surface positioning systems - ths...r2 ref r3 ref reference station gps receiver correction...

Surface Positioning SystemsHydrofest 2008

Richard Long BSC

GNSS Technical Support SpecialistVERIPOS, Subsea 7

Hydrofest – 03 April 2008

Overview

Surface Positioning Systems

Satellite Navigation Theory

Differential GNSS

Applications of GNSS in the Offshore Oil & Gas

Industry

Future Developments in GNSS

– GPS Modernisation

– GLONASS Modernisation

– Galileo


Surface Positioning for the Offshore Industry

Historically used terrestrial based radio navigation systems– Syledis– Microfix– HyperFix– Argo– Pulse-8– Decca

….All with limitations, i.e. coverage, electrical interference and limited accuracy.

Nowadays, satellite based radio navigation used

GNSS now the accepted surface positioning system for surveying in the offshore oil and gas industry

– Offshore industry was an early adopter of satellite navigation– The offshore industry is a stakeholder in all GNSS systems


GPS History

1967 – USAF Project 621B instigated

1973 – NAVSTAR development begins

1978 – First GPS satellite launched 22nd February

1993 – GPS declared IOC for civil use 8th December

1995 – GPS declared FOC on 27th April


Components of GPS


Space Segment – GPS Signals


Ground Segment

Responsible for overall satellite command and controlMaintaining exact orbits of each satelliteMonitoring the integrity and accuracy of the system


User Segment


Positioning Using GPS


GLONASS

Global’naya Navigatsionnaya Sputnikova Sistema(GLObal NAvigation Satellite System)

Similar to GPS

First launch in 1982

Constellation reached full design specification of 24 SVs in January 1995

Current constellation 16 SVs

Last launch 25th of December 2007

GLONASS-K Satellite


Characteristics of GPS and GLONASS

GPS GLONASS SATELLITES Number of satellites 21 + 3 spare 21 + 3 spare Number of orbital planes 6 3 Orbital plane inclination (degrees) 55 64.8 Orbital radius (km) 26,560 25.510 Orbital period 11h 58m 11h 15m

SIGNALS Fundamental clock frequency 10.23 MHz 5.0 MHz Signal separation technique CDMA FDMA Carrier frequencies (MHz) L1 1575.42 1602.0 - 1615.5 L2 1227.60 1246.0 – 1256.5 Code clock rate (MHz) C/A 1.023 0.511 P 10.23 5.11 Code length (chips) C/A 1023 511 P 6.187104*1012 5.11*106

C/A-CODE NAVIGATION MESSAGE Duration (minutes) 12.5 2.5 Capacity (bits) 37,500 7,500 Word duration (seconds) 0.6 2.0 Word Capacity (bits) 30 100 Number of words within a frame 50 15 Techniques for specifying satellite ephemeris Keplerian orbital Geocentric Cartesian Time reference UTC (USNO) UTC (SU) Position reference (geodetic datum) WGS84 PZ-90


GNSS Positioning

GPS & GLONASS were primarily designed for navigation and timing

Survey and Offshore community requires a higher level of accuracy

Increased accuracy can be achieved through relative positioning using DGPS (or DGLONASS) or Precise Point Positioning

Relative positioning allows for the correction or reduction of GNSS error sources that contaminate a stand-alone GNSS positioning


What Are The Errors in GPS

What are we trying to measure

Errors in satellite navigation can be divided in to 2 broad categories:

– Temporal – those that change with time

– Spatial – those that change with location

Furthermore errors can relate to:– The satellites

– The radio signal in space

– The receiver on the ground

Main Errors

Satellite orbit error (O)

Satellite clocks (C)

Atmospheric effects

Signal delay due ionosphere (I)

Signal delay due troposphere (T)

Signal reflections at user receiver (M)

Errors in user equipment

IonosphereTroposphere

I

T

O

C

M

Calc SV Pos True SV Pos

True distancesig

nal path

Measu

red di

stanc

e

we need to remove this


DGPS

Requires a number of precisely located GPS reference stations where the measurement error to each satellite is calculated by comparing known and measured range

