The Asia-Pacific Reference Frame (APREF) Project is a regional initiative that seeks to improve the geodetic infrastructure of the Asia-Pacific region. As a joint effort of the Permanent Committee on GIS Infrastructure for Asia and the Pacific (PCGIAP) and the International Association of Geodesy (IAG), APREF is developing a start-of-the-art regional geodetic reference frame. In the Asia-Pacific region there are a substantial number of state-of-the-art GNSS networks, which are commonly operated by national mapping agencies or private sector organisations. These networks represent an important and significant investment by the respective governments and industry in their own spatial infrastructure. In the APREF initiative these networks are combined to realize a high-standard regional reference frame. The GNSS data of the network is processed by different Analysis Centres (ACs). The contributions of the different ACs are combined into a weekly solution by the APREF Central Bureau. This weekly solution is the core product of the APREF; it contains weekly estimates of the coordinates of the participating Asia-Pacific GNSS tracking stations and their covariance information. The APREF product, which is available since the first quarter of 2010, gives a reliable time-series of a regional reference frame in the International Terrestrial Reference Frame and a quality assessment of the performance of the GNSS CORS stations included in the network. This contribution gives an overview of the current status of the APREF network and an analysis of the first APREF products

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

Ongoing developments in geodetic positioning towards greater accuracies with lower latency are now allowing the measurement of the dynamics of the Earth's crust in near real time. However, in the Australian circumstance a sparsity of geodetic infrastructure has limited the application of modern, geodetic science to broader geoscience research programs. Recent enhancements to the Australian geodetic infrastructure, through the AuScope initiative, offer opportunities for research into refinement of geodetic accuracies, as well as their application to measuring crustal deformation.

The GPS and Galileo systems are designed to transmit signals on three carrier frequencies, which are available freely to the GNSS user community. This provides an opportunity for users to form an optimal linear combination which has low noise, is free from ionospheric errors, has increased wavelength and will enable integer ambiguity resolution. This paper presents optimised linear combinations for GPS and Galileo which address all but the last of these factors. Based on the theoretical development, the optimal combination is free from first order ionospheric effects and has less noise than any of the other dual frequency ionosphere free combinations. The wavelength of the combination can be specified by the user at any desired length. The optimal combination was validated using authentic triple frequency data from the recently launched Block IIF GPS satellite, PRN 25. It is shown that the noise in the optimal combination is lower than the L1/L2 and L2/L5 dual frequency ionosphere free combinations and at about the same level as the L1/L5 combination. The optimal linear combination is ideal for undifferenced Precise Point Positioning (PPP) which requires an ionosphere free, low noise combination that does not necessarily have integer coefficients.

The secular aberration drift caused by the acceleration of the Solar system barycentre with respect to the reference quasars could be estimated using the recent geodetic VLBI data. The practical approach of the secular aberration drift estimation is proposed. The general equation of group delays has been modified to incorporate the barycentre acceleration vector. More than four million of group delays observed between 1980 and 2008 were adjusted to obtain the secular aberration drift estimates by three approaches. All estimates of this effect amplitude are close (about 18 ± 2 -asec/year) and correspond to the magnitude of the barycentre acceleration vector of (9+/-1)*10(-13) km/sec.

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 integrity and strengths of multi-technique terrestrial reference frames such as ITRF2000 depend on the precisely measured and expressed local tie connection between space geodetic observing systems at co-located observatories. The destructive Canberra fires of January 2003 completely destroyed the Mount Stromlo Satellite Laser Ranging observatory including the SLR, DORIS, GLONASS and GPS located at the site. Fortunately, Geoscience Australia has routinely performed classical terrestrial surveys at Mount Stromlo, including surveys in 1999, 2002 and 2003 (post-fire). These surveys have included the determination of the SLR invariant point or IVP. Using existing undamaged survey pillars a consistent stable terrestrial network has been used to compute the relationship between the pre and post fire local tie connections. This relationship includes the millimetre level accurate connections and their associated variance covariance matrix and provides an un-broken contribution of the Mount Stromlo observatory to future terrestrial reference frames and other scientific outputs. In this report observational and analysis techniques are reviewed and results are given.

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.

The number and distribution of GNSS CORS networks in the world have steadily increased over the past decades. The monument types vary range from concrete pillars to fence posts which reflects the varying requirements of the broad CORS user community. As a key element of the CORS network, site stability monitoring is a challenging topic. On the one hand, monitoring CORS site stability is important and necessary for legal traceability of measurements. The monitoring and characterization of CORS station behaviour means that users can be assured of a high quality of site data and coordinates. On the other hand, CORS monument stability is a significant issue especially for geophysical investigations because often CORS monumentations were not designed for tectonic applications. Monument instabilities may result in higher noise levels and contribute to the difficulty in discerning the long-term velocity of the site, which is a particular challenge for tectonic analysis using GPS. This paper discusses strategies for monitoring site stability at the observation and site position level. Some case studies are presented with emphasis given to the accuracy and consistency of CORS site position through both epoch by epoch solutions and daily station coordinate solutions. Initial results show: 1) in terms of standard deviations of position offsets, a significant reduction in noise can be seen when comparing the stations with the deep drill braced monuments to the sites on rooftop monuments, the biggest reductions are 4.4 mm, 4.3 mm and 3.3 mm in the east, north and vertical components, respectively; and 2) overall, those stations with small position residuals may reflect the fact that most were well designed and followed the IGS site guidelines.

The report gives an overview about activities of the Geoscince Australia IVS Analysis Center during 2010