The Department of Geodesy and Photogrammetry of KTH Royal Institute of Technology, Stockholm, participated in SWEDARP 1991/92, 1993/94 and 1997. For the 1999/2000 expedition, the Swedish team stayed in Antarctica for about 1.5 months (14 December-31 January ). Snowmobiles and helicopters provided transportation for the surveys. Geodetic tasks were incorporated in to the SCAR GPS campaign 2000, and these included making a sea-level connection from Rampen to the Wasa station, extending the gravity network around Wasa and carrying out ice-dynamic measurements. The measurements were taken by two geodesists using three dual-frequency geodetic-type GPS receivers, namely two 4000 SSE Geodetic Surveyors and one 4000 SSI Geodetic Surveyor. One W Sodin 410 gravity meter was used for taking the gravity measurements.

The goal of the SCAR GPS campaign is to study solid earth crustal motion within Antarctica, and plate motion among the Antarctic and surrounding plates. SCAR GPS campaigns have been carried out annually with varied participation from SCAR member nations since 1990/91. Some nations even operate permanent GPS stations year round; we also have plans possibly to install a permanent GPS receiver in the future.

The field-work

The SCAR GPS campaign 2000 lasted from 21 December 1999 to 29 January 2000, and consisted of more or less continuous observation around the clock. Recordings were made every 15 seconds of the L1 and L2 signals of all satellites down to an elevation angle of 5 degrees above the horizon. For the Wasa station, the number of simultaneously observed satellites varied between 4 and 9. A Trimble 4000 SSI receiver was used at Wasa, placed in a temperature-controlled hut 20m from the GPS antenna, over the marker Wasa 9201. Downloading of data from the GPS receiver to a computer was done twice a day, causing a discontinuation of 10-15 minutes. This data has been sent to the computing centre at Dresden University (Technische Universität Dresden) in the RINEX (Receiver Independent Exchange) format. The figure below shows the different stations participating in the SCAR GPS campaign.

Height determination at the Wasa station

The normal way to determine height accurately is by levelling. Levelling cannot, however, be used to determine the height of the Wasa station, as there is too little bedrock on which to install height fixes. Height above sea level is needed because topographic maps refer to this height. We were only able to determine the height using GPS technology, and GPS does not directly offer height above sea level, (i.e. orthometric height H), instead giving height above the reference geodetic height (h). This height must be corrected by the geoidal height (N) to provide the desired value, where geoid is defined as the (extended) me an sea level:

H=h-N

To solve the above equation we need to know both the geodetic and the geoidal heights. From earlier SCAR GPS campaigns we know the geodetic height at Wasa, and the geoidal height is determined by global methods and, for example, gravity measurements.

We had planned to determine the relative height difference between Was a and sea level by establishing one GPS receiver (A) close to the sea, and measuring the height difference between the water and the antenna. The next receiver (B) was placed on the shelf ice at Rampen; this site was chosen because we assumed that the shelf ice would not follow the tidal effects. The receiver on the sea-ice (A) was then assumed to follow the tidal effects. A third receiver (C) was flown up and placed at the Wasa station. The two GPS-receivers A and B were planned to run for at least 24 hours, to permit observation of tidal effects and to determine the mean sea level. Thereafter, the connection to the Wasa station was to be accomplished with the two receivers B and C. To calculate a position, and the height difference between different receivers, the survey must be done at the same time.

Transport problems meant the survey was not performed exactly as planned. Instead, receivers A and B ran for 10 hours. The receiver at the Wasa station should have been logging data over the same period, but when we later came to Wasa we saw that the receiver placed there had broken down, and it was impossible to save any data. To overcome this problem we flew to Rampen later in our stay at Wasa, and placed the receiver over the same point again. This could be done as the point had been marked in the snow. The relative ellipsoidal height difference between Wasa and sea level was computed as 459.842 m. This can be compared to the height determination for the same point produced by the SWEDARP 1997 expedition, which resulted in a different value of 460.733 m. The difficulty of determining the height is obvious. We have also seen that the point at Rampen (B) was actually in motion during the connection to Wasa, possibly explaining why we could not see any significant difference in height between the sea ice and Rampen.

To overcome this problem, the best orthometric height determination method is to take the ellipsoidal height as determined after the SCAR GPS campaign 1992 (h= 466.396 m) and then to compute the geoidal height.

The orthometric height (H) can now be calculated as H= h- N= 462.07 m, an improvement on the earlier result of 460.24 m. Other estimates will be provided when the results of the SCAR GPS campaign 1999/2000 are at hand.