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SCAR Report No 21, January 2002

VLNDEF Project for Crustal Deformation Control
of Northern Victoria Land

A. Capra*, S. Gandolfi**, F. Mancini**, P.Sarti***,L. Vittuari**
* Faculty of Engineering, Politecnico of Bari, Italy
** DISTART Dept., University of Bologna, Italy
*** ITIS, C.N.R., Matera, Italy

VLNDEF (Victoria Land Network for DEFormation control) program was developed with the aim of extending northward and southward the existing GPS network for crustal deformation control of Victoria Land.

During 1999-2000 and 2000-2001 Italian expeditions in Antarctica completely surveyed for the first time a network characterised by 25 stations, spanning an area of 800 km northward and 300 km westward, covering the area from Terra Nova Bay (TNB) to Pacific Ocean Oates Coast. There was an average distance of 70-80 km between stations (Figure 1). Because of some rather long baselines, the session duration was not less than 40 hours (15 sec. sampling rate) and the network co-ordinates were emanated from the GPS permanent stations of Terra Nova Bay Station (TNB1).

Figure 1. VLNDEF points distribution

VLNDEF project was conducted within the activity of GIANT (Geodetic Infrastructure of Antarctica) SCAR (Scientific Committee on Antarctic Research) Program. GIANT has being developed for several years with the goal to study the infrastructure and the geodynamics of Antarctica continent by the analysis of different geophysical and geodetic sources: GPS (SCAR GPS Epoch campaigns) and permanent tracking stations, DORIS, Gravimetry, VLBI, remote sensing and tide gauges. Moreover, the geodetic activities are co-ordinated and finalised within the actions of ANTEC (ANTarctic NeoTECtonics) SCAR Group of Specialists. Within the international research for the control of Antarctic crustal deformation, VLNDEF network was connected to the Transantarctic Mountains Deformation (TAMDEF) network.

VLNDEF network includes two geodetic networks previously installed around Terra Nova Bay, Italian base in Antarctica: a geodetic reference network; and a network for deformation control of Mt.Melbourne. The networks were surveyed four times with GPS techniques (Al Bayari et al., 1996, Capra A. et al., 1997, 2000).

The VLNDEF network geometry is based on the local morphology as the faulting system. The station location was planned using the tectonic map of Salvini (Salvini F. et al., 1997), which shows the distribution of faults accepted from scientific community, and using USGS maps (1:250 000 scale).

First campaign results

GPS data have been processed using Bernese version 4.2 and GIPSY - OASIS II (rel. 2.6.1) software. The complete data-set is made up by almost twenty sessions, half of which are 24 hours long while the other are 12 hours sessions.

Bernese data processing - The solution has been obtained adopting the IGS precise ephemeris and solving ambiguities with the QIF (Quasi Iono Free) method followed by the L3 (Iono-free) solution of the baselines. Ambiguities are pre-eliminated and not included in the baseline estimation procedures. The network has been fixed to the TNB1 station using as a reference co-ordinates the ITRF97 values available from the processing of the SCAR GPS data. The tropospheric delay is being modelled using Saastamoinen standard formula estimating a correction every two hours to its a priori values. Gradients for an azimuthally non-symmetric atmosphere have been taken into account and used in the processing procedure.

GIPSY data processing - The complete dataset has been analysed using the most recent version of GIPSY-OASIS II (release 2.6.1). The software uses undifferenced observations using both phase and code observables. GIPSY uses a Square Root Information Filter (SRIF) which is a modified Kalman filter where parameters have been estimated every 300 seconds. All sessions have been processed following a free network approach with fiducial orbits created at JPL. The troposphere has been modelled using Saastamoinen formula and Niell mapping function. Zenith delays a priori values have been corrected in the estimation process. Azimuth troposphere gradients have also been estimated.

The mean value of Helmert transformation between the two solutions is about at 1 cm level. The results did not show large differences, when compared to the accuracy of the technique, in terms of internal consistency and co-ordinates values. It seems that the different statistical approach and computational algorithms that the two types of software used did not influence the solutions. It is important to point out that co-ordinates evaluated with GIPSY for TNB1 (the GPS permanent station in Terra Nova Bay) are not significantly different from those computed in SCAR GPS Epoch campaigns and expressed in ITRF97. It therefore seems that fixing ITRF97 co-ordinates for TNB1 when processing data with Bernese did not introduce remarkable constraints in network solution.

It was decided to use current ITRF emanation co-ordinates of TNB1 for VLNDEF network for successive network repetition for crustal deformation investigation.

Conclusions

VLNDEF project started in 1999-2000, during the XV Italian expedition in Antarctica, when a network of 21 stations has been materialised and measured. In the following 2000-2001 expedition 5 more markers have been installed and measured with the aim to realise a southward expansion of the network.

Network solutions made by Bernese and GIPSY for 21 VLNDEF stations show good internal consistency and repeatability and are not significantly different in terms of internal consistency and co-ordinates values. It has been decided to assume the network's co-ordinates emanation from TNB1 (ITRF97 solution) for successive campaigns for the deformation control.

In the near future we want to examine the connection between VLNDEF and TAMDEF networks and with SCAR GPS Epoch solutions. It will be opportune to take into account the possible constraints introduced by the use of ITRF fixed co-ordinates for more then one GPS tracking stations, whose co-ordinates are derived form international net solutions (IGS, SCAR GPS Epoch, etc).

Acknowledgements: the research was done within the Italian PNRA (National Program of Antarctic Research).

References

Al Bayari O., Capra A., Radicioni F., Vittuari L. (1996). GPS network for the deformation control in the Mount Melbourne area (Antarctica): preliminary results of the third measurements campaign. Reports on Survey and Geodesy. M.Unguendoli ed. (DISTART), pp.64-72.

Salvini F., Brancolini G., Busetti M., Storti F., Mazzarini F., Coren F. (1997). Cenozoic geodynamics of the Ross Sea region, Antarctica: crustal extension, intraplate strike-slip faulting and tectonic inheritance. Journal of Geophysical Research, vol.02, B11, pp.24699-24696.

Capra A., Radicioni F., Vittuari L. (1998). Italian geodetic network a reference frame for geodynamics purposes (Terra Nova Bay–Victoria Land – Antarctica). IAG Symposia, Geodesy on the move, gravity, geoid, geodynamics and Antarctica. Vol. 119, IAG scientific Assembly, Rio de Janeiro, September 1997, pp.486-491.