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524 M. Mandea et al.

new generation of instruments, enabling measurements to be taken simultaneously

over different regions of the Earth. We fully expect to obtain new insights into the

interaction of the solar wind with the Earth’s internally generated dynamo ﬁeld.

The multi-satellite approach of Swarm will especially pay off for separating inter-

nal and external ﬁeld contributions. To take full advantage of the increased precision

and resolution, new mathematical methods for data analysis and improved models

of the external ﬁeld generation will be developed. The improved knowledge of the

internal ﬁeld and its temporal variations should also advance our understanding of

the dynamical processes in Earth’s liquid iron core. Finally, new opportunities to

survey the geospace environment and infer the magnetic changes will be of great

interest for our Planet.

Acknowledgments Several of our colleagues involved in different aspects of this contribution

participated in various steps of the work. We particularly thank Martin Rother, Ingo Michaelis,

Borislav Minchev, Mohamed Hamoudi, Yoann Quesnel for providing inputs to this chapter.

Alexander Jordan and Martina Krüger provided helpful support during the editorial process. We

would like to acknowledge all of them for their logistic assistance.

We appreciate the dedicated work of people and institutions carrying out magnetic measure-

ments in worldwide magnetic observatories and national repeat station networks. We also would

like to acknowledge all geological surveys and group members who provided aeromagnetic and

marine magnetic anomaly data for World Digital Magnetic Anomaly project. The CHAMP mis-

sion is operated and supported by Helmholtz Centre Potsdam GFZ German Research Centre for

Geosciences, the German Aerospace Center (DLR) and the German Federal Ministry of Education

and Research (BMBF). This work has got ﬁnancial support in the frame of GEOTECHNOLOGIEN

project 03F0442A.

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Part VIII

GGOS-D

Integration of Space Geodetic Techniques

as the Basis for a Global Geodetic-Geophysical

Observing System (GGOS-D): An Overview

Markus Rothacher, Hermann Drewes, Axel Nothnagel, and Bernd Richter

1 Motivation

In view of the alarming global change in the Earth system and the multitude of natu-

ral hazards with huge effects on humans and economy it is of greatest importance to

develop a better understanding of the processes that describe and excite these con-

tinuous as well as abrupt changes. To achieve this, the structure of all elements of the

Earth system (primarily the geosphere, the oceans, the hydrosphere and the atmo-

sphere) have to be measured and monitored exactly, and the interactions between

them have to be modeled correctly, i.e., a comprehensive global Earth observing

system has to be built up. The expected outcome of such a s ystem are consistent

geodetic-geophysical time series that refer to a highly accurate reference frame that

is stable over decades. Based on such knowledge of the past, conclusions may be

drawn for the future development of the Earth system.

On the international level, the Global Geodetic Observing System (GGOS), now

a full component of the International Association of Geodesy (IAG), is the geodetic

contribution to a comprehensive global observing system as it is presently set up by

the Group on Earth Observation (GEO) in the form of the Global Earth Observing

System of Systems (GEOSS).

The project GGOS-D ﬁts perfectly into the framework of GGOS that has as one

of its major goals the integration and combination of the various space geodetic

techniques. This integration is important not only for the realization of a highly

accurate and long-term stable reference frame as the basis of all Earth observation,

but also for the generation of homogeneous, high-quality time series of geodetic

and geophysical parameters describing the processes in the Earth system. With the

project GGOS-D, that had exactly this integration of the geometric and gravimetric

space-geodetic techniques as its goal, an important German contribution could be

made to GGOS (and to GEOSS in the broader sense).

M. Rothacher (B)

ETH Zürich, Photogrammetrie, HPV G 52, Schafmattstr. 34, 8093 Zürich

e-mail: markus.rothacher@ethz.ch

529

F. Flechtner et al. (eds.), System Earth via Geodetic-Geophysical Space Techniques,

Advanced Technologies in Earth Sciences, DOI 10.1007/978-3-642-10228-8_43,



Springer-Verlag Berlin Heidelberg 2010

530 M. Rothacher et al.

2 Goals of GGOS-D

The overall objective of the project GGOS-D has been the investigation of the

technological, methodological and information-technological realization of a global

geodetic-geophysical observing system, a system that may open up the possibility to

politics and economy to reliably identify and monitor long-term environmental pro-

cesses, to understand the course of natural hazards and, thus, to provide important

information for disaster prevention.

The main scientiﬁc goals of the project can be summarized as follows:

• Development of a data management and information system for the exchange and

the near real-time provision of data and products of a global observing system.

• Deﬁnition and implementation of standards, models and parameterizations for

the consistent processing of the data of the different space geodetic techniques

and for the representation of the products.

• Generation of a consistent reference frame for the computation and provision of

all parameters of the global observing system.

