International VLBI Service
for Geodesy and Astrometry

1. SUMMARY

1. Charter.

The International VLBI Service for Geodesy and Astrometry (IVS) is an international collaboration of organizations which operate or support Very Long Baseline Interferometry (VLBI) components. IVS provides a service which supports geodetic and astrometric work on reference systems, Earth science research, and operational activities.

IVS is an official Service of the International Association of Geodesy (IAG).

2. Objectives.

IVS fulfills its charter through the following objectives. The primary objective of IVS is to foster VLBI programs as a joint service. This is accomplished through close coordination to provide high-quality VLBI data and products.

The second objective of IVS is to promote research and development activities in all aspects of the geodetic and astrometric VLBI technique. This objective also supports the integration of new components into IVS. The further education and training of VLBI participants is supported through workshops, reports, electronic network connections, and other means.

The third objective of IVS is to interact with the community of users of VLBI products and to integrate VLBI into a global Earth observing system. IVS interacts closely with the International Earth Rotation Service (IERS) which is tasked by the IAU and IUGG with maintaining the international celestial and terrestrial reference frames and with monitoring Earth rotation.

To meet these objectives, IVS coordinates VLBI observing programs, sets performance standards for VLBI stations, establishes conventions for VLBI data formats and data products, issues recommendations for VLBI data analysis software, sets standards for VLBI analysis documentation, and institutes appropriate VLBI product delivery methods to ensure suitable product quality and timeliness. IVS closely coordinates its activities with the astronomical community because of the dual use of many VLBI facilities and technologies for both astronomy and astrometry/geodesy.

IVS accepts observing proposals for research and operational programs that conform to the IVS objectives.

3. Data Products

VLBI data products contribute uniquely to these important determinations:
• definition and maintenance of the celestial reference frame
• monitoring universal time (UT1) and length of day (LOD)
• monitoring the coordinates of the celestial pole (nutation and precession)
These results are the foundation of many scientific and practical applications requiring the use of an accurate inertial reference frame, such as high-precision navigation and positioning. IVS provides, through the collaborative efforts of its components, a variety of significant VLBI data products with differing applications, timeliness, detail, and temporal resolution, such as:
• all components of Earth orientation parameters at regular intervals
• terrestrial reference frame
• VLBI data in appropriate formats
• VLBI results in appropriate formats
• local site ties to reference points
• high-accuracy station timing data
• surface meteorology, tropospheric and ionospheric measurements
All VLBI data products are archived in IVS Data Centers and are publicly available.

4. Research

The VLBI data products are used for research in many related areas of geodesy, geophysics, and astrometry, such as:
• UT1 and polar motion excitation (over periods of hours to decades)
• solid Earth interior research (mantle rheology, anelasticity, libration, core modes, nutation/precession)
• characterization of celestial reference frame sources and improvements to the frame
• tidal variations (solid Earth, oceanic, and atmospheric)
• improvements in the terrestrial reference frame, especially in the vertical (scale) component
• climate studies
To support these activities, there are ongoing research efforts whose purpose is to improve and extend the VLBI technique in such areas as:
• improvements in data acquisition and correlation
• refined data analysis techniques
• spacecraft tracking (Earth-orbiting and interplanetary)
• combination of VLBI data and results with other techniques

2. PERMANENT COMPONENTS

IVS acquires VLBI data, correlates the data, analyzes the data to produce geodetic, astrometric, and other results, and archives and publicizes data products. IVS accomplishes its goals through the types of permanent components described in this section. IVS will accept proposals at any time for a permanent component. Such proposals will be reviewed by the Directing Board. The seven types of IVS permanent components are:

• Network Stations
• Operation Centers
• Correlators
• Analysis Centers
• Data Centers
• Technology Development Centers
• Coordinating Center

IVS acquires VLBI data, correlates the data, analyzes the data to produce geodetic and astrometric results, and archives and publicizes data products. IVS accomplishes its goals through the operational components described below.

1. Network Stations

The IVS observing network consists of high performance VLBI stations.

• Stations can be dedicated to geodesy or have multiple uses (including astronomical observations or satellite tracking applications).
• Stations comply with performance standards for data quality and operational reliability set up by the Directing Board.
• VLBI data acquisition sessions are conducted by groups of Network Stations that are distributed either globally or over a geographical region.

