Knowledge of the long-term stability of each COR is necessary to ensure internal consistency and stability of the national date, as well as reliable legal GPS position traceability (Hu and Dawson, 2013; Hu et al., 2019). Quality control of GPS position verification can be performed through long-term position time series monitoring and short-term positioning performance (Hu et al., 2013; Hu 2019). Boey, Seng Lake. and University of New South Wales. Faculty of Geomatics. (1999). A model for establishing the legal traceability of GPS measurements for cadastral surveying in Australia. Sydney: School of Geomatics Engineering, University of New South Wales This document provides an overview of Geoscience Australia`s approach to legal GPS location traceability and the legal certification process, including the quality standards and quality management system of the position verification process. Quality management includes our approach to monitoring the impact of: device configuration changes; antenna malfunctions; crust deformation; and processing strategy and modelling updates.
Some examples are based on experiences within the Asia Pacific Reference Frame (APREF) community (Hu et al. 2019). Finally, the structured program of maintenance and continuous improvement of the test laboratory is also discussed. Australia`s national measurement system provides a basis for legally understandable, consistent and internationally recognised measurements. Given society`s increasing reliance on the Global Positioning System (GPS), the need for legal traceability of GPS positions against Australia`s Geocentric Datum System (GDA), currently GDA2020 (Hu and Dawson, 2018; ICSM 2018), has become increasingly clear. To ensure consistency in derived positions from private and government continuous operating reference stations (CORS), Geoscience Australia maintains an appointment as a metrology regulatory authority under the National Measurement Act 1960 (Commonwealth of Australia) and provides legally responsible positions (Hu and Dawson, 2013; Hu 2019). Geoscience Australia`s role in the national measurement system is to operate the Australia Fiducial Network (AFN) to appropriate standards, for example to meet the highest requirements of all types of applications (Beavan 2005; Firuzabdi and King, 2011) and ensure that Australia`s major horns operated by other agencies, such as government surveying agencies, are appropriately linked to the AFN (Dawson and Woods, 2010; Hu and Dawson, 2013). Geoscience Australia may issue certificates of verification under Rule 13 of the National Measurement Regulations 1999 under the National Measurement Act 1960. These are commonly referred to as Regulation 13 certificates.
Regulation 13 certificates contain contact information and their uncertainties regarding the Recognized Value Standard (SVR) for position measurement in Australia (Hu and Dawson, 2013, 2018). In Australia, the GPS position of a station is defined with legal traceability at the time of measurement and with the specified instrumentation of a GPS monument in relation to Australia`s geocentric date. The ultimate goal of the study is to obtain reliable traceability of GPS location in Australia. We create a SINEX file with IGS discontinuities for the APREF CORS network. We update the time series of positions weekly, but associated discontinuities or lags require several weeks of data after the event or even longer to identify changes in coordinates. When estimating site speed, we introduce an offset and reestimate a set of coordinates for the location, assuming that the speed before and after the offsets is identical. Weekly solutions and combined SINEX files are the starting point for long-term monitoring of CORS performance and thus the position time series of each site. The official products of APREF consist of weekly position solutions in SINEX format (Hu et al. 2019). We update the cumulative weekly APREF position and speed estimate and associated position time series, taking into account detected position shift or outliers.
We also maintain a database for metadata and discontinuity management in SINEX format (Hu et al. 2011, 2019; Owen et al., 2018). An organizational chart of the legal traceability of the issuance of certificates under Rule 13 for a site is given in Annex 2. Feissel-Vernier, M., de Viron, O., & Le Bail, K. (2007). Stability of VLBI, SLR, DORIS and GPS positioning. Earth Planets Space, 59, 475-497. Geoscience Australia is responsible for maintaining a consistent set of geodetic estimates of position and velocity for the AFN`s 109 GNSS sites in Australia. To ensure long-term reliability and quality control of legal GPS location traceability in Australia, a rigorous site performance monitoring system was initiated and implemented in addition to metadata management and routine analysis (Owen et al. 2018; Hu et al., 2011, 2019). Measurement traceability is ensured by comparing the calculated solution with the RVS for AFN station position as well as the weekly combined solutions calculated by the International GNSS Service (IGS) in the International Terrestrial Reference Framework (ITRF), currently ITRF2014, and the individual IGS Global Analysis Centres. The validity and traceability of GPS is ensured by its connection to the global observation networks Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) via ITRF.
As the AFN is a reference for national geodetic networks, reliable and up-to-date coordinates must be available for all AFN stations. This requires us not only to detect and identify the timing of position shifts at APN sites, but also to estimate the size of the offset used for position update (Hu et al., 2019). The GPS position of a station in Australia with legal traceability is a set of point coordinates with specified instruments installed on a stable monument relative to the Australian Geocentric Reference Frame (GDA2020) referring to the GRS80 ellipsoid at the time 2020.0. Many factors affect the stability and reliability of the location of the position series, including changes in the site environment that can cause inconsistencies in the position time series and even change the repeatability of the position.