Errors remain similar for other GPS users within several hundred kilometres of reference station

Error information is delivered to the user via satellite or terrestrial radio broadcast

Robustness is improved by using data from multiple reference stations and multiple broadcasts

As the distance between the user and reference station increases the accuracy decreases, nominal accuracy is 1m within 1000Km and <3m within 2000Km of a station

Using high-precision observations accuracy of 10-20cm can be achieved


Principals of Differential GPS

R1

R2R3

R1 Ref

R2 Ref R3 Ref

ReferenceStation

GPSReceiver

CorrectionProcessor

R1 R2 R3


Principles of Precise Point Positioning

Apply calculated SV clock error correction to broadcast ephemeris value

Apply satellite orbit corrections to broadcast orbit position

Iono error is calculated using dual-frequency mobile GPS hardware

Tropo delays minimised using model plus residual error is estimated as part of the calculation process

Measurement noise and multipath minimised using carrier phase observable

Dz

XTrue SV Position

erroneous SV Position

Y

Dy

DxZ

SV Clock

iono del

ay

tropo

delay

sig reflections


Veripos Services

VERIPOS Standard L1 DGPS 1-2m accuracy

VERIPOS Standard+ L1/L2 DGPS 1-2m accuracy

VERIPOS HF L1 DGPS 1-3m accuracy (regional coverage)

VERIPOS DGLONASS L1 DGLONASS 1-3m accuracy (regional coverage)

VERIPOS ULTRA PPP 10 -20cm


VERIPOS - Standard

Differential GPS (single difference process)

Based upon CA Code and L1 phase measurements

Global network of reference stations

Accuracy based on distance from ref station, location, atmospheric conditions

Use of multiple stations improves accuracy and reliability

Typical single station accuracy is 1-2m within 1000km (<3m within 2000km)

Simple, robust and highly resilient solution


VERIPOS - Standard+

Same as VERIPOS-Standard but using dual-frequency GPS

Based upon CA / P Code and L1 /L2 phase measurements

Coverage in areas of heightened ionospheric activity

Ionospheric delays for each SV at each station transmitted to permit compensation

Typical single station accuracy is 1-2m within 2000km

Simple, robust and highly resilient solution


VERIPOS - Standard HF

available in NWECS, Mexico and Brazil

complements satellite delivery especially where:

– atmospheric conditions can affect satellite broadcast

– obstructions mask visibility to delivery satellites

– satellite low on horizon in high latitudes

system is based upon L1 DGPS

data transmitted on 2 independent frequencies

operational range is 700km over sea-path

normal system accuracy is 1-2m (2DRMS/95%)

VERIPOS HF station


VERIPOS Ultra

Based on the PPP technique providing seamless global coverage with no user reliance on reference stations

The above plots represent the horizontal and vertical position errors in Veripos Standard and Ultra solutions at a monitor site in Singapore. The reference station used

in the Standard solution was located 322Km distant at Kemaman.


VERIPOS System Architecture

Reference Station

Commsnetworks

Network Control Centre

Generate data

products

Uplink to comms

satellites

Raw measurement data – via multiple redundant communications links

broadcast to users

Monitor Reports

Service Monitoring

www.veripos.com

online user supportc.90 stations to

cover globe


Veripos Coverage Map

• 75 stations worldwide• Fully redundant state-of-the art dual-frequency GNSS

receivers• Fully redundant communications networks• 7 x delivery satellites (4 x high-power + 3 x low-power)

• Fully monitored

• 24- hour / 365 day global support


Reference Station

Reference station equipment

– 2 x dual frequency GPS receivers

– 2 x Communications servers

Uninterruptible Power Supply (UPS)

All reference stations remotely controlled from network control centre in Aberdeen

Raw GPS data and/or RTCM data returned to the control centre

Annual maintenance visits to each station

GPS rcx

Comms Server

DSL Router

UPS


Geodetic Coordination of Reference Stations

Essential to the operation of a global DGPS network is the requirement for a geodetic reference frame that will provide seamless positioning to a high level of accuracy and repeatability on a global scale

