Surface Positioning Systems

Subsea Survey & Positioning UK Ltd.

Presentation Overview

• Surface Positioning Systems

• The Constellation & Signals

• Computing Our Position

• Increasing Accuracy

• Positioning What?

Content

Surface Positioning Systems

Surface Positioning Systems

Historically?

•Sextant & Chronometer

•Network of Navigation Beacons

•Radio Based Navigation Systems

•Decca

•Argo

•Pulse-8

Surface Positioning Systems

Radio Navigation

• “Better” Than A Sextant & Chronometer

•More Robust Position

•More Accurate

•Easier To Calculate a Position

• Limitations Non-The-Less

•Decca Coverage – Max ~740km

•Interference – Multiple Sources

•Decca Accuracy – ~200m

•Upkeep Cost?

Surface Positioning Systems

What Now?

• Global Navigation Satellite System(s) – GNSS

• The ubiquitous standard positioning and

navigation system(s) for the offshore industry

• The accepted survey and positioning system in

the offshore energy sector (Oil, Gas,

Renewables)

Surface Positioning Systems

GNSS – The Main Players

• GNSS – The name given to all space based satellite

navigation system(s)

• GPS – United States of America – 31 SVs

Operational

• GLONASS – Russian Federation – 24 SVs

Operational

• GALILEO – European Union – 4 SVs “Operational”

(30 Intended)

• Beidou 2 – Peoples Republic of China – 10

Operational (75 Intended)

Surface Positioning Systems

GNSS – The Constellations

• Operational GNSS Constellations

Beidou (Compass)

GLONASS

Galileo

GPS

Surface Positioning Systems

GNSS – The Physical Realisation

USER-SEGMENT

SPACE-SEGMENT

CONTROL-SEGMENT

Surface Positioning Systems

GNSS – The Space Segment

• Constellation(s) of orbiting SVs

• Transmit to users who can only

passively receive data

• SVs signals are weak

• The user must have line-of-sight

• The signal is subject to

interference during its journey

through space and the earths

atmosphere

Surface Positioning Systems

GNSS – The GPS Control Segment

• Command & Control

• Master Control Station (MCS)

• Alternative MCS

• x4 Ground Antennas

• x6 Monitor Stations

• Monitors the system integrity and

accuracy

• Maintains SV orbits

Surface Positioning Systems

GNSS – The User Segment

• Hardware - Antenna

• Receives the GNSS signals

• Hardware – Receiver

• Decodes the GNSS signals and

determines a position

• Software

• Processes live or recorded and

stored GNSS data

The Constellation & Signals

The Constellation & Signals

GPS – The Ubiquitous System

• Baseline 24+3 satellite constellation in medium earth

orbit (MEO)

• Presently 31 operational satellites

• Global coverage, 24 hours a day, all weather

conditions

• Satellites broadcast precise time and orbit

information on L-band radio frequencies

• 6 orbital planes, 55° inclination

• 20,180 km above earth’s surface

• 11:58 Orbital Period

Computing Our Position

Computing Our Position

GNSS – How do we know where we are? • A position consists of 3 components (XYZ or Lat, Long, Height)

which will allows a user to position themselves on the Earth

• Pseudo-range ( ) is computed by the receiver

• Broadcast ephemeris contains the satellite coordinates (XS, YS, ZS) and the satellite clock offset (δtS) at the instant of satellite transmission

• Signal propagation is modelled ( )

Therefore to determine the position of a receiver, ranges to at least four satellites are required in order to solve for the four unknowns:

• Coordinates (XYZ) for user’s location and receiver clock offset

(δτR) at the instant of signal reception

SRATM∆

SRPR

Computing Our Position

GNSS – How do we know where we are?

• Calculating the Pseudo-range – Not a direct measurement of distance……

Computing Our Position

GNSS Errors – The Prime Suspects

• Sources of pseudo-range error • SV Orbit (O) • SV Clock (C) • Signal delay due ionosphere (I) • Signal delay due troposphere (T) • Signal reflections at user receiver

(M) • Errors in user equipment (δτR)

I

T

O

C

M

True SV Pos Calc SV Pos

we need to remove this

Increasing Accuracy

Increasing Accuracy

Why do we need to and how?

• GNSS was primarily designed for navigation and timing

• Survey and Offshore communities require a higher level of accuracy

• Increased dynamic accuracy can be achieved through relative positioning using Differential GNSS or Precise Point Positioning

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

Increasing Accuracy

What is DGNSS?

• Requires a number of precisely located reference stations where the

measurement error to each satellite is calculated by comparing known and

measured range

• Errors remain similar for other GNSS users within several hundred kilometres of

reference station

• Error information is generally delivered to the user via satellite

• 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 – 1m within 1000Km and <3m within 2000Km of a station

Increasing Accuracy

What is DGNSS?

R1

R2R3

R1 Ref

R2 Ref R3 Ref

ReferenceStation

GPSReceiver

CorrectionProcessor

R1 R2 R3

Increasing Accuracy

What is PPP? • Precise Point Positioning

• Apply calculated SV clock error correction to broadcast ephemeris value

• Apply satellite orbit corrections to broadcast orbit position

• Ionospheric error is calculated using dual-frequency mobile GPS hardware

• Tropospheric 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 Y

erroneous SV Position

Dx Z

Increasing Accuracy

What is PPP?

• Available Globally via x7 Broadcast SVs

• Accuracy is not reference station dependant

• Accuracy is 0.10m Horiz. & 0.20mVert.

• Must wait for system to settle to achieve maximum accuracy - >30mins

• Dual frequency antennas mitigate error

Increasing Accuracy

The elephant in the room….RTK GNSS

• Reference Station RTK

• Network RTK

• Accuracy is dependant on baseline length ~30km max…

• …or the network. The user must either be inside the network or within ~30km of the closest reference station in the network

• Accuracy is 0.010m Horiz. & 0.020mVert.

Increasing Accuracy

Long Range RTK GNSS

• Long Range RTK (Trimble RTX CentrePoint)

• Available in most of the world

• Not baseline or network dependent

• RTX is 0.040m Horiz. & 0.070mVert anywhere…On-shore ONLY!!!

Positioning What?

Positioning What?

Uses of Surface Positioning – DGNSS

• Dynamic Positioning

• ROV & Diving Support

• Precise Alongside Positioning

Positioning What?

Uses of Surface Positioning – DGNSS & PPP • Construction Support

• ROV & Diving Support

• Cable & Pipeline Laying

• Template Installation

• Jacket & Topside Positioning

• Windfarm Installation Support

• Rig Moves

• Precise Alongside Positioning

Positioning What?

• Survey

• Seabed Mapping & Charting

• Cable/Pipeline Route Surveys

• Debris Surveys

• Seismic Surveys

Uses of Surface Positioning – DGNSS, PPP & RTK

Positioning What?

• Survey

• Seabed Mapping & Charting

• Mobile Laser Scanning

Uses of Surface Positioning – RTK

Questions ?

Simon Canning MSc MRICS AMRI Dimensional Control Manager Mob; +44 (0) 7753 310716 Email; simon.canning@dofsubsea.com