Presentation on theme: "Long Range Real Time Kinematic Positioning Service Genesis Stennis Space Centre 26 th – 28 th August 2002 John Hanley Senior GPS Analyst – Norwich, UK."— Presentation transcript:
Long Range Real Time Kinematic Positioning Service Genesis Stennis Space Centre 26 th – 28 th August 2002 John Hanley Senior GPS Analyst – Norwich, UK
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 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 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 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 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 Genesis has been aimed specifically at the Offshore Market. Niche market viewed at the 20-30cm level
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 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 GPS Satellites VSAT Satellites 80+ Reference Stations Dual-Frequency Stations Single Frequency Generate, process and transmit messages 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 LES facilities Uplink SkyFix messages Including SkyFix Premier Messages 19 inches rack RIMS A X25 NETWORK 19 inches rack RIMS A High Power and Low Power satellite links Genesis & MFX3
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 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 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 ) Uplink Message Genesis Reference Station Input Genesis Hub UDb
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 Position Estimates & Quality Control Position Calculation Repair & Filter Observations Combine Synchronise Transform Weights Mobile Station Almanac/Time/Ephemeris Phase/Code/Observables Compressed Reference Station Data Uplinked from MCS
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
- Combined Genesis and SkyFix Installation - Clients NAV system Position Outputs Dual frequency DGPS data Optional inputs: 3 rd party RTCM including Type 15’s DeltaFix Corrections SkyFix RTCM (Type 1, 2, 3, 16, 55) Hardware Installation Architecture SkyFix decoder Genesis decoder GPS Receiver Genesis & MultiFix PC
Development Test-Bed Static Trials 1999-2000 Initial Test Network- History TCP/IP delivery 3/4 Station Networks 20-30cm accuracy - Planning
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 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 SV’s Number of DD’s
ABZ Rig Move Job – Repeatability over DGPS Multi Station DGPS2 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 Multi Station DGPS3 Station Genesis Solution
Aberdeen559 km Bergen 55 km Kristiansund310 km Brønnøysund647 km Sumburgh344 km Norwegian Ferry Dynamic Trial - 2001 LRTK Genesis vs. Post-Processed RTK solution
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. Snøhvit Field Dynamic Job/Trial - 2002
RTK position changed to Differential Lost RTK & Genesis Corrections Snøhvit Field – Height Accuracy
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. Snøhvit Field Trial – Preliminary Findings
LRTK Systems Importance of QC – Issues and 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 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 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 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 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 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