J Geod (2009) 83:191–198

DOI 10.1007/s00190-008-0300-3

INTRODUCTORY PAPER

The International GNSS Service in a changing landscape

of Global Navigation Satellite Systems
John M. Dow · R. E. Neilan · C. Rizos

Received: 15 December 2008 / Accepted: 17 December 2008

© Springer-Verlag 2009

Abstract The International GNSS Service (IGS) is an inter- working groups and pilot projects as the world anticipates a
national activity involving more than 200 participating truly multi-system GNSS in the coming decade.
organisations in over 80 countries with a track record of one
and a half decades of successful operations. The IGS is a
service of the International Association of Geodesy (IAG).
It primarily supports scientific research based on highly pre- 1 Introduction
cise and accurate Earth observations using the technologies
of Global Navigation Satellite Systems (GNSS), primarily The International GNSS Service (IGS) was officially estab-
the US Global Positioning System (GPS). The mission of lished in January 1994 as a service of the International
the IGS is “to provide the highest-quality GNSS data and Association of Geodesy (IAG 2008). Since June 1992, the
products in support of the terrestrial reference frame, Earth IGS—originally known as the “International GPS Service
rotation, Earth observation and research, positioning, naviga- for Geodynamics”, from 1999 simply the “International GPS
tion and timing and other applications that benefit society”. Service”, and finally since March 2005 the “International
The IGS will continue to support the IAG’s initiative to coor- GNSS Service”—has been making freely available to inter-
dinate cross-technique global geodesy for the next decade, ested users precision GPS satellite orbit and clock corrections
via the development of the Global Geodetic Observing Sys- and other products, see IGS (2008), Beutler et al. (1999) and
tem (GGOS), which focuses on the needs of global geodesy Dow et al. (2004, 2005). The origins and early development
at the mm-level. IGS activities are fundamental to scientific of the IGS are described by Beutler et al. (2008).
disciplines related to climate, weather, sea level change, and The IGS operates as a voluntary, non-commercial confed-
space weather. The IGS also supports many other applica- eration of about 200 institutions world-wide (see Fig. 1 for a
tions, including precise navigation, machine automation, and map of the global station network), self-governed by its mem-
surveying and mapping. This article discusses the IGS Stra- bers, managed on a day-to-day basis by the Central Bureau
tegic Plan and future directions of the globally-coordinated under the policy guidance of the Governing Board. Each par-
∼400 station IGS network, tracking data and information ticipating organisation contributes its own resources: there is
products, and outlines the scope of a few of its numerous no central source of funding.
Since the IGS Governing Board adopted in December
2001 its Strategic Plan, covering the years 2002–2007, a
J. M. Dow (B) number of developments taking place inside and outside the
ESA/ESOC, Darmstadt, Germany
e-mail: john.dow@esa.int
IGS has made it necessary to revise the plan. A reflection
process was initiated at the 2004 Workshop in Bern, Swit-
R. E. Neilan zerland, continued through a dedicated session at the 2006
NASA/JPL, Pasadena, USA Darmstadt Workshop, Germany, culminating in a one and a
C. Rizos
half day meeting of a specially appointed Strategic Planning
School of Surveying and Spatial Information Systems, Committee in Pasadena, California, in September 2006, and
University of New South Wales, Sydney, Australia a two day Strategic Planning Retreat of the Governing Board

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Fig. 1 Global distribution of stations of the IGS network