• Development of methods for the computation of consistent time series of the most

important parameters (station coordinates, Earth orientation parameters, quasar

coordinates, low-degree coefﬁcients of the gravity ﬁeld, troposphere parameters,

sea surface topography).

• Investigation of relationships and correlations between time series of parameters

and the comparison and validation of the time series with external geophysi-

cal data.

The major improvements (compared to the status of the international community

of today) that GGOS-D was aiming at are the following:

• Implementation of common standards for modelling and parameterization in all

software packages involved.

• Extension of the parameter space to link for the ﬁrst time geometry, Earth rota-

tion, sea surface heights and gravity ﬁeld from SLR, VLBI, GPS, altimetry and

Low Earth Orbiters (LEOs). Additional parameters (not yet considered in solu-

tions of the ofﬁcial services) are quasar coordinates, nutation offsets and rates,

tropospheric zenith path delays and gradients and low-degree coefﬁcients of the

Earth gravity ﬁeld.

• Higher resolution in time for certain parameter types: besides weekly, also daily

solutions were generated and Earth rotation parameter (ERPs) were set up with a

sub-daily time resolution of 1 h.

• Usage of the SINEX (Solution INdependent EXchange) format for the exchange

of all time series of parameters and solutions.

• The deﬁnition of a meta data standard for the SINEX format to allow the

exchange of meta data.

The goals and objectives and the list of improvements mentioned above turned

out to be very challenging, but most of them could be realized during the project.

GGOS-D: Integration of Space Geodetic Techniques 531

3 Structure of GGOS-D

The structure of the GGOS-D project is shown in Fig. 1. We see that the project

was designed to cover the processing chain of a global geodetic observing system

WP 3000

Consistent

observation series

SLR

VLBI

GPS

LEOs

Altimetry

WP 4000

Combined solutions

Computation of

consistent

geodetic

reference systems

WP 3100

Consistent standards

IERS

Technique Centres

IDS

IGS

ILRS

IVS

Data archive

single technique

SINEX

WP 5000

Generation of

time series

Coordinates

Coordinate origin

EOP

Low harmonics

Troposph. zenith delay

Kinem. sea surface

Data archive

combinations

SINEX

Data archive

time series

WP 2100

Data management

WP 2300

Meta data catalogue

IERS

Geophysical Fluid

Centres

Atmosphere

Ocean

Hydrology

Gravity

Mantle

Core

WP 6000

Analysis and validation

IERS

Product Centres

ITRF

Comparison

internal

external

-1

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

-1

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

WP 2200

Near real-time data transfer

Observing

Stations

WP 2200

Geophys.

data

series

Fig. 1 Overview of the GGOS-D project

532 M. Rothacher et al.

as completely and consistently as possible. This chain starts at the data collection

and transfer, then goes to the processing and combination of the data and includes,

ﬁnally, the validation and interpretation of the geodetic and geophysical time series.

Therefore the project can be considered as a very small and reduced version of a

global geodetic observing system or at least of the data management and process-

ing part thereof. It can be seen that the data management and information system

plays a central role in the GGOS-D structure as a place for the exchange of data,

products and other information. It should also be noted that an iteration process

is necessary to generate products of high consistency and accuracy (red dashed

arrows in Fig. 1). In the time frame of the GGOS-D project two such iterations were

performed.

The experiences made by the GGOS-D project in this respect are certainly very

valuable for similar developments on an international level as part of the GGOS

development.

4 Data Management and Information System

Figure 1 shows that the data management and information system takes a central

function in the exchange and organization of data and information. All the products

and time series of geodetic and geophysical parameters were exchanged between the

project partners through the BKG data management system. Among others a user-

friendly ﬁle browser, a discussion forum, a publication data base, and the exchange

of meta data by means of SINEX ﬁles containing a meta data block were established.

In addition, high-level tools to work with the data – like the data analysis tool to

investigate the time series or the plot tool to visualize the data sets – have been

developed.

The details of the characteristics, functionalities and options of the data man-

agement and information system realized as part of this project may be found in

Schwegmann and Richter (2009, chapter “GGOS-D Data Management – From Data

to Knowledge”, in this volume).

5 Common Modeling Standards and Parameterization

The consistency among the various solutions of the different techniques to be used

within the GGOS-D project was considered to be an issue of high importance.

Because t he solutions were the basis not only for comparisons but also for a rig-

orous combination, the deﬁnition of common standards and a unique representation

and parameterization of the relevant quantities was crucial to be able to interpret

the combined solutions in the end. As seven different software packages were used

to generate the solutions of the different techniques contributing to GGOS-D, all

these packages had to be modiﬁed to follow the same common standards. The com-

mon models used in the various software packages were not only implemented but

also validated in detail by intensive comparisons. A ﬁrst set of standards was imple-

mented during the ﬁrst phase of the project (see Steigenberger et al., 2006). These

GGOS-D: Integration of Space Geodetic Techniques 533

standards were then reviewed based on the experiences gathered with the initial

set and a second, more demanding and complete set was agreed upon by the part-

ners and subsequently implemented into the seven different software packages used

within this project. All the modeling standards and parameter representations of this

second iteration are listed in Nothnagel et al. (2009, chapter “GGOS-D Consistent

and Combined Time Series of Geodetic/Geophysical Parameters”, in this volume).

