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DGPS Services GroupC&C Technologies, Inc., (Lafayette, La)
http://www.cctechnol.com/cnav
Globally Corrected GPS (GcGPS):
C-Nav GPS System
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 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.
• 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
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
•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
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
Differential (RTCM) GPS
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
dGPS Corrections Account for??• 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
•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
R
T
C
M
One combined correction to the GPS psuedorange observation
W
A
A
S
Individual Orbit & Clock Corrections plus a regional Iono/Tropo Model
RTG
RTG
Individual Orbit Corrections and Individual Clock Corrections for each GPS satellite
WCT
One combined Orbit and Clock Correction value for each GPS satellite The C-Nav unit computes it’s own Iono/Tropo corrections from the L1/L2 measurements
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.
WCT US Network
Western Beam Central Beam
Reference/Monitor Site Processing Hub Satellite Uplink
Eastern Beam
Frame Relay with backup
ISDN
Global Network
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
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.
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 http://gipsy.jpl.nasa.gov/igdg )
WCT Reference Sites• North America (8)
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
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.
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[RM] GPS Receiver
Multi Function Antenna
L-Band Comms. Receiver
Dual-Frequency
GPS Engine
Fully Ruggedized, Masthead Mounted, Sealed Package for the
Marine Environment
Waterproof 8-Pin Connector that provides DC Power and
External Data Interfaces (RS-232 and CAN Bus)
C-Nav Block Diagram
Ground
Ign. On
D.C. In
CAN Bus
RAW GPS
Corrections
1PPS
L-BandCommunicationsDownconvertor
GPSReceiverEngine
A/DConverter
Cct.
DSP /CPU
Processor
DC to DCPowerSupply
Cct.
L1
L2
InternalTri-Freq.Antenna
Front EndRF Signal
Board
DownconverterBoard
GPS Module Digital Board
SubconConnector
Pin #1
Pin #1
Pin #2
Pin #3, #4
Pin #5, #6
Pin #7
RS232 I/F
C-Nav[RM] GPS System
C-Nav GPS Receiver
Interconnect Cable
C-Nav Control
Display Unit (CnC D.U.)
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
C-Nav[RM] GPS System Options
• 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
DP Vessel Hardware C-Nav ‘integrated’ mast-head unit
19inch Rack-Mount CnC D.U.
Desk-Top-Mount CnC D.U.
Interconnect Cable can be any length required
RS-232/422 Inline Amplifier / Converter Pair (Option)
(Optional Master/Slave Assy shown)
C-Nav GLOBAL ServiceThe 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)
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
Example SA PlotsMay 2000
Mode: Autonomous - L1
Period: 30mins
95% ~ 47.2 meters
Mode: DGPS - L1
Period: 52mins
95% ~ 0.5 meters
Example C-Nav Plots (CA Mode)
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
Example C-Nav Plots (DUAL Mode)
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
Example C-Nav Plot (Real-Time GIPSY - DUAL Mode)
Mode: RTG - L1/L2
Period: 46hrs 52mins
95% ~ 0.3 meters
America’s RTG Performance
Europe Africa RTG Performance
Asia-Australia RTG Performance
At Sea Dynamic Tests
Example C-Monitor Plots
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
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.
For additional Information see:-
www.cctechnol.com/cnav