Interferometric Synthetic Aperture Radar (InSAR) is a space-based microwave remote sensing technology that reconstructs surface elevation and monitors deformation through analysis of complex image data acquired by satellite or airborne radar systems. Since its first planetary observation applications in the 1970s, InSAR has evolved into a core technology for geological hazard warning, urban subsidence monitoring, and glacier movement analysis. Key advantages include:
- 🌍 All-Weather Monitoring Capability
Microwave signals penetrate cloud cover, rain, fog, and darkness without illumination constraints;
- 📏 Millimeter-Level Measurement Precision
Deformation monitoring accuracy reaches millimeter scale, with elevation measurement precision at sub-meter level;
- 🛰️ Hundreds of Square Kilometer Coverage
Single satellite acquisition covers hundreds of square kilometers, enabling regional-scale continuous monitoring.
Phase Interferometry and Data Processing
Phase Difference and Deformation Retrieval
InSAR extracts deformation information by comparing phase differences between two radar observations. Minute changes in surface elevation or displacement alter radar wave propagation paths, causing phase variations in return signals. Data interpretation involves:
- Image Registration (Sub-Pixel Accuracy)
Precise alignment of SAR image pixels with sub-pixel precision;
- Interferogram Generation
Multiplication of complex images to create phase-difference interferograms;
- Phase Unwrapping
Elimination of phase cycle ambiguities to recover true phase values;
- Deformation Quantification
Conversion of phase differences to vertical/horizontal displacements using orbital parameters and geometric models.
Temporal and Spatial Baseline Constraints
- Temporal Baseline
Observation intervals must match deformation rates (e.g., C-band/Sentinel-1 for short baselines <84 days monitoring earthquakes; L-band/ALOS-2 for long baselines >360 days analyzing slow crustal movements).
- Spatial Baseline
Perpendicular orbit separation must be constrained (C-band <300m) to prevent signal decorrelation. Excessive baselines require orbital optimization or algorithmic correction.
- Time-Series InSAR Evolution
Conventional D-InSAR is vulnerable to atmospheric noise. Time-series InSAR processes multi-temporal images (20-100 scenes) to separate long-term deformation, seasonal variations, and random noise.
- PS-InSAR (Persistent Scatterer Technique)
Targets stable urban reflectors (buildings, bridges) for high-precision deformation series;
- SBAS-InSAR (Small Baseline Subset Technique)
Enhances vegetated area monitoring using short-spatial-baseline interferograms;
- DS-InSAR (Distributed Scatterer Technique)
Improves complex terrain monitoring through statistical analysis of natural scatterers.
Engineering Applications: From Geological Hazards to Urban Safety
Geohazard Warning and Assessment
- Landslide Monitoring
During the 2018 Baige Landslide on Jinsha River, InSAR detected acceleration phases and predicted failure timing using inverse velocity models (INV), enabling emergency response.
- Seismic Deformation Analysis
InSAR maps coseismic deformation fields and quantifies fault slip. The 1992 Landers earthquake interferogram featured in Nature validated this capability.
Urban Infrastructure Security
- Subsidence Control
Beijing Capital International Airport achieved 0.1mm precision using corner reflectors (CR) to track ground fissure propagation;
- Reservoir Slope Stability
Patented InSAR techniques enable millimeter-level monitoring of high-gradient anti-dip bedrock, revealing reservoir-induced failure mechanisms;
- Subway Tunnel Monitoring
Megacities like Shanghai and Mexico City utilize InSAR-derived subsidence risk maps to guide tunnel reinforcement designs.
Hydrogeology and Resource Management
- Groundwater Rebound Monitoring
North China Plain studies show InSAR detects 3-6 month lag between rainfall cycles and aquifer recovery, informing elastic modulus calculations;
- Oilfield Subsidence Management
InSAR guides water injection strategies to prevent reservoir collapse by monitoring extraction-induced subsidence.
InSAR achieves continuous millimeter-level deformation monitoring, with unprecedented spatiotemporal resolution capturing crustal movements from daily to decadal scales.