1. Globally Corrected GPS (GcGPS): C-Nav GPS System
This presentation explains how Globally corrected GPS (GcGPS – a variant of the State-Space or Precise Pont Positioning approach) is different from other differential GPS techniques (Measurement Domain and Position Domain approaches – Whitehead et.al. GPS Solutions, Vol 2, No 2, 1998).
The GcGPS technique uses a network of ITRF located reference sites strategically located around the world to simultaneously monitor the entire GPS constellation.
Each reference site is equipped with dual frequency - geodetic quality GPS receivers that transmit all raw GPS observations to the Network Processing Hubs (NPH).
The NPH receives this global data and calculates orbit and clock correctors for each GPS satellite.
Corrections computed in this manner are universally valid and can be applied to GPS measurements from any location on earth.
The resulting position is very stable, accurate, and repeatable.
DGPS Services Group
C&C Technologies, Inc., (Lafayette, La)

2. GPS Space Segment
Comprises of a ‘nominal ‘network of 24 GPS satellites in orbit around the globe.
Nominal orbit height of 20,200 Kilometers.
Initial 24Hr operational capability was declared on 8 December, 1993.
Full 24 Hr operational capability was declared after testing on 17 July, 1995.
Selective Availability signal degradation was removed in May, 2000.
GPS is a satellite navigation system designed to provide instantaneous position, velocity and time information almost anywhere on the globe at any time, and in any weather. (NAVSTAR GPS stands for the NAVigation Satellite Timing And Ranging Global Positioning System) It can provide positioning accuracies ranging from 100 meters when Selective Availability (SA) and Anti-Spoofing (AS) is active (95% of the time), to 5 to 10 meters, or DIFFERENTIAL CORRECTED accuracies at the sub-meter level. Naturally the higher accuracies involve a greater infrastructure and thus greater cost. CIVILIAN usage of GPS for positioning includes personal navigation, aircraft, vessel and vehicle navigation, fleet tracking, dredging, machine control, civil engineering, surveying, GIS and Mapping, deformation analysis ... The list is almost endless. The day-to-day running of the GPS program and operation of the system rests with the US Department of Defense (DoD). Management is performed by the US Air Force with guidance from the DoD Positioning/Navigation executive Committee. This committee receives input from a similar committee within the Department of Transportation (DoT) who act as the civilian voice for GPS policy matters (NAPA, 1995). Originally designed by the US Department of Defense (DoD), GPS comprises three main components: the control, space, and user segments.

3. GPS Space Segment
The GPS Space Segment comprises a network of satellites in near circular orbits at a nominal height of 20,183 km above the Earth and with a period of approximately 12 sidereal hours. The original constellation was for 24 satellites, in 3 orbital planes and inclined to the equator (Spilker, 1980), but these plans have since been changed and the satellites are currently placed into six different orbital planes, with four satellites in each plane.
The NAVSTAR GPS satellites are at a 'nominal' altitude of 20,200 Kms with a 55 degree inclination (see diagram next).
GPS Constellation
24 satellites with a minimum of 21 operating 98% of the time
6 Orbital planes
55 degrees inclination
km above the Earth's surface
11 hours 58 minute orbital period
visible for approximately 5 hours above the horizon

4. GPS User Segment
The user segment is comprised of the GPS receivers that have been designed to decode the signals transmitted from the GPS satellites for the purposes of determining position, velocity and time.
There are two types of service available to GPS users - the SPS (Civilian) and the PPS (Military).
SPS - Standard Positioning Service is the positioning accuracy that is provided by GPS measurements based on the single L1 frequency C/A code.
PPS - Precise Positioning Service is the highest level of dynamic positioning accuracy that is provided by GPS measurements based on the second L2 frequency P-code.
GPS User Segment
The user segment comprises the receivers that have been designed to decode the signals transmitted from the satellites for the purposes of determining position, velocity or time. To decipher the GPS signals, the receiver must perform the following tasks: (Anon, 1989)
selecting one or more satellites in view
acquiring GPS signals
measuring and tracking
recovering navigational data messages (Ephemeris and code)
There are two types of service available to GPS users - the SPS and the PPS.
GPS satellites transmit two L-Band signals which can be used for positioning purposes. The reasoning behind transmitting using two different frequencies is so that errors introduced by ionospheric refraction can be eliminated.
The signals, which are generated from a standard frequency of MHz, are L1 at MHz (~19cm wavelength) and L2 at MHz (~24cm wavelength) and are often called the carriers.

