INTERNATIONAL GPS SERVICE
TERMS OF REFERENCE
(Revision, adopted December 1998)


A proof of concept for the International Global Positioning System
(GPS) Service for Geodynamics (IGS) was conducted with a three-month
campaign during June through September 1992, and was continued through a
Pilot-service until the establishment of the IGS in 1993 by the
International Association of Geodesy (IAG). The IGS began formally on 1
January 1994. IGS is a member of the Federation of Astronomical and
Geophysical Data Analysis Services (FAGS) and it operates in close
cooperation with the International Earth Rotation Service (IERS). Due to
the expansion of IGS objectives, the name of the service was changed to
International GPS Service (IGS) on 1 January 1999.

The primary objective of the IGS is to provide a service to
support, through GPS data and data products, geodetic and geophysical
research activities. Cognizant of the immense growth in GPS applications
the secondary objective of the IGS is to support a broad spectrum of
operational activities performed by governmental or selected commercial
organizations. The Service also develops the necessary standards and
specifications and encourages international adherence to its conventions.

IGS collects archives and distributes GPS observation data sets of
sufficient accuracy to satisfy the objectives of a wide range of
applications and experimentation. These data sets are used by the IGS to
generate the following data products:

  - High accuracy GPS satellite ephemerides
  - Earth rotation parameters
  - Coordinates and velocities of the IGS tracking stations
  - GPS satellite and tracking station clock information
  - Ionospheric information
  - Tropospheric information.

The accuracies of these products are sufficient to support current
scientific objectives including:

  - Realization of global accessibility to and the improvement of the
      International Terrestrial Reference Frame (ITRF)
  - Monitoring deformations of the solid earth
  - Monitoring earth rotation
  - Monitoring variations in the hydrosphere (sea level, ice-sheets, etc.)
  - Scientific satellite orbit determinations
  - Ionosphere monitoring
  - Climatological research, eventually weather prediction.

The IGS accomplishes its mission through the following components:

  - Networks of tracking stations
  - Data Centers
  - Analysis and Associate Analysis Centers - Analysis Coordinator
  - Working Groups and Pilot Projects
  - Central Bureau
  - Governing Board.


NETWORKS OF TRACKING STATIONS

IGS Stations provide continuous tracking using high accuracy
receivers and have data transmission facilities allowing for a rapid (at
least daily) data transmission to the data centers (see below). The
stations have to meet requirements, which are specified in a separate
document available from the Central Bureau. The tracking data of IGS
stations are regularly and continuously analyzed by at least one IGS
Analysis Center or IGS Associate Analysis Center. These analyses must be
available to, analyzed and published by the IERS for at least two
consecutive years. During this initial period the IGS Central Bureau can
temporarily designate new tracking stations as IGS stations.

IGS Stations which are analyzed by at least three IGS Analysis
Centers for the purpose of orbit generation, where at least one of the
Analysis Centers lies on a different continent than the station considered,
is in addition called IGS Global Stations.

All IGS stations are qualified as reference stations for regional
GPS analyses. The ensemble of the IGS stations form the IGS network
(polyhedron).


DATA CENTERS

The data centers fall into three categories: Operational, Regional,
and Global Data Centers.

The Operational Data Centers are in direct contact with the
tracking sites. Their tasks include data validation and monitoring,
suitable data reformatting into a uniform format, compression of data
files, maintenance of a local archive of the tracking data in its original
receiver and in its reformatted format, and the electronic transmission of
data to a Regional or Global Data Center. The Operational Data Center must
down load data from the receivers located at the IGS Global Stations on a
timely (e.g., daily) basis, without interruption.

The Regional Data Centers reduce traffic on electronic networks.
They collect reformatted tracking data from several Operational Data
Centers, maintain a local archive of the data received and transmits these
data to the Global Data Centers. Regional Data Centers may also meet the
operational requirements (as defined in the above paragraph) of strictly
regional network operations.

The Global Data Centers are the main interfaces to the Analysis
Centers and the outside user community. Their primary tasks include the
following:

  - Receive/retrieve, archive and provide on line access to tracking data
    received from the Operational/Regional Data Centers

  - Provide on-line access to ancillary information, such as site
    information, occupation histories, etc.,

  - Receive/retrieve, archive and provide on-line access to IGS products
    received from the Analysis Centers

  - Backup and secure IGS data and products.


ANALYSIS CENTERS

The analysis centers fall into two categories: Analysis Centers
(ACs) and Associate Analysis Centers. The Analysis Centers receive and
process tracking data from one or more data centers for the purpose of
producing IGS products. The Analysis Centers are committed to produce daily
products, without interruption, and at a specified time lag to meet IGS
requirements. The products are delivered to the Global Data Centers and to
the IERS (as per bilateral agreements), and to other bodies, using
designated standards.

