Advanced Space Technology
Terms of Reference
Section II, Advanced Space Technology of the International Association of Geodesy, is engaged in new space techniques for geodesy, geodynamics, atmospheric, oceanographic and other areas of Earth science studies. Its objectives include the participation and promotion of the research and applications using the modern space technologies for a wide variety of interdisciplinary studies in Earth and planetary sciences. Section II organizes Commissions and Special Commissions, Special Study Groups and various Services to fulfill its objectives.
SC7: Satellite Gravity Field Missions
Special Study Groups
SSG 2.162: Precise Orbits Using Multiple Space Techniques
SSG 2.183: Spaceborne Interferometry Techniques
SSG 2.192: Spaceborne GNS Atmosphere Sounding
SSG 2.193: Gravity field missions: calibration and validation
SSG 2.194: GPS Water Level Measurements
IGS International GPS Service
ILRS The International Laser Ranging Service
IVS International VLBI Service for Geodesy and Astrometry
International Coordination Of Space Techniques For Geodesy And Geodynamics (CSTG)
President: H. Drewes (Germany)
Secretary: W. Bosch (Germany)
Terms of Reference
The general objectives of the Commission on International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG) were defined during its establishment at the IUGG General Assembly in Canberra, Australia, in 1979. They may be summarized as follows:
|Develop links between various groups engaged in the field of space geodesy and geodynamics by various techniques,|
|coordinate the work of these groups,|
|elaborate and propose projects implying international cooperation, follow their progress, and report on their advancement and results.|
Besides its integration into IAG Section II, CSTG is the Subcommission B2 of the "Scientific Commission B on Space Studies of the Earth-Moon System, Planets and Small Bodies of the Solar System" of the ICSU "Committee on Space Research (COSPAR)". It is closely related to the COSPAR "Panel on Satellite Dynamics (PSD)".
The commission operates through an Executive Committee (EC) and several subcommissions and projects in carrying out the objectives stated in the charter. Each subcommission and project has its own organizational structure. Subcommissions cover long-term programs, projects generally consist of highly focused activities over a limited period of time, may be with some programmatic implications.
CSTG is initiating international projects and supporting the activities of national and international groups through the collection and dissemination of information by correspondence, publications, and international meetings. Several international services, like the International GPS Service (IGS), the International Laser Ranging Service (ILRS), and the International VLBI Service (IVS) evolved from CSTG projects or subcommissions, respectively. A strong link is maintained between the services and CSTG by mutually integrating representatives in their directing bodies.
The commission shall further promote geodetic space techniques in areas of the world in which there have been few space geodetic measurements, and it shall push the scientific efforts in comparing and combining the different space techniques. The commission will also encourage the development of new techniques and application areas.
The dissemination of information is done through the publication of the CSTG Bulletins distributed globally to all interested scientists, and in particular to the CSTG national representatives and collaborators.
The "classical" space geodetic techniques, namely Very Long Baseline Interferometry (VLBI), Satellite and Lunar Laser Ranging (SLR, LLR), and the Global Positioning System (GPS) have been very well established in the past. They are individually organized in services, planning their own measurement campaigns and schedules and providing their results, valuable observation data and estimated parameters, to the scientific community. New techniques (e.g., Glonass, Doris, Prare) are building their organizations in a similar way. An overall space geodetic service for coordinating the observations and combining the original data of the individual techniques' measurements, however, does not exist. Only the final geodetic results emerging from the individual techniques' processing centers are, among others, collected by the International Earth Rotation Service (IERS) and combined to derive the IERS Celestial Reference Frame (ICRF), the IERS Terrestrial Reference Frame (ITRF), and the Earth Rotation Parameters. Several other services and users are gaining from these products.
Other space geodetic techniques, like satellite altimetry, satellite gradiometry and interferometric synthetic aperture radar (INSAR) are scientifically not organized on an international level at all. They are mainly managed by space agencies or space research groups with a limited open international scientific cooperation.
A number of new missions of low orbiting satellites has recently been launched and will be launched in the near future. The objectives of these missions (e.g., atmospheric sounding, high resolution gravity field determination) require a precise orbit determination demanding for sophisticated algorithms and procedures combining various measurement techniques available aboard the spacecrafts.
In this environment we envisage the following main objectives for the next four years:
Coordination of Space Techniques
Combination of Data Analysis
Integrating New Techniques and Methods
Further Attendance of Microwave Techniques
The structure of CSTG is given in the following in terms of the Executive Committee, the Subcommissions and the Project.
H. Drewes (Germany)
G. Beutler (Switzerland)
International Space Geodetic Network (ISGN)
Chair: J. Bosworth (USA)
The subcommission is the successor of the former Geodetic and Geophysical Sites Subcommission which was established in 1989 as the Sites Issues Subcommission and renamed at the XXI IUGG General Assembly in Boulder, 1995. Its objective is to identify, discuss and disseminate information on the types of monumentation and local surveys of space geodetic observatories. Considering the increasing number of those sites, a selected number of sites with different observation techniques will form the International Space Geodetic Network (ISGN). The subcommission shall identify these ISGN sites according to the criteria set up by the ISGN Working Group in 1999, and it shall coordinate the activities of these stations.
Coordination and Combination of the Analysis in Space Geodesy
Chair: T. Herring (USA)
The subcommission is a follow-on of the former CSTG project with the same name which was established at the XXI IUGG General Assembly in Boulder, 1995. The objective is to study the algorithms and procedures for optimally combining measurements of different space geodetic techniques. The subcommission shall encourage common processing strategies to the individual techniques' analysis centers and develop methods and approaches for the combination of original observation data in an early stage of the adjustment.
