Artificial reflectors (or corner reflectors) are ground-based instruments that reflect radar signals back to the satellite at specific locations. There are three principle reasons for using artificial reflectors:
- To monitoring specific ground positions
- To integrate satellite-based measurements with traditional surveys, e.g. GPS, leveling, etc.
- To acquire surface displacement measurements over areas unsuitable for traditional satellite radar monitoring
Artificial reflectors are typically designed as single geometry (reflecting signals from either ascending or descending satellite orbital paths) or double geometry (reflecting signals from both ascending and descending satellite orbital paths).
Monitoring Specific Ground Positions
Artificial reflectors are installed to measure ground movement over specific features or locations of interest. Each reflector is oriented towards a targeted satellite to produce strong radar reflections which can be easily detected by the satellite. Reflected signals are then analyzed to provide a detailed time series of movement occurring at each artificial reflector location.
While artificial reflectors can be installed at any location, reflective surfaces must be visible to the satellite (i.e. not impeded by overhanging vegetation, snow or other materials).
Dataset Integration
Although SqueeSAR™ results do not require validation from additional datasets (i.e. it is a stand-alone dataset), data can be integrated with traditional ground-based survey networks or benchmarks.
SqueeSAR™ ground measurements are relative to a single reference point. By assigning the reference point to an artificial reflector and integrating the reference point with a nearby benchmark within a traditional ground-based network, measurements become absolute.
Increasing Ground Measurement Points
Synthetic Aperture Radar (SAR) systems rely on signals reflected from objects on the ground and returned to the satellite.
Although not usually the case, some areas characterized by low reflectivity (e.g. flat road surfaces, vegetation, snow, etc.) do not allow for enough natural ground points to be identified for an effective analysis of ground displacement. In areas of low reflectivity, existing features are insufficient to isolate measurement points from background noise (e.g. atmospheric effects).
Artificial reflectors can be installed to increase the number of ground measurement points by reflecting high signal-to-noise signals back to the satellite. The process of using artificial reflectors to increase the number of ground points is also referred to as corner reflector InSAR, or CRInSAR.
Each artificial reflector provides one ground measurement point and only provides data from the date of installation. As a result, CRInSAR is only available for monitoring projects.
Installation
As SqueeSAR™ detects millimeter scale ground displacement, it is important to install artificial reflectors on a foundation that will capture displacement of the phenomena of interest.
Foundations can consist of hard rock formation or concrete bases (approximately 1m3) and can be installed onto the same foundations as alternative instrumentation, e.g. GPS.
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