While I’m likely preaching to the choir here, GNSS cannot work unless we have an accurate description of the orbits of the satellites and the behavior of their atomic clocks. The accuracy with which this information is provided to a receiver or data processing software is the most important component of the error budget of GNSS positioning, navigation and timing and constitutes most of what is known as the signal-in-space (SIS) range error.
Each GNSS satellite broadcasts a description of its orbit or ephemeris along with the offset of its active clock from the system’s time standard in a navigation message decoded and used by the receiver. These data are predictions of the orbit and clock offset as computed by the system’s ground control segment and uploaded to each satellite. A recent assessment by U.S. Space Systems Command of the GPS SIS error averaged across all active satellites for a one-week period was about 50 centimeters, root-mean-square. While this is entirely adequate for many GNSS uses, it falls short of the required accuracy for high-demanding applications such as surveying, geodesy, atmospheric sensing, reference frame studies and tectonic monitoring. Which is why various organizations both private and public compute very accurate orbits and clocks and provide these to users. These computations, using data from global receiver networks, are very exacting and model the tiniest effects on the (primarily) carrier-phase measurements these receivers provide.
These effects include the offset in the electrical phase centers of a GNSS satellite’s transmitting antenna from the satellite’s center of mass and how that varies with the direction of the signal from the satellite to a receiver on Earth. Furthermore, this behavior must be calibrated and modeled for each radio frequency that the satellite transmits. Another effect that must be accounted for are the perturbations caused by non-perfect yaw-steering of a satellite’s solar panels. These panels continuously track the Sun but they have difficulty keeping up at orbit noon and midnight. Accurate models of the actual yaw angle are very important for high-precision GNSS orbits. As if these model requirements were not enough, the effect of solar radiation pressure on satellite orbits must also be modeled. While they don’t have (rest) mass, photons have energy and this can be imparted to satellites when they impinge on them. While a single photon has a negligible effect, the billions upon billions of photons making up sunlight do have a noticeable effect on a GNSS satellite’s motion and must be accounted for by orbit models.
One organization producing precise orbits for GNSS satellites – arguably the most precise in the world – is the International GNSS Service (IGS), a voluntary federation of more than 200 agencies, universities and research institutions across the globe. Several of these organizations each produce precise orbits, which they submit to the IGS to establish orbit products. One of these organizations is the Navigation Support Office (NSO) at the European Space Agency’s European Space Operations Centre. In this quarter’s Innovation column, a team of NSO engineers discusses how they have improved the orbit modeling of the GPS III satellites by around a factor of two with estimated orbit errors of about 2 centimeters or less. Wizardry? Not really – just rocket science.
The American Geophysical Union (AGU) hosts the meeting, which unites the global Earth and space science community to share findings, connect and advance the profession and its benefits for society. The conference generally has more than 25,000 attendees from more than 100 countries
A dedicated GNSS session was successful at the 2021 AGU. This year, the session is:
“As the title promises, it shall become a platform for interactions between people that are using our products and those that are generating them,” the AGU conveners wrote in an email to the CANSPACE Listserv. The conveners are Allison Craddock (IGS CB, Jet Propulsion Laboratory, USA), Felix Perosanz (CNES, France) and Rolf Dach (AIUB, Switzerland).
RINEX 4.00 (2021) is a major revision of the format document to modernize the navigation message files to be able to accommodate the new navigation messages from all the GNSS constellations, as well as system data messages such as ionospheric corrections, Earth orientation parameters and system time offsets.
The RINEX Working Group Chair Ignacio Romero provided an explanation to the GNSS community about the new RINEX 4.00 format that explains changes from previous RINEX versions.
The new format is also described in detail on the IGS Format and Standards page. For more information on the RINEX updates and activities, visit the RINEX WG Page.
The IGS adopted RINEX 4.00 during its 59th Governing Board Meeting on Dec. 7, 2021.
The IGS Workshop, scheduled for September 2021 by the International GNSS Service (IGS), has been postponed to 2022. The IGS Central Bureau voted for the postponement in December during its governing board meeting.
