Text S4: Field collection and data processing of GPS ice surface
motion measurements
Records of positional location were collected with a Trimble R7
dual-frequency global positioning system (GPS) receiver and Trimble
Zephyr Geodetic antenna (Figure S6 ). The system was installed
~750 m SSE of the moulin near the ADCP gauging site at
location 67.048°N, -49.018 °W, elevation 1211.43 m) The antenna was
affixed to a 3.3 m schedule-40 aluminum rod drilled vertically 3 m into
the ice. The aluminum rod-antenna setup was allowed to freeze overnight.
The system was powered by a 40 W solar panel attached to a weatherproof
Pelican hard case that enclosed the GPS receiver, batteries, and cables
adjacent to the antenna. The ice sheet thickness at this location is
~934 m based on Bedmachine v3 (Morlighem et al., 2017).
The entire system was provided by UNAVCO (formerly University NAVSTAR
Consortium), with protocols for field installation and GPS receiver
settings provided by UNAVCO geodetic support engineers. The GPS station
recorded positions at 5-s intervals between 5 and 13 July 2016. A base
station was also established on bedrock near the ice sheet terminus
(67.150°N, 50.058°W, elevation 581.19 m) and recorded positions at 5-s
intervals between 4 and 15 July 2016.
Trimble binary receiver files were converted to RINEX observation files
using runpkr00 v5.40 and TEQC utilities (Estey and Meertens ,
1999). On-ice kinematic GPS positions were estimated using carrier-phase
differential processing relative to the bedrock mounted reference
station (baseline of ~47 km) using TRACK v1.28
(Chen , 1998) and final International GNSS Service satellite
orbits following Andrews et al., 2018; Hoffman et al., 2011 .
During processing, kinematic station motion was constrained on an
epoch-by-epoch basis to 2,000 m yr-1 to permit rapid,
short-term velocity changes. The 5-s time series was then smoothed with
a 6-hr phase-preserving filter to eliminate spurious signals associated
with GPS uncertainties and decimated to a 15-min time series. The
smoothed x and y positions were used to calculate 6-hr velocities using
a centered time window to limit aliasing that may result from using
discrete time intervals. Uncertainties presented here are +/- one
standard deviation of the 15-min binned 5-s position data.