DREDGE POSITIONING SYSTEMS
Satellite-based C-Nav Positioning Provides
10 Centimeter Accuracy Around the World
By Edwin Danson
C-Nav Business Development, C&C Technologies, Inc.
Jan de Nul’s 11,300-cubic-meter hopper dredge Filiippo Brunelleschi is equipped with CNav positioning. When dredging far from shore, hopper dredges benefit from the vertical
accuracy of the globally corrected GPS, which provides a means for managing tidal components in the open ocean.
C&C Technologies’ C-Nav® Globally
Corrected GPS (GcGPS) system provides realtime precise point positioning (PPP) solutions
throughout the world. The system provides
sub-10-centimeter horizontal accuracy, and
better than 15-centimeter vertical accuracy.
Dredging companies Jan De Nul and Royal
Boskalis Westminster are using the system for
dredge positioning, especially internationally,
where shore control stations would be expensive to acquire and maintain.
C-Nav’s accuracy comes from its use of
real-time PPP solutions, solving for the GNSS
error sources, rather than trying to compensate
for the errors in a regional DGPS solution.
Instead of observing GNSS errors in a network of reference stations or generating
regional corrections, the C-Nav system corrects the cause of errors by generating a single
set of GNSS clock and orbit corrections having
global validity. This approach eliminates the
spatial de-correlation effects typical of DGPS,
and provides a worldwide consistent and highly accurate result.
This breakthrough in global precision is
achieved using a proprietary real-time
GIPSY® (RTG) solution. Distribution of the
RTG correction parameters is via optimized
data streams, ensuring maximum signal
strength across the L-Band communication
satellite footprints. Additional correctors
include ionosphere, troposphere, multi-path
and Earth tides.
The correction stream includes RTG-generated GNSS precise orbit and clock values for
each SV, differenced with respect to the GNSS
broadcast ephemeris. These are optimized for
distribution via six L-Band communication
satellites. The downlink signal is received
through C-Nav’s receivers using the same
antenna as the GPS L1 and L2 signals.
The C-Nav system comprises four main
segments: ground, control, space and user.
• The ground segment consists of 90 tracking
stations. Each GNSS satellite is tracked by a
minimum of seven stations. The tracking stations use two GPS L1/L2 engines operating
from a single IGS-style choke ring antenna.
The number of receivers at each ground station, combined with the over-determined
simultaneous observations to each satellite,
ensures maximum system robustness.
Reprinted from International Dredging Review | www.dredgemag.com | September/October 2008
• The control segment includes two independently-operating processing centers – in
Torrance, California and Moline, Illinois.
Each center receives the full complement of
tracking station data with a latency of less
than two seconds. High-speed, two-way communications provide continuous high-capacity feeds between the control centers. By using
primary and secondary production layers,
each processing center handles data completely independently of the other, producing
two sets of correction values per center.
Tertiary and quaternary layers within the
infrastructure allow for testing and offline
software updates for testing workflow scenarios prior to release into the higher production
layers.
• The space segment consists of six geostationary communication satellites providing global, high-power L-Band distribution between
about 75° north and south latitudes. The communication satellite constellation is uplinked
through six land earth stations, each equipped
with primary and secondary layers of equipment. Each layer receives corrections from
both control centers. Satellites are monitored
by the control centers to ensure service continuity, with backup channel capacity available
on adjacent satellites over the same regions.
The space segment is configured in two independent networks, Net-1 and Net-2, either of
which delivers the world-wide service.
• In the user domain, the C-Nav system receiver design has benefited from the collective
experience of some 40,000 field units. Every
dual frequency receiver contains Touchstone
4 ASIC precise GPS L1/L2 technology, proprietary L-Band receiver and tri-band antenna
for L1/L2/L-band reception.
The system’s positioning algorithm uses a
Kalman filter to solve for satellite and receiver
channel biases plus code-phase floating ambiguities. A least squares solution is used to calculate the position based on phase-smoothed
refraction and bias-corrected code observables.