The IGS Workshop, originally intended to take place 2020, was postponed to 2021 because of travel restrictions associated with COVID-19. The dates of the workshop will be determined later.
RINEX 3.04 contains updates to support planned GLONASS CDMA signals, as well as new BeiDou III and QZSS II signals.
The International GNSS Service (IGS) and Radio Technical Commission Maritime Service, Special Committee -104 (RTCM SC-104) RINEX Working Group, announced the availability of RINEX 3.04.
RINEX 3.04 supports all publicly available signals from existing GNSS constellations: the U.S. GPS, Russia’s GLONASS, Europe’s Galileo, China’s BeiDou, Japan’s Quasi Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite System (IRNSS).
In addition to the new signals, the RINEX 3.04 text has been edited to improve the description of messages, fields and overall readability.
The RINEX 3.04 data standard documentation is available here:
Added new signals to the GLONASS (Table 5), QZSS II (Table 8) and BeiDou III (Table 9) signal tables and updated Table A23
Added section 9.12 to describe new signals from the GLONASS, QZSS II and BeiDou III constellations and to describe the differences between QZSS I and QZSS II
Updated Appendix Table A2 SYS/#/OBS TYPES to show new signal codes for: GLONASS, QZSS II and BeiDou III
Modified Appendix Table A5 TIME SYSTEM CORR section to clarify the GNSS time system and UTC difference fields
Added numerous small corrections and text improvements as listed in the RINEX 3.04 Revision History section.
Global satellite navigation systems are continuously bathing Earth in satnav signals. As well as helping in our daily lives, these signals are also tools for cutting-edge science. A new ESA facility, based at ESA’s astronomy centre near Madrid, is championing their use for everything from Earth monitoring to fundamental physics.
A Surveyor uses a GNSS device to map urban assets with Galileo and EGNOS. (Image: ESA)
The new ESA Global Navigation Satellite Systems (GNSS) Science Support Centre is based at ESA’s European Space Astronomy Centre, ESAC, near Madrid. Run by ESA’s Galileo Science Office, the GSSC integrates IT and satnav infrastructure to deliver advanced data processing services to the scientific community.
Precisely timed to a few billionths of a second and highly stable, satnav signals can be used as a point of reference for many scientific sectors, including Earth and atmospheric sciences, astronomy, highly precise timing metrology as well as the study of relativity and other fundamental physics topics.
Current satnav infrastructure plans worldwide should see more than 120 satnav satellites in orbit in coming years. This number includes Europe’s own Galileo constellation — offering unique features such as its highly stable passive hydrogen maser atomic clocks, multiple transmission frequencies, robust modulation, wide bandwidth and onboard laser retro-reflectors, which permit exact pinpointing of the satellites’ position in space down to a few tens of centimetres.
“The potential of satnav for science has been recognised for a long time,” explains Javier Ventura-Traveset, Head of ESA’s Galileo Science Office. “The Galileo Science Office was set up in 2016 as a joint initiative between ESA’s Science and Navigation Directorates, coordinating scientific opportunities through interaction with the scientific community and the independent GNSS Science Advisory Committee.
“The opening of the new centre is the next step. It is ESA’s concrete answer to the need expressed by the scientific community for a one-stop-shop to offer researchers long-term GNSS data, products information, results of scientific experiments, plus services to enhance GNSS scientific research and collaboration.
“The future evolution of the centre will be driven by the interaction and feedback received from the scientific community, maximizing synergies with other GNSS data service providers from other institutions and research organization.”
Among the activities to be supported by the new GSSC are big data processing of large amounts of satnav data, crowdsourcing as a means of weather monitoring and a scientific assessment of satnav performance in Antarctica.
It also supports the continuing measurements of general relativity using Galileo satellites 5 and 6 and serves as a global data centre for the International GNSS Service. The long-established Navipedia website, giving technical information on satnav, is also hosted by the GSCC.
One enthusiastic early adopter is ESA’s Navigation Support Office, based at ESA’s ESOC mission control centre in Darmstadt, Germany, lending support to mission teams making use of satnav to steer satellites.
