Richard Gross* and Tom Herring†
*Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA
†Massachusetts Institute of Technology, Cambridge, USA
Unified Analysis Workshops are co-organized by the International Association of Geodesy’s (IAG’s) Global Geodetic Observing System (GGOS) and International Earth Rotation and Reference Systems Service (IERS). This was the 5th in a series of workshops that are held every two to three years for the purpose of discussing issues that are common to all the space-geodetic measurement techniques. Attendance at the Workshops are by invitation only with each IAG Service nominating 5-6 experts to attend and participate in the discussions. At this Workshop, the discussions of the 41 participants from 9 countries focused on:
- Systematic Errors and Biases in GNSS, VLBI, SLR, and DORIS
- Site Survey and Co-location
- Reference Systems and Frames
- Conventional Mean Pole
- Standards, Conventions, and Formats
- Interoperability of Portals and Metadata
Group photo of the UAW 2017 participants
Summary of Discussions and Recommendations
The discussions during the sessions about the measurement techniques concerned the source of errors and biases in the techniques and resulted in recommendations to mitigate them. For GNSS, this included recommendations to improve the force and background models including the use of modern static and time variable gravity models, improved diurnal and semi-diurnal EOP models, and improved solar radiation pressure models. Improved calibrations of GNSS antennae are also needed, particularly in situ site-dependent calibrations. In order to investigate the origin of draconitic signals in GNSS data and products it was recommended that the use of arcs longer than 24 hours be investigated.
For VLBI, besides also recommending that improved diurnal and semi-diurnal EOP models be used, it was recommended that the new mean pole model be implemented in VLBI data reduction software, that gravitational deformation of the antennae be taken into account when reducing VLBI observations, that the impact of source structure variability be investigated, that the atmospheric loading signal removed during the data reduction process be included in the SINEX file so that users can restore it when needed, and that differences in the different formulations of relativistic effects on VLBI observations be investigated.
For SLR, it was recommended that time biases be investigated and to facilitate this that time and range biases be given in SINEX files, that the ILRS provide network-fixed products as well as the loosely constrained products that they already provide, that the ILRS should establish a Pilot Project to study the impact of applying non-tidal atmospheric loading when reducing SLR observations, that complete and accurate metadata continue to be developed, and that the quality control process continue to provide feedback to station operators and analysis centers.
For DORIS, it was also recommended that improved diurnal and semi-diurnal EOP models be used as well as modern static and time variable gravity models and improved solar radiation pressure models. In addition, it was recommended that T2L2 data be used to better understand the behavior of the ultra-stable oscillators and that the IDS should continue to investigate the DORIS scale by examining the impact of low-elevation data on the scale.
Following an introduction to site ties, the Site Survey and Co-location discussions resulted in recommendations to develop in situ site-dependent GNSS antennae calibrations (as also recommended by the GNSS session), to survey sites that have not yet been surveyed, to develop an optimized strategy to employ different surveying techniques at the same site, and to examine the discrepancies between the local site surveys and the results of space-geodetic analyses.
The activities of the three IERS ITRS Combinations Centers (CCs) were presented at the Reference Systems and Frames session as well as presentations comparing the three solutions (ITRF2014, JTRF2014, and DTRF2014) to each other and the results of adopting ITRF2014 by the different Services. It was recommended that the three CCs consider the possibility of updating their frames between determinations, that the Services provide the CCs with the information that they need to update their frames, that the IERS identify reference frame users who will benefit from time series reference frames and how these reference frames will satisfy their needs, and that the IERS provide up-to-date locations of discontinuities in the coordinate time series of the four techniques.
During the Conventional Mean Pole session, consensus was reached to develop and use a linear mean pole to replace the mean pole model currently given in the IERS Conventions. In particular, it was recommended that all analysis groups, including the altimetry community, adopt a common linear mean pole model for pole tide computations and that the IERS continue to provide a filtered mean pole table for the purpose of modeling and comparing long-term trends in C21 and S21.
There was much discussion in the Standards, Conventions, and Formats session about the proposal stemming from the IGS Workshop the week before to extend station codes from 4 to 9 or more characters. It was recommended that the SINEX Working Group examine this issue. It was also recommended that a unique format for EOP data files be derived, that a formal process be developed for evaluating new models before they are adopted by the IERS Conventions, that updates to the IERS Conventions be citable, and that efforts continue to ensure that all techniques use consistent gravity field models, both static and time variable. The UAW also endorsed the recommendations given in the GGOS Bureau of Products and Standards Inventory that numerical standards should be clearly documented, that the W0 value given in IAG Resolution No. 1 (2015) be used as the new reference value for geodetic work, and that the development of a new Geodetic Reference System GRS20XX based on best estimates of the major parameters is desired.
The objective of the Interoperability of Portals and Metadata session was to begin a dialogue between the Services and GGOS to develop interoperable web portals that interface to and are discoverable by the portals of other organizations such as GEO. Following presentations about the status of the portals and metadata management systems being developed by the different Services it was recommended that the IAG Services develop web portals that are interoperable with each other and with the GGOS portal that is being developed.
Additional information about the UAW, including the Program Book and a more complete report about the discussions and recommendations, is available from the meeting web site at http://18.104.22.168/en/meetings/2017/unified-analysis-workshop
, or simply http://bit.ly/UAW2017