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