“The GSSC is a welcome addition to ESA’s activities in the science of satellite navigation,” says Werner Enderle, heading ESOC’s Navigation Support Office. “The GSSC already hosts GNSS products generated by the team at ESOC, including observations from our worldwide EGON GNSS Observation Network and precise satellite orbits generated by their state-of-the-art software. Our two teams look forward to this collaboration continuing for the benefit of ESA and the scientific community.”
The GSSC will roll out access to data, products and services over the coming months.
As Galileo, BeiDou, the Quasi-Zenith Satellite System, the Indian Regional Navigation Satellite System, and a variety of satellite-based augmentation systems join GPS and GLONASS, we help celebrate the coming 25th anniversary of the IGS as a truly multi-GNSS service.
Editor’s note: Tables 1 and 3 in the print version of this article contain some incorrect values and missing designators. These errors have been corrected in the tables below.
INNOVATION INSIGHTS by Richard Langley
A QUARTER OF A CENTURY. That is how old the International GNSS Service (IGS) will be on Jan. 1, 2019. Conceived in the early 1990s as the International GPS Service for Geodynamics, the IGS continues to be the global standard bearer in providing receiver data, satellite orbit and clock products and other resources with the highest possible precision and accuracy. I remember the discussions that took place at international conferences about the need for such a service to provide the necessary data to advance our understanding of plate tectonics and other Earth-related phenomena. And this was well before GPS was officially declared fully operational in 1995. Remember, surveyors and geodesists were early adopters of GPS, making use of the technology even when only a partial GPS constellation was in place.
The initial ideas for the IGS were laid out in an article published in GPS World in February 1993 entitled “Geodynamics: Tracking Satellites to Monitor Global Change.” But the services provided by the IGS extended well beyond the needs of the geodynamics research community, and so its name was shortened to just the International GPS Service. When GLONASS data and products became available, the name was further changed to its current moniker.
One of the IGS’s notable achievements has been in advancing GNSS standards such as the Receiver-Independent Exchange format for receiver data and other information. The need for such a standard was clear even before the formation of the IGS, and it was documented in this column in the July 1994 issue of GPS World (“RINEX: The Receiver-Independent Exchange Format”). We continued to cover the evolution of the IGS over the years with, for example, the article “The International GNSS Service: Any Questions?” in the January 2007 issue of the magazine.
And now, as Galileo, BeiDou, the Quasi-Zenith Satellite System, the Indian Regional Navigation Satellite System, and a variety of satellite-based augmentation systems join GPS and GLONASS, we help celebrate the coming 25th anniversary of the IGS as a truly multi-GNSS service.
For going on 25 years, the International GNSS Service (IGS) has carried out its mission to advocate for, and provide, freely and openly available high-precision GNSS data, as well as derived operational data products, including satellite ephemerides, Earth rotation parameters, station coordinates and clock information. The IGS is a self-governed, voluntary federation of more than 300 contributing organizations from more than 100 countries around the world that collectively operate a global infrastructure of tracking stations, data centers and analysis centers to provide high-quality GNSS data products. The IGS products are provided openly for the benefit of all scientific, educational and commercial users.
The IGS was first approved by its parent organization, the International Association of Geodesy (IAG), at a scientific meeting in Beijing, China, in August 1993. A quarter of a century later, the IGS community gathers for a workshop in Wuhan, China, this November to blaze a path to multi-GNSS through global collaboration.
As a key component of the IAG’s global geodetic infrastructure, the IGS contributes to, extends and densifies the International Terrestrial Reference Frame (ITRF) of the International Earth Rotation and Reference Systems Service (IERS). The ITRF provides an accurate and consistent spatial frame for referencing positions at different times and in different locations around the world.
In addition, IGS products enable the use of GNSS technologies for scientific applications such as the monitoring of solid Earth deformations, monitoring of Earth rotation and variations in the liquid Earth, and for scientific satellite orbit determinations, precise timing, ionosphere monitoring and water vapor measurements.
IGS products are also considered critical by surveying, geomatics and geo-information users around the world, who rely on them on a daily basis to improve efficiency. Many applications that require reliable, accurate GNSS positioning in construction, agriculture, mining, exploration and transportation also benefit from the IGS.
