Geodetic antenna calibration test in the Antarctic environment

Abstract

TransAntarctic Mountain DEFormation (TAMDEF) Monitoring Network is the NSF-sponsored OSU and USGS project, aimed at measuring crustal motion in the Transantarctic Mountains of Victoria Land using GPS carrier phase measurements. Station monumentation, antenna mounts, antenna types, and data processing strategies were optimized to achieve mm-level estimates for the rates of motion. These data contributes also to regional Antarctic frame definition. Significant amount of data collected over several years allow the investigation of unique aspects of GPS geodesy in Antarctica, to determine how the error spectrum compares to the mid-latitude regions, and to identify the optimum measurement and data processing schemes for Antarctic conditions, in order to test the predicted rates of motion (mm-level w.r.t. time). The data collection for the TAMDEF project was initiated in 1996. The primary antenna used has been the Ashtech L1/L2 Dorne Margolin (D/M) choke ring. A few occupations involved the use of a Trimble D/M choke ring. The data were processed using the antenna calibration data available from the National Geodetic Survey (NGS). The recent developments in new antenna designs that are lighter in weight and lower in cost are being considered as a possible alternative to the bulkier and more expensive D/M choke ring design. In November 2003, in situ testing of three alternative models of L1/L2 antennas was conducted at a site located in the vicinity of McMurdo Station, Antarctica (S77.87, E166.56). The antenna models used in this test were: Ashtech D/M choke ring, Trimble D/M choke ring, Trimble Zephyr, and the NovAtel GPS-702. Two stations, spaced within 30 meters, were used in the test. Both had the characteristics similar to the stations of the TAMDEF network, i.e., the UNAVCO fixed-height, force-centered level mounts with a constant antenna offset were used, ensuring extreme stability of the antenna/ mount/pin set up. During each of the four 3-day test data collection sessions, a reference station was occupied continuously with the Ashtech D/M choke ring antenna, while the second station was occupied by the tested antennas, one 3-day session for each antenna type. The coordinate differences were produced using software optimized for the analysis of data collected over short baselines. Each solution incorporated the NGS antenna calibration data appropriate for each antenna model. Hourly and 24-hour solutions were analyzed for repeatability and compared to the standard baseline coordinate differences. No significant variation was observed when comparing the same type of antennas and when switching antennas at the test site using daily solutions. An mm-level scatter can be observed comparing different antennas over the 1-hour solutions; it is smaller for the horizontal components, as compared to the vertical direction. At this point, it can be concluded that the standard antenna calibration models from NGS used for each antenna involved in this test did not result in any significant variation in the daily results, but with some in the hourly results. Thus, based on this fact, the antenna types tested here could be used in the future TAMDEF campaigns, where 24-hour solutions are normally used for deformation monitoring. These results can serve as good guidance to any future use of GPS equipment in Antarctica.