AN AUSTRALIAN PILOT PROJECT FOR A REAL TIME
KINEMATIC GPS NETWORK USING THE VIRTUAL REFERENCE STATION CONCEPT
Matthew B HIGGINS, Australia
Key words: GPS, Surveying, Real Time Kinematic,
Virtual Reference Station.
ABSTRACT
The Virtual Reference Station (VRS) concept from Trimble Navigation
is designed to support high-precision positioning over a wide area.
The system involves permanently running GPS reference stations, at
spacings up to 70km, then feeding GPS data to a central processing
computer via a computer network. The central processing facility then
models spatial errors that limit GPS accuracy. Corrections are then
generated and made available for roving receivers as required. VRS
systems support a multitude of positioning applications and enable
roving receivers to be positioned anywhere inside the network with an
accuracy better than a few centimetres in real time. The concept is an
extension of the so-called real time kinematic (RTK) technique
developed for GPS surveying and other forms of high precision
positioning. VRS overcomes three main limitations of the current RTK
technique. Firstly, operators no longer need to establish and run
their own base GPS receiver and base radio every time they want to
work. Secondly, the use of mobile phone technology overcomes the
limitation of the range of radio communications. Thirdly, multiple
base stations increase the redundancy and thus the confidence in the
resulting rover positions.
A pilot network has been established over the south-east corner of
the state of Queensland as a partnership between the Queensland
Department of Natural Resources and Mines (NRM), Trimble Navigation
and Ultimate Positioning. The pilot network is the first of its kind
in the Southern Hemisphere and capitalises on existing infrastructure
using four NRM offices and the computer network linking them and on
NRM's existing GPS equipment and expertise. At the time of writing
this paper, the pilot network has been established but full testing of
rover receivers is yet to begin. Therefore, this paper outlines the
establishment of the network and plans for investigating the technical
and financial feasibility of the VRS approach. The project is planned
for completion prior to this conference and latest progress will be
presented at the conference.
INTRODUCTION: THE VIRTUAL REFERENCE STATION CONCEPT
The Virtual Reference Station (VRS) concept from Trimble Navigation
is an extension of the so-called real time kinematic (RTK) technique
developed for GPS surveying and other forms of high precision
positioning. With RTK, one can establish a base (or reference) station
at a known point and broadcast the data from the reference station to
one or more roving receivers. The computer processor at the roving
receivers (usually in the handheld data recorder or survey controller)
combines the reference station data with the rover data. With modern
equipment, only a few epochs of data are typically required to fix the
ambiguities associated with the GPS phase data observable and compute
a GPS baseline; the difference in latitude, longitude and height
between the reference and rover positions. RTK enables the roving
receivers to be positioned with accuracy better than a few centimetres
relative to the reference station. Before RTK, GPS baselines had to be
post-processed in the office. The ability of RTK to process and
display results in real time is further revolutionising the
productivity achievable with GPS.
VRS takes the productivity increase a step further by overcoming
three main limitations of the current RTK technique. Firstly,
operators no longer need to establish and run a GPS receiver and radio
at their own reference station every time they want to work. Secondly,
the use of mobile phone technology overcomes the limitation of the
range of radio communications. Thirdly, multiple reference stations
increase the redundancy and thus the confidence in the resulting rover
positions.
The VRS concept involves permanently running GPS reference
stations, at spacings up to 70km. They feed their GPS data to a
central processing computer via a computer network. The central
processing computer can use the reference station data to model
spatial errors that limit GPS accuracy and generate appropriate
corrections. From the user's perspective, a roving receiver makes a
mobile phone call into the central processing facility, supplying its
approximate position (based on a GPS navigation position) and
requesting corrections. The central processing computer then generates
corrections as though there was a reference station at the coordinates
of the rover's approximate position and the rover is positioned
relative to this virtual reference station.
For more information on the technical background to VRS see Vollath
et al (2000a), Vollath et al (2000b) and Trimble (2000).