Veripos uses International Terrestrial Reference Frame (ITRF) 2000 geodetic reference frame

All stations are coordinated using dual frequency GPS data and precise orbit information processed using the JPL/NASA developed software package GIPSY-OASIS II

Accuracy is typically at the cm level

Multipath audits also conducted for each site


Network Control Centre (NCC)

Centralised control of all reference stations

Processing and dissemination of data

Monitoring the integrity of the communications network

GNSS integrity monitoring

Provision of real-time performance monitoring and QC information

Secure activation and deactivation of users

24-hour manned facility

Customer support


Service Monitoring

Used for QC and client support

Using raw data and RTCM corrections from reference stations

Every station in the Veripos network is monitored 24/7


User Hardware

Demodulator– used for receiving broadcast augmentation data– supplied as turnkey unit or OEM cards

GNSS receiver– for establishing range measurements from GPS, GLONASS and SBAS and other

satellite ranging systems– RTCM compatibility – user choice of receiver (Turnkey or OEM)


Operational Implementation of Satellite Navigation

In offshore operations there are 2 x types of requirement

Navigation and positioning for survey applications including:– Seismic survey, Hydrographic survey– Construction and pipe-lay support– Positioning of vessels and structures– Requires sophisticated quality control processes and software,

high levels of accuracy and redundancy to ensure high-quality data

Navigation and positioning for vessel station-keeping– Dynamic positioning / mooring monitoring– Requires simple to operate, robust stable positioning– Stability of position - more important than accuracy– Other reference systems such as acoustics, taut-wire –mean less

dependence - unless in deep water– Critical to vessel operation


Applications

Precision underwater metrology

Seabed mapping, route and site surveys

Offshore construction

Pipeline and structures inspection


Responsibility of the Surveyor

Positioning is an essential part of all operations within the offshore oil and gas industryWithout it surveying would not be possibleAlso essential for safe navigation

Offshore Surveyor– works as part of a team– responsible for the running of surveys– responsible for the real time collection of survey data– ensuring the quality of the data is correct plus all quality control checks and calibrations

are performed and documented– responsible for ensuring that all positioning systems are operating


Vessel Fleet Management

A growing opportunity resulting from use of low cost GPS receivers and telemetry systemsUtilising GPS, GSM and terrestrial radio technologiesCombining precise positioning, communications, data collection and displayMultiple applications e.g. asset management, safety systems, security systems, logistics and land seismic

Benefits– Monitoring of all assets– Increased operations efficiency– Increased safety & security


Future Trends

DGNSS users will see significant changes in the satellite navigation over the next 10 to 20 years

– Modernisation of GPS– Modernisation of GLONASS

– Introduction of Galileo (European Satellite service)– Introduction of CNSS (Chinese Navigation Satellite System)

This will include the availability of more satellites and also more navigation signals which will provide greater positional accuracy and reliability

GNSS will have more redundancy because of the multiple constellations and no reliance on one nation

The actual combination of signals to be used will be determined by the application and will be a trade-off between cost, accuracy and receiver design

The modernised constellations of GPS and GLONASS coupled with Galileo and CNSS clearly show that satellite navigation will continue to be the system of choice for navigation and surveying in the offshore industry


The Future for the DGNSS Service Provider

The impact of the modernised and new satellite constellations means significant changes to the service provider network

Additional signals will impact:– data collection – more signals means more information to be collected– data transfer from network – increased communication bandwidth required– data processing – more information to process– message generation – more corrections / service information to generate– transfer of service to user – more information means more bandwidth

Interoperability between GPS / GLONASS / Galileo / CNSS– geodetic reference frame– time systems– signal structure

C/AOS/GPS III

L1L1(1575.42 MHz)

E6(1278.75 MHz)

L2(1227.6 MHz)

L5/E5AL5/E5A(1164-1214

M P(Y)PRS

L2C

M P(Y)

L2C

M P(Y)PRS

CS

E5bMHz)

L5 E5a E5b


Questions?

Thank You

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