The Analysis Centers provide, as a minimum, the core products of
ephemerides, earth rotation parameters, station coordinates, and clock
information, as well as other recommended products, such as rapid and
predicted orbit and earth rotation solutions. (Clock estimates are not
required of those ACs which process differenced observables but are
encouraged.)

Associate Analysis Centers are organizations that produce
specialized products, e.g., ionospheric information or station coordinates
and velocities for a global or regional sub-network, as recognized by the
Governing Board.

The Charter for Analysis Centers and Associate Analysis Centers is
contained in the document 'Charter For IGS Analysis Centers and Associate
Analysis Centers' available from the Central Bureau.


ANALYSIS COORDINATOR

The Analysis Coordinator assists the Analysis Centers. The Analysis
Coordinator monitors the Analysis Centers activities to ensure that the IGS
objectives are carried out.

Specific expectations include quality control, performance
evaluation, and continued development of appropriate analysis standards.
The Analysis Coordinator is also responsible for the appropriate
combination of the Analysis Centers products into a single set of products.
As a minimum a single IGS ephemeris for each GPS satellite is to be
produced. In addition, IERS will produce ITRF station coordinates/
velocities and earth rotation parameters to be used with the IGS orbits.

The Analysis Coordinator is a voting member of the IGS Governing
Board (see below) and interacts regularly with the Central Bureau and the
IERS. The Analysis Coordinator or his designee is one of the two IGS
representatives to the IERS Directing Board.

Generally the responsibilities for the Analysis Coordinator shall rotate
among the Analysis Centers with appointments and terms specified by the
Governing Board.


WORKING GROUPS AND PILOT PROJECTS

A Working Group works on a particular topic related to the IGS
components according to goals and schedule specified in the working group's
charter. A Pilot Project aims at the development of particular IGS
product(s) or service(s) relying on the IGS infrastructure.

Working Groups and Pilot Projects are established and terminated by
the Governing Board at one of its regular meetings. The IGS policy to
establish Working Groups and Projects is contained in the document 'IGS
Policy for the Establishment of IGS Projects and Working Groups', available
from the Central Bureau.

The Chairpersons of the Working Groups and Pilot Projects are
non-voting members of the IGS Governing Board (see below).


CENTRAL BUREAU

The Central Bureau (CB) is the executive arm of the IGS Governing
Board and as such is responsible for the general management of the IGS
consistent with the directives, policies and priorities set by the
Governing Board.

In this role the CB, within available resources, coordinates IGS
activities, facilitates communications, coordinates general aspects of the
IGS network operations, promotes compliance to IGS standards, monitors
network operations and quality assurance of data, maintains documentation,
and organizes reports, meetings and workshops.

The CB actively coordinates with the IGS Pilot Projects, Working
Groups and committees and insures the compatibility of IGS and IERS by
interfacing with the IERS. The CB acts as the outreach office and promotes
use of IGS data and products, maintaining and expanding the visibility of
the IGS. To accomplish these tasks the CB fully works closely with the
independent Analysis Coordinator described above.

The CB operates the information system for the IGS and produces the
IGS Annual Reports and directories. The CB coordinates the publication of
other documents required for the satisfactory planning and day-to-day
operation of the Service, including standards and specifications regarding
the performance, functionality and configuration requirements of all
elements of the Service.

Although the Chairperson of the Governing Board is the official
representative of the IGS to external organizations, the CB, consonant with
the directives established by the Governing Board, is responsible for the
day-to-day liaison with such organizations.

Through the existing reciprocity agreement between IGS and IERS the
CB serves as the GPS Technique Center for IERS, and as such it's designated
representative, subject to Governing Board approval, is a member of the
IERS Directing Board (together with the Analysis Coordinator). This
representative is a voting member of the IGS Governing Board. In turn, the
IERS Directing Board designates a representative to the IGS Governing
Board. This arrangement is to assure full cooperation between the two
services.

The long term functioning of IGS is assured through redundancy and
emergency contingency for all of its components except for the CB. For this
reason the performance of the Central Bureau is to be formally reviewed by
the GB at least every five years. On these occasions the Central Bureau
will either reconfirm its commitment to the IGS for the next period of five
years or announce to stop operations after a transition phase of at least
one year. The GB will formally approve the Central Bureau's proposal, or,
alternatively issue a Call for Proposal for a new IGS Central Bureau to
take over responsibilities after the transition phase.

In summary, the Central Bureau performs primarily a long-term
coordination role to ensure that IGS participants contribute to the Service
in a consistent and continuous manner and adhere to IGS standards.


