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Long Range Real Time Kinematic Positioning Service Genesis. John Hanley Senior GPS Analyst – Norwich, UK. Stennis Space Centre  26 th – 28 th August 2002. Outline of Presentation. Introduction. Overview - General Background & Applications. Basic Thales DGPS Infrastructure.

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long range real time kinematic positioning service genesis

Long Range Real Time Kinematic Positioning ServiceGenesis

John Hanley

Senior GPS Analyst – Norwich, UK

Stennis Space Centre

 26th – 28th August 2002

outline of presentation
Outline of Presentation
  • Introduction.
  • Overview - General Background & Applications.
  • Basic Thales DGPS Infrastructure.
  • Genesis Reference Station and Hub Processes.
  • Genesis User Processes and Data Flow.
  • Status and Operational & Trial examples.
    • Early Trials (Rig Moves and Ship & Ferry Trials).
    • Recent operations.
  • Quality Control of High accuracy systems.
introduction
Introduction

Thales GeoSolutions:

  • Supplier and a User [Commercial View]
  • Built reputation in Surveying and Positioning services offshore worldwide
  • Genesis System
    • Why develop Genesis?
    • How and Where?
    • What for?
  • The Future – Refinement and Development
  • New group with Thales Navigation [Ashtech GPS Technology]
code improved code or carrier phase
Code / Improved Code or Carrier Phase
  • Technical basis is to improve on DGPS by using the Carrier-Phase information
  • Carrier gives better repeatability and accuracy
  • Trade-Offs with improved code-based methods.
      • Initialisation
      • Data transmission
      • Filtering and QC
  • Combination of both approaches
lrtk genesis system
LRTK - Genesis System
  • Increased level of accuracy using latest GPS technologies
  • Regional high performance solution (North Sea Region)
  • For specific applications requiring better accuracies than standard DGPS
  • Targeted at Offshore applications – 20-30cm
  • QC information as per DGPS (Is more needed? Standards?)
  • Delivered via satellite
  • Long ranges and multiple stations
genesis applications
Genesis Applications
  • Genesis has been aimed specifically at the Offshore Market.
  • Niche market viewed at the 20-30cm level
  • Offshore Positioning
  • Vertical control
  • Emphasis on the Z- component
  • Vessel Passage with draught close to navigable depth
  • High Accuracy Navigation
  • AUV navigation
  • Real-Time vertical use – Multi Beam
  • High Accuracy 3D Control – Rig Moves
conventional rtk attributes
Conventional RTK Attributes
  • High accuracy systems (1-5 centimetres)
  • Essentially Carrier phase based
  • Operate over relatively short range
  • Single baseline approach
  • Maximum range varies from 10 to 40 km
  • Dependant on a reliable radio link
  • Require local geodetic point for installation
  • “Black Box” systems with little QC
  • Expensive 2-Receiver system
genesis lrtk attributes
Genesis LRTK Attributes
  • Satellite delivery based LRTK system
  • Accuracy of 20-30 cm
  • Operational over long baselines
  • PC based software with multi-station computations
  • PC based software allows for added system QC
  • Provides a highly accurate solution
  • Refinement and Development – No Limitations
thales gps infrastructure

RIMS A

19 inches rack

RIMS A

19 inches rack

RIMS A

Thales GPS Infrastructure

VSAT

Satellites

High Power and

Low Power

satellite links

GPS

Satellites

  • 2 MCC facilities
  • Aberdeen and Singapore
  • 2 remote MCC facilities at Perth and Reston
  • Monitor and Control
  • Archive
  • Maintain and Plan
  • Manage external entities
  • Interface

Genesis & MFX3

  • LES facilities
  • Uplink SkyFix messages
  • Including SkyFix Premier Messages

X25NETWORK

  • 80+ Reference Stations
  • Dual-Frequency Stations
  • Single Frequency
  • Generate, process and transmit messages
thales reference station configuration
Thales Reference Station Configuration