5. Positioning With GPS GPS is a ‘one way’ TIME BASED measurement system.
Full three dimensional (3D) navigation or positioning uses a minimum of four range measurements to four satellites.
With these pseudorange measurements the user is able to solve time, and then the three-dimensional coordinates (x, y, z) of their GPS receiver antenna electrical phase center.
Over determined solution calculations (>4 SVs) provide redundant measurements that provide for better positioning and also the ability to determine any possible erroneous conditions in calculating the final 3D surface position.
Positioning with GPS: There are essentially two broad categories of GPS positioning which can be described as real-time code-based navigation and high precision carrier phase positioning.
Navigation uses a minimum of four pseudorange measurements to four satellites which are used to solve for the three-dimensional coordinates of the receiver and the clock offset between the GPS receiver oscillator and GPS system time. An extension to this mode is differential GPS (DGPS) which again uses the pseudorange observable for positioning, but also incorporates real-time corrections for the errors inherent in the measurements.
Combining the pseudorange with the phase data reduces the noise error within the pseudorange measurement resulting in a much higher positioning accuracy.

6. GPS Error Sources User Independent User Dependent
Ephemeris Data - Errors in the transmitted location of the GPS satellite
Satellite Clock - Errors in the transmitted clock (including SA)
Ionosphere - Errors in the corrections of the measured pseudorange caused by ionosphere signal path effects or delays
Troposphere - Errors in the corrections of pseudorange caused by troposphere signal path effects or delays
User Dependent
Multipath - Errors caused by reflected signals entering the receiver antenna from local surfaces (longer travel times)
Receiver - Errors in the GPS receiver's measurement of range caused by thermal noise, software accuracy, and inter-channel biases
User – Errors caused by the operator of the GPS receiver
Error Sources
GPS user position calculations are degraded by satellite range error and bias in the measurement of the distance from each GPS satellite to the user’s GPS receiver antenna. Generally, the errors and biases that influence the GPS measurement fall into three categories: Satellite, Observation and Station biases.
GPS User Independent errors can be reduced by the use of GPS Measurement Correction techniques.
The traditional approach has been by the use of Differential Pseudorange GPS (DGPS ) as implemented by the Radio Technical Commission for Maritime Services Special Committee 104 (RTCM CS-104) standard. This technique combines the User Independent errors into once correction value based on the Reference Base Station observations.
Wide Area GPS (WAGPS) techniques such as the FAA Wide Area Augmentation System WAAS - (for North America) are an alternative methodology that provides each individual GPS Error observation correction in ‘real-time’ to be transmitted to the user.

7. GPS Errors (Diagram) The User
Uncorrected GPS Observation RANGE MEASUREMENT Error Budgets:
L1 Error model ( one-sigma ) L1/L2 Error model
Ephemeris data =
Satellite clock = (no SA) or 20.0 to 100 (with SA)
Ionosphere = 3.0 to
Troposphere = <1m with model)
Multipath =
Receiver =
User equivalent range error *
(UERE), rms =
Filtered UERE, rms =
10.2 m Horizontal errors (HDOP=2.0) 6.6 m
12.8 m Vertical errors (VDOP=2.5) m
* This is the statistical ranging error (one-sigma) that represents the total of all contributing sources. The dominant error is usually the ionosphere for L1 only measurements.

8. What Does This Mean?
The accuracy and stability of ‘real-time’ corrected GPS navigation solutions are dependant on:-
How well the GPS corrections are computed and measurements are applied by the GPS user
The location of the GPS antenna to reduce signal blockages and multi-path effects
The quality of the GPS receiver and it’s operation to reduce ‘noise’ and operator errors
GPS Satellite Error Sources:
Orbit Position Error
Clock Error
** Selective Availability – Epsilon & Dither of SV Clock Time
Environment Error Sources:
Ionospheric Delays / Refraction – Can be computed from differencing L1/L2 signal measurements
Tropospheric Delays / Refraction – Use of a standard model
User Environment Error Sources:
Multipath Affects
GPS Receiver being used
Operator

9. Differential (RTCM) GPS
Traditional DGPS
The reference station (or network) computes a Pseudorange Correction (PRC) for each satellite, and a rate of change Range Rate Correction (RRC). Thus, the mobile user is able to model the time varying characteristics of the corrections over the time intervals in which they are 'periodically' generated at the reference station and applied at the mobile user GPS receiver location (the age of correction).
The mobile user receives the Pseudorange and Range Rate Corrections as a binary message format. The corrections are then translated and applied to the individual Pseudorange Measurements observed by the mobile user's GPS receiver.

