Globally Corrected GPS (GcGPS):
C-Nav GPS System
GPS Services Group
C&C Technologies, Inc., (Lafayette, La)
www.cctechnol.com
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.
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GPS Space Segment
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.
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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.
GPS Error Sources
•
User Independent
• 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
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GPS Errors (Diagram)
The User
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
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Differential (RTCM) GPS
The C-Nav Methodology
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Does not use the ‘traditional’ (RTCM) measurement
and/or position domain correction methodology
Uses a state-space ‘style’ 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 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
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StarFire 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
StarFire US Network (WCT)
( Original )
Frame Relay
with backup
ISDN
Western Beam
Reference/Monitor Site
Central Beam
Processing Hub
Eastern Beam
Satellite Uplink
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Wide Area Correction Transform
• Use of four (4) ‘continental’ Global Reference Network (GRNs) sites that
transmit all RAW GPS dual frequency observations to two independent
Network Processing Hub locations over high speed data links.
• The NPH’s performs the task of breaking down the GPS range error sources
into their component, User Independent, parts in real-time.
• A single Refraction Corrected range correction is computed for each GPS
satellite tracked in the ‘continental’ GRN’s, all of which are transmitted to
the C-Nav GPS end user in real-time over L-Band communication satellite.
• 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 WCT corrections plus the internally
computed, Refraction Corrected, GPS Satellite pseudorange observations to
compute a 3D surface position.
WCT Reference Sites
• North America (8)
Redondo Beach, CA / Portland, OR / Fargo, ND / Kansas City, MO /
WestLaCo, TX / Moline, IL / Belleglade, FL / Syracuse, NY
• South America (3)
Rosario, Argentina / Horizontina, Brazil / Catalao, Brazil
• Europe (4)
Tampere, Finland / Madrid, Spain / Goonhilly, U.K. / Zweibruken, Germany
• Australia (5)
Perth (2 sites) / Sydney / Brisbane / Melbourne
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Real Time Gipsy Corrections
• Use of a ‘worldwide’ Global GPS Network (GGN) reference stations that
transmit all of their RAW GPS dual frequency observations to three
Network Processing Hub locations (SF & JPL) over the ‘open 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 in ‘real-time’, and transmitted via Land Earth Stations for
uplink over geo-stationary communication satellite(s) to the remote
user(s).
• 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
plus the internally computed, Refraction Corrected, GPS Satellite
pseudorange observations to compute an accurate 3D surface position.
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
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StarFire Global Network
StarFire 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.
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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
C-Nav GPS Receiver
Multi
Function
Antenna
Fully Ruggedized, Masthead
Mounted, Sealed Package for
the Marine Environment
L-Band
Comms.
Receiver
DualFrequency
GPS
Engine
Waterproof 8-Pin Connector
that provides DC Power and
External Data Interfaces (RS232 and CAN Bus)
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C-Nav GPS System
C-Nav
GPS
Receiver
C-Nav
Control
Display Unit
(CnC D.U.)
Interconnect
Cable
C-Nav GPS User System
• Basic System Hardware ‘Bundle’
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1 x C-Nav GPS Receiver
1 x 100 ft 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 Software Utilities
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C-Nav GPS System Options
• Options
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50 ft Interconnect Cable
200 ft Interconnect Cable
28Vd.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
C-Nav/StarFire Offshore Service
• The StarFire OFFSHORE subscription service is
available in the following regions:
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North & Central America (WCT & RTG)
Western Europe & Mediterranean Sea (WCT & RTG)
Australia (WCT & RTG)
South America (WCT & RTG)
Eastern Europe, Mediterranean & Black Sea (RTG)
African Continent (RTG)
Middle East, Indian Ocean and Asian Continent (RTG)
Atlantic and Pacific Oceans (RTG)
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C-Nav GPS Receiver Features
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‘Global corrected’ GPS Positioning ( RTG, WCT & WAAS )
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 ( less than 10 Watts )
5Hz positioning and data output ( w/o CnC Display Unit )
Automatic Restart based on last operating configuration
WCT(conus) Results XY Plot
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RTG Results XY Plot
RTG Results Scatter Plot
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Example SA Plots
May 2000
Mode: Autonomous - L1
Mode: DGPS - L1
Period: 30mins
Period: 52mins
95% ~ 47.2 meters
95% ~ 0.5 meters
Example C-Nav Plots
(DUAL Mode)
Mode: Autonomous - L1/L2
Mode: WCT(Conus) - L1/L2
Mode: WAAS - L1/L2
Period: 0hr 55mins
Period: 1hr 0mins
Period: 1hr 00mins
95% ~ 1.2 meters
95% ~ 0.2 meters
95% ~ 0.5 meters
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WCT(Conus) Dynamic Tests
Example C-Monitor Plots
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Applications
• Offshore applications for the C-Nav GPS equipment and the StarFire
correction signal services include:
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Dynamic Vessel positioning
Jacket and Template positioning
Work Boat operations
Dredging operations and surveying
Geophysical, Geotechnical, and Geodetic surveying
Hydrographic surveying
Oceanographic surveying
Offshore construction surveying
Pipeline construction, maintenance, and route surveying
ROV support positioning
Commercial diving support positioning
Underwater cable route, installation, maintenance surveying
Conclusion
• The StarFire correction service and dual-frequency GPS equipment
provides ‘real-time’, 24-hour, refraction corrected, GPS satellite
pseudoranges, that provide accurate, stable, and precise user positioning
solutions.
• C-Nav and the StarFire RTG corrections are a truly Globally corrected GPS
solution for the offshore industry.
• 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 are available on a regional basis – North America,
Western Europe, and Australia (S.America unreliable) < 50 cm
performance.
• The RTG corrections provide 1-2 meter performance and ongoing work is
underway to provide decimeter performance by mid 2002 to all users.
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