LRTK Genesis

North Sea Coverage Region

  • 7 stations in the North Sea
  • Dual Frequency Enabled
  • Choke Ring Antenna fitted
  • Pre-processing carried out
  • Data sent to MCS (Master Control Station) in Aberdeen.
  • Tromso & Hammerfest added
reference station pre processing
Reference Station Pre-Processing
  • Raw data taken from Geodetic GPS Receivers
  • Clock Corrections
  • Cycle Slip on L1 and L2
  • Code – Carrier Filtering
  • Multipath Mapping
  • SNR used to assess measurement Quality
  • Observations Compressed
  • Transmitted to MCS
  • Major Upgrade Process Underway [2001-2003]
mcs hub configuration current

Genesis Reference Station Input

Genesis Hub

UDb

Uplink Message

MCS Hub Configuration - Current
  • Data output from the pre-processing functions is compressed prior to transmission (reduce the bandwidth requirement)
  • Input Compressed Long Range Real-Time Kinematic messages
  • Interface to Unit Database (Udb) for User Control information
  • Uplink to the delivery satellite
  • Visual displays
  • Operator configurable settings
  • Simple error handling and printout facility
  • Bandwidth and Need for compression ( 8 to 1 reduction )
genesis user processes 1
Genesis User Processes (1)
  • Data Reconstruction
    • Proprietary Compressed data

received by Decoder

  • User Pre-Processing
    • User Dual Frequency GPS receiver board
    • Data pre-processed in similar manner to Reference Stations
  • Observation Combination/Differencing [@ User]
    • The key issue in the use of carrier-phase ranges. High emphasis on carrier data:
      • reduces sensitivity to geometry (DOP chimneys)
      • reduces sensitivity to code anomalies
    • Observation differencing (single or double) can be used to reduce the contribution of various error components.
genesis user data flow

Data RX

Genesis User Data Flow

Repair & Filter Observations

  • Combine
  • Synchronise
  • Transform

Position Calculation

Repair & Filter Observations

Weights

Compressed Reference Station Data Uplinked from MCS

Mobile Station

Almanac/Time/Ephemeris

Position Estimates & Quality Control

Phase/Code/Observables

genesis user processes 2
Genesis User Processes (2)
  • Network Approach
    • Network approach is very much at the centre of this system
    • More than one reference station provides additional observations and increases system availability and integrity
  • Position Determination and Quality Control
    • The position computation is built around the use of double differenced carrier-phase observations
    • Use of ionospheric delay free data addresses ionospheric error
    • Enhanced code and tropospheric weighting improves solution robustness
hardware installation architecture
Hardware Installation Architecture
  • Optional inputs:
  • 3rd party RTCM including Type 15’s
  • DeltaFix Corrections

Genesis decoder

Dual frequency DGPS data

GPS Receiver

Position

Outputs

Clients NAV system

Genesis & MultiFix PC

SkyFix decoder

SkyFix RTCM (Type 1, 2, 3, 16, 55)

- Combined Genesis and SkyFix Installation -

development test bed static trials 1999 2000
Development Test-BedStatic Trials 1999-2000
  • Initial Test Network- History
  • TCP/IP delivery
  • 3/4 Station Networks
  • 20-30cm accuracy - Planning
slide22

Operational Performance Examples

  • Selection of operational examples.
  • Rig-Moves and Survey Jobs/Trials.
  • Slow dynamic and High dynamic applications
  • Various baselines considered on different trials.
  • ‘Truth’ required for performance comparison. Initially assessed against DGPS.
  • Became clear that higher accuracy ‘truth’ was required to assess performance and QC elements
  • Algorithm improvement & ongoing trials from 2000-2002.
aberdeen rig move job slow dynamics 2001
Aberdeen Rig Move Job - Slow Dynamics - 2001

Number of SV’s

  • Rig-Move greatly affected by the convoluted structural environment
  • Baselines 150km East - West
  • High Repeatability can be clearly seen
  • Factors affecting positioning
    • Obstructions
    • Constellation - Geometry of SV’s
    • Constellation - Number of SV’s
    • Loss of signal = Loss of Double Difference sets

Number of DD’s

abz rig move job repeatability over dgps
ABZ Rig Move Job – Repeatability over DGPS

Multi Station DGPS

2 Station Genesis Solution

early dynamic ship trials 2001 repeatability

Multi Station DGPS

3 Station Genesis Solution

Early Dynamic Ship Trials - 2001 - Repeatability
  • Dynamic trials show repeatability
  • Accuracy harder to assess due to the problem of finding a suitable ‘truth’ system
  • Post-Processing of raw data to obtain ‘truth’ will be required
  • Increase in Repeatability over DGPS
  • Lower noise in LRTK solution
slide26

Norwegian Ferry Dynamic Trial - 2001

LRTK Genesis

vs.