Community Collaboration
At the heart of the IGS is a strong culture of sharing expertise, infrastructure and other resources for the purpose of encouraging global best practices for developing and delivering GNSS data and products all over the world. The collaborative nature of the IGS community leverages this diversity to integrate and make full use of all available GNSS technologies while promoting further innovation.
More than 15,000 geodetic community members, some of whom comprise the backbone of the worldwide geodetic community, ensure that new technologies and systems are integrated into operational IGS products. Responsive to this innovation, the IGS develops and publicly releases standards, guidelines and conventions for the collection and use of GNSS data and the aforementioned products.
The IGS strives to maintain an international federation with committed contributions from its members. Participation of individuals and organizations is often driven by user needs, a key characteristic of the inclusive culture within the IGS.
Structure of the IGS
The IGS consists of a central bureau, a global network of GNSS stations, data and analysis centers and a number of working groups all coordinated and overseen by a governing board.
Central Bureau. The IGS Central Bureau (CB) functions as the secretariat of the IGS, providing continuous management and technology to sustain the multifaceted efforts of the IGS in perpetuity. The CB responds to the directives and decisions of the IGS governing board. It coordinates the IGS tracking network and operates the CB information system, the principal information portal where the IGS web, FTP and mail services are hosted (www.igs.org). The CB also represents the outward face of IGS to a diverse global user community, as well as the general public. The CB office is hosted at the California Institute of Technology/Jet Propulsion Laboratory in Pasadena, California. It is funded principally by the U.S. National Aeronautics and Space Administration (NASA), which generously contributes significant resources to advance the IGS.
The IGS Network. The foundation of the IGS is a global network of more than 500 permanent and continuously operating stations of geodetic quality. These stations track signals from GPS, and increasingly also track signals from GLONASS, Galileo, BeiDou, the Quasi-Zenith Satellite System (QZSS), the Indian Regional Navigation Satellite System (IRNSS; also known as NavIC: Navigation with Indian Constellation), as well as space-based augmentation systems (SBAS).
FIGURE 1 shows the recent state of the IGS network, indicating which stations are GPS only, GPS+GLONASS and multi-GNSS. FIGURE 2 is a photo of the IGS station ARHT at McMurdo Station, Antarctica.
FIGURE 1 . The extent of the IGS network in 2017, showing the locations of stations monitoring just GPS, GPS and GLONASS, and GPS and GLONASS plus at least one other constellation. (Map: IGS)FIGURE 2. The consistency of the final GPS satellite orbit solutions from individual IGS analysis centers over the past 25 years. Each line depicts the solution of one analysis center, as compared to the weighted mean. COD: Center for Orbit Determination in Europe, EMR: Natural Resources Canada (formerly Energy, Mines and Resources Canada), ESA: European Space Agency, GFZ: GeoForschungsZentrum (German Research Centre for Geosciences); GRG: Centre National d’Etudes Spatiales (Groupe de Recherche de Géodésie Spatiale); JPL: Jet Propulsion Laboratory; MIT: Massachusetts Institute of Technology; NGS: National Geodetic Survey; SIO: Scripps Institution of Oceanography; IGR: IGS rapid product. (Graph courtesy of T. Herring, MIT and M. Moore, Geoscience Australia)
The IGS is a critical component of the IAG’s Global Geodetic Observing System (GGOS), where it encourages and advocates for geometrical linkages of GNSS with other precise geodetic observing techniques, including satellite and lunar laser ranging, very long baseline interferometry and Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS). These linkages are fundamental to generating and accessing the ITRF.
Data and Analysis Centers. Lots of hard work and dedication from IGS contributing organizations goes into the fabrication of IGS products, which start at the tracking network, then are collected by data centers and sent to analysis centers. At these centers, the data are compared and combined by the analysis center coordinator, and finally made available as IGS products.
The IGS ensures high reliability by building redundancy into all of its components. In 1994, the IGS started with a network of about 40 stations; today, more than 500 receivers are included in the network. Critical to this activity are three categories of data center — operational, regional and global. At the ground level are operational data centers, which are in direct contact with IGS tracking sites and are responsible for such efforts as station monitoring and local archiving of GNSS tracking data. Operational data centers also validate, format, exchange and compress data. Regional data centers then collect tracking data from multiple operational data centers or stations, maintaining a local archive and providing online access to their data.