There also the most interesting results concerning state-of-the-art modeling can be

found.

6 Consistent Long-Term Time Series of the Space

Geodetic Techniques

As input for the later combination of the space geodetic techniques (e.g. for a TRF

or/and CRF) the global data, available from the various space techniques, were pro-

cessed covering as long a period of time as possible using the standards, models and

parameterizations agreed-upon within the project (see Sect. 5). A list of the time

series (SINEX ﬁles) produced in the framework of the GGOS-D project is given in

Table 1. Compared to the international setup of t oday, a considerable extension of

the set of parameters estimated for each of the technique was realized and all the

parameters common to more than one technique were saved in the SINEX format.

As additional parameter types the quasar coordinates, nutation offsets and rates, tro-

pospheric zenith path delays and gradients and low-degree coefﬁcients of the Earth

gravity ﬁeld were set up to allow, for the ﬁrst time, the linking of geometry, Earth

rotation and gravity ﬁeld. The decision to include as many parameters as possible

and to go for a high temporal resolution for some of the parameters was also moti-

vated by the fact, that parameters can easily be eliminated but not set up on the

level of the normal equation systems and that the temporal resolution can always

be reduced but not increased on the normal equation level. Therefore, daily normal

equation systems were generated (daily resolution for site coordinates) and ERPs

were set up on an hourly basis for VLBI and GPS.

Table 1 Consistent time series of SINEX ﬁles generated as part of the GGOS-D project

Technique Institution Software Time interval

Time

resolution Status

SLR DGFI DOGS 1992–2007 7 d Complete

SLR GFZ EPOS 1993–2006 7 d Complete

VLBI DGFI OCCAM 1984–2006 1 d Complete

VLBI IGG/BKG CALC/SOLVE 1984–2006 1 d Complete

GPS GFZ EPOS 1994–2007 7 d Almost

complete

GPS GFZ Bernese 1994–2007 1 d Complete

LEOs GFZ EPOS 2000–2007 1 d Complete

Altimetry DGFI DGFI software 1992–2007 7 d Complete

534 M. Rothacher et al.

For each of the major techniques (i.e., VLBI, GPS and SLR) two series were

produced using two different software packages. This allows the comparison and

validation of results generated with different algorithms and follows the strategy

used in the IAG Services to have more than one solution available for each of the

techniques with a subsequent combination of the solutions. Such a combination was

done for the two solution series available for VLBI and SLR. For GPS this was not

possible because one of the series could only be completed towards the very end of

the project.

It should be mentioned that the series generated within the GGOS-D project were

not only optimized with respect to consistency, i.e., using common modeling stan-

dards and parameterizations, see Sect. 5, but also with respect to accuracy. Thus the

most recent, state-of-the art modeling features were implemented as, e.g., the use of

6-hourly ECMWF grids for the computation of the hydrostatic delay and the use of

the hydrostatic mapping function VMF1 by Boehm et al. (2006) for VLBI and GPS,

the correction for higher-order ionospheric terms in GPS according to Fritsche et al.

(2005), and the modeling of thermal deformation f or the VLBI telescopes given in

Nothnagel et al. (2009, chapter “GGOS-D Consistent and Combined Time Series of

Geodetic/Geophysical Parameters”, in this issue) and Skurikhina (2001).

To study the contribution of the data from the recent gravity ﬁeld missions

CHAMP and GRACE to reference frame issues (e.g., geocenter variations and prin-

ciple axes of inertia of the Earth), combined daily solutions including the data of

the global GPS ground station network and the data of the CHAMP and GRACE

satellites (including the accelerometer and K-band measurements) were produced.

In addition, time series of geocenter variations and degree 2 gravity ﬁeld coefﬁcients

were also generated from altimetry data for comparison and integration studies. For

this purpose all the altimetry missions present during the lifetime of Topex were

calibrated against each other (see Fig. 2).

Fig. 2 Range biases resulting from the common calibration of all altimtery missions, that were

measuring during the complete lifetime of Topex. The observation periods of the individual mis-

sions are given in the bar chart. The range biases may reach up to 45 cm (for ENVISAT and ERS-1)

and are not stationary at all for some of the missions. Please note the obvious drifts of ERS-1 and

ERS-2 that might be caused by an insufﬁcient calibration of auxiliary sensors