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Geotechnical News • June 2012
GEOTECHNICAL INSTRUMENTATION NEWS
Remote monitoring of deformations using
Differential Global Positioning System (D-GPS)
Jason Bond and Rob Nyren
Principal of operation
The Global Positioning System is a
tool for determining terrestrial posi-
tion from satellites. The system itself
consists of 3 main components: Space
Segment; Control Segment; and User
Segment. The Space Segment consists
of the GPS satellites orbiting the earth
approximately 20,000 km above its
surface. The Control Segment com-
prises control and monitoring station
infrastructure on the earth for man-
aging each GPS satellite. The User
Segment comprises the GPS receivers
designed to track GPS satellite signals.
The basis of GPS is ‘trilateration’ or
the use of intersecting range/distance
measurements to determine position.
GPS receivers measure the elapsed
time from when the GPS signal is
transmitted to when the GPS receiver
is received, from which the distance
from a receiver to the satellite is
determined. The locations of the satel-
lites are determined by the Control
Segment and this data (ephemeris) is
logged by the GPS receiver.
GPS is based on signal transmit time
that necessitates very precise time syn-
chronization of GPS receiver/satellite
clocks. Error sources impact the time
measurement of signal travel. These
include: satellite and receiver clock
errors; atmospheric delay errors; sig-
nal reflection (“multipath”) and signal
bending (“diffraction”) effects.
Differential GPS (or D-GPS) is
used to mitigate error sources. To do
this, one receiver is established as a
‘reference’ and measured differences
between the calculated and ‘true’
position allow observation errors to
be estimated. GPS observations made
at locations close together on the
earth will experience similar errors.
As distance between the reference
and monitored stations increases, the
correlation in measurement errors is
likely to decrease. For the best accu-
racy and precision, these distances are
kept less than 10 km.
For geotechnical applications, D-GPS
can be used for monitoring move-
ments of any structure (e.g. dams,
bridges, buildings, earth embank-
ments, etc). The primary output for
these applications is a time series of
3D coordinates. Resonant frequencies
of structures can also be extracted for
GPS observations using GPS receivers
capable of measuring up to 100 Hz.
Accuracy
It is not uncommon to achieve instan-
taneous positioning for a GPS antenna
at accuracies of ± 1 cm horizontally
and ± 1.5 cm vertically (one sigma).
Using advanced signal processing
techniques, mm and sub-mm level
trends can be extracted from the real-
time solution time series. The highest
obtainable accuracy is on the order of
0.5mm. The time required to achieve
the highest accuracy varies according
to the software package and can vary
from hours to several days.
Advantages and limitations
D-GPS has favorable characteristics as
a monitoring technology when care-
fully implemented: a) 3-dimensional
position information is provided to
mm accuracy; b) position is referenced
outside of the deformation zone; c)
position updates can be provided at
frequencies as high as 100 Hz; d) line
of sight is not required between sta-
tions; and e) by isolating information
of interest from the GPS measure-
ments (mainly in the measurement
domain), GPS can also be used to
determine orientation and vibration.
Challenges associated with using
D-GPS technology for monitoring
applications include: a) GPS receivers
collect data continuously and therefore
must be powered at all times, increas-
ing power demands; b) receivers
must have good satellite visibility. In
order to achieve the highest accuracy,
there must be few obstructions near
the GPS antenna and six or more
satellites should be visible from all
sections of the sky; c) the monitoring
network requires a stable reference
point for the base station. Finding
satisfactory locations can be challeng-
ing; and d) readings can be affected
by signal multipath (the arrival of the
same GPS signal via multiple paths
at the antenna, caused by nearby or
remote reflectors) and signal diffrac-
tion (occurs when the GPS signal is
obstructed but still arrives at the GPS
receiver and is processed). Identify-
ing and troubleshooting these effects
requires both specialized knowledge
and experience.
Jason Bond
Gemini NavSoft Technologies Inc.,
20 Barrett Court, Fredericton, NB
Canada
E:
Rob Nyren
Geocomp Corporation,
125 Nagog Park, Acton, MA, USA
E:
