Journal of Global Positioning Systems (2002)
Vol. 1, No. 2: 144-150
Network RTK Research and Implementation - A Geodetic Perspective
C. Rizos
School of Surveying and Spatial Information Systems, The University of New South Wales, Sydney NSW 2052 AUSTRALIA
e-mail: mailto:[e-mail]
Received: 3 January 2003 / Accepted: 10 January 2003
Biography
Prof. Chris Rizos, is a Professor at the School of
Surveying & SIS, UNSW, Australia, and leader of the
Satellite Navigation and Positioning (SNAP) Group
(http://www.gmat.unsw.edu.au/snap/). He has been
engaged in GPS research since the mid-1980s, which was
at first focused on geodetic applications. More recently
Chris has broadened the SNAP group’s research across a
wide range of positioning applications that can be
addressed by GNSS and various ground-based wireless
location technologies. He is currently secretary of Section
1 “Positioning” of the International Association of
Geodesy (IAG).
Background: Why Complicate Matters?
The standard mode of precise differential positioning is
for one reference receiver to be located at a base station
whose coordinates are known, while the second receiver's
coordinates are determined relative to this reference
receiver. This is the principle underlying pseudo-range-
based differential GPS (or DGPS for short) techniques.
However, for high precision applications, the use of
carrier phase data must be used, but comes at a cost in
terms of overall system complexity because the
measurements are ambiguous, requiring that ambiguity
resolution algorithms be incorporated as an integral part
of the data processing software. Such high accuracy
techniques are the result of progressive R&D innovations,
which have been subsequently implemented by the GPS
manufacturers in their top-of-the-line “GPS surveying”
products. Over the last decade or so several significant
developments have resulted in this high accuracy
performance also being available in real-time -- that is, in
the field, immediately following the making of
measurements, and after the data from the reference
receiver has been transmitted to the (second) field
receiver for processing via some sort of data
communication links (e.g., VHF or UHF radio, cellular
telephone, FM radio sub-carrier or satellite com link).
Real-time precise positioning is even possible when the
GPS receiver is in motion. These systems are commonly
referred to as RTK systems (“real-time-kinematic”), and
make feasible the use of GPS-RTK for many time-critical
applications such as engineering surveying, GPS-guided
earthworks/excavations, machine control and other high
precision navigation applications.
The limitation of single base RTK is the distance between
reference receiver and the rover receiver due to distance-
dependent biases such as orbit error, and ionospheric and
tropospheric signal refraction. This has restricted the
inter-receiver distance to 10km or less. On the other hand,
Wide Area Differential GPS (WADGPS) and the Wide
Area Augmentation System (WAAS) use a network of
master and monitor stations spread over a wide
geographic area, and because the measurement biases can
be modelled and corrected for, the positioning accuracy
will be almost independent of the inter-receiver distance
(or baseline length). However, these are pseudo-range
based systems intended to deliver accuracies at a metre
level. Continuously operating reference stations have
been deployed globally to support very high accuracy
geodetic applications for well over a decade. How can
GPS surveying take advantage of such developments in
geodesy and global navigation? The answer is to take
advantage of multiple reference station networks, in such
implementations as Network RTK.
Network RTK is a centimetre-accuracy, real-time, carrier
phase-based positioning technique capable of operating
over inter-receiver distances up to many tens of
kilometres (the distance between a rover and the closest
reference station receiver) with equivalent performance to
current single base RTK systems (operating over much

Rizos: Network RTK Research and Implementation - A Geodetic Perspective 145
shorter baselines). The reference stations must be
deployed in a dense enough pattern to model distance-
dependent errors to such an accuracy that residual
double-differenced carrier phase observable errors can be
ignored in the context of rapid ambiguity resolution.
Network RTK is therefore the logical outcome of the
continuous search for a GPS positioning technique that
challenges the current constraints of cm-accuracy, high
productivity, carrier phase-based positioning.
