long range real time kinematic positioning service genesis stennis space centre 26 th – 28 th...
TRANSCRIPT
Long Range Real Time Kinematic Positioning Service
Genesis
Stennis Space Centre
26th – 28th 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.
IntroductionThales 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]
Overview of Thales LRTK Genesis
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
GPSSatellites
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
RIMS A
X25NETWORK
19 inches rack
RIMS A
High Power and
Low Power satellite links
Genesis & MFX3Genesis & 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
Repair & Filter Observations
CombineSynchroniseTransform
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:
•3rd 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-BedStatic 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 DGPS 2 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 DGPS 3 Station Genesis Solution
• Aberdeen 559 km
• Bergen 55 km
• Kristiansund 310 km
• Brønnøysund 647 km
• Sumburgh 344 km
Norwegian Ferry Dynamic Trial - 2001
LRTK Genesis
vs.
Post-Processed RTK solution
Norwegian Ferry Horizontal Performance
Norwegian Ferry – 3D Position Error
Delta East 0.06m (1- 68%)
Delta Height 0.10m (1- 68%)
Delta North 0.09m (1- 68%)
Norwegian Ferry – 3D Position Error
Norwegian Ferry – Height Comparison
Snøhvit Field Dynamic Job/Trial - 2002
3 Station networkHammerfest Tromso Kristiansund
Coincident Projects Aberdeen Pipe-lay Project Aberdeen Rig Move
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
Raw Antenna Height (JD175)
47.0
48.0
49.0
50.0
51.0
52.0
53.0
124005 134005 144005 154005 164005
GPS Time
Genesis (Hamm; Trom; Kris)
RTK
Genesis (Hamm; Trom)
-2
-1
0
1
2
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 SystemsImportance 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