CORS, NRTK and Monitoring - Experiences from Australia

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


PART 1 – 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 Examples of CORS Installations

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)


• 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


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


PART 2 – Network RTK

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 - VRS

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 - MAC

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)


PART 3 – Monitoring

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.


• Monitoring is important because things move!


• 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

)


• 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++ Portal

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

e: [email protected]

mailto:[email protected]

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.

CORS, NRTK and Monitoring - Experiences from Australia

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