10. The C-Nav Methodology
Does not use the ‘traditional’ (RTCM) measurement and/or position domain correction methodology
Uses a state-space, ‘precise point positioning’ solution (Wide Area dGPS) whereby the actual physical properties that comprise each of the errors in pseudorange observations are computed (the User Independent Errors) –similar to SatLOC and WAAS, EGNOS etc…
C-Nav is a further development from existing WADGPS solutions in that the use of Dual Frequency GPS receivers to compute the ‘local user’ ionospheric pseudorange observation errors (differencing of the L1/L2 code derived pseudorange measurements) is employed
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)
Elements of GPS Precise Point Positioning – A THESIS, Witchayangkoon, Univ. Of Maine, 2000
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) -

11. dGPS Corrections Account for??
User Independent W A S
Individual Orbit & Clock Corrections plus a regional Iono/Tropo Model R T C M
One combined correction to the GPS psuedorange observation
WCT
One combined Orbit and Clock Correction value for each GPS satellite
RTG
Individual Orbit Corrections and Individual Clock Corrections for each GPS satellite
Ephemeris Data - Errors in the transmitted location of the GPS satellite
Satellite Clock - Errors in the transmitted clock (including SA)
Ionosphere - Errors in the corrections of the measured pseudorange caused by ionosphere signal path effects or delays
Troposphere - Errors in the corrections of pseudorange caused by troposphere signal path effects or delays
The C-Nav unit computes it’s own Iono/Tropo corrections from the L1/L2 measurements
User Dependent
So to revisit the GPS Error Sources, we can identify what types of corrections account for which error either together as a group or otherwise.
DGPS using RTCM corrections:
Provide a single pseudorange correction for each GPS SV observation ie; combines the User Independent Errors into one correction BASED ON THE REFERENCE SITE (network) observations.
Wide-area Correction Transform:
Provides a single correction value for the Ephemeris and Clock errors as computed by the StarFire ‘regional network’– while the C-Nav L1/L2 GPS receiver calculates the Iono/Tropo errors at the user location.
Real-Time Gypsy:
Provides an Ephemeris correction and a Clock correction as computed by the StarFire ‘GLOBAL ITRF network’– while the C-Nav L1/L2 GPS receiver calculates the Iono/Tropo errors at the user location.
Wide Area Augmentation System (S.B.A.S. – Satellite Based Augmentation System):
Provides an Ephemeris correction, a Clock correction, and an Iono (Tropo) as computed by the ‘regional continental network’.
Multipath - Errors caused by reflected signals entering the receiver antenna from local surfaces (longer travel times)
Receiver - Errors in the GPS receiver's measurement of range caused by thermal noise, software accuracy, and inter-channel biases
User – Errors caused by the operator of the GPS receiver

12. dGPS Corrections Account for??
RTCM (1) = Combined (Ephemeris + Clock + Iono + Tropo)
S.B.A.S (3) = Independent (Ephemeris) + (Clock)
[WAAS / EGNOS] Combined (Iono + Tropo) ‘Model/Grid’
RTG (2)** = Independent (Ephemeris) + (Clock)
to the broadcast GPS SV ‘almanac’ information
WCT (1)** = Combined (Ephemeris + Clock)
** C-Nav GPS Engine calculates, from the GPS L1 and L2, DUAL FREQUENCY, signal measurements, the local user area ‘Atmospheric’ delays for each GPS observation to correct for the Ionosphere delay errors.
So to revisit the GPS Error Sources, we can identify what types of corrections account for which error either together as a group or otherwise.
DGPS using RTCM corrections:
Provide a single pseudorange correction for each GPS SV observation ie; combines the User Independent Errors into one correction BASED ON THE REFERENCE SITE (network) observations.
Wide-area Correction Transform:
Provides a single correction value for the Ephemeris and Clock errors as computed by the StarFire ‘regional network’– while the C-Nav L1/L2 GPS receiver calculates the Iono/Tropo errors at the user location.
Real-Time Gypsy:
Provides an Ephemeris correction and a Clock correction as computed by the StarFire ‘GLOBAL ITRF network’– while the C-Nav L1/L2 GPS receiver calculates the Iono/Tropo errors at the user location.
Wide Area Augmentation System (S.B.A.S. – Satellite Based Augmentation System):
Provides an Ephemeris correction, a Clock correction, and an Iono (Tropo) as computed by the ‘regional continental network’.

13. Frame Relay with backup ISDN
WCT US Network
Frame Relay with backup ISDN
The overall topology of one of the StarFire Ground Reference Networks (GRN) for WCT (CONUS), in the continental U.S. It is comprised of seven reference/monitor sites, two redundant network-processing-hubs and an uplink facility for the geo-stationary communications satellite.
 Each of the reference/monitor sites is configured with an identical set of equipment including:
a) Two redundant GPS reference receivers which send a full set of dual frequency observables for all satellites in view to both of the redundant processing hubs,
b) A fully packaged production StarFire GPS user equipment unit which serves as an independent monitor receiver,
c) Communications equipment (routers, ISDN modems),
d) A remotely controlled power switch and UPS module.
The main communication lines used to link the reference sites with the network processing hubs are frame relay private virtual circuits (orange and blue lines in the Figure). Each frame relay circuit is backed up with an ISDN dial up line that is activated automatically from either of the network processing hubs in the event any frame relay connection fails.