Post-Processed RTK solution

  • Aberdeen 559 km
  • Bergen 55 km
  • Kristiansund 310 km
  • Brønnøysund 647 km
  • Sumburgh 344 km
norwegian ferry 3d position error
Norwegian Ferry – 3D Position Error

Delta East 0.06m (1- 68%)

Delta North 0.09m (1- 68%)

Delta Height 0.10m (1- 68%)

slide31

Snøhvit Field Dynamic Job/Trial - 2002

  • 3 Station network
    • Hammerfest
    • Tromso
    • Kristiansund
  • Coincident Projects
    • Aberdeen Pipe-lay Project
    • Aberdeen Rig Move
slide32

Snøhvit Field Dynamic Job/Trial - 2002

  • Objective was to establish whether LRTK Genesis could provide height accuracy (in this high Latitude North Sea location) to determine a tide value for vessel.
  • Vessel reference position computed using numerous sensors:
    • GPS antenna position (from Genesis)
    • Pitch, Roll and Heave (from Motion Unit)
    • Vessel draught sensor
  • Performance compared against short-range Thales Ashtech RTK and using Tide information logged at Hammerfest Tidal station.
  • Data currently being processed and evaluated.
slide33

Snøhvit Field – Height Accuracy

Lost RTK & Genesis Corrections

RTK position changed to Differential

slide34

Snøhvit Field Trial – Preliminary Findings

  • Preliminary results on previous slide show that LRTK Genesis performance was comparable to short range RTK system.
  • Further processing by Thales Norway required.
  • Reception of corrections in high latitude areas is an issue. This is the operational reality !
  • Re-initialisation is therefore inevitable. Must not be excessive and as seamless as possible.
  • Quality Control process MUST be able to supply useful information to the user.
lrtk systems requirements
LRTK Systems Requirements
  • Continuous and high quality L1 & L2 GPS data
  • Continuous reference station data / Corrections
  • > 5 satellites
  • good geometry = DOP

LRTK Systems Issues to Consider

  • Interruptions in local GPS data (masking / poor tracking)
  • Loss of Correction link
  • Latency of Reference station data
  • Number of satellites drop
  • DOP Holes
  • Convergence Time at Start-up (Initialisation) and on Re-Initialisation
ukooa guidelines
UKOOA Guidelines
  • Published by ‘United Kingdom Offshore Operators Association (Surveying and Positioning Committee)’.
  • Installation and Operation of DGPS Equipment
  • Quality Measures
  • Minimum Training Standards
  • GPS Receiver Outputs
  • Data Exchange Format

Are similar standards suitable for LRTK

ukooa quality measures
UKOOA Quality Measures
  • Designed to produce a universal set of quality measures for ‘DGPS’ positioning software
    • Unit Variance
    • Marginal Detectable Error (MDE)
    • Internal reliability
    • External Reliability
    • F-test on Unit Variance
    • W-test for Outliers
statistical measures
Statistical Measures
  • Unit Variance of the Position Fix Computation (Least Squares Weights)
  • F-Test on the Position Fix Unit Variance
  • W-Test on the Position Fix Residuals to detect Outliers

Still carried out on Least Squares Residuals, even within LRTK

lrtk genesis statistical measures
LRTK Genesis - Statistical Measures

F-Test

  • The F-Test is a test of the overall consistency of the observations (double differences) and the resulting position solution.
  • Upper Test looks at poorer than expected measurements. [a priori = optimistic]
  • Lower Test looks at better than expected measurements [a priori = pessimistic]

W-Test

  • The W-test is a statistical test applied to an individual observation (double difference). The aim is to identify a faulty measurement.
  • The test is conducted using the residuals from the least squares position calculation.
high accuracy positioning recommendations
High Accuracy Positioning - Recommendations
  • It is essential to assess the reliability and precision of each position fix to ensure the quality of GPS measurements
  • W-Test for outliers to be carried out for each fix
  • F-Test for on Unit Variance to be carried out for each fix.
  • When no more outliers are identified in any fix, precision and reliability measures are to be computed
  • Estimate of Precision – a posteriori error ellipse