From 1 - 10 / 1735
  • 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.

  • Applications dated 18 August 2010 for verification of a reference standard of measurement under Regulation 12 of the National Measurement Regulations 1999 was received from the Land and Property Management Authority, NSW for verification of GDA94 position on their CORSnet monuments. This report documents the processing and analysis of GPS data observed by the CORSnet-NSW GPS stations during a 7-day period from 08 August to 14 August 2010 (day of year 220 to 226) for 4 stations (CSNO, IHOE, PBOT, and TBOB) to satisfy the position verification requirements.

  • The gravitational attraction of the Galactic centre leads to the centrifugial acceleration of the Solar system barycentre. It results in secular aberration drift which displaces the position of the distant radio sources. The effect should be accounted for in high-precision astrometric reductions as well as by the corresponding update of the ICRS definition.

  • In 1994, the United Nations Regional Cartographic Conference for Asia and the Pacific resolved to establish a Permanent Committee comprising of national surveying and mapping agencies to address the concept of establishing a common geographic information infrastructure for the region. This resolution subsequently led to the establishment of the Permanent Committee for GIS Infrastructure for the Asia and Pacific (PCGIAP). One of the goals of the PCGIAP was to establish and maintain a precise understanding of the relationship between permanent geodetic stations across the region. To this end, campaign-style geodetic-GPS observations, coordinated by Geoscience Australia, have been undertaken throughout the region since 1997. In this presentation, we discuss the development of an Asia Pacific regional reference frame based on the PCGIAP GPS campaign data, which now includes data from 417 non-IGS GPS stations and provides long term crustal deformation estimates for over 200 GPS stations throughout the region. We overview and evaluate: our combination strategy with particular emphasis on the alignment of the solution onto the International Terrestrial Reference Frame (ITRF); the sensitivity of the solution to reference frame site selection; the treatment of regional co-seismic and post-seismic deformation; and the Asia-Pacific contribution to the International Association of Geodesy (IAG) Working Group on "Regional Dense Velocity Fields". The level of consistency of the coordinate estimates with respect to ITRF2005 is 6, 5, 15 mm, in the east, north and up components, respectively, while the velocity estimates are consistent at 2, 2, 6 mm/yr in the east, north and up components, respectively.

  • The new Australian geodetic VLBI network operated by the University of Tasmania (UTAS) started regular observations in October, 2010. Three 12-meter "Patriot" radio telescopes are dedicated to the improvement of the celestial and terrestrial reference frames in the southern hemisphere. We present first results from the analysis of an eight-month set of geodetic VLBI data. The data were processed within a global VLBI solution by the least squares collocation method using the OCCAM software. The geodetic positions of the AuScope radio telescopes were estimated with accuracy less than 10 millimetres, and the first sign of their motion due to tectonic plate movement was indicated.

  • The purpose of this paper is to investigate and quantify the accuracy with which hydrological signals in the Murray-Darling Basin, southeast Australia can be estimated from GRACE. We assessed the extent to which the Earth's major geophysical processes contaminate the gravitational signals in the Basin. Eighteen of the world's largest geophysical processes which generate major gravitational signals (e.g. melting of the Greenland icesheet, hydrology in the Amazon Basin) were simulated and the proportion of the simulated signal detected in the Murray - Darling Basin was calculated. The sum of the cumulative effects revealed a maximum of ~4 mm (equivalent water height) of spurious signal was detected within the Murray - Darling Basin; a magnitude smaller than the uncertainty of the basin-scale estimates of changes in total water storage. Thus, GRACE products can be used to monitor broad scale hydrologic trends and variability in the Murray-Darling Basin without the need to account for contamination of the estimates from external geophysical sources.

  • Many of the compact extragalactic radio sources that are used as fiducial points to define the celestial reference frame are known to have proper motions detectable with long-term geodetic/astrometric Very Long Baseline Interferometry (VLBI) measurements. These changes can be as high as several hundred micro-arcseconds per year for certain objects. When imaged with VLBI at milli-arcsecond (mas) angular resolution, these sources (radio-loud active galactic nuclei) typically show structures dominated by a compact, often unresolved ``core'' and a one-sided ``jet''. The positional instability of compact radio sources is believed to be connected with the changes in their brightness distribution structure. We test this assumption, not for individual objects, but in a statistical sense on a large sample. We investigate a sample of 68 radio sources for which reliable long-term time series of astrometric positions as well as detailed 8-GHz VLBI brightness distribution models are available. We compare the characteristic direction of their extended jet structure and the direction of their apparent proper motion. We present our data and analysis method, and conclude that the correlation between the two characteristic directions is weak. This can be naturally explained if the VLBI jet directions are misaligned between the 1-10 mas and sub-mas angular scales in a significant fraction of sources. It is also possible that systematic all-sky effects are present in the measured apparent proper motion data.

  • This report gives an overview about activities of the Geoscience Australia IVS Analysis Center during 2010

  • 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}.

  • This report gives an overview of the activities of the Geoscience Australia IVS Analysis Center during 2011.