Precise Satellite Microwave Systems
Chair: P. Willis (France)
The subcommission was established at the XXI IUGG General Assembly in Boulder, 1995. Its objective was to coordinate activities with the new satellite microwave techniques (Glonass, Doris, Prare) and to maintain the link to the International GPS Service (IGS). As the mentioned microwave techniques may be integrated into own new services or existing scientific entities, the subcommission shall only be continued as long as a technique is not incorporated in an own structure (service or project).
Multi-Mission Satellite Altimetry
Chair: W. Bosch (Germany)
This subcommission was newly installed at the XXII IUGG General Assembly in Birmingham, 1999. Its main objectives are to promote free scientific access to all satellite altimetry data, to study synergies among different altimetry missions as well as with respect to other remote sensing techniques, and to set up the requirements for a stable, unified multi-mission long-term record of altimeter data. It shall study new techniques and application areas, such as off-nadir altimetry, altimeter profiles over land, laser altimeters etc. Another objectives is the work on mission independent data structures and standards. The subcommission shall investigate the possibilities for the establishment of an international altimeter service as an element of an Integrated Global Geodetic Observing System.
Precise Orbit Determination for Low Earth Orbiting Satellites
Chair: M. Rothacher (Germany)
This subcommission was newly installed at the XXII IUGG General Assembly in Birmingham, 1999. Its general objectives are to coordinate and to study mission-independent aspects related to precise orbit determination (POD) of low orbiting satellites (LEO) using spaceborne GPS receivers, to study different empirical LEO orbit models, and to work on mission-independent data structures. For the next four-years time period, special studies shall be carried out on the interface between GPS and Glonass determination of orbits and of other parameters (e.g., troposheric zenith delay). Guidelines for the scientific community concerning the use of LEOs in geodesy, geodynamics, and atmospheric sciences shall result from the work of this subcommission.
Project on Doris
Chair: G. Tavernier (France)
This project was established at the XXII IUGG General Assembly in Birmingham, 1999, considering the increasing importance of the Doris technique and the ongoing activities to install an International Doris Service (IDS). The project shall coordinate the establishment of such a service in cooperation with CSTG and the other international space geodetic services. If an IDS is installed, it will be discontinued like the other techniques' subcommissions (GPS, SLR/LLR, VLBI) in the past.
Special Commission SC7
Satellite Gravity Field Missions
Chair: Karl-Heinz Ilk (Germany)
Co-Chair: Pieter Visser (The Netherlands)
Secretary: Jürgen Kusche (Germany)
Terms of Reference.
The investigation of the Earth's gravity field enters a new era at the turn of the new millenium. Despite the fact that a remarkable improvement of our knowledge of the gravity field has been achieved during the past decades, the coming years promise another giant step in better understanding the Earth system. During the last three decades, two satellite-borne gravity measurement concepts have been discussed which are based on the same physical principle: Satellite-to-satellite Tracking (SST) and Satellite Gravity Gradiometry (SGG). In case of SST the relative motion is measured along the line-of-sight(s) of two (or more) satellites.
The concept is possible either in the so-called low-low or in the high-low mode. In the former case, the satellites have approximately the same altitude (200 to 400km). In this case, both satellites are equally sensitive to gravity field irregularities. In the latter case, only one (the gravity field sensitive) satellite is placed into a low orbit while the other (observing) satellite(s) describe orbits with high altitudes. In case of SGG, the elements of the gravity gradient or linear combinations thereof are intended to be measured simultaneously, depending on the sensitivity axes realized in the gradiometer instrument. It can be shown that the observations in these three cases can be related to the
|gravitational potential V in case of high-low SST, to the|
|gradient of the potential grad V, in case of low-low SST, and to the|
|gradient of the gradient of the gravitational potential grad grad|
|V(gravitational tensor), in case of SGG|
After careful selection procedures, three mission concepts were successful and will be realized in the coming years: CHAMP, a German multi-sensor satellite mission with international contributions will be launched in April 2000 and GRACE, a combined high-low/low-low SST mission as a joint American-German mission will follow in June 2001. The European SGG - mission concept GOCE was successful in a hard mission selection process of ESA and put at the first place out of four candidate Earth Explorer Core missions in October, 1999 in Granada/Spain. The launch is envisaged for the year 2004. CHAMP, GRACE and GOCE have the potential to revolutionize the knowledge of the system earth. Not only the static part of the gravity field can be determined with unattained accuracy also an eventual time dependency can be derived.
Despite the fact that all three missions have the potential to measure the gravity field by sort of relative measurements between free falling sensors, they are not redundant. Indeed, the characteristics of high-low SST, low-low SST and SGG are rather complementary than competitive. SST is superior in the lower harmonics below degree and order 50 to 60. A mission like GRACE, therefore, is optimal for studying time-varying gravity effects at moderate wavelengths. The static part of the gravity field up to approximately degree 50 can be expected with high accuracy. A condition to detect temporal effects is a corresponding mission duration of several years. Satellite gradiometry is superior for obtaining high spatial resolution from a moderate mission length. A recent study showed that increase of measurement precision or decrease of altitude results in a clear gain of spatial resolution in case of SGG, while this effect is very moderate in case of SST. A SGG mission like GOCE is superior in the short wavelengths parts of the gravity field up to a spherical harmonics degree of 250. The results of a mission like GOCE start to be better than those of a low-low SST mission from degree 60 to 80 on. A high-low SST mission like CHAMP can provide an improvement in the knowledge of the gravity field of approximately one order of magnitude over present models for wavelengths between 400 to 2000km. The recent decisions to realize these satellite gravity field missions represents an enormous challenge for the geo-sciences.