2. Operation Centers

The IVS Operation Centers coordinate the routine operations of one or more networks. Operation Center activities include:

• planning network observing programs,
• establishing operating plans and procedures for the stations in the network,
• supporting the network stations in improving their performance,
• making correlator time available at an IVS Correlator,
• generating the detailed observing schedules for use in data acquisition sessions by IVS Network Stations,
• posting the observing schedule to an IVS Data Center for distribution and to the Coordinating Center for archiving.
IVS Operation Centers follow guidelines from the Coordinating Center for timeliness and schedule file formats. Operation Centers cooperate with the Coordinating Center in order to define:
• the annual master observing schedule,
• the use of antenna time,
• tape availability and shipping,
• the use of other community resources.

3. Correlators

The IVS Correlators process raw VLBI data and station log files following a data acquisition session. Their other tasks are to:

• provide immediate feedback to the Network Stations about problems that are apparent in the data,
• jointly maintain the geodetic/astrometric community's tape pool,
• make processed data available to the Analysis Centers,
• regularly compare processing techniques, models, and outputs to ensure that data from different Correlators are identical.

4. Analysis Centers

The IVS coordinates VLBI data analysis to provide high-quality products for its users. The analyses are performed by Analysis Centers and by Associate Analysis Centers.

Analysis Centers are committed to produce series of Earth Orientation Parameters (EOP) or series of individual EOP components, without interruption and at a specified time lag to meet IVS requirements. In addition, Analysis Centers produce station coordinates and source positions in regular intervals.

The Analysis Centers place their final results in IVS Data Centers for dissemination to researchers and other users. They adhere to IVS recommendations for the creation of high-quality products and their timely archiving and distribution. Any deviations that an Analysis Center makes from IVS recommendations are properly documented. Analysis Centers provide timely feedback about station performance. In addition to these regular services, Analysis Centers may also perform any task of an Associate Analysis Center.

Associate Analysis Centers are committed to regularly submit specialized products using complete series or subsets of VLBI observing sessions. The analysis is performed for specific purposes as recognized by the Directing Board such as exploitation of VLBI data for new types of results, investigations of regional phenomena, reference frame maintenance, or special determinations of Earth orientation parameters. The Associate Analysis Centers place their final results in IVS Data Centers for dissemination to researchers and other users. They adhere to IVS recommendations for the creation of high-quality products and their timely archiving and distribution. Any deviations that an Associate Analysis Center makes from IVS recommendations are properly documented.

5. Data Centers

The IVS Data Centers are repositories for VLBI observing schedules, station log files, and data products. Data Centers may mirror other Data Centers to make the distribution and maintenance of data more efficient and reliable.

• Data Centers are the primary means of distributing VLBI products to users.
• Data Centers work closely with the Coordinating Center and with the Analysis Centers to ensure that all the information and data required by IVS components are quickly and reliably available.

Data Centers provide the following functions:

• receive and archive schedule files from Operation Centers,
• receive and archive log files and ancillary data files from the Network Stations,
• receive and archive data products from the Analysis Centers,
• provide access and public availability to IVS data products for all users.

6. Technology Development Centers

The IVS Technology Development Centers contribute to the development of new VLBI technology. They may be engaged in hardware and/or software technology development, or evolve new approaches that will improve the VLBI technique and enhance compatibility with different data acquisition terminals. They will:
• design new hardware,
• investigate new equipment,
• develop new software for operations, processing or analysis,
• generate new information systems,
• develop, test, and document prototypes of new equipment or software,
• assist with deployment, installation, and training for any new approved technology.
• After dissemination of the new hardware or software, the centers may continue to provide maintenance and updating functions.

7. Coordinating Center

The IVS Coordinating Center is responsible for coordination of both the day-to-day and the long-term activities of IVS, consistent with the directives and policies established by the Directing Board. Specifically, the Coordinating Center monitors, coordinates, and supports the activities of the Network Stations, Operation Centers, Correlators, Data Centers, Analysis Centers, and Technology Development Centers. The Coordinating Center works closely with the Technology Coordinator, the Network Coordinator, and the Analysis Coordinator to coordinate all IVS activities.

