Geoinformation for Disaster and Risk Management - ISPRS

Satellite based positioning techniques are very
commonly used for geodetic monitoring of objects
such as landslides. Currently a low cost (LC) PDGNSS
monitoring system on a commercial off-the-shelf
basis, has been developed at the Institute of Geodesy,
University of the Federal Armed Forces of Germany
in Munich (Pink 2007, Günther et al. 2008, Glabsch
et al. 2009) as a cost-effective readily available
technique for monitoring tasks.
GNSS NRTP approach - brief theory
To meet the required accuracies of a few millimetres
for point position in monitoring applications, only
carrier phase (CP) based GNSS methods come into
consideration, which is the so-called precise
differential positioning (PDGNSS). By using PDGNSS
techniques to observe discrete points permanently
in a monitoring network, the results should be
available with a minimum of delay, involving near
real-time processing (NRTP). This approach is based
on the evaluation of CP measurements recorded over
certain, in principle freely and individually selectable
time spans at different locations. Depending on the
receivers, the satellite visibility and the expected
velocities of the points, usually a time interval of
about 15min referred to as an 'epoch' in the
following with a recording frequency of 1Hz or
similar can be considered for the CP raw data
acquisition for landslide monitoring tasks. Once the
raw data from several locations at least one should
be on stable ground and the others are spread on the
slope is available at a central computing location the
baseline processing can start immediately.
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Standard WLAN can be chosen for permanent
wireless communication between the different
locations and the central computing station. Thus the
combination of sensing, communication and
affiliated computing forms a geo-sensor network
(GSN) and the locations the reference and object
points are designated as sensor nodes (SNs). Since
the distances range up to several kilometers,
normally special WLAN antennas or alternatively
repeater stations are needed on site.
For economical reasons simple LC L1 navigation
receivers have been investigated at the Institute of
Geodesy (Pink 2007, Günther et al. 2008, Glabsch et
al. 2009) as the sensor component at the SNs. The
main prerequisite of these receivers is that they must
have the possibility to read-out the CP raw data, for
instance via RS232 or USB. Most of the simple
navigation receivers make use of these data for some
internal smoothing operations, but do not have the
ability of an autonomous phase-based positioning, as
do the customary rovers commonly available in the
real-time kinematic (RTK) systems. Finally, the
chosen NRTP concept opens up the possibilities of
the well-known options of highly sophisticated postprocessing
for all kinds of simple receivers in a
geodetic monitoring network adjustment, which are
of course not solely restricted to low cost equipment.
In addition to the length of an epoch, a short delay
due to gathering and evaluating all data at the
computing station has to be accepted in applying this
concept. Considering the expected landslide
movement rates and the essential advanced warning
times, the concept of “near real-time” should present
no limitations on its intended use for early warning
of landslides.
System layout - example of GSN Aggenalm
(project alpEWAS)
Aggenalm Landslide monitoring concept
The Aggenalm Landslide will be discussed as a
practical realization of the above briefly described LC
PDGNSS NRTP approach. This work is embedded in a
comprehensive development of a widespread and
cost-effective GSN for landslide monitoring in the
alpEWAS project. The project is funded by the
German Federal Ministry of Education and Research
(BMBF) in the geo-scientific research and
development program “Geotechnologien”. For more
details please see Thuro et al. (2009a, 2009b) and
refer to www.alpewas.de.
Innovative geodetic and geotechnical measuring
techniques are applied for monitoring the landslide
mechanisms at the test site Aggenalm in the
Bavarian Alps. Time Domain Reflectometry (TDR)
which is an automated low cost inclinometer, low
cost Global Navigation Satellite System receivers
(GNSS) and a recently developed reflectorless Video-
Tacheometric Positioning System (VTPS) are being
investigated in this project. By combining the data of
all these different measuring systems, 3D
deformation information can be derived not only
with high spatial, but also with high temporal
resolution, as all three systems can operate
continuously. However, at present, the VTPS is used
only temporarily at the Aggenalm Landslide site. The
three techniques are combined in a multifunctional
GSN as shown schematically in Fig. 1. All sensor
nodes can be read at two places which are connected
to the base station. All data traffic is handled in a
WLAN supplemented by some cabled connections
for economical and practical reasons. A central
computer manages all system operations, e.g. data
collection and logging and controlling of the sensor