The main objective of SC7 is to create a forum that distributes information and integrates allcurrent international activities related to gravity field determination by satelllite-to-satellite tracking and satellite gravity gradiometry. The Special Commission shall
|inform the geodetic community about all activities related to these missions,|
|investigate scientific and commercial applications of a veryprecise high resolution gravity field, and|
|suggest alternative techniques and discuss future developments.|
To achieve this the home page of SC7 will be redesigned and will give links to the most important addresses related to these missions. We will include a discussion board where suggestions can be made to various topics. National and international activities related to the gravity field missions will be distributed to the interested community. It is intended to complete a bibliography and a list of references related to these topics.
The active members of the Special Commission will be defined by the members actively involved in projects. As a first step we define a couple of projects; but the list of projects shall be extended, or certain projects renounced if there are no interest in them.
There are three main problem areas; each of it consists of several Special topics:
|Analysis of the observation system: on the flight validation and calibration of satellite data of various missions, in cooperation with SSG 2.193, integrated sensor analysis, new sensors (laser interferometers, alternate gradiometers and accelerometers),|
|Modelling and data analysis aspects: comparison of analysis techniques (global and regional),|
|gravity field modelling aspects with view to the time dependency of the gravity field -cooperation with SSG 4.187: Wavelets in geodesy and geodynamics (W. Keller)|
|combination of satellite data and terrestrial data - cooperation with SSG 3.185: Merging data from dedicated satellite missions with other gravimatric data (N. Sneeuw)|
|calibration of satellite derived data, including downward/upward continuation|
|Applications in geosciences, oceanography, climate change studies and other inter-disciplinary research topics in earth sciences:|
|inversion of the gravity field,|
|structure of atmosphere and ionosphere,|
|temporal variations of the gravity field and the cryosphere,|
|temporal variations of the gravity field and the hydrosphere.|
Everybody is welcome to become member of SC7. To bring together those which are interested in a certain topic we will relate the members to certain groups, those who are interested to receive only news or informations, those who are interested to contribute actively in one of the project topics; those are automatically members of the first group.The definition of the projects and the members which are interested in a cooperation will be defined afterthe response of the first circular letter.
Special Study Group 2.162
Precise Orbits Using Multiple Space Techniques
Chair: R. Scharroo (The Netherlands)
Terms of Reference
Modern satellites that require precise positioning are equipped with several independent tracking devices. The ERS satellites were the first to combine Satellite Laser Ranging (SLR) and Doppler tracking with the Precise Range And Range-rate Equipment (PRARE) for precise orbit determination in support of the radar altimeter (RA). It was soon shown that the RA itself proves an important tracking device. Interferometric Synthetic Aperture Radiometry (InSAR) has recently developed to become another demanding consumer of precise satellite orbits.
TOPEX/POSEIDON (T/P) carries, apart from its RA, four independent tracking systems including SLR, Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS), Global Positioning System (GPS), and the Tracking and Data Relay Satellite System (TDRSS). For the first time, the force model errors, especially gravity, have been reduced to a point where a comparison of the various satellite tracking systems at or near their noise level is possible.
Results, as expected, show that each system has its own strengths and weaknesses. Therefore, recent precise orbit determination improvements for ERS-2 and T/P have been obtained using a combination of multiple tracking techniques. With PRARE on ERS-1 and GPS on Geosat Follow-On (GFO) on the limb, orbits for these satellites will likely remain to be based partly on altimeter tracking data.
The next generation of altimeter satellites (Jason-1, Envisat and Cryosat) will also be equipped with several tracking systems to support their altimeter, either DORIS or GPS in combination with SLR. There are great expectations for achieving orbits with sub-centimeter precision with a latency of about a month. Operational near real-time orbit determination is rapidly gaining interest and precision. With the approach of DIODE real-time orbits will be at hand.
In the future navigation and tracking satellites (GPS, GLONASS, and TDRS) will start demanding higher precision orbit determination, because they are and will be used as reference for Low Earth Orbiters (LEOs) in high-low satellite-to-satellite tracking configurations. Some of these navigation satellites are equipped with more than one tracking system. An important aspect is also to assess the respective tracking station coordinate solutions and evaluate misfits between the solutions.
GRACE will provide precise satellite-to-satellite tracking in a low-low configuration. Since precise orbit information for this satellite is so important, it will be wise to combine this tracking data type with e.g. the readings of the accelerometers. This is a joint research topic with IAG SSG 2.193.
The focus of this study group will be to further evaluate and characterize the various tracking systems, develop and assess new tracking techniques, and apply the products to improve the state-of-the-art in precision orbit determination.
Characterize the strengths and weaknesses of all of the current and proposed precise tracking techniques including SLR, DORIS, GPS, TDRSS, GLONASS, PRARE, and satellite altimetry.
Where possible, assess the impact of multiple tracking techniques on a single spacecraft (i.e. T/P, GPS-35, GPS-36, ERS-2, GFO, CHAMP, and future spacecraft).
Refine techniques of high-low GPS or GLONASS tracking for LEOs; doubly-differenced measurements versus direct phase measurements, etc.
Study the possibilities to use GRACE SST in combination with the accelerometer measurements for precise orbit determination (in collaboration with SSG 2.193.
Attempt to resolve discrepancies between the various techniques: e.g. the unexplained "Z-bias" observed between the SLR/DORIS and GPS based T/P orbits and peculiar static biases and variant drifts between coordinates of collocated SLR, DORIS and GPS stations.
Develop and evaluate alternative tracking techniques to further improve satellite positioning: e.g. use multi-mission altimeter cross-overs in simultaneous orbit determination, or use SLR/DORIS orbits from T/P to refine the TDRS and TDRSS-user satellite ephemerides.
