Abstract
Continuously operating Global Navigation Satellite Systems (cGNSS) can be used to convert relative values of vertical land motion (VLM) derived from Interferometric Synthetic Aperture Radar (InSAR) to absolute values in a global or regional reference frame. Artificial trihedral corner reflectors (CRs) provide high-intensity and temporally stable reflections in SAR time series imagery, more so than naturally occurring permanent scatterers. Therefore, it is logical to co-locate CRs with cGNSS as ground-based geodetic infrastructure for the integrated monitoring of VLM. We describe the practical considerations for such co-locations using four case-study examples from Perth, Australia. After basic initial considerations such as land access, sky visibility and security, temporary test deployments of co-located CRs with cGNSS should be analysed together to determine site suitability. Signal to clutter ratios from SAR imagery are used to determine potential sites for placement of the CR. A significant concern is whether the co-location of a deliberately designed reflecting object generates unwanted multipath (reflected signals) in the cGNSS data. To mitigate against this, we located CRs >30 m from the cGNSS with no inter-visibility. Daily RMS values of the zero-difference ionosphere-free carrier-phase residuals, and ellipsoidal heights from static precise point positioning GNSS processing at each co-located site were then used to ascertain that the CR did not generate unwanted cGNSS multipath. These steps form a set of recommendations for the installation of such geodetic ground-infrastructure, which may be of use to others wishing to establish integrated InSAR-cGNSS monitoring of VLM elsewhere.
Highlights
Introduction and BackgroundVertical land motion (VLM), i.e., subsidence or uplift, arising from natural and/or anthropogenic phenomena affects geodetic benchmark heights and complicates the sea level record at tide gauges [1,2]
The latter may be achieved by transforming all line of sight (LoS) displacements into the vertical direction under the assumption that no relative horizontal motion is occurring [7]; through combination of Interferometric Synthetic Aperture Radar (InSAR) measurements made from different satellite geometries or “look” directions; or through removal of any relative horizontal motion determined by GNSS [9,10]
Using the example of Perth, Australia, we have described the practical considerations necessary for installing ground-based infrastructure to monitor vertical land motion (VLM) using Continuously operating Global Navigation Satellite Systems (cGNSS) and SAR satellites
Summary
Vertical land motion (VLM), i.e., subsidence or uplift, arising from natural (e.g., tectonic) and/or anthropogenic (e.g., subsurface resource extraction) phenomena affects geodetic benchmark heights and complicates the sea level record at tide gauges [1,2]. The second key difference is that GNSS provides VLM in an absolute sense with respect to some defined reference frame [11,12,13,14], whereas InSAR provides VLM estimates relative to the time of the first SAR acquisition and a local spatial reference within the imaged area This may be the mean VLM of all pixels in the image [15], or a far-field point or reference region in the images, where it is assumed that no VLM is occurring [16,17]. We use the example of Perth, Australia, to describe a series of tests conducted before new permanent installations of co-located ground infrastructure to support integrated cGNSS and InSAR monitoring of VLM In documenting these efforts, we aim to provide some framework for efficiently establishing operational integrated satellite-based VLM monitoring that may be applicable elsewhere. The improvements to geodetic infrastructure resulting from this study will help to overcome the challenges identified by Featherstone et al [39] of measuring small magnitude (
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