Satellite-based C-Nav Positioning Provides 10 Centimeter Accuracy Around the World
C&C Technologies’ C-Nav® Globally Corrected GPS (GcGPS) system provides real-time 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.
• 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 40000 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.
BUILT-IN RTK
C-Nav’s delivery of 10cm accuracy in real-time 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 C-MONITOR 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 leads to efficient and cost-effective dredging operations. In particular the less than 15-centimeter vertical accuracy of C-Nav provides a means for managing tidal components when far from shore. For example combining C-Nav’s ellipsoid vertical component with real-time (or post-processed) tidal observations or in some areas of the world national vertical reference systems such as VORF reduces the vertical error while also keeping costs down.
However many dredging operations require vertical accuracy better than five centimeters. In such cases the high accuracy solution from the C-Nav 2050 receivers in their RTK mode is used. For example Jan De Nul uses their 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 C-Nav 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.