14. 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.

15. Global Reference Network
Reliability based on redundancy
Two independent/redundant Network Processing Hubs
Redundant communication links (Frame Relay, ISDN)
Dual modulation racks at each LES uplink facility
Redundant Reference Receivers at WCT sites
Redundant Reference Sites (more than minimally required)
Dual frequency GPS reference receivers
Refraction corrected pseudoranges observations
Extended smoothing to minimize multipath measurement
Phase tracking to aid with C/A code measurement processing
Two major advantages result from having one consolidated set of corrections for the entire service area:
a) Bandwidth requirements on the geo-stationary communications satellite are minimized. This results in a significant cost savings since the price of leased satellite channels is roughly proportional to the broadcast power required which is directly proportional to the bandwidth required.
b) The correction computation algorithm, including the final weighting, is done at a centralized facility (at the network processing hubs) instead of being performed by the user equipment based on location dependent models. This enables improvements and upgrades to the WCT to be made, in most cases, without requiring changes to the algorithms in the mobile user equipment. This is a significant logistic benefit when, as is the case now with StarFire, thousands of user equipment units are deployed across the continental U.S.

16. Real Time Gipsy Corrections
‘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.

17. ( see JPL Live Demo WEB Site at http://gipsy.jpl.nasa.gov/igdg )
RTG Reference Sites
Global GPS Network (30+)
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 / plus others…
( see JPL Live Demo WEB Site at )
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:
Torrance, CA
Moline, IL
JPL Pasadena, CA as tertiary backup
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
Obsolete:
The L.E.S. for N.A Spot Beam Coverage (Conus) located in Reston, NJ has been discontinued

18. WCT Reference Sites North America (8) Europe (4) Australia (5)
Redondo Beach, CA / Portland, OR / Fargo, ND / Kansas City, MO / WestLaCo, TX / Moline, IL / Belleglade, FL / Syracuse, NY
Europe (4)
Tampere, Finland / Madrid, Spain / Goonhilly, U.K. / Zweibruken, Germany
Australia (5)
Perth (2 sites) / Sydney / Brisbane / Melbourne
South America (3)
Rosario, Argentina / Horizontina, Brazil / Catalao, Brazil
The dual frequency observables are used to form smoothed, refraction corrected pseudoranges, which are free of ionosphere delay and, due to extended smoothing, virtually free of multipath. These are then normalized with respect to receiver clock offsets and modeled site troposphere delays. Finally, the normalized pseudoranges for each satellite are combined in a weighted average to form a single, wide area pseudorange correction for that satellite. A similar process is performed using the finite difference of the carrier phase to generate pseudorange rate corrections. The ensemble of these corrections for all satellites in view is formatted into a tightly packed, binary message and sent from the hub to the uplink facility for broadcast on the geo-stationary communications satellite.
Because the WCT uses refraction corrected pseudoranges, the resulting corrections are free of the errors caused by spatial decorrelation of ionosphere delays that are inherent in single frequency corrections. When dual frequency mobile receivers are used which employ the same refraction corrected techniques, a single set of corrections can be used across the entire continental service area with uniform, high accuracy.

19. C-Nav Global Positioning
Both RTG and WCT corrections are optimized for dual frequency GPS user equipment.
The GPS SV Orbit and Clock corrections for the RTG process are globally uniform.
One set of RTG corrections for all GPS SV’s worldwide.
WCT corrections provide back-up, secondary positioning in regional areas.
SBAS corrections provide regional tertiary positioning.
NavCom Technology, Inc., has teamed with JPL in a joint effort to merge the RTG technology and JPL/NASA GGN with the StarFire system. The major benefits and elements of synergy between the two systems include:
1)     Both systems are optimized for dual frequency GPS user equipment capable of producing refraction corrected measurements.
2)     The orbit and clock corrections produced by the RTG process are globally uniform. One set of corrections is generated for all active satellites, which is applicable worldwide. This provides a bandwidth advantage similar to the WCT method.

20. 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.

