SCEC Award Number 20092 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Update of operational GNSS products and development of integrated products for the CGM
Investigator(s)
Name Organization
Michael Floyd Massachusetts Institute of Technology Thomas Herring Massachusetts Institute of Technology
Other Participants
SCEC Priorities 1a, 2a, 3e SCEC Groups Geodesy, CXM, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/16/2021
Project Abstract
 This project is a continuation of our work to develop GNSS products for the Community Geodetic Model, and to guide the InSAR component towards a rigorous and compatible combination with GNSS. Over a few years we have developed the tools necessary to ingest products from analysis centers, which process the raw data operationally, to rescale, reweight and combine the products in a self-consistent manner and produce an average time series which does not suffer from any of the potential problems with a single analysis center's processing. A major consideration is the treatment of scale by some analysis centers and how this ultimately manifests in the vertical (height) component, which is of increasing interest to those studying hydrological signals related to climate and when requiring the full three-dimensional coordinates solution for differentiation between physical mechanisms, such as in the aftermath of an earthquake. We have concluded that it is critical to restore any estimated and removed scale estimates to avoid suppression of vertical signals that may represent real deformation rather than a shifting reference frame definition. We are now at the stage where our CGM (cGNSS) time series are operationally generated, lagging about a week behind real time. We have also demonstrated a method for estimating tropospheric delays for potential application to InSAR based on principles and models used to calculate a priori values for GNSS processing. This may have utility in creating a consistent tropospheric delay correction when combining InSAR with GNSS time series.
Intellectual Merit The Community Geodetic Model (CGM) is a core product of SCEC, along with other Community Models. Its goal is to provide the most consistent time series and velocity products from GNSS and InSAR based on best current practices and understanding to reduce the need for community members to produce their own data processing and results.

Major goals of this effort are the freely and publicly available products in a format that is intuitive and easily useable. Throughout multiple years of our SCEC CGM (GNSS) projects, we have developed combination strategies with corresponding corrections to source time series from analysis centers, each with their own preferred strategy and final reference frame, with particular focus and interest on the vertical component. This component is commonly considered a weaker component of GNSS results because of the worse precision and difficulty in determining a rigorous reference frame for continental-scale processes. However, it is critical to understand such processes, like the effect of climate on hydrological loading, and to distinguish between some hypothetical mechanisms for post-earthquake deformation.

Therefore our continued communication with and has ensured that SCEC's combined CGM product will be as compatible as possible between analysis centers, whose averaging reduces the potential consquence of outliers to analysis. The feedback to date that we have had so far from preliminary "testers" of the CGM (GNSS) product has been that the time series improve fits to models and reduce error, such as in the post-seismic study of the Ridgecrest earthquakes.

Furthermore, a critical part of the design of the CGM, as opposed to the GNSS-only products of many analysis centers, is the inclusion of compatible InSAR time series. The CGM is proving to be a key discussion forum and testing ground for GNSS principles being applied to InSAR, where previously InSAR has dealt with much larger signal-to-noise phenomena, such as coseismic displacements, which have not required the same application of small perturbation sources as GNSS for tectonics. Such phenomena include the solid Earth and ocean tide loading signals and propagation delay effects through the atmosphere (troposphere and ionosphere), both of which have long been included in GNSS processing. Therefore we have become involved closely with the InSAR Working Group of the CGM to ensure that the ultimate direction of the InSAR processing follows the precedents set by GNSS, which are widely accepted as best practice for such geodetic satellite signals, in their own time series processing. This will ultimately produce geodetic time series which rely on the same fundamental principles and models for their correction, which should reduce any discrepancies between them and provide a more rigorous and complementary set of products.
Broader Impacts This work contributes to one of SCEC's Community Models, which are necessarily designed to be both publicly accessible and easy to use. This presents opportunities beyond the scientific community for educators and the public with general interest in our ability to measure tectonic motions across southern California and investigate the data themselves.
Exemplary Figure Figure 1: Pre-Ridgecrest raw, adjusted and combined CGM time series for site NOTA site P617, which acts as the reference pixel for descending tracks 64 and 71 currently being finalized for the CGM (InSAR) time series. Left column: Original source analysis center products before scaling of uncertainties (see Table 1) and restoration of scale (Gipsy-processed products from NGL (UNR), MEaSUREs (JPL) and USGS only) for input to the CGM combination; the up component also shows the height variation that is effectively removed when a scale adjustment is estimated and applied. Right columns: Reweighted and scaled products and the final CGM combination.
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