Latest Developments in High Accuracy DGPS

Dr David RussellSenior GNSS Technical Specialist

VERIPOS, Subsea 7

Overview

Introduction

Satellite Positioning and the Offshore Industry

Methods of Improving Accuracy

– Relative GNSS

– PPP

Advantages/Disadvantages of the Two Methods

Veripos Systems

The Future

Satellite Positioning – a mature utility…still developing?

Offshore industry was an early adopter of satellite navigation technology

GNSS technology has made positioning and navigation more accessible to the wider community plus provided global coverage

GNSS is fundamental to all positioning and navigation offshore and is considered a mature technology

The offshore industry is a key stakeholder in all GNSS systems

The correct terminology?

– DGPS term is no longer representative

different satellite navigation systems (GLONASS, Galileo)

different techniques (e.g. PPP)

– We will refer to GNSS, DGNSS

DGNSS In The Offshore Oil and Gas Industry

Navigation and positioning for survey applications– Seismic survey, hydrographic survey, construction and

pipe-lay support– Positioning of vessels and structures– Requires high levels of accuracy and redundancy to

ensure high-quality data

Navigation and positioning for vessel station-keeping– Dynamic positioning / mooring monitoring– Stability of position - more important than accuracy– Other reference systems such as acoustics, taut-wire –

mean less dependence on DGNSS except in deep water where there is likely to be greater dependence on DGNSS

– Critical to vessel operation

In offshore operations there are 2 x types of requirement

Set the scene - what are the errors in GNSS?

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

True SV PosCalc SV Pos

True distancesignal p

athMea

sured

distan

ce

we need to remove this

Methods of Improving GNSS Accuracy

There are two key yet very different methods in use today for improving GNSS

accuracy:

– Relative GNSS (a.k.a. Differential, RTK)

– Precise Point Positioning (a.k.a. PPP, State-Space, Globally Corrected GPS, SDGPS)

Each method also provides integrity information on the GNSS constellation:

– Independent check on each GNSS satellite signal

– Determine whether data is good or bad

Principals of Relative GNSS (DGPS)

Requires a number of precisely located GNSS 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 delivered to the user via satellite or terrestrial radio broadcast

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

Nominal accuracy is 1-2m within 1000Km and <3m within 2000Km of a station

As the distance between the user and reference station increases, accuracy decreases:– De-correlation of orbit error– De-correlation of ionosphere delays

DGNSS

R1

R2R3

R1 Ref

R2 Ref R3 Ref

ReferenceStation

GPSReceiver

CorrectionProcessor

R1 R2 R3

High Accuracy DGNSS

Accuracy improvements to the standard DGNSS solutions have been made through the following:

– More precise correction information using measurements approaching carrier phase accuracy

– Improved troposphere modelling– Removal of Ionosphere effects– Inclusion of satellite orbit corrections leading to reduced spatial de-correlation

Other techniques include extending the range of the real time kinematic systems:– Double differencing– Centimetre accuracy – Raw data transmissions lead to high-bandwidth requirements and prevent operation over

satellite links

Examples include Thales Genesis, Fugro HP, VERIPOS-Extra

Principals of PPP

Data from a network of GNSS stations to calculate satellite-based errors

– A single set of correction information is calculated for each satellite

Correction to broadcast satellite orbitCorrection to satellite clock

– The information is valid for each satellite wherever the user is – therefore no dependence on proximity to reference stations

Ionosphere errors at user location are removed

– Dual-frequency GPS receiver at the user location

Troposphere errors are removed – High-accuracy model– Mathematical estimation

Range Measurements – precision / accuracy improved by use of carrier phase observations (L1/L2)– Reduction in multi-path

Nominal accuracy of this approach is 10-20cm (horizontal) 15-30cm (vertical)

PPP - Overview

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

PPP Mathematical Approaches

Kalman-Filter Approach

– use iono-free observable

– filter over time to resolve carrier phase integer ambiguities

– filter also used to resolve other residual errors (e.g. Tropo)

– model to account for user dynamics

Non Kalman-Filter Approach

– uses iono-free observable

– epoch-epoch calculation (no dynamic model required)

– carrier phase integer ambiguities are estimated

PPP Convergence

With a Kalman-Filter approach convergence of the PPP solution is variable

Convergence linked to change in relative satellite geometry

– more observations and measurements help with the estimation of parameters

Certain values that are being estimated vary slowly over time (e.g. troposphere)

Convergence time can be reduced by using

– known start position

– more accurate a-priori information

– More satellites/observations

Advantages & Disadvantages of Each Method

DGNSS PPP

Advantages- simple and robust solution

- redundancy with many independent stations

Advantages- user has no reliance on reference stations

- corrections are valid globally

Disadvantages- reliance on reference stations

- error de-correlation with distance leading to degradation in accuracy

Disadvantages- complex single process (glitch can have global effect)

- geodetics and the relationship to DGNSS

Veripos Services

VERIPOS Standard L1 DGPS 1-2m accuracy

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

VERIPOS Ultra PPP 20cm accuracy

VERIPOS Extra HA-DGNSS 20cm accuracy (launch 2006)

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

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

Veripos Standard+

Same as VERIPOS-Standard (DGNSS) but dual-frequency GNSS

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

Standard DGNSS v Standard+

veripos-standard+veripos-standard

Typical single station accuracy (2.16m) – baseline 800km (Brazil) Typical single station accuracy (0.74m) – baseline 800km (Brazil)

• Standard graph shows degradation of dgps solution due to none-cancellation of ionosphere

• Standard+ plot shows the reinstatement of iono-cancellation achieved by iono measurement

Veripos Ultra

Singapore

Hz error 0.13m (95%) – including convergence

Vt error 0.32m (95%) – including convergence

Comparison of Veripos Ultra & Standard

Future Developments & the Impact on Positioning

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

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 system

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

Modernised constellations and Galileo clearly show that satellite navigation will continue to the system of choice for navigation and surveying in the offshore industry

What the Future Holds for the User

Combined GPS / GLONASS / Galileo constellations will provide up to 80 satellites

Availability of new signals

Greater position accuracy – attainable accuracy is finite (~5cm)

Faster, more robust initialization

Greater integrity and availability providing more robust positioning

Different signal combinations to suit different market requirements

Development of new positioning techniques?

Questions?