Information Technology Solutions Information Technology Solutions
Eldar Rubinov Accra, Ghana 23-01-2015
Ghana National GNSS Workshop
CORS, NRTK and Monitoring
Experiences from Australia
Information Technology Solutions Information Technology Solutions Outline of Presentation
• Introduction
• Part 1 – CORS Networks
• Part 2 – Network RTK
• Part 3 - Monitoring
• Conclusion / Questions
Information Technology Solutions Information Technology Solutions GNSS Positioning
• GNSS positioning works by measuring ranges to a number of
satellites to compute receiver position
• Number of global systems available in various stages of
development:
- GPS (USA) fully deployed
- GLONASS (Russia) fully deployed
- Galileo (EU) in progress
- Beidou (China) in progress
Information Technology Solutions Information Technology Solutions Regional Systems
• Number of Regional Navigational Satellite Systems (RNSS) are
also being developed
• QZSS – Japan (3 satellites)
IRNSS – India (7 satellites)
QZSS IRNSS
Information Technology Solutions Information Technology Solutions Introduction
• Various modes of positioning available – absolute vs relative,
static vs kinematic, real-time vs post-processed
• Mode used depends on the application and the accuracy
requirements
• Real-time techniques include – DGNSS (m), PPP (dm), RTK (cm)
• NRTK is a new form of positioning NRTK based on
Continuously Operating Reference Stations (CORS) networks
Information Technology Solutions Information Technology Solutions CORS Networks
• CORS networks have been established all over the world to
support the mission of modern geodesy:
- Scientific research, geodynamics
- Reference frame establishment
- Atmospheric modelling
- Network RTK
• Networks range from global to regional
and local
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Global
• IGS Network - ITRF establishment and maintenance
Geodynamic applications
Precise orbit and clock products
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Global
• Omnistar - Offshore survey, mining, agriculture,
mapping, GIS, aviation etc.
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Regional
• AFREF - African Reference Frame
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Regional
• AUSCOPE - Asia-Pacific Reference Frame
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Local
• Country-wide networks in Africa
TrigNet
South Africa
Repangol
Angola
NigNet
Nigeria
Information Technology Solutions Information Technology Solutions Examples of CORS Networks - Local
• State-wide networks in Australia
Victoria New South Wales Western Australia
Information Technology Solutions Information Technology Solutions CORS Infrastructure
CORS Components
• Geodetic GNSS Receiver
• Geodetic Antenna
• Antenna Monument
• Uninterrupted Power Supply
• Modem
• Surge Protection
• Solar (optional)
Information Technology Solutions Information Technology Solutions CORS Site Selection
• Clear sky view free from obstructions
• Security
• No nearby signal reflectors (multipath)
• No nearby signal transmitters (interference)
• Stability – soil expansion, building movement etc.
• Power supply
• Internet connection
Information Technology Solutions Information Technology Solutions CORS Hierarchy
CORS installations can be classified according to a tiered system:
• Tier 1 – IGS sites or equivalent, support geoscientific research
and ITRF realisation
• Tier 2 – national mapping agencies, support regional and national
geodetic datums
• Tier 3 – state governments networks, commercial services, NRTK
Rizos (2010)
Information Technology Solutions Information Technology Solutions CORS Guidelines
A number of publications for CORS best practice guidelines are
publicly available:
• NGS NOAA – Guidelines for New and Existing Continuously
Operating Reference Station (2013)
• ICSM – Guideline for Continuously Operating Reference Stations,
Special Publication 1 (2013)
• LPI – Guidelines for CORSnet-NSW Continuously Operating
Reference Stations (2011)
Information Technology Solutions Information Technology Solutions CORS Commercialisation
Why do we need CORS networks in the first place?
• Cost savings and productivity gains for users. No need to purchase
two receivers and setup base stations
• Support surveying, construction, mining, agriculture etc.
• Have a positive impact on the economy
• More homogeneous datum realisation
Information Technology Solutions Information Technology Solutions CORS Commercialisation
Number of models available:
• Model 1: Institutional CORS infrastructure, no commercial services
• Model 2: Government CORS infrastructure, operates commercial
services
• Model 3: Government CORS infrastructure, licenses data to private
sector
• Model 4: Cooperative privately-owned CORS infrastructure,
commercial services
• Model 5: Privately-owned CORS infrastructure, commercial services
Rizos (2007)
Information Technology Solutions Information Technology Solutions Examples from Australia
• Nationwide institutional CORS infrastructure – Auscope (Model 1)
• All states and territories (with exception of Tasmania) have a
government owned CORS infrastructure
• Most states provide commercial NRTK services (Model 2)
• Most states also licence data to private sector (Model 3), eg.