Study possibilities for precise near real-time and real-time orbit determination for altimeter satellites and other satellites with important real-time applications like altimeter.
Boudewijn Ambrosius (The Netherlands)
Per-Helge Andersen (Norway)
Jean-Paul Berthias (France)
Willy Bertiger (USA)
John Dow (Germany)
Ramesh Govind (Australia)
Bruce Haines (USA)
Jaroslav Klokocnik (Czech Republic)
Scott Luthcke (USA)
F.-H. Massmann (Germany)
François Nouel (France)
Erricos Pavlis (USA)
John Ries (USA)
Markus Rothacher (Switzerland)
Remko Scharroo (The Netherlands)
Ernst Schrama (The Netherlands)
Ladislav Senhal (Czech Republic)
C.K. Shum (USA)
Tim Springer (Switzerland)
Mike Watkins (USA)
René Zandbergen (Germany)
Shengyuan Zhu (Germany)
Pieter Visser (The Netherlands)
Special Study Group 2.183
Spaceborne Interferometry Techniques
Chair: Ramon Hanssen (The Netherlands)
Terms of reference
The work of SSG 2.183 will focus on identifying practical procedures, as well as mathematical techniques that can be applied to describe the quality of the interferometric products.
--The objectives of the SSG are:
--to develop techniques and algorithms that allow extracting unambiguously topographic, deformation, and atmospheric signal from spaceborne repeat-pass radar interferometry,
--to develop methods that allow describing the quality, in terms of accuracy and reliability, of the interferometric products taking the most significant error sources into account, and
--to validate topographic and deformation maps for various applications and under various environmental conditions.
The above mentioned terms of reference result into a series of tasks:
--Develop phase unwrapping methodologies. Description of the quality of each pixel, quality measures, proper choice of the norm, quality description of the unwrapped phases, phase unwrapping strategies in highly decorrelated interferograms (patch unwrapping).
--Investigate the atmospheric signal. Role of weather radar data, synergy of various sensors on board of future satellite missions (e.g., MERIS on board ENVISAT), role of permanent GPS tracking networks, assessment of the spatial and temporal characteristics of the ionosphere on short spatial scales (below 100 km). Statistical (model) based approach to the atmospheric signal (Atmospheric Phase Screen).
--Evolve applications of water vapor mapping using InSAR.
--Develop a quality description of interferometric products (Bayesian approach)
--Develop methods for integrated deformation analysis using InSAR, GPS, levelling, and geophysical data.
--Analyze the effects of temporal decorrelation: further assessment of the relation between temporal decorrelation and various types of terrain, expected decorrelation due to weathering, effect of ground cover, phase stability of man-made and natural permanent scatterers.
--Define data and instruments requirements of science users for orbiting and planned SAR instruments.
--Exploiting the potential of InSAR in the commercial and industrial sector (near real-time data processing, continuous data flow, mosaicking).
--Prepare a report on the SSG's activities and recommendations.
Falk Amelung (USA)
Richard Bamler (Germany)
Alessandro Ferretti (Italy)
Satoshi Fujiwara (Japan)
Linlin G. E. (Australia)
Rick Guritz (USA)
Ramon Hanssen (The Netherlands)
Johan Mohr (Denmark)
David Sandwell (USA)
Andrew Wilkinson (South Africa)
Howard Zebker (USA)
Special Study Group 2.192
Spaceborne GNS Atmosphere Sounding
Chair: Rob Kursinski (USA)
Co-chair: Klemens Hocke (Germany)
Terms of Reference
Global Navigational Systems (GNS) are continuously sounding the Earth's atmosphere. By means of GNS receivers onboard of low Earth orbit (LEO) satellites and the radio occultation method, high resolution atmospheric profiles of density, temperature, geopotential height, water vapour and electron density are obtained. These profiles are attractive for climate monitoring, weather forecast, space weather and atmospheric research on a global scale.
Originally the radio occultation method has been developed and applied for discovery of planetary atmospheres and observation of solar wind and corona. In the 90s, the well-known technical report by Melbourne et al. (1994) and observational results of the GPS/MET satellite mission convinced an increasing number of scientists from Africa, Asia, Australia, Europe and South America to join the radio occultation community, mainly consisting of US-american and Russian scientists.
Numerous GNS/LEO satellite missions are in various stages of evolution from data acquisition (Oersted, Sunsat), launch in 2000 (CHAMP, SAC-C and IOX) or in the slightly more distant future (e.g., Metop, COSMIC, ACE). GNS atmosphere sounding covers the fields of atmospheric sciences, geodesy, radio engineering and remote sensing.
Based on recent interest and developing research, GNS reflection science will also be included as a study topic. This involves using the GNS signals reflected off Earth's surface to study surface properties such as ocean height and wind speed. The GPS receivers on SAC-C and CHAMP will have antennas directed at the surface to perform some initial surface reflection observations.