Network-based Positioning: The Geodetic Perspective
All GPS-based positioning techniques operate under a set
of constraints (Rizos, 2002). These constraints may be
baseline length, attainable accuracy, assured reliability,
geometrical strength, signal availability, time-to-solution,
instrumentation, operational modes, and so on. GPS
product designers must develop systems (comprising
hardware, software and field procedures) that are
optimised for a certain target user market, by addressing
only those constraints that are crucial to the most
common user scenarios. For example, current single base
RTK systems are capable of high performance when
measured in terms of such parameters as accuracy, time-
to-solution (i.e. speed of ambiguity resolution after signal
interruption), utility (due to the generation of real-time
solutions), flexibility (being able to be used in static and
kinematic applications), ease-of-use, and cost-
effectiveness. As a result the sale of RTK systems is
booming. However, the authors believe that the 10km
baseline constraint will increasingly become an issue.
RTK GPS users will demand an infrastructure of base
stations to support them, in much the same way that
DGPS users have for many years been able to take
advantage of free-to-air or fee-based differential services.
However, it is generally unrealistic to deploy reference
receivers across a country, or even just within a city, at
such a density that all users are within 10km of a
reference receiver transmitting RTK messages. Network
RTK techniques use base station separations of several
tens of kilometres, hence requiring fewer reference
receivers. This significantly reduces the infrastructure
investment required. The development of Network RTK
can viewed from three distinct perspectives:
1. The evolution of the high productivity GPS
Surveying technique in order to preserve single base
RTK performance, but to permit much greater GPS
inter-receiver distances. The change from single base
to multi-base allows for the empirical modelling of
the distance-dependent measurement biases. It is this
modelling (and the transmission of ‘corrections’ for
the normally unaccounted for biases) that overcomes
the distance constraint, with no requirement for an
upgrade to the user equipment software. The same
GPS surveying user functionality is preserved.
2. The use of sparse networks of base stations is the
basis of WADGPS and WAAS positioning
techniques (Lachapelle et al., 2002). Data from the
base station network are sent to a central computing
facility, and empirical models of the distance-
dependent biases are generated in the form of
‘corrections’ (which may be in the form of
proprietary messages, or an industry standard RTCM
or WAAS message type). These corrections are
transmitted to user across a wide geographic area
(most commonly via satellite communication links).
However, because such Augmented GPS Navigation
techniques use pseudo-range data, and the separation
of the base stations is typically many hundreds to
several thousands of kilometres, sub-decimetre level
positioning accuracy is unattainable. The evolution
to Network RTK would require a significant
improvement in accuracy, through the use of carrier
phase data and a much denser deployment of
reference receivers.
3. GPS Geodesy has evolved since the early 1980s into
a powerful, ultra precise positioning technique that is
used for a range of applications, including the
definition of the fundamental geodetic framework
and the measurement for tectonic motion. GPS
Geodesy uses a multi-receiver data processing
methodology in which all measurement biases, no
matter how small, are carefully accounted for in the
functional and stochastic models of the double-
differenced carrier phase observables. Continuously
operating reference station (CORS) networks have
been established around the world to support a range
of geodetic applications. Positioning accuracy at the
few parts-per-billion (ppb) are now routinely
obtained, using sophisticated data processing
algorithms in packages such as the Bernese software
(Rothacher & Mervart, 1996). (One ppb is equivalent
to 1mm relative accuracy over a baseline one
thousand km in length.) Clearly GPS Geodesy could
evolve into the Network RTK technique, if receivers
were permitted to be in motion and data processing
could be undertaken in real-time. Both of these are
significant challenges. Furthermore, the Network
RTK strategy could be used to densify high precision
CORS networks for certain geodetic applications.