SmartNet-AUS, AllDayRTK
• In some states private companies also set up their own infrastructure
to provide commercial services, eg. SST GPS (Model 5)
Information Technology Solutions Information Technology Solutions Examples from Australia – SmartNet-AUS
Information Technology Solutions Information Technology Solutions Examples from Australia – AllDayRTK
Information Technology Solutions Information Technology Solutions CORS Coordination
• With a large number of CORS being setup by different agencies
coordination becomes important
• Without proper coordination, duplication of resourses can happen
• This is also true in Africa, MTWA & MTVE in Tanzania are 3km apart
(Wonnacott, 2013)
Information Technology Solutions Information Technology Solutions CORS Coordination
• To address a problem of coordination ThinkSpatial has created an
interactive national CORS webmap (Hausler, 2014)
• Webmap shows all the CORS in Australia along with all relevant
metadata – agency, type, coordinates etc.
• Promotes co-operation, not duplication
• Similar solution can be implemented in Africa
Information Technology Solutions Information Technology Solutions CORS Coordination
http://thinkspatial.com.au/maps/natgnss
Information Technology Solutions Information Technology Solutions Regulation
• Due to a high number of CORS sites and providers, the government
introduced accreditations and standards for CORS
• Regulation 13 – is a certificate given to to each CORS to verify that
it is up to the national standard
• Valid for 5 years
• Assists users in establishing legal traceability of GNSS positions
Information Technology Solutions Information Technology Solutions Regulation
• Example Reg 13 certificate
Information Technology Solutions Information Technology Solutions Network RTK Technology - Background
• Traditional RTK
- Requires own base station
- Data transfer via UHF radio
- Single base solution
- Range < 20km
- Positioning quality decreases with distance
Information Technology Solutions Information Technology Solutions Network RTK Technology - Background
• Network RTK
- Based on CORS network, no need to own a base station
- Data transfer via mobile internet (Ntrip)
- Networked solution
- Increased range up to 70km between CORS sites
- Consistent positioning quality, as long as user is inside the network
Information Technology Solutions Information Technology Solutions Network RTK Technology – Data Transfer
• In NRTK data transfer is done via the Networked Transfer of RTCM
via Internet Pprotocol (NTRIP)
• Consists of 3 parts – Ntrip Server, Ntrip Client, Ntrip Caster
• Ntrip server – sends data to Caster (e.g. CORS site)
• Ntrip client – receives data from Caster (e.g. NRTK rover)
• Ntrip caster – HTTP server/splitter, integrates data between Ntrip
servers and Ntrip clients
Information Technology Solutions Information Technology Solutions Network RTK Technology – Data Transfer
Information Technology Solutions Information Technology Solutions Network RTK Technology - Applications
• Network RTK is used to support the following industries
- Land surveying
- Engineering Surveying
- Precision Agriculture
- Construction
- Mining
- GIS and mapping
Information Technology Solutions Information Technology Solutions Network RTK Concept
• Network RTK workflow:
1) Computation of network biases at the references sites
2) Bias interpolation throughout the network
3) Dissemination of interpolated corrections to user
• Two widely used methods – Virtual Reference Station and Master-
Axilliary Concept
Information Technology Solutions Information Technology Solutions Network RTK Concept - VRS
• Ionospheric and geometric models computed over the network area
from CORS data
• These models used to compute the biases at approximate user
position
• Simulated (virtual) reference station is created at user’s location and
used to send corrections to rover
• Rover is ‘fooled’ into thinking its receiving corrections from a real
reference station
• Standard RTCM messages used
Information Technology Solutions Information Technology Solutions Network RTK Concept - MAC
• Ambiguities from all CORS sites resolved at network level
• A subset of stations is chosen to generate corrections for the rover
• To save bandwidth full observations are sent from only one station
(master)
• Between-station single differences are sent from remaining stations
(auxiliary)
• The rover uses information provided to determine network biases,
resolve ambiguities and compute position
• New set of RTCM messages was implemented in RTCM v3.1
Information Technology Solutions Information Technology Solutions Network RTK Concept – VRS vs MAC
VRS
• Network biases computed at
NCC
• Corrections from virtual
station
• Standard RTCM messages
used
MAC
• Network biases computed at
rover
• Corrections from real stations
• Special NRTK RTCM
messages used
• Both techniques empirically shown to provide ±2cm in horizontal at
70km CORS spacing (Edwards, 2008)
• Vertical is not as consistent with larger differences observed
Information Technology Solutions Information Technology Solutions Network RTK Heighting
• Empirical studies showed that NRTK heighting can reach errors of
up to 5-10cm in a typical 70km inter-station CORS network
• To minimise the chances of erroneous position, checks on known
points should be carried out throughout the day
• Raw data logging should also be enabled on the rover receiver
• If raw data is present, problems can be fixed by post-processing
analysis
• If no raw data available, in case of problems the survey will need to
be repeated
Information Technology Solutions Information Technology Solutions Network RTK Heighting Accuracy
• Some countries chose to densify their CORS networks to achieve
more reliable results in height
• Sweden has 300 CORS at average spacing of 35km!