THE AUSTRALIAN PILOT PROJECT
The Department of Natural Resources and Mines (NRM) of the
Queensland Government is responsible for the surveying and geodetic
infrastructure of the Australian state of Queensland. Given the
potential impact of VRS on future development of that infrastructure,
NRM approached Trimble Australia and its agent in the area, Ultimate
Positioning, to establish a pilot project. The location for the
proposed VRS network is shown in Figure 1 and the following points are
relevant:
- The corridor between Brisbane and the Gold Coast is the fastest
growing urban area in Australia. That is significant for testing
commercial viability of the VRS network.
- The station locations are NRM offices, all of which are linked
by high-speed Wide Area Network (WAN) with each building also
having a Local Area Network (LAN) enabling all sites to be linked
by frame relay. While not directly relevant for this pilot, NRM
has more than 30 such District Offices spread across the state.
- The stations at Ipswich, Beenleigh and Gold Coast are District
Offices.
- The Brisbane station is at the Landcentre building, which houses
Survey Infrastructure Services (the section in NRM responsible for
the project).
- The Landcentre is also home to the Information Technology
Section for NRM, which oversee the operation of the WAN and LANs.
- As such, the Landcentre is the logical site for GPS Network
Control Centre.
- The Landcentre site also takes advantage of existing
infrastructure associated with a permanently running receiver
participating in the International GLONASS Service (IGLOS).
- The distance between Ipswich and Gold Coast (76km) is at the
limits of station spacing recommended for VRS. This, along with
the smaller triangle formed by Ipswich, Brisbane and Beenleigh
will enable rover testing under conditions ranging from ideal to
limiting.
- There is also the possibility of running with and without the
Beenleigh station to simulate problems with station drop out etc.
- The Beenleigh site can also be run as though it is a rover. That
will enable long duration rover data to be gathered.
- In relation to rover testing, the VRS network runs over a 3 to
5km density network of existing control stations observed with
static and fast static GPS.
Figure 1 Proposed Location of VRS Base Stations
GOALS OF THE PILOT PROJECT
The goals of the pilot project have been broken into three main
areas and are detailed below.
ESTABLISHING AND RUNNING THE VRS NETWORK
The project will investigate the technical and financial aspects of
establishing and running a VRS network, including:
- Establishing Stations on NRM Buildings
- Connections between network base stations using LAN/WAN.
- Connection of the GPS Network Control Centre to the mobile phone
network.
Figure 2 shows how the architecture for linking the GPS reference
stations, the computer network and the mobile phone network. The pilot
project uses personal computers running Microsoft Windows NT 4 at each
of the reference stations. Their basic purpose is simply to translate
the GPS data from the serial port to network protocol (TCP) and send
it to the central processing computer as required. As such, personal
computers have much more power than is necessary for a production
situation but has been used for the pilot project to facilitate remote
control of reference stations (eg to simulate outages) and to
facilitate monitoring of load on the WAN/LAN.
Figure 2 Information Technology Architecture for
VRS
At each site fibre optic cable is used between the GPS PC and the
rest of the LAN/WAN. This is to protect against lightning strike
damaging the LAN.
In terms of the GPS component, Trimble equipment is used at all
sites. At the Landcentre, Ipswich, and Gold Coast sites, 4700
receivers and choke ring antennae, supplied by NRM, are being used. At
the Beenleigh site, Trimble has supplied the newly released 5700
receiver and Zephyr Geodetic antenna.
GPS ROVERS WITHIN A VRS NETWORK
The project will also investigate the technical and financial
aspects of running GPS rovers within a VRS network. A detailed testing
plan will include matters such as:
- Establishing that the VRS solution allows the use of RTK
positioning at all suitable locations within the Network with
accuracy and initialisation times that are independent of distance
from a physical reference station.
- Establish how quality measures degrade as rovers move outside
the network coverage.
- Testing RTK positioning across a range of applications,
including stop and go survey applications and on dynamic
platforms, such as for earth moving.
- As well as RTK (centimetre) positioning, the ability of the
network to support DGPS (sub-metre) will be tested. This will
include long-range DGPS outside the network coverage recommended
for RTK.