in San Francisco in December 2006. A new IGS Strategic 3 The changing GNSS landscape
Plan for the years 2008–2012 has resulted from this process.
This article outlines some of the current central concerns While the IGS product range has been mainly concerned with
of the IGS and points to the key issues for the coming years. GPS, since 1998 GLONASS products were developed, ini-
tially in connection with the International GLONASS Exper-
iment (IGEX) of 1999 (Slater et al. 2004). This continued
seamlessly, from 2001, through the International GLONASS
2 The IGS products Service (IGLOS), a pilot project of the IGS, which reached a
successful conclusion in December 2005, when the GLON-
The IGS collects, archives, and distributes GPS and GLON- ASS products (raw data and derived products) were inte-
ASS observation data sets of sufficient accuracy to meet the grated into the mainstream IGS product flow (see Table 1).
objectives of a wide range of scientific and engineering users. The participation during the past 2 years of the Russian Insti-
These data sets are analysed and combined to form the IGS tute IAC in the IGS GLONASS orbit combination has con-
products shown in Table 1 (IGS 2008). tributed to improvements in the quality of these products.
IGS products support scientific activities such as improv- Improvements in the GLONASS geodetic reference frame
ing and extending the International Terrestrial Reference PZ 90 are in part due to assimilation of IGS stations with
Frame (ITRF) maintained by the International Earth Rotation coordinates well-determined in the ITRF.
and Reference Systems Service (IERS); monitoring defor- More recently, the IGS has been actively following the
mations of the solid Earth and variations in the liquid Earth development of the European Galileo system, in three gen-
(sea level, ice sheets, etc.) and in Earth rotation; determining eral areas (Dow et al. 2007).
orbits of scientific satellites; and monitoring the troposphere First, the IGS GNSS Working Group and its individual
and ionosphere. members are involved in bringing to the attention of the new
Typical accuracies and latencies of the various products GNSS system providers the experience gained over the past
are indicated in Table 1. The primary IGS products are the decade and a half in the IGS concerning orbit models, antenna
GPS satellites’ IGS Final Orbit and Clock corrections, now phase calibrations, standardisation of data formats and other
at the cm accuracy level (see Fig. 2 for the recent evolution matters. This applies to future generation GPS as well as to
of orbit accuracy). The accuracy of a recent orbit solution, Galileo, and to other systems such as China’s Beidou/Com-
together with some relevant ancillary information, can be pass, India’s Regional Navigation Satellite System (IRNSS),
seen in Fig. 3. and Japan’s Quasi-Zenith Satellite System (QZSS).

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The International GNSS Service in a changing landscape of Global Navigation Satellite Systems 193

Table 1 IGS product summary

Accuracy Latency Updates Sample interval

GPS satellite ephemerides/satellite and station clocks

Broadcast
Orbits ∼160 cm
Real time – Daily
Sat. clks ∼7 ns
Ultra-rapid (predicted half)
Orbits ∼10 cm
Real time Four × daily 15 min
Sat. clks ∼5 ns
Ultra-rapid (observed half)
Orbits <5 cm
3h Four × daily 15 min
Sat. clks ∼0.2 ns
Rapid
Orbits <5 cm 15 min
17 h Daily
Sat. and Stn. clks 0.1 ns 5 min
Final
Orbits <5 cm 15 min
∼13 days Weekly
Sat. and Stn. clks <0.1 ns 5 min
GLONASS satellite ephemerides
Final 15 cm 2 weeks Weekly 15 min
Geocentric coordinates of IGS Tracking Stations (>130 sites)
Final positions
Horizontal 3 mm
12 days Weekly Weekly
Vertical 6 mm
Final velocities
Horizontal 2 mm/year
12 days Weekly Weekly
Vertical 3 mm/year
Earth rotation parameters
Ultra-rapid (predicted half)
PM 0.3 mas
PM rate 0.5 mas/day Real time Four × daily Four × daily (00,06,12,18 UTC)
LOD 0.06 ms
Ultra-rapid (observed half)
PM 0.1 mas
PM rate 0.3 mas/day 3h Four × daily Four × daily (00,06,12,18 UTC)
LOD 0.03 ms
Rapid
PM <0.1 mas
PM rate <0.2 mas/day 17 h Daily Daily (12 UTC)
LOD 0.03 ms
Final
PM 0.05 mas
PM rate <0.2 mas/day ∼13 days Weekly Daily (12 UTC)
LOD 0.02 ms
Atmospheric parameters
Final tropospheric zenith path delay 4 mm <4 weeks Weekly 2h
Ultra-rapid tropospheric zenith path delay 6 mm 2–3 h Every 3 h 1h
Final ionospheric TEC grid 2–8TECU ∼11 days Weekly 2 h, 5 deg (lon) × 2.5 deg (lat)
Rapid ionospheric TEC grid 2–9TECU <24 h Daily 2 h, 5 deg (lon) × 2.5 deg (lat)

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Fig. 2 Improvement of IGS

final orbits with time (since GPS
week 700)

Fig. 3 Accuracy of a typical recent Ultra Rapid IGS orbit combination, by satellite

Second, European IGS participants contributing to the The third area in which IGS has been able to contribute its
global IGS ground station network (ESOC and GeoFors- expertise is in the Galileo Geodetic Service Provider (GGSP)
chungsZentrum Potsdam—GFZ) have been working with Prototype. This is a project funded by the European GNSS
European industry and ESA to set up and operate the network Supervisory Authority (GSA), with technical management
of sensor stations (GIOVE Experimental Sensor Stations— support from ESA, with the objective of designing and devel-
GESS) to track the experimental GIOVE satellites. This net- oping a system capable of providing and maintaining over
work, totalling 13 stations, including 11 on well-tested IGS the projected 20 year lifetime of the Galileo system a geo-
sites, has been fully operational since early 2007. detic reference frame, to be known as the Galileo Terrestrial

