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  • The IAG Working Group (WG) 'Integration of Dense Velocity Fields in the ITRF' was created in 2011 as follow-up of the WG 'Regional Dense Velocity Fields' (2007-2011). The goal of the WG group is to densify the ITRF (International Terrestrial Reference Frame) using regional GNSS solutions as well as global solutions. This was originally done by combining several cumulative position/velocity solutions as well as their residual position time series submitted to the WG by the IAG regional reference frame sub-commissions (APREF, EUREF, SIRGAS, NAREF) and global (ULR) analysis centers. However, several test combinations together with the comparison of the residual position time series demonstrated the limitations of this approach. In June 2012, the WG decided to adopt a new approach based on a weekly combination of the GNSS solutions. This new approach will mitigate network effects, have a full control over the discontinuities and the velocity constraints, manage the different data span and derive residual position time series in addition to a velocity field. All initial contributors have agreed to submit weekly solutions and in addition initial contacts have been made with other sub-commissions particularly Africa in order to extent the densified velocity field to all continents. More details on the WG are available from http://epncb.oma.be/IAG/.

  • The national geodetic program in Australia is undertaken by the National Geospatial Reference System (NGRS) Section within Geoscience Australia. The NGRS is a continually evolving system of infrastructure implemented through the existing geodetic techniques such as Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI). The NGRS serves the broader community by providing an accurate foundation for positioning, and consequently all spatial data, against which every position in Australia is measured and can be legally traced. In Australia, the sparsity of geodetic infrastructure has limited the developments of geodetic applications. For instance, the Geocentric Datum of Australia 1994 (GDA94) was based on observations (1992 - 1994) from a sparse network of Continuously Operating Reference Station (CORS) called the Australian Fiducial Network (AFN). Since that time the demand for higher accuracies has resulted in GDA94 no longer adequately serving user demand. The adoption of a fully dynamic datum will ensure that Australians can use positioning technology to its fullest capability, whereas at present when using GDA94 they are limited to the accuracy that was achievable in 1994 when GDA94 was created. Consequently, national infrastructure development programs, such as AuScope, have been implemented to improve the geodetic accuracies by contributing to the next generation of the Global Geodetic Observing System (GGOS). This presentation reviews the national geodetic activities in Australia, especially the AuScope program, a recent enhancement to the Australian geodetic infrastructure.

  • Advice to National Measurement Institute regarding update to the recognized-value standard of measurement for position, June 2011

  • Tide gauge data forms the basis for determining global or local sea level rise with respect to a global geocentric reference frame. Data from repeated precise levelling connections between the tide gauge and a series of coastal and inland benchmarks, including a Continuous GPS (CGPS) benchmark, is used to determine the stability of tide gauges at 12 locations in the South Pacific. The method for determining this is based on a constant velocity model which minimises the net movement amongst a set of datum benchmarks surveyed since the installation of the tide gauges. Tide gauges were found to be sinking, relative to the CGPS benchmark, in Pohnpei (FSM), Samoa, Vanuatu, Tonga, Nauru, Tuvalu, Fiji and Cook Is; listed in order of the sinking rate, with a maximum of -1.01 - 0.63 mm/yr at Pohnpei (FSM) and a minimum of -0.03 - 0.81mm/yr at Cook Is. The tide gauge was rising, relative to the CGPS benchmark, in Solomon Is, Manus Is (PNG), Kiribati and Marshall Is, with a maximum of 3.12 - 0.49mm/yr in Solomon Is and a minimum of 0.01 - 0.91mm/yr in Marshall Is. However, these estimates are unreliable for the Solomon Is and Marshall Is, which have recently established CGPS benchmarks and have been surveyed less than 3 times. In Tonga and Cook Is, the tide gauge was found to be disturbed or affected by survey errors whereas the Vanuatu results were affected by earthquakes. It was also found that the constant velocity model did not fit the observations at the tide gauges in Tonga, Cook Is, Fiji, Marshall Is and Vanuatu, which had large variations in their velocities. This is an indicator of the high frequency (short period) motion of the tide gauge structure, which cannot be measured by the levelling method since these have a higher frequency than the time interval between levelling surveys.

  • This research utilises metadata from GA's centralised metadata store containing the history of the equipment changes which have taken place at all GNSS stations; such as antenna or receiver swaps, firmware upgrades and removal/ alteration of antenna domes and cables. Several change detection algorithms have been implemented for automatic detection of discontinuities in the coordinate time series. Once offsets are detected, their position in time is correlated with equipment changes or earthquake occurrences nearby the station. If a correlation is found and the offset is visibly evident, the offset is introduced into a database. This information is used in the routine combination of weekly SINEX solutions using the CATREF software to produce an enhanced set of coordinates and velocities. It is shown that after cleansing the offsets in time series using this approach, the quality of the combined APREF solution is improved in terms of WRMS. By analysing time series coordinates at a few stations using CATS software, it is shown that the uncertainty of velocity estimates is improved after offsets are detected and removed from the time series.

