Specifications

12
taken when the available equipment or other technical circumstances prevent operations
in DGPS mode.
The detailed procedure for taking DGPS measurements vary with the specific receiver
model used, see the instrument documentation for details.
Whatever the instrument used, the model, configuration and setup must be recorded and
special care must be paid in properly configurating the antenna parameters and in
positioning the antenna on the stake. Unobstructed view of the sky must be ensured.
GPR surveys
Snow depth profiles are surveyed with a 500 or 800 MHz shielded antenna and either
constant time (0.25 sec) or constant distance (0.5 m) settings by setting up the antenna on
a fiberglass sled towed by a snowmobile. The monitor should be setup so that the
snowmobile driver can see the data being acquired and adjust the speed consequently.
Every few hundred meters, and also every time unclear features are seen on the display,
the snowmobile must stop and a few measurements of the snopac
k depth must be taken
with the avalanche probe, recording all the values and not just the average.
Data files will be frequently downloaded from the GPR instrument to a USB stick and
daily backe
d-up to a second storage support to be kept at Zackenberg Station.
The tracks to be followed during the GPR survey should cover the glacier as evenly as
feasible, but avoiding dangerous crevassed areas unless the snow cover is abundant and
safe. As an example, Fig. 2 shows the tracks followed in 2008. GIS files with the tracks
surveyed during previous years are available from within GEUS intranet in the folder:
\\Geusnt1\glaciologi\GlacioBasis\GlacioBasis Manual\GIS files\