The six global data centers receive, retrieve, archive and provide online access to tracking data from operational and regional data centers. These global data centers are also responsible for archiving and backing up IGS data and products, and maintaining a balance of data holdings across the IGS network.
Analysis centers then receive and process tracking data from one or more data centers to generate IGS position, orbit and clock products. These products are produced in ultra-rapid, rapid, final and reprocessed versions for each analysis center.
FIGURE 3 shows the huge improvement in the precision and accuracy of the final orbit submissions from the analysis centers over the past 25 years.
Associate analysis centers produce specialized products, such as ionospheric information, tropospheric parameters or station coordinates and velocities for global and regional sub-networks. Regional and global network associate analysis centers complement this work as new capabilities and products emerge within the IGS.
FIGURE 3. The antenna of IGS station ARHT at McMurdo Station, Antarctica. (Photo: IGS)
Products from each analysis center are then combined into a single set of orbit and clock products by the analysis center coordinator, who monitors and assists the activities of analysis centers to ensure IGS standards for quality control, performance evaluation and analysis are successfully executed. The different analysis solutions ultimately verify the accuracy of IGS products, provide important redundancy in the case of errors in a particular solution, and average out modeling deficiencies of a particular software package.
TABLE 1 shows the quality of service characteristics of the various IGS GPS and GLONASS orbit and clock products. Similarly, TABLES 2, 3 and 4 show the characteristics of the tracking station coordinates, Earth rotation parameters and atmospheric parameters. See www.igs.org/products for further details.
TABLE 1. Quality of service characteristics for IGS orbit and clock products relating to GPS and GLONASS satellite orbits and satellite (sat.) and station (stn.) clocks as of 2017. (Data: IGS)TABLE 2. Quality of service characteristics for tracking station positions and velocities. (Data: IGS)TABLE 3. Quality of service characteristics for Earth rotation parameters: polar motion coordinates and rates of change and length-of-day (µas = microarcsecond). (Data: IGS)TABLE 4. Quality of service characteristics for atmospheric parameters: tropospheric zenith path delay and gradients and global grids of total electron content. (Data: IGS)
Working Groups and Projects
The IGS technical working groups (WGs) focus on topics of particular interest to the IGS, and consider various aspects of product generation and monitoring. The current working groups of the IGS span topics from antennas to tide gauges.
Antenna Working Group. To increase the accuracy and consistency of IGS products the Antenna WG coordinates research on GNSS receiver and satellite antenna phase-center determination. The group manages official IGS receiver and satellite antenna files and their formats.
Bias and Calibration Working Group. Different GNSS observables are subject to different satellite biases, which can degrade the IGS products. The Bias and Calibration WG coordinates research in the field of GNSS bias retrieval and monitoring.
Clock Products Working Group. This group is responsible for aligning the combined IGS products to a highly precise timescale traceable to the world standard: Coordinated Universal Time (UTC). The IGS clock product coordinator forms the IGS timescales based on the clock solutions of IGS analysis centers, and IGS rapid and final products are aligned to these timescales.
Data Center Working Group. The Data Center WG works to improve the provision of data and products from the operational, regional and global data centers, and recommends new data centers to the IGS governing board.
Joint GNSS Monitoring and Assessment Working Group. This working group, in conjunction with a joint trial project with International Committee on GNSS’s (ICG) International GNSS Monitoring and Assessment (IGMA) Task Force, seeks to install, operate and further develop a GNSS Monitoring and Assessment Trial Project.
GNSS Performance Monitoring ICG-IGS Joint Trial Project. The quality of navigation signals enables numerous applications, including worldwide time and frequency transfer and GPS meteorology. This project of the IGMA task force, coordinated in partnership with the IGS, focuses on monitoring GNSS constellation status.