The primary functions of the Coordinating Center are to:

• coordinate observing programs approved by the Directing Board,
• maintain the master schedule of observing sessions, coordinating the schedule with astronomical observing programs and with IVS networks,
• foster communications among all components of the IVS,
• define the best use of community resources,
• develop standards for IVS components,
• provide training in VLBI techniques,
• organize workshops and meetings, including an annual IVS technical meeting,
• produce and publish reports of activities of IVS components,
• maintain the IVS information system and archive all documents, standards, specifications, manuals, reports, and publications,
• provide liaison with the IERS, IAG, IAU, FAGS, and other organizations,
• provide the Secretariat of the Directing Board.

3. COORDINATORS

1. Network Coordinator

The IVS Network Coordinator is selected by the Directing Board from responses to an open solicitation to all IVS components. The Network Coordinator represents the IVS Networks on the Directing Board and works closely with the Coordinating Center. The Network Coordinator is responsible for stimulating the maintenance of a high quality level in the station operation and data delivery. The Network Coordinator performs the following functions:
• monitors adherence to standards in the network operation,
• participates in the quality control of the data acquisition performance of the network stations,
• tracks data quality and data flow problems and suggests actions to improve the level of performance,

The Network Coordinator works closely with the geodetic and astronomical communities who are using the same network stations for observations. The Coordinator takes a leading role in ensuring the visibility and representation of the Networks.

2. Analysis Coordinator

The IVS Analysis Coordinator is selected by the Directing Board from responses to an open solicitation to the IVS Analysis Centers. The Analysis Coordinator is responsible for coordinating the analysis activities of IVS and for stimulating VLBI product development and delivery. The Analysis Coordinator performs the following functions:
• fosters comparisons of results from different VLBI analysis software packages and different analysis strategies,
• encourages analysis software documentation,
• participates in comparisons of results from different space geodetic techniques,
• monitors Analysis Centers' products for high quality results and for adherence to IVS standards and IERS Conventions,
• ensures that analysis products from all Analysis Centers are archived and available for the scientific community, and
• forms the official products of IVS, as decided by the IVS Directing Board, using a suitable combination of the analysis results submitted by the Analysis Centers.

The Analysis Coordinator works closely with the geodetic and astronomical communities who are using some of the same analysis methods and software. The Analysis Coordinator plays a leadership role in the development of methods for distribution of VLBI products so that the products reach the widest possible base of users in a timely manner. The coordinator promotes the use of VLBI products to the broader scientific community and interacts with the IVS Coordinating Center and with the IERS.

3. Technology Coordinator

The IVS Technology Coordinator is selected by the Directing Board from responses to an open solicitation to the IVS Technology Development Centers. The Technology Coordinator is responsible for coordinating the new technology activities of IVS and for stimulating advancement of the VLBI technique. The Technology Coordinator performs the following functions:
• maintains cognizance of all current VLBI technologies and ongoing development
• coordinates development of new technology among various IVS components
• helps promulgate new technologies to the geodetic/astrometric community
• strives to ensure the highest degree of global compatibility of VLBI data acquisition systems

The Technology Coordinator works closely with the astronomical community because of the many parallels between the technology development required for both groups.

4. DIRECTING BOARD

1. Roles and Responsibilities

The Directing Board determines policies, adopts standards, and approves the scientific and operational goals for IVS. The Directing Board exercises general oversight of the activities of IVS including modifications to the organization that are deemed appropriate and necessary to maintain efficiency and reliability.

A specific function of the Board is to set scientific goals for the IVS observing program. The Board will establish procedures for external research programs and will review any proposals thus received.

The Board may determine appropriate actions to ensure the quality of the IVS products and that the IVS components maintain the adopted standards.

2. Membership

The Directing Board consists of appointed members who serve ex officio, members elected by the Directing Board, and members elected by the IVS components. The members are:

Appointed members ex officio:

• IAG representative
• IAU representative
• IERS representative
• FAGS representative
• Coordinating Center Director

Through a reciprocity agreement between IVS and IERS the IVS serves as the VLBI Technique Center for IERS, and as such its designated representative(s) serve on the IERS Directing Board. In turn, the IERS Directing Board designates a representative to the IVS Directing Board. This arrangement is to assure full cooperation between the two services.

