Processed GPS trajectories from sites on the Totten Glacier, 2016-2019

Full description

This metadata record pertains to processed dual-frequency geodetic quality Global Positioning System (GPS) data from six sites deployed on the surface of the Totten Glacier between approximately late 2016 and early 2019. The data captures the three-dimensional motion/trajectory of the glacier over a period of approximately 2 years, with a temporal sampling rate of 1 coordinate estimate every 5 minutes. The raw GPS data is covered in a separate metadata record.
Site description:
•    At each site, aluminium towers were erected on the glacier surface – these towers housed a GPS antenna connected to GPS receiver, batteries and solar panel. The approximate height of the GPS antenna was ~3 m above the snow surface at the time of deployment. The GPS receivers to logged data to all GPS satellites in view at a data rate of 1 observation every 15 seconds.
•    The sites were revisited at the midpoint of the project during the 2017/18 austral summer field season. Given the expect large snow accumulation rate in the region of the deployment, the aluminium towers were extended to elevate the antenna ~3 m above the snow surface at that time.
•    The GPS equipment used were Trimble NETR9 receivers and Trimble TRM57971.00 antennas.
Processing description:
•    Dual frequency code and carrier phase GPS data from each site was processed using a kinematic Precise Point Positioning (kPPP) approach with the NASA/JPL Gipsy software package (v 6.3). Processing was undertaken by Christopher Watson (University of Tasmania).
•    The processing approach followed the standard conventions in the geodetic community (IERS2010 conventions for tidal deformation of the solid Earth, VMF1 mapping function for the resolution of tropospheric zenith delay, JPL final orbits and clocks). Site coordinates were computed once every 5 minutes.
•    Processed site trajectories are provided at a temporal resolution of 1 sample per 5 minutes. Coordinates are relative to the ITRF2014 terrestrial reference frame.
•    The data archive with file name AAS_4287_GPS_txyz_llh_sigma is considered the first level of processing it contains the most basic output of antenna position for each epoch in time. These files provide Earth-Centred Earth-Fixed (ECEF) cartesian coordinates (XYZ) as well as latitude, longitude and ellipsoidal height coordinates (expressed on the GRS80 ellipsoid). The formal uncertainty from Gipsy v6.3 processing is provided for each epoch in the form of a 3D sigma (expressed in units mm). No outlier detection has been undertaken. The time standard used is UTC, and the temporal sampling is 1 coordinate estimate per 5 minutes. The change in antenna height at the mid-point of the data collection (to raise the tower given snow accumulation) has not been corrected for in any way.
•    The data archive with file name AAS_4287_GPS_tllh_al_ac_pos_vel is considered the next level of post-processed data as it contains quantities derived from the GPS trajectories and appropriate correction of the antenna height change at the mid-point of the data collection. These files contain the site latitude, longitude and ellipsoidal height (expressed on the GRS80 ellipsoid), the along flow position and velocity, the across flow position and velocity. Outlier detection using a threshold of 25 mm on the 3D sigma (see previous file description) has been undertaken. To resolve the along and across flow coordinate transformation, coordinates were first transformed into a topocentric (north, east) system. The origin of this transformation is provided in the header of each data file. From the local topocentric system, an along and across flow system was derived using piecewise linear fitting of n and e components and temporal knots every 3 months. The temporal knots used in the piecewise linear fitting are provided in the header of each data file. The antenna height correction was estimated using a regression process – offsets and their uncertainty in north, east and up directions are provided in the header of each data file. Again, the time standard used is UTC, and the temporal sampling is 1 coordinate estimate per 5 minutes.

Six GPS sites were deployed in the 2016/17 austral summer season. The sites were revisited in the 2017/18 austral summer season and then retrieved in the 2018/19 austral summer field season. Dates for specific sites are below (yyyy/mm/dd):
Site TG01: 2016/11/22 – 2018/12/23
Site TG02: 2016/11/25 – 2019/01/09
Site TG03: 2016/12/03 – 2019/01/28
Site TG04: 2016/12/03 – 2018/12/24
Site TG05: 2016/11/25 – 2019/01/08
Site TG06: 2016/12/03 – 2018/12/24
Note that data has temporal gaps due to a) lack of solar power over winter and b) equipment failure at TG03.

Lineage

Progress Code: completed

Statement: All sites have expected gaps over winter as they were solar powered. The receiver at TG03 failed after an initial ~7 days of data collection at the start of the record, hence there is a 1 year gap until the site was restarted during the 2017/18 site visit. There were no specific problems with processing of the data. It is important to note that after deployment in the 2016/17 summer season, the sites were revisited in the 2017/18 summer season prior to being retrieved in the 2018/19 field season. During the visit in 2017/18, the antenna at each site was elevated by ~3 m to remain clear of the snow surface. This offset is not corrected in the archived files: AAS_4287_GPS_txyz_llh_sigma.zip as this is considered the first level of processing. The offset is estimated and removed in the archived files: AAS_4287_GPS_tllh_al,ac_pos_vel.zip as this is considered a higher level post-processing of these data. The method for removing this offset is the section above (Question Six).