M. Pearlman

The International Laser Ranging Service

The International Laser Ranging Service (ILRS) was established in September 1998 to support programs in geodetic, geophysical, and lunar research activities and to provide the International Earth Rotation Service (IERS) with products important to the maintenance of an accurate International Terrestrial Reference Frame (ITRF). Now in operation for nearly two years, the ILRS develops (1) the standards and specifications necessary for product consistency, and (2) the priorities and tracking strategies required to maximize network efficiency. The Service collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data to satisfy a variety of scientific, engineering, and operational needs and encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products. The ILRS works with (1) new satellite missions in the design and building of retroreflector targets to maximize data quality and quantity, and (2) science programs to optimize scientific data yield. The ILRS is organized into permanent components: (1) a Governing Board, (2) a Central Bureau, (3) Tracking Stations and Subnetworks, (4) Operations Centers, (5) Global and Regional Data Centers, and (6) Analysis, Lunar Analysis, and Associate Analysis Centers. The Governing Board, with broad representation from the international Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) community, provides overall guidance and defines service policies, while the Central Bureau oversees and coordinates the daily service activities, maintains scientific and technological data bases, and facilitates communications. Active Working Groups in (1) Missions, (2) Networks and Engineering, (3) Data Formats and Procedures, (4) Analysis, and (5) Signal Processing provide key operational and technical expertise to better exploit current capabilities and to challenge the ILRS participants to keep pace with evolving user needs. The ILRS currently includes more than 40 SLR stations, routinely tracking about 20 retroreflector-equipped satellites and the Moon in support of user needs.

Global Geodetic Observing System

As the observing system of the IAG, GGOS facilitates a unique and essential combination of roles centering upon advocacy, integration, and international relations. GGOS also promotes high‐level outcomes, such as the realization of the International Terrestrial Reference Frame through developing and maintaining working relationships among a variety of internal and external groups and organizations.

The future Global Geodetic Observing System

In this Chapter, we focus on the design of the geodetic observing system that will meet the specifications summarized in Section 7.7 and be able to sustain the products listed in Section 7.5. Thus, this Chapter treats GGOS as an observing system (see Section 1.3 for a discussion of the two different meanings of “GGOS”). In Chapter 10, the main focus will be on GGOS as an organization and the integration of GGOS in the global context of Earth observation. GGOS has been organized by the IAG to work with the established IAG Services in order to provide the geodetic contribution to global Earth monitoring, including the metrological and reference system basis for many other Earth observing systems. GGOS is therefore one of the basic observing systems comprising GEOSS. GGOS is complex, addressing relevant geodetic, geodynamic and geophysical problems, which have deep impact on vital issues for humankind, such as global change, sea level rise, global water circulation, water supply, natural disasters, risk reduction, etc.(see Chapter 5 for details). It is a visionary concept based on the requirements and specifications given in Chapter 7 and on the assessment of what components are needed to meet the very demanding goals. In order to address the ambitious GGOS goals, we will integrate a multitude of sensors into one global observing system. In the following sections the focus will be on the technical design and rationale for the proposed GGOS. The individual components of the system will be discussed and the interaction between the components will be outlined, from the geodetic observations and the interfaces to the products for the users.