Figure 3 Existing Geodetic Network and the VRS
Network
As mentioned earlier, the VRS network runs over a 3 to 5km density
network of existing control stations observed with static and fast
static GPS. The homogeneity of the network and its direct connection
to the networks forming the backbone of the Geocentric Datum of
Australia enable testing against high quality three dimensional
coordinates (including consistent ellipsoidal height). Figure 3 shows
this existing geodetic network in relation to the VRS network. A least
squares adjustment of the GPS baselines between the reference stations
and static GPS baselines connecting each to the local geodetic network
ensure that VRS network maintains a high level of homogeneity with the
underlying geodetic network. Stations in this geodetic network will be
occupied with VRS rovers to quantify the accuracy achievable inside
and outside the VRS network. Geodetic network stations of comparable
quality are also available further away from the VRS network and these
will be occupied to quantify the achievable DGPS accuracy.
COMMERCIAL VIABILITY
As well as the technical aspects, the project will investigate the
commercial viability of a VRS network, including potential clients,
revenue etc. Sound assessment of commercial viability will be
facilitated through involvement of key players in government and
industry in the fields of surveying, earth moving, mining and
agriculture.
- These will include players directly interested in the trial
network coverage such as Main Roads and Railways Dept, the three
local authorities and large surveying firms covered.
- There will also be exposure to players outside the direct trial
network coverage who have an interest in VRS for their areas of
operation. These include mining and precision agriculture
operators and NRM's equivalents in other Australian jurisdictions
and New Zealand.
CONCLUSION
This paper has described a pilot project to establish a network of
permanently running GPS reference stations to support high precision
positioning using the Virtual Reference Station (VRS) concept. VRS
overcomes many of the limitations of the current real time kinematic (RTK)
GPS surveying technique. The network covers the southeast corner of
the Australian state of Queensland and represents the first time that
VRS has been deployed in the Southern Hemisphere. At the time of
writing, the pilot network has been established but full testing of
rover receivers is yet to begin. Therefore, this paper has
concentrated on the establishment of the network and outlined what is
planned for investigating the technical and financial feasibility of
the VRS approach. The presentation at the conference will present
latest progress and future papers will report the outcomes of the
pilot project.
REFERENCES
Trimble, 2000, GPS Reference Station Networks, Product Brochure,
Spectra Precision Terrasat GmbH, Trimble Navigation Limited, September
2000.
(available on-line at http://www.terrasat.de/applications/refvirtual.htm)
Vollath U., A. Buerchl, H. Landau, C. Pagels and B. Wagner, 2000a,
Multi-Base RTK Positioning Using Virtual Reference Stations,
Proceedings of ION GPS 2000, 13th International Technical Meeting of
the Satellite Division of the Institute of Navigation, September 2000,
Salt Lake City, Utah.
Vollath U., A. Buerchl, H. Landau, C. Pagels and B. Wagner, 2000b,
Long-Range RTK Positioning Using Virtual Reference Stations,
Proceedings of ION GPS 2000, 13th International Technical Meeting of
the Satellite Division of the Institute of Navigation, September 2000,
Salt Lake City, Utah.
BIOGRAPHICAL NOTE
Matt Higgins is a Senior Surveyor with the Department of
Natural Resources and Mines, Queensland, Australia. He received
Bachelor and Masters degrees in Surveying in 1983 and 1988,
respectively. He is presently undertaking a PhD at the University of
New South Wales working in GPS Heighting. Matt is Vice Chair of FIG
Commission 5 on Positioning and Measurement for the period 1998 to
2002. He is on the Editorial Advisory Board of GPS World Magazine.
Matt is also a member of the Geodesy Sub-Committees of both the
Australian Academy of Sciences and the Inter-Governmental Committee on
Surveying and Mapping for Australia and New Zealand.
CONTACT
Matt Higgins
Department of Natural Resources and Mines
Locked Bag 40
Coorparoo Delivery Centre
Bribane
Queensland 4151
AUSTRALIA
Tel. + 61 7 3896 3754
Fax + 61 7 3891 5168
E-mail: matt.higgins@dnr.qld.gov.au
17 April 2001
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