21. C-Nav[RM] GPS Receiver
Fully Ruggedized, Masthead Mounted, Sealed Package for the Marine Environment
Multi Function Antenna
L-Band Comms. Receiver
The Figure shows the major components of the StarFire network GPS user equipment as packaged for C-Nav.
 a) A multi-function antenna assembly is used which is capable of receiving the L1 and L2 GPS frequencies as well as the Inmarsat receive frequency band. The gain pattern of this antenna is designed to be relatively constant even at lower elevation angles. This allows for an efficient link budget when the unit is operated at higher latitudes where the elevation of the geo-stationary communications satellite is low.
 b) An L-band receiver was developed to acquire, track, down convert, sample and demodulate the StarFire data stream broadcast from the geo-stationary communications satellite. The receiver is frequency agile across the Inmarsat receive band under software control.
c) A state-of-the-art, dual frequency GPS receiver module, designed and produced by NavCom, provides the most important enabling technology in the user equipment.
Connections for the external interfaces of the C-Nav user equipment are provided through a sealed 8-pin connector.
Power requirements are DC, with an input range of between 10 to 40 volts.
Data interfaces include both RS232 serial data ports and also a CAN Bus.
Dual-Frequency GPS Engine
Waterproof 8-Pin Connector that provides DC Power and External Data Interfaces (RS-232 and CAN Bus)

22. C-Nav Block Diagram Internal Tri-Freq. Antenna Front End RF Signal
Ground
Ign. On
D.C. In
CAN Bus
RAW GPS
Corrections
1PPS
L-Band
Communications
Downconvertor
GPS
Receiver
Engine
A/D
Converter
Cct.
DSP /
CPU
Processor
DC to DC
Power
Supply L1 L2
Internal
Tri-Freq.
Antenna
Front End
RF Signal
Board
Downconverter
GPS Module
Digital Board
Subcon
Connector
Pin #1
Pin #2
Pin #3, #4
Pin #5, #6
Pin #7
RS232 I/F
The Main user interface for command, control and PVT data is provided by the RS-232 Interface (with a secondary CAN Bus (Controller Area Network) Interface.
The user can also obtain RAW GPS measurement information from the C-Nav GPS receiver from the RS232 RAW GPS observations data port. The proprietary data format can be decoded by the use of a purpose built software utility. The use of the software allows the ‘binary’ information to be stored to a file that can subsequently be converted to a RINEX ASCII format file for post-processing analysis.
The C-Nav GPS receiver provides position, velocity, and time ( PVT) information at either a 1Hz data rate or a and 5Hz data rate. This is selectable by the user.
Extensive Quality Assurance and Control information is available from the C-Nav GPS System. The primary source of information to achieve this is by the use of the C-Nav Control Display Unit (CnC Display Unit). Alarm conditions and warnings are provided to the user, as is the ability to obtain all of the information on the operation, and performance of the C-Nav GPS receiver and the StarFire network. Additional information is provided by Receiver Autonomous Integrity Monitoring (RAIM) compliance NMEA GPS Statistic (GST) message.

23. C-Nav Control Display Unit (CnC D.U.)
C-Nav[RM] GPS System
C-Nav GPS Receiver
C-Nav Control Display Unit (CnC D.U.)
The C-Nav system provides the user with industry standard NMEA sentence information. These are:-
GGA - Global Positioning System Fix Data
GLL - GPS Latitude and Longitude Data
GSA - GPS Mode, Satellites used for navigation, and DOP Data
GST - GPS Satellites Position Error Statistics (RAIM compliance)
RMC - GPS Recommended Minimum Specific Data
VTG - GPS Velocity, Track made good and Ground speed Data
ZDA - GPS UTC Date & Time Data
In addition, the C-Nav system provides additional NMEA sentence information in the form of ‘proprietary’ messages, as follows:-
NAVQ - StarFire Navigation Quality
NETQ - StarFire Network Quality ( to be implemented )
RXQ - StarFire Correction Signal Quality
SATS - GPS Dual Frequency (L1/L2) Satellites in view (similar to GSV message that only provide L1 frequency signal strength information).
Interconnect Cable

24. C-Nav[RM] GPS User System
Basic System Hardware ‘Bundle’
1 x C-Nav GPS Receiver
1 x 30m Interconnect Cable
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 C-Nav Operations Manual
1 x Qa/Qc and Application Software Utilities
The CnC Display Unit functions as a key element of the C-Nav GPS System. Besides providing the information and control functions for the user, in an easy to read LCD format and by simple keypad control, the CnC Display Unit also ensures that all NMEA messages are immediately output to the user interface, upon receipt from the GPS receiver with zero latency time, and that the C-Nav recomputed, local position, RTCM ‘binary’ correction messages are re-routed to a separate RS232 data port so that the user can use separately as required.
The C-Nav GPS Receiver and the CnC Display Unit are firmware driven. Thus tools are supplied to allow the user to upgrade the operating firmware for both units in the field. By simple or ftp receipt of any required ‘binary’ flash memory program files, the user can quickly and easily reprogram the internal processors operating program code from their Windows 95/NT operating system running on a computer or laptop.