International research, cooperation and projects will lead to improved technological solutions, data analysis methods, applications and discovery of new potentials of GNS atmosphere sounding. This may further establish the role of spaceborne atmospheric GNS sounding as accurate and efficient remote sensing technique for the Earth's atmosphere. Impacts of our research activities and occultation data on other fields such as telecommunications, SAR, aviation and navigation are likely. Our study group shall function as focal point in the discussions and developments in the following areas:
1.Coordination of ongoing and planned LEO observations
2.Hardware and software developments for sounding the lower troposphere
3.Use of Y-code and/or additional carrier frequencies
4.Radio wave propagation in the Earth's atmosphere: Theory, Simulation, and Observation
5.Data analysis, Data assimilation and Tomography
6.Accuracy, Resolution and Validation of GNS sounding data
7.Weather forecast, Atmospheric monitoring and research by use of spaceborne atmospheric GNS sounding data
8.Space Weather, Ionospheric monitoring and research by use of spaceborne atmospheric GNS sounding data
9.Surface reflection research by use of spaceborne GNS surface reflection data
10.Operational aspects and planning of LEO constellations, future experiments
Dave Anderson (USA)
Mikhail Gorbunov (Russia)
Jennifer Haase (France)
Sean Healy (UK)
Benjamin Herman (USA)
David P. Hinson (USA)
Klemens Hocke (Germany)
Per Hoeg (Denmark)
Cheng Huang (China)
Kiyoshi Igarashi (Japan)
Nobert Jakowski (Germany)
Joanna Joiner (USA)
Gottfried Kirchengast (Austria)
Rob Kursinski (USA)
Sanjay S. Limaye (USA)
Juha-Pekka Luntama (Germany)
Garth Milne (South-Africa)
Antonio Rius (Spain)
Pierluigi Silvestrin (NL)
Toshitaka Tsuda (Japan)
Ming Yang (Taiwan)
Xiaolei Zou (USA)
Bechir Belloul (France)
Benjamin F. Chao, (USA)
John Eyre (UK)
E. Tuna Karayel (USA)
Luis Kornblueh (Germany)
Lou Lee (Taiwan)
Jann-Yeng Liu (Taiwan)
Mette Dahl Mortensen (Denmark)
William G. Melbourne (USA)
Christoph Reigber (Germany)
Victor H. Rios (Argentina)
Giulio Ruffini (Spain)
M.N. Sasi (India)
Kaoru Sato (Japan)
C.K. Shum (USA)
Wim Spakman (NL)
Andrea Steiner (Austria)
Paul R Straus (USA)
Makoto Suzuki (Japan)
Stig Syndergaard (Denmark)
Jens Wickert (Germany)
Jim Yoe (USA)
David Bromwich (USA)
Alex Flores (Spain)
Ramesh Govind (Australia)
George A. Hajj (USA)
Stefan Heise (Germany)
Ying-Hwa (Bill) Kuo (USA)
John LaBrecque (USA)
Christian Marquardt (Germany)
Manuel Martin-Neira (NL)
Thomas K. Meehan (USA)
Alexander Pavelyev (Russia)
Christian Rocken (USA)
Torsten Schmidt (Germany)
William Smith (USA)
Sergey Sokolovskiy (Germany)
Thomas P. Yunck (USA)
Changyin Zhao (USA)
Cinzia Zuffada (USA)
Special Study Group 2.193
Gravity field missions:
calibration and validation
Chair: Pieter Visser (The Netherlands),
Co-chair: Christopher Jekeli (USA)
Terms of Reference
At least three future missions will be flown by the middle of the next decade that will employ advanced and unprecedented techniques for global gravity field mapping. These missions are the German CHAMP, the US/German GRACE and ESA GOCE satellites. CHAMP will produce the first consistent long-wavelength mean gravity field model employing a geodetic-quality GPS receiver for high-low Satellite-to-Satellite Tracking (SST) in combination with a high-quality accelerometer measuring non-conservative forces. GRACE aims at monitoring long- to medium-wavelength gravity field variations, but will also enable the mapping of the mean global gravity field with a resolution significantly surpassing existing models. To this aim GRACE will consist of two low-flyers enabling high-accuracy low-low SST tracking in combination with GPS high-low SST and accelerometers. Finally, GOCE will be the first satellite equipped with a spaceborne gravity gradiometer (SGG) together with a high-quality GPS/GLONASS receiver. GOCE aims at recovering the global mean gravity field with unprecedented resolution.
The focus of this study group will be to evaluate and characterize the possibilities and methods for calibration and validation of the different levels of CHAMP, GRACE and GOCE gravity field related products and measurements, ranging from the high-low, low-low SST, accelerometer and the SGG measurements to the final high level gravity field products, for example, spherical harmonic models, geoid or gravity anomaly maps, and their accuracy assessments.
The calibration entails the conversion of the raw instrument measurements into engineering units within known limits of accuracy and precision, for example cm and mm/s for the SST measurements, m/s^2 for the accelerometer and m/s^2/m for the differential accelerometer measurements by the gradiometer. The validation concerns the conversion of these engineering quantities into geophysical units with sufficient accuracy, for example cm for geoid undulations, mgal for gravity anomalies and E (Eotvos Units) for the gravity gradients.
The activities will be in close coordination with Special Commission SC7: Gravity Field Determination by Satellites, chaired by K-H. Ilk. It is anticipated that this SSG will contribute and complement the ongoing activities in this Special Commission. In addition, a cross-fertilization is expected to take place with Section III: Determination of the Gravity Field, especially in relation to the validation part of the activities of this SSG. Finally, in many cases a precise orbit determination of the gravity field satellites is a prerequisite, linking the activities of this SSG with those of SSG 2.162 Precise Orbits Using Multiple Space Techniques, chaired by R. Scharroo.
A number of different categories of possibilities for calibration and validation can be distinguished:
--pre-flight calibration of the science instruments; on-the-fly calibration and validation;
--use of ground truth data;
--comparison with existing state-of-the-art gravity field models;
--intercomparison between gravity field products from different missions, but also based on different instruments within one mission.
For each of the above categories, with possible additions in due course, the special study group will focus on
--identifying methods and tools for calibration and validation of the different levels of gravity field products;
--assessing the merit of these methods and tools;
--assessing the maturity of these methods and
--tools and (additional) steps required for implementation.