  • An increasingly important requirement for Australia's geodetic reference system is that the relationships between the International Terrestrial Reference Frame (ITRF) and the national horizontal and vertical datums are well understood. To support the development of improved geodetic infrastructure in Australia, we have analysed GPS data observed at 2310 survey marks. These data, observed between 1995 and 2009, across continental Australia were processed with consistent standards to generate a combined solution with an estimated uncertainty of better than 5 and 20 mm (1 sigma) in the horizontal and vertical components, respectively. Our combined solution, which was mapped to ITRF2005 at the reference epoch of 2000, is the first unified single-epoch solution with sufficient resolution to support datum modernisation in Australia. We review the considerable work undertaken to determine the optimum analysis procedure, including comparisons of solutions using different antenna phase centre variations (PCV) calibration models, and find that the heights determined using relative PCV models differ from those determined using absolute PCV models by a maximum of 27 mm and an average of 6 mm. Also, we assess the impact of both observation session lengths and crustal velocity modelling. There will be two important applications for this new GPS solution. First, will be the development of an improved model for the estimation of Australian Height Datum (AHD) values from GNSS observations, and the solution will be an important input into the Australian Height Modernisation Project. Second, will be its use as constraining dataset for the readjustment of the terrestrial geodetic observations used in GDA94 as part of the creation of the Geodetic Model of Australia, and will potentially lead to a new national datum.

  • Australia's National Geospatial Reference System (NGRS) is a continually evolving system of infrastructure, data, software and knowledge. The NGRS serves the broader community by providing an accurate foundation for positioning, and consequently all spatial data. The NGRS is administered by the Intergovernmental Committee on Surveying and Mapping (ICSM) and maintained by its Federal and State jurisdictions. Increasingly, the role of Global Navigation Satellite Systems (GNSS) in positioning has required the globalisation of national coordinate systems. In the early 1990's ICSM endorsed the adoption of the Geocentric Datum of Australia (GDA94) which was aligned to the International Terrestrial Reference Frame (ITRF) with a stated uncertainty of 30mm horizontally and 50mm vertically. Since that time crustal deformation and the demand for higher accuracies has resulted in GDA94 no longer adequately serving user requirements. ITRF has continued to evolve in accuracy and distribution to the extent that it now requires very accurate modelling of linear and non-linear crustal deformation. Even the Australia plate, which has long been considered to be rigid, is now considered to be deforming at levels detectable by modern geodesy. Consequently, infrastructure development programs such as AuScope have been implemented to ensure that crustal deformation can be better measured. The Auscope program also aims to improve the accuracy of the ITRF by contributing to the next generation of the Global Geodetic Observing System in our region. This approach will ensure that the ITRF continues to evolve and that Australia's NGRS is integrally connected to it with equivalent accuracies. Ultimately this will remove the need for National Reference Systems, with a globally homogenous and stable reference system (e.g., ITRF) being far more beneficial to society. This paper reviews Australia's contribution to GGOS and how this impacts on positioning in Australia.

  • A key element to the determination of International Reference Frame (ITRF) is a sufficient number of well distributed co-location sites between the major geodetic observation techniques SLR, VLBI and GPS. Today GPS plays a major role connecting both techniques at the ITRF combination stage. However any GPS bias in a co-located site may have an impact on the ITRF quality and its defining scale and origin parameters. For GPS the scale is highly dependent on ground and satellite PCV. Therefore the presence of an uncalibrated radome at a collocated station is likely to have an impact on the scale estimation. The International GNSS Service (IGS) station YAR2, located at Yarragadee, has an uncalibrated radome 'JPLA' and is collocated with a SLR observatory with a reported residual of 14mm in height. This contribution looks at analyses of recent local tie surveys carried out at Yarragadee, which included a week of GPS observations at YAR2 without the radome installed, as well as additional observations on other local tie monuments. In particular we give an estimation of the bias that is introduced by the unmodelled JPLA radome, and look at other possible sources of discrepancy between SLR and GPS derived ITRF solutions.

  • Analysis of very long baseline interferometry (VLBI) records of distant radio source signals allows one to determine the proper motions of extragalactic objects with an accuracy of a few tens of microseconds of arc per year. Such an accuracy is sufficient to investigate the aberration in proper motions of distant bodies due to the rotation of the Solar system barycenter around the Galactic center, as well as higher degree systematics of the velocity field. We analyzed geodetic and astrometric VLBI data of 1979--2010 to produce radio source coordinate time series. The velocity field made up of the proper motions of 497 sources of good observational history is investigated by fitting the vector spherical harmonic components of degree 1 and 2. Within error bars, the magnitude and the direction of the dipole component agree with predictions made by using the most recent estimates of the Galactic parameters. The acceleration vector, estimated together with a non significant global rotation, has an amplitude of 5.8+/-1.4 microseconds of arc per year and is directed towards equatorial coordinates alpha = 266+/-8 deg and delta = -18+/- 18 deg. Degree 2 harmonics of the velocity fields appear to be less significant. It yields that the primordial gravitational wave density integrated over a range of frequencies less than 10^{-9} Hz is lower than 0.0031+/- 0.0002h^{-2}.

  • The annual Asia Pacific Regional Geodetic Project (APRGP) GPS campaigns are an important activity of the regional geodesy working group of the Permanent Committee on GIS Infrastructure for Asia and the Pacific Region (PCGIAP). The major objective of these campaigns is the densification of the International Terrestrial Reference Frame (ITRF) in the Asia-Pacific region. The APRGP GPS campaigns consist of 7-day observation sessions and have been undertaken from 1997 to 2008. In this work, we focus on the assessment of realistic uncertainty estimates of the derived crustal velocities, which is still an important unresolved issue. Although assessments of the quality of Continuous GPS (CGPS) determinations of crustal velocity have previously been undertaken, little research has been conducted on the quality of the velocity estimates derived from campaign-based coordinate time series. We have compared our velocity estimates with those published by the International GNSS service (IGS) at common sites and found that they are consistent at 1.4, 1.7, 3.9 mm/yr level in the east, north and up components, respectively. Also, we find that a minimum of 3 years of campaign data is required before reliable velocity estimates can be derived from campaign-based GPS, which is mostly due to the increased possibility of outliers.