25. C-Nav[RM] GPS System Options
15m Interconnect Cable
60m Interconnect Cable
Universal a.c. to d.c Power Supply
RS-232 to RS-422 In-line Amplifier/Converter System
19 inch Rack Mount C-Nav Control Display Unit
Rugged Transport / Shipping Case
Note: Multiple Interconnect Cables can be connected ‘end-to-end’ as required.
For long distances, the RS-232 to RS-422 In-Line Amplifier/Convertor is available, and simply plugs ‘in-line’ to the Interconnect Cable 8-Pin connectors at the C-Nav GPS Receiver at the ‘masthead’ and at the ‘CnC Display Unit.

26. C-Nav ‘integrated’ mast-head unit
DP Vessel Hardware
C-Nav ‘integrated’ mast-head unit
RS-232/422 Inline Amplifier / Converter Pair (Option)
(Optional Master/Slave Assy shown)
Interconnect Cable can be any length required
The C-Nav[RM] GPS System is designed to be a simple plug-n-play installation.
The system design allows for a single CnC D.U. to be used to communicate with the C-Nav[RM] masthead unit via an Interconnect Cable (I.C.).
Optional RS232-to-RS422 ‘inline converter’ can be installed if required for use on long I.C. cable runs to ensure reliable serial communications.
In addition, a second CnC D.U. SLAVE unit can be connected to the ‘inline converter’ below deck unit to allow additional system interconnection interface requirements and Remote Alarm display functionality.
Desk-Top-Mount CnC D.U.
19inch Rack-Mount CnC D.U.

27. C-Nav GLOBAL Service
The C-Nav correction service is available in the following regions:
North & Central America (RTG, WCT, and WAAS)
Western Europe & Mediterranean Sea (RTG, WCT, and EGNOS)
Australia (RTG & WCT)
South America (RTG)
Eastern Europe, Mediterranean & Black Sea (RTG)
African Continent (RTG)
Middle East, & Asian Continent (RTG)
Atlantic, Indian and Pacific Oceans (RTG)
RTG corrections are available ‘globally’ within the Inmarsat communication satellite signal footprint (72deg N to 72deg South)
WCT corrections are available in North America, South America, Northern Europe and Australia.
WAAS corrections are available in North America
EGNOS corrections will be available for Europe
MSAS corrections will be available for Japan

28. C-Nav Features
‘Global corrected’ GPS Positioning ( RTG, WCT & S.B.A.S. )
NMEA Data Msgs ( GGA, GLL, GSA, GST, RMC, VTG, ZDA ) - 1Hz
Proprietary NMEA Data Msgs ( SATS, NAVQ, RXQ, NETQ )
RTCM Output ( Standard RTCM Type 1 Message Format – 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 )
Optional 5Hz positioning and data output ( w/o CnC Display Unit )
Automatic Restart based on last operating configuration
Other information and data provided by the C-Nav GPS receiver are the calculation of RTCM SC-104 Type 1 pseudorange correction (PRC) binary messages for all satellites ‘in view’. These RTCM Type 1 messages are provided so that the user can output to 3rd party interface devices or standard DGPS L1 only receivers. The RTCM Type 1 binary messages are computed by the C-Nav GPS receiver based on the current PVT solution and re-computation of the errors from the L1 range observations. This can be accomplished due to the efficiency and processing power, embedded, in the GPS engine. The RTCM Type 1 PRC’s are output every 5 seconds when enabled.

29. Example SA Plots May 2000 Mode: Autonomous - L1 Period: 30mins
Examples show a commercial, survey grade GPS receiver ‘scatter plot’ during early 2000 with Selective Availability (SA) ENABLED.
The ‘scatter plot’ to the left shows the effects of Autonomous GPS with SA over a 30 minute period. Notice how the navigation position ‘wanders’. 95% error circle = 47.2 meters
The ‘scatter plot’ to the left shows the effects of Differentially corrected GPS with SA over a 50 minute period. Notice how the corrections compensate for the ‘intentional clock drift (dither) effect of SA. 95% error circle = 52 centimeters
Mode: Autonomous - L1
Period: 30mins
95% ~ 47.2 meters
Mode: DGPS - L1
Period: 52mins
95% ~ 0.5 meters