Miguel Aguirre (The Netherlands)
Dimitri Arabelos (Greece)
Srinivas Bettadpur (USA)
Richard Coleman (Australia)
Rene Forsberg (Denmark)
Cheng Huang (China)
Cheinway Hwang (Taiwan)
Karl-Heinz Ilk (Germany)
Chris Jekeli (USA)
Steve Kenyon (USA)
Gerard Kruizinga (USA)
Juergen Mueller (Germany)
Felix Perosanz (France)
Tadahiro Sato (Japan)
Martijn Smit (The Netherlands)
Dru Smith (USA)
Hans Suenkel (Austria)
Peter Schwintzer (Germany)
Pieter Visser (The Netherlands)
John Wahr (USA)
Alberto Anselmi (Italy)
Georges Balmino (France)
Stefano Cesare (Italy)
Reinhard Dietrich (Germany)
Yoichi Fukuda (Japan)
Johnny Johannessen (Norway)
Helmut Oberndorfer (Germany)
Christian LeProvost (France)
John Manning (Australia)
Reiner Rummel (Germany)
Jens Schroeter (Germany)
Avri Selig (The Netherlands)
C.K. Shum (USA)
Christian Tscherning (Denmark)
Pierre Touboul (France)
Phil Woodworth (United Kingdom)
Changyin Zhao (USA)
Yaozhong Zhu (China)
Special Study Group 2.194
GPS Water Level Measurements
Chair: Gerry Mader (USA)
Co-chairs: Tilo Schöne (Germany)
Doug Martin (USA)
Terms of Reference
The determination of absolute sea level and its changes requires an integrated approach based on several techniques. Most relevant among those are satellite altimetry, tide gauges, GPS equipped buoys and land based GPS stations tied to tide gauge benchmarks.
GPS-buoy water level measurements represent one new technique which encompasses distinct advantages over satellite altimetry (radar and laser) and tide gauges. It represents geocentric measurements and has the potential ability to be deployed for various applications to enhance spatial and temporal sampling of water level measurements, i.e., deep/coastal ocean, lakes, river, semi-enclosed seas, littoral regions, harbors, etc.
The potential applications of GPS-buoy water level measurements include absolute calibrations of radar (and laser) altimeters, coastal circulation, coastal tide modeling, lake/river water level monitoring, ship harbor navigation, wave height and direction measurements, etc.
A number of technical issues, including automation requirements (power, communication bandwidth, robustness), hardware design (receiver, pressure sensor, tiltmeter or gyro, batteries, etc.), GPS kinematic solution techniques and potential Real-Time Kinematic (RTK) considerations, are among a list of difficulties for this unique space geodesy technique to mature. In addition, the issues of techniques to account for wave or directional wave modeling to reduce GPS to sea level measurements, calibration of buoy measurements, error estimates, influence of sea state, etc. will also be considered.
The objectives of this study group include the addressing of technical issues of the use of a robust GPS water level measurement system, its reliability, error budgets, and to exploit the increasing vast areas of its applications, including altimeter calibration, coastal circulation and tides measurements.
The proposed list of activities and research topics is as follows:
--Intercomparison between the wave spectra from GPS buoys and wave rider buoys
--Studying the behavior of different buoy types (Question: to which degree a buoy type represents the correct msl and/or wave spectra.)
--How bottom mounted pressure sensors may contribute to a relative, and long term calibration and drift estimation of an altimeter
--Develop or improve GPS kinematic software products to give stable solution for buoys under high motion conditions
--Develop/test low cost / reliable data transfer techniques for GPS data from off shore buoys
--Intercomparison of SWH and wind estimates
--How can GPS buoys contribute to atmospheric soundings, weather prediction models and tide models
--How can a GPS buoy contribute to develop 'transfer functions' between satellite crossovers and shore based tide gauge systems
Juan-Jose Benjamin (Spain)
Pascal Bonnefond (France)
Richard Coleman (Australia)
Reinhard Dietrich (Germany)
Xiaoli Ding (Hong Kong)
Richard Francis (NL)
Roberto Gutierrez (USA)
Bruce Haines (USA)
Guenter W. (Germany)
Cheng Huang (China)
Hans-Gert Kahle (Switzerland)
Gerard L.H. Kruizinga (USA)
Pete Lessing (USA)
Tom Lippmann (USA)
Yves Menard (France)
A. R. de Mesquita(Brazil)
Terry Moore (UK)
Michael Parke (USA)
Antonio Ruis (Spain)
John Blaha (USA)
George Born (USA)
Alexander Braun (Germany)
Gunter Liebsch (Germany)
Kevin Key (USA) Bruce Parker(USA)
Matthias Rentsch (Germany)
Monica Roca (NL)
International GPS Service (IGS)
Chair: Christopher Reigber (Germany)
Director of the Central Bureau: Ruth Neilan (USA)
Deputy Director of the Central Bureau:
Angelyn Moore (USA)
The IGS global system of satellite tracking stations, Data Centers, and Analysis Centers puts high-quality GPS data and data products on line in near real time to meet the objectives of a wide range of scientific and engineering applications and studies.
The 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 data products mentioned above which are made available to interested users through the Internet. In particular, the accuracies of IGS products are sufficient for the improvement and extension of the International Terrestrial Reference Frame (ITRF), the monitoring of solid Earth deformations, the monitoring of Earth rotation and variations in the liquid Earth (sea level, ice-sheets, etc.), for scientific satellite orbit determinations, ionosphere monitoring, and recovery of precipitable water vapor measurements.
The primary mission of the International GPS Service, as stated in the organization's Terms of Reference, is
"to provide a service to support, through GPS 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/specifications and encourages international adherence to its conventions."