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The International GNSS Service in a changing landscape of Global Navigation Satellite Systems 195

Reference Frame (GTRF). This frame shall be within 3 cm The current IGS pilot projects are:
(2-sigma) of the ITRF (IERS 2008), to which the IGS con-
tributes on a routine basis the consolidated GPS input. Three • Tide Gauge Benchmark Monitoring Project for Sea Level
European IGS Analysis Centres (University of Bern, GFZ Studies (TIGA)
Potsdam, ESA/ESOC), as well as key European institutes • Real Time Pilot Project
involved in the IERS (BKG, IGN), supported by Canadian
and Chinese organisations (NRCan, University of Wuhan), The charter of each WG and PP describes the goals and objec-
are working in a consortium led by the GFZ. A first realisation tives, see IGS (2008). The chairs of the WGs and PPs report
of the GTRF, based on GPS data from selected IGS sites, has to the IGS Governing Board on a regular basis.
been successfully validated. Updates will be based on inclu-
sion (in addition) of GPS data from the GIOVE network
and then GIOVE data itself. As the Galileo Sensor Stations 5 Towards real time products
(GSS) planned for the Galileo In-orbit Validation Phase are
established, data from those sites will also be processed and The IGS has been developing the capability for real time
included. The GGSP will have an important two-way inter- data streaming from the ground station network for some
face with the Galileo Ground Mission Segment (GMS), to years. Currently up to 60 stations are providing data, with a
retrieve the necessary GSS data and to provide the resulting latency of the order of a few seconds (IGSRT 2008). A recent
reference frame information. It will also provide the inter- development was the initiation of a Real Time Pilot Project
face between the GMS and the International Laser Ranging (RT-PP), which has the following objectives:
Service, in order to facilitate provision of satellite laser rang-
ing data for the Galileo spacecraft for calibration purposes. • Manage and maintain a global IGS real time GNSS track-
Further details of the GGSP can be found in Gendt et al. ing network.
(2007). • Enhance and improve selected IGS products.
IGS and IAG representatives are active in the recently • Generate new real time products.
established US National Space-based Position Navigation • Investigate standards and formats for real time data col-
and Timing Advisory Board, which provides to the PNT lection, data dissemination and delivery of derived prod-
Executive Committee advice relating to positioning, naviga- ucts. Both the RTIGS and the NTRIP protocols will be
tion and timing (PNT) policy and capabilities (PNT 2008). assessed as to their suitability.
In addition, the IGS (as well as the IAG and other inter- • Monitor the integrity of IGS predicted orbits and GNSS
national and intergovernmental organisations) are members status.
of the International Committee on GNSS (ICG), convened • Distribute observations and derived products to real time
by the U.N. Office of Outer Space Affairs (UNOOSA 2008), users, and support Network DGPS/RTK operations.
see below. • Encourage cooperation among real time activities, par-
ticularly in IGS densification areas.

The call for participation in the RT-PP (see IGS 2008) requ-
4 IGS working groups and pilot projects
ested proposals for:
The IGS has a number of working groups, focused on differ-
• Real time Tracking Stations
ent aspects of current GNSS product generation, as well as
• Real time Data Centres
pilot projects investigating future developments which could
• Real time Analysis Centres
lead to the generation of new IGS products. The current WGs
• Real time Associate Analysis Centres
are:
• Real time Analysis Centre Coordinator
• Real time Network Management and Monitoring
• Ionosphere Working Group • Real time Users for Assessment, Evaluation and Feed-
• Troposphere Working Group back
• IGS Reference Frame Working Group
• Low Earth Orbiter (LEO) Working Group The RT-PP will gather and distribute real time data and prod-
• Real Time Working Group ucts associated with GNSS satellite constellations. The pri-
• GNSS Working Group mary products envisioned are multi-frequency observation
• Data Center Working Group data and precise satellite clocks and orbits made available
• Clock Products Working Group in real time. These products will be freely available to par-
• Calibration and Biases Working Group ticipants, and eventually to external users, for any purpose,