The author describes below several developments in GPS
Geodesy that could be viewed as being predecessors to
the development of the Network RTK concept. In fact,
network-based positioning techniques have been an
interest of geodesists for some time. During the past
decade the International Association of Geodesy (IAG)
has established several Special Study Groups (SSG) to
research several topics concerned with permanent GPS
networks. In 1999 the IAG established SSG1.179 “Wide
Area Modelling for Precise Satellite Positioning”. The
Chair. SSG1.179, Dr. Shaowei Han, will report to the

146 Journal of Global Positioning Systems
IAG at the next General Assembly in 2003, in Sapporo
(Japan).
Kinematic Geodesy: An Evolutionary 'Deadend'?
Colombo et al. (1995) describes an experiment in which a
moving vessel in Sydney Harbour (Australia) was
positioned to sub-decimetre accuracy relative to several
GPS reference receivers deployed at distances up to
1000km from the mobile receiver. This was a dramatic
new GPS Geodesy technique that challenged the
requirement that geodetic accuracy over long inter-
receiver distances was only possible for a static receiver
that was collecting carrier phase data over many hours. It
was indeed a geodetic technique because: (a) all
measurement biases were accounted for in the functional
model, (b) sub-part-per-million relative accuracy was
obtained, and (c) a simultaneous multi-receiver solution
was performed. The first author coined the expression
“kinematic geodesy” to describe this technique. In 1995
an Australian Research Council grant was obtained to
support graduate studies into high precision, long- and
medium-range, kinematic GPS positioning, as reported in
Han (1997).
The data processing algorithm used by Colombo et al.
(1995) was particularly innovative, consisting of a
partitioned Kalman filter that estimated the slow-
changing biases such as due to satellite orbit error and
atmospheric effects, at the same time generating epoch-
by-epoch kinematic coordinate solutions for the mobile
receiver, using carrier phase data from several reference
receivers (as well as the mobile receiver). The
observation biases were carefully modelled, as in the
‘standard’ geodetic methodology used by GAMIT and the
Bernese software, and 3-D accuracy is of the order of 3-
5cm, for any length of baseline. During the last ten years,
this technique has been used in Australia, Denmark,
Japan, Spain, The Netherlands and the U.S. for
applications as diverse as sea buoys, boats, aircraft,
trucks, and altimetric satellites. Recent publications
reporting on “kinematic geodesy” projects include
Colombo et al. (2000, 2001, 2002).
However, the promise shown by this technique has not
led to its widespread adoption by geodesists.
Nevertheless this technique can lay claim to having
demonstrated, for the first time, the feasibility of carrier
phase-based positioning of a moving platform over very
long baselines. Amongst its shortcomings are the
simultaneous analysis of all GPS data (from reference
receivers and the mobile receiver), and the difficulty in
implementing this technique in real-time.
Low-cost Deformation Monitoring: The Utility Of
Mixed Networks
Deformation monitoring of structures (such as bridges,
buildings, etc.) and ground monumentation (in volcanic,
ground subsidence and geological faulting zones) are
ideal geodetic applications of GPS (Rizos et al., 1997).
To keep the cost of such monitoring systems low, single-
frequency GPS receivers are often used (see, e.g., the
theses by Chen, 2001; Roberts, 2002). However, data
from single-frequency GPS receivers cannot be corrected
for ionospheric delay, as is the case with dual-frequency
data. Therefore a combination of single- and dual-
frequency instrumentation in a mixed-mode network is a
feasible methodology for ensuring high accuracy
coordinate results using a large number of static receivers
must be deployed permanently across a region
experiencing deformation, while keeping hardware costs
as low as possible. This is possible by augmenting the
single-frequency receivers with a small number of dual-
frequency receivers surrounding the zone of deformation.
The primary function of this fiducial network is to
generate empirical ‘correction’ terms to the double-
differenced phase observables within the deformation
monitroing network. This research was funded by the
Australian Research Council (1999-2001), in address the
need for a low-cost Indonesian volcano monitoring
system.
This methodology has been tested in many networks, and
results reported in a large number of papers, including
Rizos et al. (2000a, 2000b), Chen et al. (2001). Dai et al.