The IGS Terms of Reference (comparable to the by-laws of the organization) describes in broad terms the goals and organization of the IGS. These are available from the internet at: http://igscb.jpl.nasa.gov/organization/bylaws.html
To accomplish its mission, the IGS has a number of components: an international network of nearly 200 continuously operating dual-frequency GPS stations, more than a dozen regional and operational data centers, three global data centers, seven analysis centers and a number of associate or regional analysis centers. The Central Bureau for the service is located at the Jet Propulsion Laboratory, which maintains the Central Bureau Information System (CBIS) and ensures access to IGS products and information. An international Governing Board oversees all aspects of the IGS. The IGS is an approved service of the International Association of Geodesy since 1994 and is recognized as a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS) since 1996.
The IGS has developed a worldwide system comprising satellite tracking stations, Data Centers, and Analysis Centers to put high-quality GPS data and data products on line within a day of observations. For example, following the Northridge, California, earthquake in January 1994, analysis teams using IGS-supplied data and products were able to quickly evaluate the disaster's immediate effects by determining station displacements accurately to within a few millimeters.
The IGS global network of permanent tracking stations, each equipped with a GPS receiver, generates raw orbit and tracking data. The Operational Data Centers, which are in direct contact with the tracking sites, collect the raw receiver data and format them according to a common standard, using a data format called Receiver Independent Exchange (RINEX). The formatted data are then forwarded to the Regional or Global Data Centers. To reduce electronic network traffic, the Regional Data Centers are used to collect data from several Operational Data Centers before transmitting them to the Global Data Centers. Data not used for global analyses are archived and available for online access at the Regional Data Centers. The Global Data Centers archive and provide on-line access to tracking data and data products.
The IGS collects, archives, and distributes GPS observation data sets of sufficient accuracy to meet the objectives of a wide range of scientific and engineering applications and studies. These data sets are used to generate the following products:
|GPS satellite ephemerides|
|Earth rotation parameters|
|IGS tracking station coordinates and velocities|
|GPS satellite and IGS tracking station clock information|
IGS products support scientific activities such as improving and extending the International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF), monitoring deformations of the solid Earth and variations in the liquid Earth (sea level, ice sheets, etc.), and in Earth rotation, determining orbits of scientific satellites and monitoring the ionosphere. For example, geodynamics investigators who use GPS in local regions can include data from one or more nearby IGS stations, fix the site coordinates from such stations to their ITRF values, and more importantly, use the precise IGS orbits without further refinement. Data from an investigator's local network can then be analyzed with maximum accuracy and minimum computational burden. Furthermore, the results will be in a well-defined global reference frame.
An additional aspect of IGS products is for the densification of the ITRF at a more regional level. This is accomplished through the rigorous combination of regional or local network solutions utilizing the Solution Independent Exchange Format (SINEX) and a process defined in the densification section.
In the future, the IGS infrastructure could become a valuable asset for support of new ground-based applications hand could also contribute to space-based missions in which highly accurate flight and ground differential techniques are required.
Governing Board Members:
The International Laser Ranging Service (ILRS)
Chairman of the Governing Board:
John Degnan (USA)
Director of the Central Bureau:
John Bosworth (USA)
Secretary: Michael Pearlman (USA)
For many years, international SLR activities had been organized under the Satellite and Lunar Laser Ranging (SLR/LLR) Subcommission of the CSTG. The Subcommission provided a venue for organizing tracking campaigns, adopting data formats, reporting on network status, and sharing technology. However, membership and commitment to the Subcommission were informal, and the main focus was on systems and data acquisition rather than on the production of the most meaningful data products for end users.
With strong encouragement from Gerhard Beutler, then President of the CSTG, the CSTG SLR/LLR Subcommission Steering Committee undertook the formation of the ILRS. A draft Terms of Reference, detailing the mission and the organization of the new service was written and accepted by the CSTG Executive Board in May 1997. A joint CSTG/IERS Call for Participation in the new ILRS was drafted by the SLR/LLR Subcommission Chairman, John Degnan, and the SLR Representative to the IERS Directing Board, Bob Schutz, and issued on 24 January 1998. Institution proposals in response to the Call were evaluated at a special meeting of the CSTG SLR/LLR Subcommission Steering Committee and subsequently approved by both the CSTG Executive Board and the IERS Directing Board on 18 April 1998. ILRS approval was granted to 46 tracking stations, 4 Operations Centers, 3 Analysis Centers, 4 Lunar Analysis Centers, 18 Associate Analysis Centers, 2 Global Data Centers and 1 Regional Data Center. The Central Bureau was established at the NASA Goddard Space Flight Center with John Bosworth as Director and Michael Pearlman of the Harvard-Smithsonian Center for Astrophysics as Secretary. Appointments and elections of Governing Board members were carried out during the summer of 1998. On 22 September 1998, the CSTG SLR/LLR Subcommission was officially disbanded, and replaced by the First ILRS General Assembly, held in conjunction with the 11th International Workshop on Laser Ranging in Deggendorf, Germany. The first ILRS Governing Board meeting was held on 25 September 1998; John Degnan was elected by the Board as Chairperson, and the Coordinators and Deputy Coordinators for the various Working Groups were also selected.