Ionosphere Working Group. This group produces global ionosphere maps of ionosphere vertical total electron content (TEC). A major task of the Ionosphere WG is to make available global ionosphere maps from the TEC maps produced independently by ionosphere associate analysis centers within the IGS.
FIGURE 4 shows an example TEC map recomputed from data collected on March 17, 2015. The large values of TEC in the ionosphere’s equatorial anomaly are plainly visible.
FIGURE 4. An example total electron content map recomputed from data collected on March 17, 2015. TECU: total electron content units. (Image: IGS)
Multi-GNSS Working Group. This group supports the Multi-GNSS Experiment (MGEX) Project by facilitating estimation of intersystem biases and comparing the performance of multi-GNSS equipment and processing software. The MGEX Project was established to track, collate and analyze all available GNSS signals including those from BeiDou, Galileo and QZSS in addition to GPS and GLONASS.
Reference Frame Working Group. This working group combines solutions from the IGS analysis centers to form the IGS station positions and velocity products, and Earth rotation parameters for inclusion in the IGS realization of ITRF. A new reference frame, called IGS14, was adopted on Jan. 29, 2017 (GPS Week 1934). At the same time, an updated set of satellite and ground antenna calibrations, igs14.atx, was implemented.
Real-Time Working Group. The Real-Time WG supports the development and integration of real-time technologies, standards and infrastructure to produce high-accuracy IGS products in real time. The group operates the IGS Real-Time Service (RTS) to support precise point positioning (PPP) at global scales, in real time.
RINEX Working Group. The RINEX-WG jointly manages the Receiver-Independent Exchange (RINEX) format with the Radio Technical Commission for Maritime Services Special Committee 104 (RTCM-SC104). RINEX has been widely adopted as an industry standard for archiving and exchanging GNSS observations, and newer versions support multiple GNSS constellations. Recently, the IGS governing board agreed to adopt the official RINEX V3.04 format, handling the ability for nine-character station ID and fixing the definition of GNSS reference time scales.
Space Vehicle Orbit Dynamics Working Group. This group brings together IGS groups working on orbit dynamics and attitude modeling of spacecraft. This work includes the development of force and attitude models for new GNSS constellations to fully exploit all new signals with the highest possible accuracy.
Troposphere Working Group. The Troposphere WG supports development of IGS troposphere products by combining troposphere solutions from individual analysis centers to improve the accuracy of PPP solutions. The goal of the Troposphere WG is to improve the accuracy and usability of GNSS-derived troposphere estimates.
Tide Gauge (TIGA) Working Group. When studying sea level changes, where the GPS height of the benchmark is used for defining an absolute sea-level datum, problems occur when correcting the time series for height changes of the benchmark. TIGA is a pilot study for establishing a service to analyze GPS data from stations at or near tide gauges in the IGS network to support accurate measurement of sea-level change across the globe.
A Multi-GNSS IGS Network
The development of a multi-GNSS sub-network within the greater IGS network, led by the MGEX Project, develops the IGS’s capability to operate with multiple GNSS constellations. It has 223 multi-GNSS-capable (GPS + GLONASS + at least one other constellation) stations. Also, the number of IGS stations capable of real-time data streaming in support of the IGS Real-Time Project has increased to 195.
MGEX was founded in 2012 to build a network of GNSS tracking stations, characterize the space segment and user equipment, develop theory and data-processing tools, and generate data products for emerging satellite systems. The stations within its network contain a diverse assortment of receiver and antenna equipment, which are recognized and characterized by the IGS in equipment description files. Other than GPS and GLONASS, no combination process has yet been implemented within IGS for precise orbit and clock products of the other, newer, constellations. Despite this, cross-comparison among analysis centers, as well as with satellite laser ranging, has been used to assess the precision or accuracy for various products.
The growing role of multi-GNSS within the IGS network was benchmarked by the transition of MGEX to official IGS project status in 2016. For the sake of consistency, and as a nod to its heritage, use of the acronym “MGEX” has been retained.
Making Strides in Real Time
Through the Real-Time Service (RTS), the IGS extends its capability to support applications requiring real-time access to IGS products. The RTS is a GNSS orbit and clock correction service that enables PPP and related applications, such as time synchronization and disaster monitoring, at worldwide scales. The RTS is based on the IGS global infrastructure of network stations, data centers and analysis centers that provide world-standard high-precision GNSS data products.