• Japan has 1200 CORS at average spacing of 20km!
Swepos Network GeoNet Network
Information Technology Solutions Information Technology Solutions Network RTK Guidelines
A number of NRTK best-practice guidelines are publicly available:
• Emardson et al. – Measurement accuracy in Network-RTK (2009)
• Edwards et al. – An examination of commercial network RTK GPS
services in Great Britain (2008)
• ICSM – Guideline for Control Surveys by GNSS, Special Publication 1
(2014)
Information Technology Solutions Information Technology Solutions Monitoring
• Monitoring is an important part of running a CORS network
• Network operators need to be aware of what is happening with
their CORS installations
• If a CORS site is not stable, it could have an effect on network
solution
• Number of things need to be monitored
- Coordinates – daily, weekly and monthly solutions
- Multipath – can tell about the changes happening around the site
- Signal-to-Noise Ratio (SNR) – can tell if there is a problem with a
cable (water getting through etc.)
- Other quality indicators – data availability, latency, ionosphere etc.
Information Technology Solutions Information Technology Solutions Monitoring
• Monitoring is important because things move!
Information Technology Solutions Information Technology Solutions Monitoring
• Ghana is moving ~30mm/year north-east
• If you set up a permanent base station 10 years ago, where is it
now?
-50
-30
-10
10
30
50
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
(mm
)
Information Technology Solutions Information Technology Solutions Monitoring
• Coordinate monitoring needs to be done using scientific GNSS
software such as Bernese, GAMIT or GIPSY
• Specifically designed for processing large networks of reference
stations
• Require extensive inputs such as solid earth tides, earth rotation
parameters, differential code biases, precise orbits and clocks etc.
• Require high-skilled staff to operate
• Normally reserved for the use by universities and national mapping
agencies due to high level of complexity
Information Technology Solutions Information Technology Solutions Monitoring Challenges
• Free online processing services such as Auspos provide good results,
but not practicable for daily use, especially for large networks
• Scientific software packages are expensive, difficult to operate,
require skilled staff and provide complex outputs
• Typical Bernese output consists of over a hundred of text files
• Up to the user to extract the relevant information
Information Technology Solutions Information Technology Solutions Position++
• To address the challenges of monitoting ThinkSpatial has developed a
new cloud-based service based on Bernese software
• Bernese engine runs in the background on the Amazon EC2 server
• All data and results are available on the Position++ portal for
authorised users
• Specifically designed to take out the complexity of Bernese processing
for CORS operators
• Designed to work with any number of reference stations
Information Technology Solutions Information Technology Solutions Position++ Unique Features
• Users can choose up to 5 reference frames for coordinate output
(eg. ITRF2008, ITRF2005, local datum)
• Continuous monitoring or short campaigns
• Combined weekly and monthly solutions
• Multipath and SNR monitoring
• Customised alerting based on user-defined thresholds
• Deliverables are available via simple download from the website in
various formats for inclusion in spreadsheets, reports etc.
Information Technology Solutions Information Technology Solutions Position++ Progress
• Currently being trialled on the Victorian CORS network of 130
stations
• Successfully deployed to monitor a number of oil platforms in the
Caspian Sea
• Used by Bureau of Meteorology in Victoria to compute Zenith
Tropospheric Delays (ZTDs)
Information Technology Solutions Information Technology Solutions Questions
Eldar Rubinov
Information Technology Solutions Information Technology Solutions References
• Edwards S, Clarke P, Goebell S, Penna N (2008). An examination of
commercial network RTK GPS services in Great Britain. Technical
Report, Newcastle University.
• Hausler G (2014). National Positioning Infrastructure – Technical,
Institutional and Economic criteria for coordinating access to
Australia’s GNSS CORS infrastructure. PhD Thesis, University of
Melbourne.
• Rizos C (2007). Alternatives to current GPS-RTK services & some
implications for CORS infrastructure and operations. GPS Solutions,
11(3): 151-158.
Information Technology Solutions Information Technology Solutions References
• Rizos C, Satirapod C (2011). Contribution of GNSS CORS
Infrastructure to the mission of modern geodesy and status of GNSS
CORS in Thailand. Engineering Journal, 15(1).
• Wonnacott R (2013). Permanent GNSS networks in Africa.
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