1. C-Nav GPS System & the Seamless Vertical Datum
GPS Services Group
C&C Technologies, Inc., (Lafayette, La)

2. Real Time Gipsy (RTG)
Current release provides sub-meter level horizontal accuracies.
Version 12.2 (beta) test indicates decimeter level horizontal accuracy.
We will be discussing Version 12.2

3. The C-Nav RTG Methodology
Does not use the ‘traditional’ (RTCM) measurement domain or position domain correction methods, nor is it RTK.
Corrects each source of error.
Broadcasts correctors for orbits and satellite clocks.
Dual-frequency code and carrier phase measurement are used to form pseudoranges free from ionospheric delays.
Three approaches (Abousalem, 1996) to solving the wide area differential GPS problem:
The measurement domain approach solves for the mean of the individual DGPS Reference sites correction values
The position domain approach solves for the mean of the actual GPS position solutions resulting from using the individual DGPS reference site corrections.
The ‘State-Space’ method solves the problem more elegantly by computing the actual physical quantities comprising the pseudorange error
Whitehead et.al. state that the advantages are:-
Superior spatial de-correlation properties, such that the end user solution performance is independent of the reference station location or distance.
Fewer reference sites are required over the large coverage area
Minimal bandwidth is required to transmit the correction data
Performance degradation is insignificant for any single Reference Site loss or failure and degrades gracefully for multiple-reference site loss.
Some References:
A Close Look at SatLoc’s Real-Time WADGPS System – Whitehead et.al. (GPS Solutions, Vol 2, No.2, 1998)
A Real-Time Wide Area Differential GPS System – Bertinger et.al. (Revised February 1998)
Precise Post-Processing of GPS Data: Products and Services from JPL – Zumberge & Webb (ION 2001)
ION Proceedings over the past ‘several years’
IGDG (Internet-based Global Differential GPS) -
NavCom Technology, Inc., Technical Archives -

4. Real Time Gipsy (RTG)
‘Worldwide’ Global GPS Network (GGN) reference stations transmit all of their RAW GPS dual frequency observations to three Network Processing Hub locations (SF & JPL) via TCP/IP and the ‘Internet’.
The NPH’s performs the task of breaking down the GPS range error sources into their component, User Independent, parts in real-time.
Independent Refraction Corrected Orbit and Atomic Clock Offset corrections (to the broadcast ephemeris) for all GPS satellites are computed (by the NPH), and transmitted via Land Earth Stations for uplink over StarFire L-Band communication satellites.
The user requires a Dual-Frequency GPS receiver to be used at their remote location so that computation of the ‘local’ Refraction Corrected pseudorange observations can be obtained.
The GPS receiver applies the received RTG Orbit and Clock corrections along with the internally computed, Refraction Corrected, GPS Satellite pseudorange observations to compute a 3D surface position.
Over the past 20 years the California Institute of Technology’s Jet Propulsion Laboratory has evolved into one of the premier centers for research in precise orbit determination. The venerable GIPSY-OASIS software suite, used by research teams worldwide for geodetic analysis and orbit determination was developed at JPL.
Internet based Global Differential Gps - IGDG was used to control and operate the SATLOC commercial differential service from 1995 until 2000 (when the system was bought by a competitor and then shut down). While operating, the SATLOC service had the best reliability and accuracy record of any wide area differential service. IGDG's impressive performance in the SATLOC service was probably key to the FAA's decision to adopt IGDG and its concept of operations for its wide area augmentation system.
IGDG was successfully implemented in the FAA WAAS by the prime contractor, Raytheon, and later was also implemented in a similar system Raytheon built in Japan.
Over the last six years, the GPS group at JPL has created a system, based on adaptations and refinements of the core GIPSY algorithms, which operates in real time to produce high precision GPS corrections suitable for broadcast to navigation users. This system, called Real Time GIPSY (RTG), accurately estimates and models many parameters and error sources in the GPS satellite system using real time data received via the Internet from a worldwide network of ‘dual-frequency’ GPS reference receivers, and utilizes the dual-frequency GPS receiver at the user location to resolve for the local area Iono/Tropo errors (Refraction Correction) by ‘differencing’ the C/A code measurements of the L1 and L2 frequencies.

