What Is Radar Interferometry?

Radar interferometry is a signal processing technique that extracts topographic or displacement information from a scene by comparing the phase of two or more radar observations of the same area. When a coherent radar illuminates a surface from two slightly different positions or at two different times, the difference in round-trip path length between the two observations produces a measurable phase shift. This phase difference, combined with knowledge of the imaging geometry, yields centimeter-scale measurements of surface elevation or deformation across areas hundreds of kilometers wide. The technique is most commonly applied using synthetic aperture radar (SAR), in which case it is called interferometric SAR, or InSAR.

The approach draws on interferometric principles established in radio astronomy and optical holography, adapted to the microwave frequencies used by imaging radar. Its application to Earth observation became practical in the early 1990s when researchers demonstrated that spaceborne SAR data from the ERS-1 satellite could resolve millimeter-scale ground displacement associated with seismic events.

InSAR Phase Processing

In InSAR processing, two SAR acquisitions of the same scene are co-registered to sub-pixel accuracy and then multiplied together (one conjugated) to form an interferogram: a map of phase differences that encodes topography, displacement, atmospheric delay, and noise. The topographic component can be removed using an existing digital elevation model, leaving a residual phase signal proportional to the displacement of the ground surface along the satellite's line of sight. The U.S. Geological Survey's InSAR overview describes how InSAR makes high-density measurements by using radar signals from Earth-orbiting satellites to detect changes in land-surface altitude at fine spatial detail. Phase unwrapping converts the wrapped phase, constrained to the range minus pi to pi, into a continuous deformation map.

Differential Interferometry

Differential InSAR (DInSAR) isolates surface displacement by subtracting the expected topographic phase contribution from an interferogram. The technique was first applied to measure co-seismic displacement from the 1992 Landers earthquake in California, revealing a spatially continuous deformation field that was impossible to obtain from sparse GPS networks. DInSAR achieves line-of-sight displacement accuracy of roughly one centimeter over a full scene when atmospheric conditions are favorable. The ESA guidelines on SAR interferometry processing provide the processing chain for generating differential interferograms from raw SAR data, covering co-registration, interferogram formation, filtering, and phase unwrapping.

Persistent Scatterer and Multi-Temporal Techniques

Single-pair InSAR is limited by atmospheric phase screen artifacts and temporal decorrelation from changes in vegetation or soil moisture between acquisitions. Multi-temporal methods address these limitations by analyzing stacks of interferograms formed from many acquisitions spanning months or years. Persistent scatterer interferometry (PSI) identifies pixels, typically artificial structures such as buildings and infrastructure, that maintain stable radar backscatter across the entire time series and uses them as reference points to separate atmospheric delay from true surface motion. The small baseline subset (SBAS) method applies a similar principle to distributed scatterers such as bare soil and rock. A review published in Progress in Physical Geography surveys the advances in InSAR methodology and its growing role in geophysical monitoring from continental-scale strain accumulation to millimeter-per-year subsidence.

Applications

Radar interferometry has applications in a wide range of fields, including:

• Earthquake deformation mapping and fault slip estimation
• Volcanic ground inflation and deflation monitoring
• Landslide kinematics and slope stability assessment
• Urban subsidence monitoring from groundwater extraction or tunneling
• Glacier and ice-sheet flow velocity measurement
• Infrastructure monitoring for bridges, dams, and buildings