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; [email protected]