GOVERNING BOARD

The principal roles of the Governing Board (GB) are to set policy
and to exercise broad oversight of all IGS functions and components. It
also controls general activities of the Service, including restructuring,
that would be appropriate to maintain efficiency and reliability, while
taking full advantage of the advances in technology and theory.

The Governing Board (GB) consists of sixteen voting members and a
number of non-voting members. They are distributed as follows:

  Elected by IGS Associates (see below):
    Analysis Centers' representatives                   3
    Data centers' representative                        1
    Networks' representatives                           2

  Elected by the Governing Board upon recommendations from the Central
  Bureau, for the next term:
    Representatives of Analysis, Data Centers or
      Networks                                          2
    Members at large                                    2

  Appointed members:
    Director of the Central Bureau                      1
    Analysis Coordinator                                1
    IGS/CB Representative to the IERS                   1
    Representative of the IERS                          1
    IAG representative                                  1
    President of IAG or his designee from the IAG
      Executive Committee                               1

  Total voting members                                 16

The Working Group and Pilot Project chairpersons as well as the FAGS
representative are the GB members with voice but without vote.

The appointed members are considered ex officio and are not subject
to institutional restrictions. The elected ten persons must be members of
different organizations. All ten candidates have staggered four- year
terms, which are renewable once. The GB membership should be properly
balanced with regard to supporting organizations as well as geographically.

For the first six of the above positions representing components of
the IGS, the nominating committee conducts the elections. The nominating
committee consists of three members, the chair of which is appointed by the
Chair of the GB and who is a member of the GB not currently up for
re-election. The nominating committee presents to the Associate Members,
for each position to be filled, a list of at least two candidates for
election. The election will be by the number of votes received by the
Associate Members. In the case of a tie the election is by the members of
the Governing Board.

For the next four elected positions, four weeks prior to the
election by the Governing Board, the Central Bureau presents its
candidate(s) to the Governing Board. Based on the reactions of the GB
members, or on the outcome of the elected positions, the CB may present a
revised list at the GB meeting where the election of the new Board members
takes place.

In case of a resignation from the Board, after consulting with the
appropriate IGS components, the CB will nominate replacement candidates for
election by the GB. The replacement will serve until the end of the term of
the resigned Board member.

The Chairperson is one of the members of the GB elected by the
Board for a term of four years with the possibility of reelection for one
additional term. The Chairperson does not vote, except in case of a tie.
He/she is the official representative of IGS to external organizations.

The IAG and FAGS representatives are appointed respectively by the
IAG Bureau and by FAGS for a maximum of two four-year terms.

Members of the GB become IAG Fellows with the appropriate rights
and privileges after an initial two- year period.

Most GB decisions are to be made by consensus or by a simple
majority vote of the members present, provided that there is a quorum
consisting of at least ten members of the GB. In case of lack of a quorum
the voting is by mail. Changes in Terms of and Chairperson of the GB can be
made by a 2/3 majority of the members of the GB, i.e., by ten or more votes.

The Board shall meet at least annually and at such other times as
shall be considered appropriate by the Chairperson or at the request of
five members. The Central Bureau provides the secretariat of the GB.

For the GB to effectively assess the value of IGS services to the
user communities, and to ensure that the service remains up to date and
responsive to changing user needs, the GB will organize reviews of the IGS
components at appropriate intervals. The Governing Board will decide, on an
annual basis, those groups (Analysis Centers, Data Centers, Network
Stations, and the Central Bureau) that are to be reviewed and from time to
time may select other activities for review as it deems appropriate.


IGS ASSOCIATE MEMBERS

Associate Members are persons representing organizations, which
participate in any of the IGS components. The Central Bureau maintains the
list of Associate Members, which is revised annually and is subject to the
approval of the Governing Board. The membership is balanced with respect to
IGS components, organizational representation and geography, and is meant
to include those people affiliated with institutions which contribute
significantly to the IGS on a continuous basis.

Associate Members vote for the incoming members of the GB (except
for those nominated by the CB). The list of Associate Members eligible for
voting in the elections must be approved by the GB at least three months
prior to the election process. For the purposes of the election current and
former GB members are also considered Associate Members.

IGS Associate Members are considered IAG Affiliates with the
appropriate rights and privileges.


IGS CORRESPONDENTS

Correspondents are persons on a mailing list maintained by the
Central Bureau, who do not actively participate in the IGS but express
interest in receiving IGS publications, wish to participate in workshops or
scientific meetings organized by the IGS, or generally are interested in
IGS activities. Ex officio IGS Correspondents are the following persons:

  - IAG General Secretary
  - President of IAG Section II
  - President of IAG Commission VIII (CSTG)
  - President of IAG Section V