The RTS is currently offered as a GPS-only operational service, but GLONASS is initially being offered as an experimental product for the development and testing of applications. GLONASS will be included within the service when the IGS is confident that a sufficient number of analysis centers can ensure solution reliability and availability. Other GNSS constellations will be added as they become available.
Engagement with the United Nations
The IGS engages with diverse organizations, outside of the immediate precise GNSS community, that have an interest in geodetic applications of GNSS. Notably, the IGS has supported the development of the Global Geodetic Reference Frame resolution, roadmap and implementation plan within the United Nations Global Geospatial Information Management (GGIM) Committee of Experts.
The IGS also works with the United Nations Office for Outer Space Affairs (UNOOSA) International Committee on GNSS (ICG) to develop common understandings of the requirements for multiple system monitoring through the joint pilot project with the ICG’s IGMA subgroup. The IGS also co-chairs ICG Working Group D, which focuses on reference frames, timing and applications.
A Multi-GNSS Future
Though the accuracy of current IGS multi-GNSS products lags behind standard IGS products for GPS and GLONASS, multi-GNSS paves the way for complete exploitation of new signals and constellations in navigation, surveying, geodesy and remote sensing.
IGS also looks externally to other techniques through its participation in the IAG’s GGOS, which has illuminated how satellite laser ranging observations to GNSS satellites improves our understanding of observational errors and thus drives further improvement of IGS position, clock and orbit products.
As it enters its second quarter-century, the IGS is evolving into a truly multi-GNSS service. For 25 years, IGS data and products have been made openly available to all users for use without restriction, and continue to be offered free of cost or obligation. In turn, users are encouraged to participate within the IGS, or otherwise contribute to its advancement.
Acknowledgements
The authors gratefully acknowledge the contributions of the IGS governing board and associate members in the drafting of this article. Special thanks to Anna Riddell and Grant Hausler, who, along with Gary Johnston, have an extensive chapter on IGS in the Springer Handbook of Global Navigation Satellite Systems, published in 2017 by Springer (see Further Reading). This book chapter is the new recommended official citation for publications referencing IGS data, products and other resources.
Allison Craddock a member of the Geodynamics and Space Geodesy Group in the Tracking Systems and Applications Section at the NASA Jet Propulsion Laboratory in Pasadena, California. She is the director of the IGS Central Bureau, manager of external relations for the International Association of Geodesy’s Global Geodetic Observing System, and staff member of the NASA Space Geodesy Program.
Gary Johnston is the head of the National Positioning Infrastructure Branch at Geoscience Australia. Johnston is the chair of the IGS governing board and the co-chair of the Subcommittee on Geodesy under the United Nations Global Geospatial Information Management committee of experts.
FURTHER READING
GNSS Handbook Chapter on IGS
“The International GNSS Service” by G. Johnston, A. Riddell and G. Hausler, Chapter 33 in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
International GNSS Service Technical Report 2017 (IGS Annual Report), edited by A. Villiger and R. Dach, published by IGS Central Bureau and University of Bern, Bern Open Publishing, Bern, Switzerland, 2018, doi: 10.7892/boris.116377. Includes reports from analysis centers, data centers and working groups.
“The Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS) – Achievements, Prospects and Challenges by O. Montenbruck. P. Steigenberger, L. Prange, Z. Deng, Q. Zhao, F. Perosanz, I. Romero, C. Noll, A. Stürze, G. Weber, R. Schmid, K. MacLeod and S. Schaer in Advances in Space Research, Vol. 59, No. 7, April 1, 2017, pp. 1671–1697, doi: 10.1016/j.asr.2017.01.011.
“Data Formats” by O. Montenbruck and K. MacLeod, Annex A in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
A new version of the open source PPP-Wizard user software has been published. The link to download the wizard is being provided on request, so the International GNSS Service (IGS) can keep track of interested users.