30. Example C-Nav Plots (CA Mode)
Examples show a C-Nav GPS Receiver CA MODE (SINGLE L1 FREQUENCY) position solution ‘scatter plot’ during January 2002 with Selective Availability (SA) DISABLED.
The left ‘scatter plot’ shows the performance of a C-Nav unit in Autonomous (‘standalone’) mode. Notice how the data shows the effects of the visible GPS satellite ‘geometry’ changing in relation to time for the static observation. 95% error circle = 3.5 meters
The middle ‘scatter plot’ shows the performance of a C-Nav unit with the Wide area Correction Transform (WCT-Conus) for North America. 95% error circle = 66 centimeters
The right ‘scatter plot’ shows the performance of a C-Nav unit with the FAA Wide Area Augmentation System correction signal for North America. 95% error circle = 58 centimeters
Mode: Autonomous - L1
Period: 1hr 00mins
95% ~ 3.5 meters
Mode: WCT(Conus) - L1
Period: 1hr 12mins
95% ~ 0.7 meters
Mode: WAAS - L1
Period: 1hr 00mins
95% ~ 0.6 meters

31. Example C-Nav Plots (DUAL Mode)
Examples show a C-Nav GPS Receiver DUAL MODE (L1 and L2 FREQUENCY) position solution ‘scatter plot’ during January 2002 with Selective Availability (SA) DISABLED.
The left ‘scatter plot’ shows the performance of a C-Nav unit in Autonomous (‘standalone’) mode. Notice how the data shows the effects of the visible GPS satellite ‘geometry’ changing in relation to time for the static observation. 95% error circle = 1.2 meters
The middle ‘scatter plot’ shows the performance of a C-Nav unit with the Wide area Correction Transform (WCT-Conus) for North America. 95% error circle = 16 centimeters
The right ‘scatter plot’ shows the performance of a C-Nav unit with the FAA Wide Area Augmentation System correction signal for North America. 95% error circle = 53 centimeters
Mode: Autonomous - L1/L2
Period: 0hr 55mins
95% ~ 1.2 meters
Mode: WCT(Conus) - L1/L2
Period: 1hr 0mins
95% ~ 0.2 meters
Mode: WAAS - L1/L2
Period: 1hr 00mins
95% ~ 0.5 meters

32. Example C-Nav Plot (Real-Time GIPSY - DUAL Mode)
Mode: RTG - L1/L2
Period: 46hrs 52mins
95% ~ 0.3 meters
Example shows a C-Nav GPS Receiver DUAL MODE (L1 and L2 FREQUENCY) position solution ‘scatter plot’ during August 2002 with Selective Availability (SA) DISABLED.
The ‘scatter plot’ shows the performance of a C-Nav unit located at the C&C Technologies, Houston Office site (static location) over a period of 2 days. The 95% error circle = 29 centimeters
The C-Nav firmware revision in use was SF_V12r2.hex.

33. America’s RTG Performance
Cumulative RTG Horizontal Solution performance analysis for October, 2002 for the Americas region
The 95% confidence horizontal accuracy is 30 to 35 centimeters

34. Europe Africa RTG Performance
Cumulative RTG Horizontal Solution performance analysis for October, 2002 for the Europe Africa region
The 95% confidence horizontal accuracy is 25 to 30 centimeters

35. Asia-Australia RTG Performance
Cumulative RTG Horizontal Solution performance analysis for October, 2002 for the Asia-Australia region
The 95% confidence horizontal accuracy is 30 to 35 centimeters

36. At Sea Dynamic Tests
The Figure shows a comparison of a C-Nav GPS receiver unit and a DGPS ‘Coast Guard Beacon receiver as compared to a ‘commercially available’ DGPS system conducted onboard a C&C Technologies hydrographic survey vessel project.
From the Figure, the difference of the Trimble DGPS Receiver is approximately 2+ meters, 95% confidence level (2dRMS).
The C-Nav GPS receiver using global RTG corrections is approximately 1.2 meters at the 95% confidence level (2dRMS).
Remember, all three systems onboard the vessel, were used in a dynamic operating mode and as such, vessel movement (pitch, heave, and roll) and offset layback alignments affect the analysis shown for all systems and these effects are not accounted for in the example provided.
The errors in the ‘commercially available’ DGPS system measurement used as a reference add to the above system performance as it has been assumed to be the reference for the vessel. Thus if we assume a meter or so performance (95% confidence level) of the ‘commercially available’ DGPS system then the C-Nav RTG solution is a decimeter solution and the Trimble DGPS Receiver is a near sub-meter solution.
The accuracy and repeatability of the C-Nav GPS equipment, with the global RTG correction signal service, surface positioning can be seen to be more than comparable with existing DGPS operations.