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in accordance with the IGS open data policy. An important • How to manage the potentially disruptive process of sta-
objective of the RT-PP will be to support and promote the tion upgrade (in particular changes in antenna type) as
development of real time applications. The IGS will work new receivers, tracking next generation GNSS signals,
closely with standards setting bodies such as the RTCM, to are progressively installed?
ensure appropriate real time capabilities are implemented in • How to encourage the establishment of more IGS stations
the next generation of GNSS receivers. The RT-PP will oper- in parts of the world where there is currently a lower den-
ate for a period of up to 3 years. Annual reviews will be con- sity? In particular in Africa, Russia, and parts of East and
ducted by the IGS Governing Board to assess the project’s S.E. Asia, with a real time data streaming capability.
progress towards achieving its goals and objectives. • Can there ever be “too many” IGS stations? What station
spacing does that correspond to?
• How to best integrate the operations of IGS stations in
6 The IGS network areas where there are already dense receiver networks,
such as the western United States and parts of Europe?
The IGS may be unique in its commitment to inclusiveness
and a vendor-neutral stance toward instrumentation. This per-
7 IGS and the Global Geodetic Observing System
mits participation by a wide variety of agencies, universities,
and individuals. It also results in a large and heterogeneous
In parallel with these IGS internal developments, the IGS has
network of equipment that presents certain calibration, stan-
been working with the IAG on the design of the Global Geo-
dardization, and coordination challenges. A typical IGS site
detic Observing System (GGOS 2008), which would inte-
consists of a monument, antenna, receiver, an ultra-stable
grate the activities and products of the IAG Services and
clock (in many cases a hydrogen maser or rubidium or cesium
Commissions (IAG 2008) in order to provide the contribu-
atomic clock) and optionally a computer and communica-
tion of geodesy to the Global Earth Observing System of
tions device. The monument is the stable connection of the
Systems (GEOSS) now being established by the inter-gov-
antenna to a point on the ground. Monument types include
ernmental Group on Earth Observations (GEO). The ITRF
pillars, braced-rod types, and building mounts. Antennas and
(in particular, its future evolution, and its correct and consis-
receivers from a variety of commercial manufacturers are
tent use) is a central issue of the GGOS initiative. The IGS,
dual-frequency and record both code and phase from the GPS
with its prime concern for high accuracy and high reliability
and optionally GLONASS satellites. Not all types of equip-
processing of the signals of the GNSS constellations and as
ment are suitable for the most demanding precise geophysical
provider of the consolidated inputs of the GNSS contribution
applications; more information is available at the IGS station
to the ITRF, will necessarily play a key role in GGOS.
guidelines (IGS 2008). Since IGS stations must return data
The work of the IGS and its constituent elements is becom-
on a daily basis, they are connected to communications suit-
ing even more relevant to global societal issues which are
able to support this. Internet, telephone, radio, and satellite
driving the need for a better understanding of the “Earth Sys-
communications are variously employed according to each
tem” in which we live. The IGS, though its participation in the
site’s characteristics. Obviously real time data streaming by
GGOS effort, can contribute in areas such as climate change,
stations participating in the RT-PP is a significant communi-
global mass transport, sea level rise, measuring surface geo-
cations challenge for some stations.
dynamics at a range of spatial scales, geohazard prediction
What are some of the issues facing the IGS tracking net-
and monitoring, and natural disaster mitigation (earthquakes,
work in the context of the “changing landscape” of GNSS?
volcanoes, tsunamis, etc.)
The authors list the following:
8 The International Committee on GNSS
• What will be the characteristics of the future IGS-type
geodetic receiver? What transmitted signals will it track? Another recent development is the establishment of the Inter-
• What is the most suitable station monument for future national Committee on GNSS (ICG), which was officially
IGS stations? Need they all be constructed to the highest established through the United Nations Office of Outer Space
possible stability standards? Or is it likely that there may Affairs (UNOOSA) in December 2005, following extensive
be several “tiers” of IGS station types? preparatory meetings and actions over several years in which
• How to define “minimum operational requirements” for the IGS played an active role. The members of the ICG are the
IGS stations for an era where the quality of derived IGS developers (or “providers”) of the GNSS systems and several
products must increase significantly in order to satisfy the other interested UN member states, while associate mem-
goals of GGOS (see below)? Are there generally accepted bers are mainly inter-governmental and non-governmental
standards for data quality, or other quality metrics, that organisations representing primarily users of GNSS, such as
can be adhered to? the IGS, IAG, etc. (UNOOSA 2008).

