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SCAR Report 23

Geodesy Activities in the Prince Charles Mountains
Expedition of Germany and Australia (PCMEGA) 2003

Gary Johnston1, John Manning1, John Dawson1, Paul Digney2
and Michael Baessler3
1.Geoscience Australia
2. Formerly Geoscience Australia
3. Institut fuer Planetare Geodaesie, Dresden University of Technology, Germany

Abstract

During the Antarctic summer 2003/2004 a major Geodetic Geological Geophysical expedition to the southern Prince Charles Mountains was undertaken jointly between Germany and Australia. This project was termed PCMEGA.
The joint expedition studied geochronology and metamorphism of the area. Extensive airborne gravity and ice radar survey flights were undertaken over the ice cap to the immediate south of the mountains. Aeromagnetic transects over the Southern PCMs were also flown.
As part of the structural investigation of the area several first epoch GPS sites were established, together with occupations on previously established geodetic survey points, and a number of new mapping control were surveyed. An ice field calibration site for ICESAT, at a satellite crossover point near Mt Cresswell, was surveyed using kinematic GPS and a DORIS beacon was deployed on the glacial stream of the Lambert Glacier.
The GPS points were subsequently processed and the precision achieved on the first epoch occupations assessed. Additionally a GPS point on the nearby Grove Mountains was reoccupied for the third summer in a row to provide information to the ANTEC program from this part of East Antarctica. The preliminary results of these occupations will be presented in this paper.

Figure 1. GPS point on Seavers Nunatak looking south East to Mt Menzies

1.Introduction

In the Austral summer of 2002-3 a geodesy team participated in the Prince Charles Mountains Expedition of Germany and Australian (PCMEGA). The expedition’s objective was to undertake a geological and geophysical survey of the Southern Prince Charles Mountains in East Antarctica, including terrestrial and airborne gravity surveys, aeromagnetics, an ice radar survey and the enhancement of the geodetic network. The geodesy component was undertaken by a team consisting of surveyors from Australia (Geoscience Australia) and Germany (Dresden University of Technology).

2. Historical Background

The Prince Charles Mountains (PCM’s) were first sighted as distant horizon features on the trimetrogon aerial photographs of the 1947-48 United States Operation Highjump. It was first reached by ground parties from Mawson in 1954 and then explored with aircraft support in the second half of the 1950s, when aerial photography was flown and controlled by sparse astronomical position fixes for reconnaissance mapping.

From the late 1960s the application of distance measuring equipment and the transition to use of helicopter support in the summer enabled a terrestrial geodetic network to be gradually developed through the northern and southern Prince Charles Mountains. This network, know as the Australian Antarctic Geodetic Network (AAGN), was been observed using angles and distances. It was later extended to the east and the west. By January 1976 the geodetic network stretched continuously from Davis to Molodezhnaya. This was a major feat but the positional accuracy suffered from a lack of traditional azimuthal control in poorly conditioned geometric figures, as it was difficult to observe Laplace azimuths in the summer daylight.

With the development of early satellite positioning techniques several Doppler Satellite fixes were observed in the Northern Prince Charles in 1988 and the next year surveyors’ trialled the use of GPS for Antarctic geodesy. This quickly became the standard technique used for positioning during summer expedition projects. To provide a framework for these summer campaigns permanent GPS base stations were established at Casey, Davis, Mawson and Macquarie Island from 1993 onwards.

In 1995 and 1997 positional upgrades were observed with GPS at some geodetic network sites in the Southern PCM’s. These were relatively short occupations on tripods but produced key tie points and significant improvements were achieved in the absolute accuracy of the geodetic network. Observations continued in 2000/01 along the northeastern edge of the Amery Ice shelf thus strengthening the single line traverse connection to Davis.

From 2000 Geoscience Australia entered into a multi year project under the Australian Antarctic Science Advisory Committee (ASAC) process. The Objectives of this project (ASAC 1159) are:

3. PCMEGA 2003

The primary geodetic objective was to obtain accurate ITRF2000 coordinates and gravity values on a network of points across the southern PCM’s (SPCM’s). This was done by:

In addition other tasks completed were

All GPS observations were taken on rock mounted antenna mounts (see Figure 1 for example) which eliminated the uncertainty of using tripod setups. Where existing AAGN points existed on the feature the new mark was place adjacent to it and connected using terrestrial survey techniques allowing the adjustment of the AAGM network onto the new GPS control. Where no existing AAGN points were present the site was chosen for optimum stability and ease of access. Subsequent re-adjustment of the AAGN using these new points and the connections to the existing control has resulted in significant improvements in the accuracies of the network throughout the area.

More importantly the new GPS network bridges the Lambert Glacier with a number of sites either side of the main stream and a good latitudinal distribution as well (Figure 2).

Figure 2. Distribution of GPS sites in the Southern Prince Charles Mountains

The observational strategy of running three continuous trackers in the vicinity of the survey while the shorter seven day occupations occurred has resulted in very high coordinate precisions, even on the shorter occupations. Figure 3 and 4 illustrate the daily coordinate repeatability’s achieved. Figure 3 shows one of the longer occupations at Mt Creswell and Figure 4 shows Harbour Headland which is one of the shorter occupations.

Figure 3. Aus367 (Mt Creswell) time series

This first epoch observation at the network of new geodynamic style points has established the framework for future tectonic studies in the region. Subsequent observations on these points will allow a structural investigation of the region, and may give some insight into the processes which formed the Lambert Graben.

Figure 4. Scatter plot of AUS380 (Harbour Headland) results

Additionally, a GPS point on rock in the nearby Grove Mountains was reoccupied for a third summer in a row to provide horizontal movement information to the ANTEC program from this part of East Antarctica. The results of these occupations indicates that the horizontal motion is consistent with the east Antarctic plate, but a small differential vertical motion on this inland site compared to the coastal margin as represented by the IGS sites of Mawson and Davis is present. Figure 5 illustrates the time series for the Grove Mountains site. The continuation of this time series with longer observational spans will improve the understanding of this feature.

Relative gravity observational loops were taken between Davis and Mt Creswell, as well a Mt Creswell and Mawson. These loops were later extended to all sites visited in the area so that gravity values can be computed relative to the fundamental gravity stations and Davis and Mawson. Most points were visited twice for redundancy.
The secondary tasks completed will not be discussed here, but will be recorded at more length in a full technical report being prepared at Geoscience Australia.

Figure 5. AUS351 (Grove Mountains) time series

4. Conclusions

The 2002-03 PCMEGA campaign was an unprecedented success for all disciplines of Geoscience studied. While extremely fortuitous weather conditions certainly contributed to the success of the campaign, the logistical and organisational support provided by the Australian Antarctic Division contributed more.

The geodesy program established 23 new geodynamic style GPS points with a resultant coordinate precision in the order of 1-2mm horizontal and 2-3mm vertical. Many of these were also connected to the existing AAGN network allowing significant improvements to it. Gravity observations were also taken adjacent to these points enhancing the usability of the gravity values. They can now be used for ground control of airborne or satellite gravity missions.