37. Example C-Monitor Plots
Examples show a C-Nav GPS Receiver DUAL MODE (L1 and L2 FREQUENCY) as recorded by the C-Monitor Software Application during January 2002 with Selective Availability (SA) DISABLED.
The left panel shows the current position solution information (top of panel) plus ‘history time line’ graphs of several of the NMEA data message information that can be selected by the user. In this case:-
L-Band SNR from the C-Nav RXQ NMEA message
Differential correction age from the GGA NMEA message
HDOP from the GGA NMEA message
GPS SV’s used in the current solution from the GGA NMEA message
These individual graphs can be assigned WARNING and ALARM limits as desired.
Note: Other attributes can be graphed as desired by the user
The next panel show the NMEA GST ‘Error Ellipse’ information as provided by the C-Nav GPS Receiver and also by any other NMEA GST message device (in the example a Trimble AgGPS 132 and a SatLoc GPS receiver are shown).
The 3rd panel displays the ‘scatter plot’ view of the NMEA position and is referenced to a Reference Position for station keeping monitoring.
The right panel displays the GPS satellites in view for the C-Nav GPS Receiver. This information is obtained from the NMEA ‘SATS’ message. Note the L1 and also the L2 signal strength va;ues can be seen. For other GPS receiver types, the C-Monitor application can decode the standard NMEA GSV message for L1 only GPS tracking.

38. Applications
Offshore applications for the C-Nav GPS equipment and the Global correction signal services include:
Hydrographic surveying
Oceanographic surveying
Dynamic Vessel positioning
Jacket and Template positioning
Work Boat operations
Dredging operations and surveying
Geophysical, Geotechnical, and Geodetic surveying
Offshore construction surveying
Pipeline construction, maintenance, and route surveying
ROV support positioning
Commercial diving support positioning
Underwater cable route, installation, maintenance surveying

39. Conclusion
The RTG correction service and C-Nav GPS System provides ‘real-time’, 24-hour accurate, stable, and precise user positioning solutions.
C-Nav and the RTG corrections are a truly Globally corrected GPS solution and provides decimeter performance to all C-Nav users between 72deg North and 72deg South..
The C-Nav GPS equipment is rugged, reliable and able to withstand the offshore environment.
The C-Nav GPS system provides industry standard NMEA sentence messages and can also provide RINEX L1/L2 code and phase observations.
Comprehensive QA and QC is available from the C-Nav GPS System to enable the user to monitor the navigation solution performance.
The WCT corrections provide sub-meter performance and are available on a regional basis – North America, Western Europe, and Australia (S.America) and is a backup positioning source to RTG.
S.B.A.S signals are also available (where applicable) for tertiary backup.
The operational StarFire Network Processing Hubs with their over redundancy of GPS observations, real-time network monitoring and backup processing and communication links are providing 100% user correction utilization and availability. The StarFire network correction signals are providing accurate, stable, and precise user positioning solutions in real-time. This is giving existing GPS users reliable and repeatable GPS navigation positioning that ensures the efficiency of their business and operations
C-Nav and the RTG correction network are a truly Globally corrected GPS solution. All the required components essential to accurate real-time positioning have been packaged into an integrated ‘masthead’ unit with a ‘below deck’ control display unit. The equipment can quickly and easily be installed on any offshore vessel, vehicle, or platform. Comprehensive QA and QC information are available from the C-Nav GPS System that allows the user to monitor the navigation solution performance and accuracies.
Other GPS user packaging is available. Development is ongoing to provide new hardware designs for a variety of GPS applications. Ongoing development is underway to continue to improve the accuracies of the ‘global’ RTG corrections (especially the vertical), for all suitably equipped GPS users. The goal is to provide sub-decimeter level positioning accuracies, at any time, and at any geographic location within the current worldwide geo-stationary communication satellite footprints.

40. For additional Information see:- www.cctechnol.com/cnav
The operational StarFire Network Processing Hubs with their over redundancy of GPS observations, real-time network monitoring and backup processing and communication links are providing 100% user correction utilization and availability. The StarFire network correction signals are providing accurate, stable, and precise user positioning solutions in real-time. This is giving existing GPS users reliable and repeatable GPS navigation positioning that ensures the efficiency of their business and operations
C-Nav and the StarFire RTG correction network are a truly Globally corrected GPS solution. All the required components essential to accurate real-time positioning have been packaged into an integrated ‘masthead’ unit with a ‘below deck’ control display unit. The equipment can quickly and easily be installed on any offshore vessel, vehicle, or platform. Comprehensive QA and QC information are available from the C-Nav GPS System that allows the user to monitor the navigation solution performance and accuracies.
Other GPS user packaging is available. Development is ongoing to provide new hardware designs for a variety of GPS applications. Ongoing development is underway to provide ‘global’ RTG corrections, for all suitably equipped GPS users. The goal is to provide decimeter level positioning accuracies, at any time, and at any geographic location within the current worldwide geo-stationary communication satellite footprints.