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Three meetings of the ICG have taken place, the first at the ence frame, Earth rotation, Earth observation(s) and research,
United Nations center in Vienna, Austria, in November 2006, positioning, navigation and timing and other applications that
the second in Bangalore, India, in early September 2007, and benefit society.
the third in Pasadena, California, in early December 2008.
Significant issues (from the point of view of the IGS) that 9.2 Long-term goals
were discussed included standardisation of geodetic and time
reference frames. Three recommendations relevant to these 1. Serve as the premier source of the highest-quality
issues were adopted in the final plenary session. GNSS related standards (conventions), data and products,
The IGS is thus playing an even more active role in the openly available to all user communities.
international context of GNSS. The latter itself is changing 2. Attract leading-edge expertise to pursue challenging,
rapidly, with further improvements of the GPS system (GPS innovative projects in a collegial, collaborative and crea-
IIM, GPS IIF, GPS III satellite constellations), the revival tive culture.
of the Russian GLONASS (likely to reach a complete con- 3. Incorporate and integrate new systems, technologies,
stellation again in the next year or two, with additional sys- applications and changing user needs into IGS products
tem developments on the horizon, including a possible move and services.
to CDMA signals), the European Galileo system of 30 sat- 4. Facilitate the integration of IGS into GGOS and other
ellites, and global or regional systems being developed by more broadly based earth observing and global naviga-
Japan, China and India, among others. High on the inter- tion systems and services.
national agenda is the compatibility and (where possible) 5. Maintain an international federation with committed con-
interoperability of these systems. The IGS will continue to tributions from its members, and with effective leadership,
take an active role in monitoring the progress of these sys- management and governance.
tems, investigating their utility for the highest-quality GNSS 6. Promote the value and benefits of IGS to society, the
products and services, and in providing its experience and broader scientific community, and in particular to policy
expertise, in particular with regards to the support of high- makers and funding entities.
accuracy research and applications based on the analysis of
GNSS signals.
Based on these, the new plan identifies three key strategies:
The IGS, as a source of independent expertise on matters
related to GNSS technology and its application to reference
frame definition, will be an important partner in global initia- 1. Deliver world-standard quality GNSS data and prod-
tives such as AFREF. This project seeks to establish a single, ucts to all users globally with leading-edge expertise and
high quality geodetic reference frame for the whole of the resources.
African continent. In partnership with Africa’s national sur- 2. Develop, integrate, and participate with new and chang-
veying and mapping agencies, and international associations ing GNSS systems and user needs to continuously
such as the IAG and the International Federation of Surveyors improve IGS services and to provide value to a broad
(FIG), the IGS is encouraging the establishment of a network range of users.
of continuously operating GNSS reference stations across the 3. Continuously improve the effectiveness of IGS manage-
continent. This network will provide the raw observations ment and governance to support future growth of the
which, when processed together with IGS products such as service.
precise GNSS orbits, will underpin all future geospatial data
gathering, geoscientific studies and navigation operations in The broad objectives remain unchanged, however a signifi-
Africa. cant number of the derived actions are new. The full plan will
be available shortly on the IGS website, as well as in printed
form.
9 The IGS strategic plan 2008–2012 The implementation of the Strategic Plan will be aided
by the formulation and execution of annual Implementation
Although much of the IGS Strategic Plan 2002–2007 remains Plans, in which the principal targets for the various elements
valid, a new plan was developed during 2006–2007. A mis- and projects will be defined for each calendar year.
sion statement and six long-term goals were formulated.

9.1 Mission 10 Concluding remarks

The International GNSS Service provides the highest-quality The IGS is continuing its mission of providing highest-qual-
GNSS data and products in support of the terrestrial refer- ity GNSS data and products in support of the terrestrial

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reference frame; Earth observations and research; position- Dow JM, Gendt G, Moore A, Neilan RE, Weber R (2004) The Inter-
ing, navigation and timing; and other applications that benefit national GPS Service—what’s next? 10th anniversary assembly
charts future directions. In: Proceedings of ION GNSS 2004, Long
society. The changing landscape of global navigation satellite Beach, CA, USA, 21–24 September, 1741–1748
systems necessitates that the IGS become even more involved Dow JM, Neilan RE, Gendt G (2005) The International GPS Service:
with international developments. Long-standing activities are celebrating the 10th anniversary and looking to the next decade.
being consolidated and new directions defined. This is doc- Adv Space Res 36:320–326. doi:10.1016/j.asr.2005.05.125
Dow JM, Neilan RE, Weber R, Gendt G (2007) Galileo and the IGS:
umented and supported by the recently developed IGS Stra- Taking advantage of multiple GNSS constellations. Adv Space Res
tegic Plan 2008–2012. 39:1545–1551. doi:10.1016/j.asr.2007.04.064
Gendt GS, Rothacher M, GGSP Prototype Team (2007) Realisation and
Acknowledgments The continuing contribution of very many indi- maintenance of the Galileo Terrestrial Reference Frame (GTRF).
viduals from many organisations world-wide to maintaining the high In: Proceedings of 1st colloquium on scientific and fundamental
quality of the IGS products is gratefully acknowledged. aspects of the Galileo programme, Toulouse, France, 1–4 October
GGOS (2008) http://www.ggos.org
IAG (2008) http://www.iag-aig.org
IERS (2008) http://www.iers.org
IGS (2008) http://igs.org
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