The PPP-Wizard is defined as a precise point positioning with integer and zero-difference ambiguity resolution demonstrator. Available for non-commercial purposes, it performs real-time PPP using corrections streams provided by the IGS Real Time Service. It features:
GPS and GLONASS code and phase measurements (mono or dual frequency)
Ambiguity resolution on GPS thanks to the new standardized phase biases messages (with the compatible CLK91 stream)
Advanced RAIM
SBAS iono for single-frequency receivers (u-blox and nvs receivers)
Fast reconvergence using iono estimation
Compatibility with rtklib and BNC (rtrover interface)
Multiple receivers processing
Preparation for augmented regional networks (tropo & iono interface)
C/C++ portable and light implementation
The PPP-Wizard demonstrator is a “proof of concept” of the zero-difference ambiguity resolution method developed in the orbit determination service at CNES.
The International GNSS Service (IGS) has published a new version of the Open Source BKG NTRIP Client (BNC) for real-time GNSS applications and RINEX3 QC.
Follow this IGS link to download precompiled binaries for Linux and Windows systems. Version 2.11.0 includes a bug fix for GLONASS code biases as well as RTCM SSR message updates for PPP. The maximum number of GNSS observations has been increased and the loss of lock handling modified. Also, BNC now supports raw stream output through TCP/IP port.
The source code is available via SVN software archive. A Mac binary is under preparation.
Knowledge-base sections have been created for IGS Working Groups to maintain content for public access. The site administrators ask Working Group chairs to provide updated content to their sections.
Other sections include IGS Real-Time Service, IGS Presents (videos), IGS Multi-GNSS Experiment, news and events.
Because content has been extensively reorganized with this revision, links from many external websites will need to be updated, and the IGS apologizes for any inconvenience.
On June 2, GPS World reported that GPS tracking stations co-sponsored by U.S. interests but located in Russia had stopped making their data available to scientists and others.
Now, “It looks like the tap has been turned back on, at least at slow flow,” reports Richard Langley, GPS World Innovation editor and University of New Brunswick professor. “Hourly and daily data files from the affected stations are once again being sent to IGS data archive centres. Grigory Steblov of the Geophysical Survey of the Russian Academy of Sciences has reported that ‘the transmission of the data from NEDA [North Eurasian Deformation Array] GPS sites [had] been temporarily suspended due to technical reasons.’
Now, after reorganization, the data flow is being resumed on hourly basis.”
Langley stresses, however, that the real-time flow of data from the NEDA stations has not been turned back on, which is important for some applications.
As announced by Russian Deputy Prime Minister Dmitry Rogozin on May 13, 2014, GPS tracking stations co-sponsored by U.S. interests have stopped making their data available to scientists and others.
The tap on the flow of data from 11 stations was turned off starting on May 31. The data flow included hourly and daily data files from the stations as well as the real-time flow of data over the Internet.
In an item entitled “On Execution of the Instructions of the Government of the Russian Federation,” the website of Roscosmos, the Russian Space Agency, reported:
“In accordance with the instructions of the Government of the Russian Federation, the Russian Space Agency in conjunction with the Federal Agency scientific organizations on June 1, 2014, implemented measures to avoid the use of information from the global seismographic network stations operating on the signals of the GPS system and located in the Russian Federation, for purposes not covered by existing agreements, including military uses.” (As translated by Google Translate.)
It should be pointed out that none of the affected stations contribute to the day-to-day running of GPS; that is, they are not part of the GPS command and control network. They are stations participating in the work of the International GNSS Service, which provides data and products to scientists and other researchers for different purposes including geodesy, geodynamics, orbital mechanics, and atmospheric studies.
It is believed that the Russian move is a tit-for-tat exercise in response to sanctions by western countries following recent events in Ukraine. However, the Russians say that the action was initiated by the refusal of the U.S. to enter into negotiations on the placement of Russian-operated GLONASS tracking stations on U.S. territory. Russia wishes to expand its global network of differential correction and monitoring stations, which could conceivably be also used to supply data for GLONASS command and control purposes.
What isn’t widely known is that Roscosmos already uses sites on U.S. territory for monitoring the availability and health of the GLONASS satellites as the map below clearly shows.
