A DORIS determination of the absolute velocities of the Sorsdal and Mellor glaciers in Antarctica

Abstract

The Lambert–Amery System is the largest glacier–ice shelf system in East Antarctica, draining a significant portion of the ice sheet. Variation in ice sheet discharge from Antarctica or Greenland has an impact on the rate of change in global mean sea level; which is a manifestation of climate change. In conjunction with a measure of ice thickness change, ice sheet discharge can be monitored by determining the absolute velocities of these glaciers. In order to demonstrate the capability of the DORIS system to determine glacier velocities, Geoscience Australia undertook a Pilot Project under the auspices of the International DORIS Service. A DORIS beacon was deployed on the Sorsdal (November 2001 – January 2002 and November 2003 – January 2004) and Mellor (December 2002 – January 2003) glaciers. The DORIS data, transmitted from the autonomously operating ground beacon for each satellite pass, were stored in the receiver on-board the satellite and later downlinked to the DORIS control centres for processing. This paper describes the campaigns that were conducted at the Sorsdal and Mellor glaciers, the data processing standards for modelling the Doppler measurements, precise orbit determination of the satellites using the data from the globally distributed DORIS network, tracking station position and reference frame modelling, the point positioning mode employed for determining the position and velocities of the transmitting beacon antennas located on the glaciers and provides the velocity estimates that have been determined from the analysis of these tracking data. For the Sorsdal 2001/2002 campaign, using SPOT-4 data only, the measured effective horizontal ice motion was estimated to be 30 ± 0.4 cm/day (azimuth of N246°E ± 0.1°). The inferred velocities for the Sorsdal 2003/2004 campaign, using SPOT-4 and SPOT-5 data, was 5.7 ± 0.8 cm/day (azimuth of N264°E ± 7.5°) for the first eight days and 11.4 ± 1.4 cm/day (azimuth of N241°E ± 1.5°) for the subsequent 21 days. There was a noted decrease in the inferred velocities between the beginning and the end of the observing period. A sub-division of the latter 21 day observing period into three segments showed a decrease in 2D velocity from 18.3 ± 0.7 to 11.2 ± 0.7 cm/day and then to 7.4 ± 0.9 cm/day for the first, second and third segments, respectively. In comparison, a GPS derived velocity over the time-span of the 2001/2002 Sorsdal campaign gave a mean ice flow rate of 31 cm/day. The GPS velocity was derived from two daily position estimates 65 days apart. The DORIS determination from 26 days of continuous SPOT-4 and SPOT-5 data compared well with the GPS derived velocity. For the 2002/2003 Mellor glacier campaign, using SPOT-4 and SPOT-5 data, the estimated average ice velocity was 104 ± 25 cm/day (azimuth of N33°E ± 0.1°); which compared well with an InSAR derived velocity of between 110 and 137 cm/day. The point positioning technique as implemented in this study was further validated and assessed by replicating the computational process to determine the position and velocity of the permanent International DORIS Service site at Terre Adélie, Antarctica. Through these experiments, it has been successfully demonstrated that the DORIS system is capable of determining the velocities of glaciers with an accuracy of a few cm/day over a period of several weeks; operating in remote regions in an autonomous mode. With an increasing number of DORIS-equipped satellites and multiple daily passes, it has the potential to measure glacial velocities at a high temporal resolution (sub-daily).