5. StarFire Global Network
The figure shows an overview of the StarFire WADGPS network. At a conceptual level, it is similar to other wide-area DGPS systems such as :
WAAS - Wide Area Augmentation System (FAA North America)
EGNOS – European Geo-stationary Navigation Overlay System
MSAS – Multifunctional transport Satellite-based Augmentations System (Japan)
SNAS – Satellite Navigation Augmentation System (China)
GRAS – Ground-based Regional Augmentation System (Australia – VHF)
For the WCT networks, a number of reference/monitor sites are distributed across the continental U.S., Europe, South America and Australia. (sites in green)
For the RTG network, another set of reference/monitor sites are distributed across the entire world (sites in red).
Each reference site sends dual frequency code and phase observables for all GPS satellites in view as well as system integrity information to two redundant network processing hubs (NPH) via terrestrial communication links (sites in blue) in North America.

6. RTG Reference Sites Global Network (26+)
Brewster, USA
Cordoba, Argentina
Christiansted,Virgin Islands
Fairbanks, USA
Galapagos Island, Ecuador
Greenbelt, USA
Goldstone, USA
Dededo, Guam
Krugersdorp, South Africa
Bangalore, India
JPL Pasadena, USA
Kokee Park, USA
Robledo, Spain
Ross Island, Antarctica
Mauna Kea, USA
Moscow, Russia
Franceville, Gabon
Norilsk, Russia
Lamont, USA
Quezon City, Phillipines
Bishkek, Kryghystan
Santiago, Chile
Tidbinbilla, Australia
USNO, USA
Usuda, Japan
Yakutsk, Russia
Two key correction factors are computed for transmission to the user navigation receivers:
1) Clock corrections for each active GPS satellite are computed every few seconds. Like the WCT method, these corrections are based on refraction corrected measurements and are therefore optimized for dual frequency user equipment.
2) Orbit corrections for each active GPS satellite are computed every few minutes. Computation of these corrections is facilitated by measurements from a globally distributed network of reference receivers that provide observability of the orbit errors with sufficient geometry.
The Network Processing Hubs are located at:
Redondo Beach, CA
Moline, IL
The L.E.S. for WCT(Conus) is located in Reston, NJ
The L.E.S. for the Inmarsat Americas is located in Laurentides, Canada
The L.E.S. for the Inmarsat Europe/Africa is located in Goonhilly, U.K.
The L.E.S. for the Inmarsat Asia is located in Auckland, New Zealand

7. C-Nav GPS User System Basic System Hardware ‘Bundle’:
1 x C-Nav GPS Receiver
1 x C-Nav Control Display Unit (CnC D.U.)
1 x C-Nav GPS Receiver Data and Power Y-Cable
1 x DC Power Cable
1 x Power Supply
1 x C-Nav Operations Manual
1 x Software Utilities

8. C-Nav GPS Receiver Design
Multi-function L-Band antenna
12 channel dual-frequency, geodetic grade GPS engine
L-Band communications receiver and embedded microprocessor
Patented multi-path reduction signal processing capability and P code recovery algorithm
Dual-frequency code and carrier phase measurement are used to form smooth refraction corrected code pseudoranges
Compact size and integrated package design
The GPS engine has twelve (12) dual frequency GPS channels, ten (10) of which are allocated for GPS signal tracking and the remaining two (2) for WAAS, L-Band, signal tracking. It produces GPS observables of the highest quality suitable for use in the most demanding applications including millimeter level static surveys.
Key features of the GPS engine include:
 ·        A patented multipath reduction technique is built into the digital signal processing ASICs of the receiver. This greatly reduces the magnitude of multipath distortions on both the CA code and P2 code pseudorange measurements. When combined with extended, dual frequency code-carrier smoothing, multipath errors in the code pseudorange measurements are virtually eliminated.
 ·        A patented technique is used to achieve near optimal recovery of the P code from the anti-spoofing Y-code resulting in more robust tracking of the P2/L2 signals.
 ·        The compact size (4” x 3”x 1”) of the Geodetic Grade, Dual Frequency, GPS engine allows it to be readily integrated into the StarFire GPS User package.
 ·        The GPS engine provides a high-resolution 1pps output signal, synchronized to GPS time. This signal is used by the L-band communications receiver to calibrate its local oscillator and thus accelerate acquisition of the StarFire correction signal. NavCom Technologies has also patented this technique.