(2001) extended this methodology to include integrated
GPS/GLONASS reference receiver networks. This
methodology can address geodetic applications where a
CORS network of geodetic quality GPS receivers exists.
Furthermore, this data processing strategy is identical to
what we now know as the Network RTK, or multiple
reference station, class of techniques. That is, there are
three distinct processes: reference station network data
processing to generate ‘corrections’, correction of double-
differenced phase data involving user receiver(s), and
(static or kinematic) baseline processing using the
corrected GPS phase observables. It is this separation of
processes that sets this class of techniques apart from the
conventional multi-station geodetic technique, and the
“kinematic geodesy” approach described earlier. The
extension of this methodology to operate in real-time,
though an engineering challenge, is relatively
straightforward.
Network RTK Issues: Theoretical & Practical
Challenges
Many investigators have contributed to the definition of
the appropriate functional and stochastic models for

Rizos: Network RTK Research and Implementation - A Geodetic Perspective 147
medium-range and long-range GPS/GLONASS survey-
type positioning (as opposed to geodetic techniques)
using CORS networks. Research has addressed topics
such as: multipath mitigation algorithms, troposphere
model refinement, regional ionosphere modelling
algorithms, phase centre calibration, and orbit bias
modelling. The authors would be unable to do justice to
all contributions in this review paper and refer the reader
review papers such as Rizos & Han (2002). Although
most of these research topics are of general interest to
precise GPS positioning, several are explicitly related to
the processing of CORS network data in order to generate
the empirical ‘correction’ data that must be transmitted to
users in Network RTK type implementations. Some of
these topics include: rapid ambiguity resolution for the
network receivers, validation of the ambiguities so
resolved, the nature of the model for the distance-
dependent biases across the CORS network, the method
of interpolation of the corrections for the user-base
station baseline, and the format for the transmitted
‘correction’ data.
After the double-differenced ambiguities associated with
the reference station receivers have been fixed to their
correct values, the double-differenced GPS/GLONASS
residuals can be generated. The spatially correlated errors
to be interpolated could be the pseudo-range and carrier
phase residuals for the L1 and/or L2 frequencies, or other
linear combinations. One core issue for multi-reference
receiver techniques is how to interpolate the distance-
dependent biases generated from the reference station
network for the user's location? Over the past few years,
in order to interpolate (or model) the distance-dependent
residual biases, several interpolation methods have been
proposed. They include the Linear Combination Model
(Han & Rizos, 1996; Han, 1997), the Distance-Based
Linear Interpolation Method (Gao & Li, 1998), the Linear
Interpolation Method (Wanninger, 1995), the Low-Order
Surface Model (Wübbena et al., 1996; Fotopoulos &
Cannon, 2001), and the Least Squares Collocation
Method (Raquet, 1998; Marel, 1998). The theoretical and
numerical comparison of the various interpolation
algorithms has been made by Dai et al. (2003), and there
is no obviously ‘superior’ technique. The essential
common formula has been identified: all use n-1
coefficients and the n-1 independent ‘correction terms’
generated from a n reference station network to form a
linear combination that mitigates spatially correlated
biases at user stations.
While theoretical and numerical studies have contributed
to the development of the Network RTK class of
techniques, there are a host of ‘practical’ issues that must
be addressed in order to implement a RTK service that
operates ‘24/7’. For example, the Network RTK system
needs a data management system and a data
communication system. It needs to manage corrections
generated in real-time, the raw measurement data,
multipath template for each reference stations (for
multipath mitigation), precise/predicted IGS orbits, etc.
There are two aspects to the data communication system:
(a) between the master control station (MCS - where all
the calculations are undertaken) and the various reference
stations, and (b) communication between the MCS and
users. Furthermore, from the Network RTK
implementation point of view, there are three possible
architectures: (1) generation of the Virtual Reference
Station (VRS) and its corrections, (2) generating and
broadcasting Network RTK corrections, or (3)
broadcasting raw data for all the reference stations. The
debate about the ‘best’ architecture is still raging, and it is
likely that combinations of some or all may be
implemented, with the appropriate RTCM/RTK messages
being defined. However, research into all aspects of
Network RTK, theoretical and practical, is difficult to
undertake in universities because of the expense of
establishing and operating ‘test networks’.
