise construction platform at golden hour, backlit by a low sun sinking behind a distant city skyline. In the foreground, the skeletal concrete floor slab is edged with upright steel reinforcing bars, their dark vertical lines punctuating the glowing scene. Two construction workers in hard hats and high-visibility jackets stand apart on the deck—one at left appears to be checking a device or plans, the other at right gazes toward the skyline. Overhead, two tower cranes stretch their booms across the frame: the nearer crane’s lattice mast rises from the deck and supports a hanging load line, while the second crane looms softly blurred in the background. The entire image is suffused with warm oranges and ambers, lens-flare streaks and bokeh highlights dancing across the shot, contrasting with the cool silhouettes of steel and concrete. The city’s softened high-rise outlines form a muted backdrop, emphasising the interplay of human scale, heavy machinery, and the radiance of the setting sun.
A High Rise Construction Site in Auckland is the perfect case for InSAR Satellite Monitoring

Implementation roadmap for smart In-SAR Project Plannnig and Deployment


  1. Baseline screening – Compile a multi-year Sentinel-1 deformation map to locate “hot spots” along transport and utility networks.

  2. Priority monitoring zones – Set up rolling updates over critical slopes, embankments, or coastal zones; integrate alerts into GIS or asset-management systems.

  3. Sensor fusion – Co-locate low-cost GNSS, tiltmeter or fibre-optic DAS sensors on the highest-risk assets to capture sub-daily dynamics.

  4. Digital-twin integration – Publish deformation layers through BIM or SCADA dashboards, closing the loop between remote sensing and maintenance scheduling



Swapping conventional ground instruments with country-wide, millimetre-precision deformation grids via InSAR, infrastructure owners can move from reactive inspections to predictive maintenance. This is great for reducing risk from earthquakes, landslides, subsidence and climate-driven sea-level rise while optimising capital spend.

 
 
 

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