9. C-Nav Features ‘Global corrected’ GPS Positioning ( RTG, WCT & WAAS )
1Hz NMEA Msgs ( GGA, GLL, GSA, GST, RMC, VTG, ZDA )
Proprietary NMEA Data Msgs ( SATS, NAVQ, RXQ, NETQ )
RTCM Output ( Standard RTCM Type 1 PRC – every 5 seconds )
Dual Frequency, Geodetic GPS Engine to resolve local Ionospheric delay observation errors
Multipath Mitigation Algorithm
Rugged and waterproof Single Integrated Package
Low Power Consumption ( < 10 Watts – 9v to 40v d.c.)
5Hz positioning and data output ( w/o CnC Display Unit )
Automatic Restart based on last operating configuration
The GPS engine has twelve (12) dual frequency GPS channels, ten (10) of which are allocated for GPS signal tracking and the remaining two (2) for WAAS, L-Band, signal tracking. It produces GPS observables of the highest quality suitable for use in the most demanding applications including millimeter level static surveys.
Key features of the GPS engine include:
 ·        A patented multipath reduction technique is built into the digital signal processing ASICs of the receiver. This greatly reduces the magnitude of multipath distortions on both the CA code and P2 code pseudorange measurements. When combined with extended, dual frequency code-carrier smoothing, multipath errors in the code pseudorange measurements are virtually eliminated.
 ·        A patented technique is used to achieve near optimal recovery of the P code from the anti-spoofing Y-code resulting in more robust tracking of the P2/L2 signals.
 ·        The compact size (4” x 3”x 1”) of the Geodetic Grade, Dual Frequency, GPS engine allows it to be readily integrated into the StarFire GPS User package.
 ·        The GPS engine provides a high-resolution 1pps output signal, synchronized to GPS time. This signal is used by the L-band communications receiver to calibrate its local oscillator and thus accelerate acquisition of the StarFire correction signal. NavCom Technologies has also patented this technique.

10. C&C (C-Nav) Locations

11. Vertical Accuracy IHO SP 57, 1987 0.3 meters at 90% confidence
Assuming Gaussian:
0.36 meters at 95% confidence

12. Vertical Accuracy IHO S 57, 1998, category 1:
Depth error of 0.5 m at 95% confidence.
Assume sounding error of 0.36 meters at 95%:
Allowable “Tide” meters at 95% confidence.

13. Vertical Accuracy IHO S 57, 1998, category “special”:
Depth error of 0.25 m at 95% confidence.
Assume sounding error of 0.15 meters at 95%:
Allowable “Tide” meters at 95% confidence.

14. Vertical Accuracy NOS Specifications and Deliverables January 2002:
Tidal errors range from 0.2 m to 0.45 m
at 95% confidence.

15. Vertical Accuracy
A vertical accuracy of 0.35 meters at 95% confidence is sufficient for all IHO categories except “special” and is about as accurate as a tide-based datum.
A vertical accuracy of 0.2 meters at 95% confidence is sufficient for IHO “Special” surveys and is about as good as the best tide based datum.

16.   Figure 1: SD = m
Figure 2: SD 0.277
Figure 3: SD = m

17. High Multipath

19. Good Data 95% of data is within 0.41 meters of the mean.
Max HDOP = 3, max speed = 6 m/s; Discard stand alone GPS.

20. Good Data

21. How Good? Vertical accuracy of about 0.41 meters at 95%.
About the same as NOS zoned tides in the most difficult areas.
Can be used for IHO Category 1, but sounding errors must be limited to 0.3 meter at 95%.
Further testing needed to confirm accuracy and understand restrictions.

22. Conclusions Vertical accuracy of about 0.41 meters
Multipath needs to be controlled.
About ten minutes to sub-meter data.
About 2 hours to decimeter data.