Selected by Directing Board upon review of proposals from IVS Member Organizations:

• Technology, Network, and Analysis Coordinators (3 total)

Elected by Directing Board upon recommendation from the Coordinating Center (see below):

• Members at large (3)

Elected by IVS Components (see below):

• Correlators and Operation Centers representative (1)
• Analysis and Data Centers representative (1)
• Networks representatives (2)
• Technology Development Centers representative (1)

Total number:    16

The five appointed members are considered ex officio and are not subject to institutional restrictions. The FAGS representative is a non-voting member in accordance with FAGS requirements.

The five members of the Directing Board who are elected by IVS Permanent Components must each be a member of a different IVS Member Organization. All elected members serve staggered four-year terms once renewable.

At large members are intended to ensure representation on the Directing Board of each of the components of IVS and to balance representation from as many countries and institutions and IVS interests as possible. At large members serve 2-year terms once renewable.

A Board member who departs before the end of his/her term is replaced by a person selected by the Directing Board. The new member will serve for the remainder of the original term.

The three Coordinators are selected by the Directing Board on the basis of proposals from IVS Member Organizations. On a two-thirds vote the Directing Board may call for new proposals for any Coordinator when it determines that a new Coordinator is required. Coordinators are encouraged to give at least three months notice before resigning.

3. Elections

Election of Board members by the IVS components shall be conducted by a committee of three Directing Board members, the chair of which is appointed by the chair of the Directing Board. The committee solicits nominations for each representative from the relevant IVS components. For each position, the candidate who receives the largest number of votes from the Associate Members will be elected. In case of a tie the Directing Board will make the decision.

4. Chair

The chair is one of the Directing Board members and is elected by the Board for a term of four years with the possibility of reelection for one additional term. The chair is the official representative of IVS to external organizations.

5. Decisions

Most decisions by the Board are made by consensus or by simple majority vote of the members present. In case of a tie, the chair shall vote but otherwise does not vote. If a two-thirds quorum is not present, the vote shall be held later by electronic mail. A two-thirds vote of all Board members is required to modify the Terms of Reference, to change the chair, or to change any of the members elected by the Directing Board before the normal term expires.

6. Meetings

The Board meets at least annually, or more frequently if meetings are called by the chair or at the request of at least three Board members. The Board will conduct periodic reviews of the IVS organization and its mandate, functions, and components. The reviews should be done every four years.

5. DEFINITIONS

1. Member Organizations

Organizations that support one or more IVS components are IVS Member Organizations. Individuals associated with IVS Member Organizations may become IVS Associate Members.

2. Affiliated Organizations

Organizations that cooperate with IVS on issues of common interest, but do not support an IVS component, are IVS Affiliated Organizations. Affiliated Organizations express an interest in establishing and maintaining a strong working association with IVS to mutual benefit. Individuals affiliated with IVS Affiliated Organizations may become IVS Correspondents.

3. Associate Members

Individuals associated with organizations that support an IVS component may become IVS Associate Members. Associate Members are generally invited to attend non-executive sessions of the Directing Board meetings with voice but without vote. Associate Members take part in the election of the incoming members of the Directing Board representing the IVS components.

4. Corresponding Members

IVS Corresponding Members are individuals on a mailing list maintained by the Coordinating Center. They do not actively participate in IVS but express interest in receiving IVS publications, wish to participate in workshops or scientific meetings organized by IVS, or generally are interested in IVS activities. Ex officio corresponding members are the following:
• IAG General Secretary
• President of IAG Section II -- Advanced Space Technology
• President of IAG Section V -- Geodynamics
• President of IAU Division I -- Fundamental Astronomy
• President of IAU Commission 19 -- Rotation of the Earth
• President of IAU Commission 8 -- Positional Astronomy
• President of IAU Commission 31 -- Time
• President of IAU Commission 40 -- Radio Astronomy
• President of URSI Commission J -- Radio Astronomy
Individuals are accepted as IVS Corresponding Members upon request to the Coordinating Center.