Singapore Integrated Multiple Reference Station
Network
Due to the complexity (and cost) involved in establishing
fully functioning reference receiver networks, the data
links and the data processing/management servers at the
master control station (MCS), there have been
comparatively few university-based Network RTK
systems established to support research. During the last
few years, to the best of the authors’ knowledge, only the
Singapore Integrated Multiple Reference Station Network
(SIMRSN) has been operating both as a research facility
and an operational Network RTK that can be used by
surveyors. The SIMRSN is a joint research and
development initiative between the Surveying and
Mapping Laboratory, of the Nanyang Technological
University (NTU), Singapore (http://gis.ntu.edu.sg/
generaterinex/index.htm), the Satellite Navigation and
Positioning group, of the University of New South Wales
(UNSW), Australia (http://www.gmat.unsw.edu.au/snap/
work/singapore.htm), and the Singapore Land Authority
(SLA). In Singapore the project was funded by the
National Science and Technology Board (1998-2001),
while in Australia it was funded by the Australian
Research Council (1999-2001).
The SIMRSN consists of five continuously operating
reference stations (tracking satellites 24 hours a day),
connected by high speed data lines to the MCS at NTU
(Figure 1). It is a high quality and multi-functional
network designed to serve the various needs of real-time
precise positioning, such as surveying, civil engineering,
precise navigation, road pricing etc. The SIMRSN also
serves off-line non real-time users via the Internet. The
inter-receiver distances are of the order of several tens of
kilometres at most. However, tests conducted in 2001

148 Journal of Global Positioning Systems
have shown that even a network with such comparatively
short baselines had difficulty in modelling the disturbed
ionosphere in equatorial regions, during the last solar
maximum period of the 11 year sunspot cycle (Hu et al.,
2002a; 2002b; 2002c).
10
5
0
#
#
#
#
#
Fig. 1 The Singapore Integrated Multiple Reference Station Network - supporting R&D into Network RTK
and other network-based positioning concepts.
The ‘roots’ of Network RTK can be found in geodesy,
surveying and precise navigation. Each sub-discipline can
claim some credit for the development of the Network
RTK concept. The author in this paper has emphasised
the geodetic perspective, and shown how geodetic
methodology and applications were a driver for multi-
reference receiver techniques that ultimately led to the
development of Network RTK. The paper has also
highlighted the contributions of Australian and
Singaporean researchers to the development and
implementation of the data processing algorithms, and
associated data management and communication systems,
that underpin the totally university-developed Network
RTK service.
Concluding Remarks
Network RTK is best implemented by a service provider,
an organisation that operates the receiver network
infrastructure, the necessary data communication links
and the MCS facility. This is a radically different scheme
to the standard single base RTK where the GPS Surveyor
owns and operates all of the equipment. At present there
are very few continuously operating Network RTK
systems. However, with the likely upgrade of CORS
networks around the world to offer RTK services over the
next few years, the author believes that there will be a
boom in Network RTK implementations.
There is currently only one commercial product, the
Trimble VRS (Vollath et al., 2002), although the Leica
company has also developed a Network RTK system
(Euler et al., 2001). A number of test networks have been
operating in Europe, the U.S., Australia, New Zealand,
China and Japan. However, a unique university-led
Network RTK system has been operating in Singapore
for a number of years. Australian and Singaporean
researchers have gained invaluable insight into the
challenges of operating such an infrastructure on a ‘24/7’
basis. It is intended to mirror this facility in Sydney
during 2003, supporting independent research into
Network RTK algorithms, products, operational issues,
and business models, carried out outside North America,
Europe and east Asia.
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Missouri, 17-20 September, 1845-1852.