Unlike terrestrial DGPS augmentation methods, which suffer from distorting Earth tides,
the system’s state-space solution removes tides
through an algorithm accessing the proprietary
Sinko Earth-tide model.
The C-Nav 2050 dual frequency receiver
and antenna.
DREDGE POSITIONING SYSTEMS
BUILT-IN RTK
C-Nav’s delivery of 10cm accuracy in realtime has led to it being widely adopted as the
system of choice throughout the offshore and
maritime communities. The same accuracy has
also led to its extensive use on land. For those
users who demand even greater accuracy, the
C-Nav 2050 dual frequency receiver comes
with inbuilt RTK capabilities. The RTK solution can be used either stand-alone, in the normal configuration of a base station and rover,
or combined with C-Nav GcGPS.
The stand-alone C-Nav ULTRA delivers
0.005 meter 1ppp RTK solutions at distances in
excess of 60 kilometers, while C-Nav
EXTEND is a unique solution combining
GcGPS with RTK.
C-Nav Extend provides high accuracy RTK
in congested or obstructed environments where
radio communications between base station
and the rover are liable to interruption. Extend
can also maintain RTK accuracy for up to
twenty minutes with no radio communications.
For high dynamic situations such as airborne surveys, the C-Nav VuStar system has
been designed to deliver real-time PPP performance on high platform velocities, eliminating the need for costly ground control.
C-Nav has recently
introduced a fully
compliant UKOAA /
IMO QC package,
embedded in the CMONITOR display.
DREDGING
APPLICATIONS
Minimizing
the
horizontal and vertical
uncertainties in the
error budget significantly improves multibeam and swath
bathymetry. Improving the resolution and
reducing depth uncertainties of multibeam
and swath systems The Auckland, New Zealand land earth station satellite uplink.
leads to efficient and
cost-effective dredgreference systems such as VORF, reduces the
ing operations. In particular, the less than 15vertical error while also keeping costs down.
centimeter vertical accuracy of C-Nav provides
However, many dredging operations require
a means for managing tidal components when
vertical accuracy better than five centimeters.
far from shore. For example, combining CIn such cases, the high accuracy solution from
Nav’s ellipsoid vertical component with realthe C-Nav 2050 receivers in their RTK mode is
time (or post-processed) tidal observations or,
used. For example, Jan De Nul uses their
in some areas of the world, national vertical
Circle Reader Reply No. 210
Reprinted from International Dredging Review | www.dredgemag.com | September/October 2008
DREDGE POSITIONING SYSTEMS
2050’s for controlling dredge cutting heads and excavator buckets, and
for managing the vertical tidal elements during dredging surveys. The
next stage in these specialized applications will be the RTK Extend and
Ultra features discussed above.
Boskalis has C-Nav installed on many of its large fleet of vessels, particularly for the international fleet where satellite-delivered accuracy
eliminates the need for costly shore control stations. Other benefits are
C-Nav’s rapid start up and no need for complex switching and commonality.
Dredging contractors and their customers are focused on the vertical;
the most critical of the measurements for efficient operations and accurate dredge profiles. Of particular interest to the dredging industry are the
recent developments that exploit the stability of the C-Nav ellipsoid vertical with long range RTK.
SUMMARY
Developments in C-Nav are an ongoing process. A team of C-Nav
technologists are dedicated to meeting customer’s demands and setting
the industry’s agenda for future developments. The recent advances in CNav QC, the first package designed specifically for real-time PPP, is one
of many examples where C & C Technologies sets the goals for others to
follow.
New features and ground-breaking products are continuously under
review and production. Customers and prospective customers of C-Nav
are encouraged to put forward suggestions for how we can improve our
services and products. As one great scientist once put it “evolving technology is just an act of will… the ideas come from listening…”
The author thanks Royal Boskalis Westminster and Jan De Nul for
allowing us to cite them as C-Nav satisfied customers.
Reprinted from International Dredging Review | www.dredgemag.com | September/October 2008