The ILRS collects, merges, analyzes, archives and distributes Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) observation data sets of sufficient accuracy to satisfy the objectives of a wide range of scientific, engineering, and operational applications and experimentation. The basic observable is the precise time-of-flight of an ultrashort laser pulse to and from a satellite, corrected for atmospheric delays. These data sets are used by the ILRS to generate a number of fundamental data products, including but not limited to:
|Centimeter accuracy satellite ephemeredes|
|Earth orientation parameters (polar motion and length of day)|
|Three-dimensional coordinates and velocities of the ILRS tracking stations|
|Time-varying geocenter coordinates|
|Static and time-varying coefficients of the Earth's gravity field|
|Fundamental physical constants|
|Lunar ephemeredes and librations|
|Lunar orientation parameters|
The ILRS accomplishes its mission through the following permanent components:
|Tracking Stations and Subnetworks|
|Global and Regional Data Centers|
|Analysis, Lunar Analysis, and Associate Analysis Centers|
|Central Bureau (Director, J. M. Bosworth)|
|Governing Board and Working Groups (Chairperson, 1998, J. J. Degnan)|
Information on these permanent components via the ILRS Home Page at the following URL address:
The products of the Analysis, Lunar Analysis, and Associate Analysis Centers are made available to the scientific community via the two Global Data Centers:
|Crustal Dynamics Data Information System (CDDIS) at the NASA Goddard Space Flight Center, Greenbelt, MD, USA,|
|European Data Center (EDC), Munich, Germany, and one Regional Data Center|
|Shanghai Observatory, Shanghai, PRC.|
The accuracy of SLR/LLR data products is sufficient to support a variety of scientific, engineering, and operational applications including:
|Realization of global accessibility to and the improvement of the International Terrestrial Reference Frame (ITRF)|
|Determining the precise location of the geocenter relative to the global network and its time variations|
|Monitoring three-dimensional deformations of the solid Earth|
|Monitoring Earth rotation and polar motion|
|Monitoring the static and dynamic components of the Earth's gravity field and geoid.|
|Supporting, via precise ranging to altimetric satellites, the monitoring of variations in the topography of the liquid and solid Earth (ocean circulation, mean sea level, ice sheet thickness, wave heights, vegetation canopies, etc.)|
|Tidally generated variations in atmospheric mass distribution|
|Calibration and validation of microwave tracking techniques (e.g., GPS, GLONASS, DORIS, and PRARE)|
|Picosecond global time transfer experiments|
|Determination of non-conservative forces acting on the satellite|
|Astrometric observations including determination of the dynamic equinox, obliquity of the ecliptic, and the precession constant|
|Gravitational and general relativistic studies including Einstein's Equivalence Principle, the Robertson-Walker ? parameter, and time rate of change of the gravitational constant, G|
|Lunar physics including the dissipation of rotational energy, shape of the core-mantle boundary (Love Number k2), and free librations and stimulating mechanisms|
|Solar System ties to the International Celestial Reference Frame (ICRF)|
Many ILRS and related publications and reports can now be accessed online via the ILRS Home Page at the following URL address: http://ilrs.gsfc.nasa.gov/ilrs_home.html and include:
|ILRS Terms of Reference and Working Group Charters|
|ILRS Annual Report (first volume to be published in first quarter 2000)|
|ILRS General Assembly Minutes and Reports|
|ILRS Governing Board Minutes|
|ILRS Working Group Minutes and Reports|
|ILRS Associates Telephone and Email Directory|
|ILRS Organizations and Technical Contacts|
|Science and Engineering References and Reports|
International VLBI Service
for Geodesy and Astrometry (IVS)
Directing Board Chair: W. Schlüter (Germany)
Coordinating Center Director: N. Vandenberg (USA)
Encouraged by the success of the International GPS Service (IGS), established in 1992, the president of the CSTG, Gerhard Beutler, University of Berne, proposed in May 1997 to organize the SLR/LLR and the VLBI subcommissions of the CSTG into comparable services. His vision was twofold: first, a service will guarantee much more strongly the provision of highly reliable products and second, the SLR/LLR and the VLBI techniques urgently needed more acceptance and support in the scientific community. The coordination of the activities within services of the SLR/LLR and VLBI communities will strongly concentrate resources and increase the potential to improve the techniques.
Tomas A. Clark, in his function as chairman of the CSTG VLBI Subcommission, drafted the Terms of Reference for the IVS in October 1997. The final version was worked out by a Subcommission Steering Committee whose members were James Campbell (chairman), Yasuhiro Koyama, Chopo Ma, Arthur Niell, Axel Nothnagel, Jim Ray and Nancy Vandenberg. The Terms of References were presented and approved at the CSTG Executive Committee Meeting in Nice/France in April 1998. The Call for Participation was distributed on June 1, 1998. The Steering Committee evaluated the proposals and accepted all of them in October 1998.
In November 1998, the Steering Committee, in accordance with the Terms of Reference, released the call for nominations for representative positions on the Directing Board (due to December 10, 1999) and the call for proposals for the positions of the Coordinators (Analysis, Network, Technology). The elections were carried out in the period from December 15, 1998 to January 1999 via e-mail by the Associate Members. The Steering Committee formed the Directing Board on the basis of the elections, the proposals for Coordinators, and the nominations for the At Large members. The initial Directing Board held its first meeting on February 11, 1999 at Wettzell.
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. The primary objective of IVS is to foster VLBI programs as a joint service to support geodetic, geophysical, astrometric, and other research and operational activities. 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.
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.
IVS accomplishes its goals through seven types of components. Today the IVS has: 30 Network Stations, 3 Operation Centers , 7 Correlators , 6 Data Centers , 18 Analysis Centers , 9 Technology Development Centers, 1 Coordinating Center .
All together there are: 74 components, representing 30 institutions in 15 countries , 229 individuals who are Associate Members forming the IVS today.
The current IVS Directing Board consists of the following members:
Alan Whitney (USA)
Publications and Meetings
IVS published an Annual Report in 1999, shortly after its inauguration, to inform the international community about IVS and to serve as a baseline from which to measure future progress. The report is available from the Coordinating Center.