Scientific objectives of current and future WEGENER activities

Abstract The WEGENER group has promoted the development of scientific space-geodetic activities in the Mediterranean and in the European area for the last fifteen years and has contributed to the establishment of geodetic networks designed particularly for earth science research. WEGENER currently has three scientific objectives which are related to plate-boundary processes, sea-level and height changes, and postglacial rebound. In a full exploitation of the space-geodetic techniques, namely SLR, VLBI and GPS, the individual scientific projects do not only pursue these objectives but also contribute to improving and developing the observation techniques as well as the modelling theories. In the past, particularly SLR observations within WEGENER-MEDLAS have provided a fundamental contribution to determine the regional kinematics of the tectonic plates in the Mediterranean with high precision. With GPS, spatially denser site distributions are feasible, and in several WEGENER projects detailed studies of tectonically active areas were possible on the basis of repeated episodic GPS observations. Current projects associated with WEGENER are successful in separating crustal movements and absolute sea-level variations as well as in monitoring postglacial rebound. These tasks require high-precision height determinations, a problem central to all of the present WEGENER activities. In these projects, continuously occupied GPS sites are of increasing importance. Time series of heights observed with continuous GPS can be determined with a few centimeters RMS error thus enabling the reliable estimates of vertical rates over relatively short time intervals. Regional networks of continuous GPS sites are already providing results relevant, for example, for the study of postglacial rebound. The Mediterranean area is an extraordinary natural laboratory for the study of seismotectonic processes, and the wealth of observations acquired in previous WEGENER projects together with new space-geodetic observations will allow the test of geophysical hypotheses linking three-dimensional deformations of the Earths surface to the dynamics of the Earths interior. In particular, it is anticipated that WEGENER projects will aim at a test of the slab-detachment hypothesis. The complex investigations on sea-level fluctuations presently carried out at basin scale from the Strait of Gibraltar to the Black Sea make it possible to study the present and recent past interactions of ocean, atmosphere and solid Earth, as well as to develop appropriate models to assess future aspects.

GGOS working group on ground networks and communications

Properly designed and structured ground-based geodetic networks materialize the reference systems to support sub-mm global change measurements over space, time and evolving technologies. Over this past year, the Ground Networks and Communications Working Group (GN&C WG) has been organized under the Global Geodetic Observing System (GGOS) to work with the IAG measurement services (the IGS, ILRS, IVS, IDS and IGFS) to develop a strategy for building, integrating, and maintaining the fundamental network of instruments and supporting infrastructure in a sustainable way to satisfy the long-term (10–20 year) requirements identified by the GGOS Science Council. Activities of this Working Group include the investigation of the status quo and the development of a plan for full network integration to support improvements in terrestrial reference frame establishment and maintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geodetic and gravimetric applications required for the long-term observation of global change. This integration process includes the development of a network of fundamental stations with as many co-located techniques as possible, with precisely determined intersystem vectors. This network would exploit the strengths of each technique and minimize the weaknesses where possible.

The goals, achievements, and tools of modern geodesy

Friedrich Robert Helmert (1843-1917) defined geodesy as the science “of measurements and mappings of the Earth’s surface”. Over time, this definition of geodesy has been extended, mainly as a consequence of technological developments allowing geodesy to observe the Earth on global scales with high accuracy. Today, geodesy is the science of determining the geometry, gravity field, and rotation of the Earth and their evolution in time. This understanding of modern geodesy has led to the definition of the “three pillars of geodesy”, namely (1) Geokinematics, (2) Earth Rotation and (3) the Gravity Field (see Figure 1.1 on page 4). These three pillars are intrinsically linked to each other, and they jointly change as a consequence of dynamical processes in the Earth system as a whole. The changes in Earth’s shape (including the surface of the water and ice bodies), i.e. the geokinematics, are the result of dynamic processes in the solid Earth and its fluid envelope, affecting mass distribution and angular momentum, and thus changing the gravity field and Earth rotation. Traditionally, geodesy has been a service science, providing an important utility to other sciences and many applications. This aspect has remained unchanged, and a principal tool and output of geodesy is a reference frame allowing the determination of the position of points relative to each other. But geodesy has developed into a science that can no longer satisfy this service aspect without encompassing and monitoring the whole Earth system, its kinematic and dynamics. As an additional benefit, geodesy is increasingly forced not only to “measure” the geokinematics, gravity field, and rotation, but also to “model” these quantities on the basis of mass transport and dynamics. The instruments (or measurement tools) are of crucial importance in geodesy. They in essence define the scope of the problems, which may be addressed by geodesy. Before the advent of the space age the geometrical aspects were studied mainly by measuring angles and time (time-tagging of the observations). In the best

Linking the Global Geodetic Observing System (GGOS) to the Integrated Global Observing Strategy Partnership (IGOS-P) through the Theme ‘Earth System Dynamics’

When setting up GGOS as a project, the IAG Executive Committee asked the GGOS Steering Committee to establish a relationship with IGOS-P. IGOS-P addresses a number of problems and components of Earth observing systems in the frame of specific Themes. The IGOS-P Theme process will also be an important mechanism for the development of the components of the Global Earth Observation System of Systems (GEOSS).