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\section*{Space Geodesy}

\subsection*{Definitions and Principles}
{\it Geodesy} is the science studying the size and the figure of the Earth
including the determination of the Earth's gravity field. {\it Geodetic
astronomy} is that part of astronomy dealing with the definition and
realization of a terrestrial and a celestial reference frame
({\footnotesize cf. TERRESTRIAL COORDINATE SYSTEMS \& FRAMES}). By {\it
Space Geodesy} we mean, then, those aspects of geodesy and geodetic
astronomy studied by using natural or artificial celestial bodies as
observed objects or as observing platforms. In the older literature the
term {\it Cosmic Geodesy} is sometimes used as a synonym. Space geodesy is
thus defined through the observation techniques, below referred to as
space geodetic techniques, or methods.

Space geodesy evolved rapidly in the second half of the twentieth century.
The {\it space age} was initiated by the launch of the first artificial
satellite, Sputnik I, on October 4 of the International Geophysical Year
1957. In the space age it became possible to deploy and use artificial
satellites {\it either} to study size and figure of the Earth from space
{\it or} to observe them as targets from the surface of the Earth. The use
of artificial Earth satellites for geodetic purposes is also referred to
as {\it satellite geodesy}. The second essential development consists of
the {\it Very Long Baseline Interferometry} (VLBI) technique as a new tool
to realize an extraordinarily accurate and stable inertial reference
system and to monitor Earth rotation using quasars ({\footnotesize cf.
EXTRAGALACTIC REFERENCE FRAMES}).

Today, space geodetic techniques are the primary tools to study size,
figure and deformation of the Earth, and its motion as a finite body in
the inertial reference system ({\footnotesize cf. SPACE\&TIME REFERENCES:
CONCEPTS}). Space geodetic techniques thus are the fundamental tools for
geodesy, geodetic astronomy, and geodynamics.

Space geodetic observations contain information about the position (and
motion) of the observed object and the observer. Therefore, space geodetic
observations also contain information concerning the transformation
between the terrestrial and the inertial systems. The Earth orientation
parameters, {\it i.e.}, polar motion, UT1, precession and nutation
({\footnotesize cf. EARTH ROTATION: THEORY, POLAR MOTION AND LENGTH OF
DAY}) define this transformation.

\subsection*{The role of the Earth's atmosphere}

In space geodesy the signals of the observed or observing celestial bodies
have to cross the Earth's atmosphere. This changes the path and the travel
times of the signals. These are referred to as {\it refraction effects}.
Refraction is usually considered a nuisance in astronomy, geodesy and
geodynamics --- as a matter of fact it is the motivation for many
spaceborne experiments related to this field of science. In recent years
refraction effects are more and more considered and understood as a
primary source of information for atmosphere science and are monitored
through space geodetic methods. Let us point out that the same signals and
space geodetic analysis methods are used to study the Earth's atmosphere
as for geodetic and geodynamics purposes. Interdisciplinary studies and
projects have become important aspects in modern space geodesy.

Whether the atmosphere related signal is useful depends on the wavelengths
of the analyzed signals. If we measure, {\it e.g.}, distances or distance
differences to satellites using optical signals, refraction effects may be
computed with sub-centimeter accuracy using pressure, temperature and
humidity registrations at the observing sites. We may therefore conclude,
{\it e.g.}, that laser ranging is not capable of contributing to
atmosphere monitoring. This fact may also be formulated in the positive
way: Laser observations are well suited for calibrating other techniques,
which are more prone to atmospheric effects.

For microwave techniques (Doppler, GPS, VLBI) we have to distinguish
between ionospheric refraction stemming from the ionized upper part of the
atmosphere (extending up to about 1500km) and tropospheric refraction,
stemming from the lower, neutral layers of the atmosphere. Ionospheric
refraction is wavelength-dependent and may be (almost completely)
eliminated if coherent signals are sent through the atmosphere on
different carrier wavelengths. In the VLBI technique this is achieved by
observing the quasars in different wavelengths, in the Doppler or GPS
technique the same is achieved by using two different wavelengths for
signal transmission.

For microwave techniques tropospheric refraction is the ultimate
accuracy-limiting component in the error budget. As opposed to range
observations in the optical band, we have to take into account the
so-called ``wet component'' of tropospheric refraction, which is highly
variable in time and space. This fact forces analysts using microwave
observations to introduce station and time specific parameters (or to
model the effect as a random process). It allows, on the other hand,
analysts to determine the water vapor content above an observatory with
high accuracy and high temporal resolution (Bevis et al., 1992).

\subsection*{Optical period}

For centuries optical (astrometric) observations were the only observation
type available in astronomy. In the pre-space era a series of astrometric
instruments was used for the purpose of defining a terrestrial reference
frame and for monitoring Earth rotation. The photographic zenith tube and
the {\it Danjon astrolabe} were probably the most advanced of these
instruments. They were widely used by observatories contributing to the
International Polar Motion Service (IPMS) and the Bureau International de
l'Heure (BIH) to determine the geographic latitude of a station with a
precision of about 10-40mas (milliarcseconds) in one night. We refer to
(Moritz and Mueller, 1988) for more information.

Optical observations where already made of the first generation of
artificial Earth satellites, like Sputnik 2 and Explorer 1. The balloon
satellites Echo 1 and 2 and PAGEOS (passive geodetic satellite), which
could even be seen ``by naked eye'', were observed by a worldwide optical
tracking network. These satellites were (supposedly) spherical, consisted
of layers of aluminized mylar foil, and, thanks to their brightness, their
tracks could easily be photographed against the star background. It was
not trivial to assign time-tags to specific points of the track. Much
better suited from this point of view, although more difficult to track,
were smaller satellites like Geos 1 (Explorer 29) and Geos 2 (Explorer 36)
equipped with flash lamps allowing for tens of thousands of high-precision
optical observations. Obviously, the quasi-simultaneity of observations
from different sites was possible.

Fascinating results came out of this first phase of satellite geodesy. The
geodetic datums on different continents could be related to the geocenter
and thus to each other with an accuracy of about 5m. First reliable
coefficients of the gravity field (spherical expansion up to degree and
order 12-15) could be also derived.

The astrometric technique, when applied to artificial satellites in the
1960s and 1970s, had serious disadvantages. The star catalogues were not
of sufficiently good quality and the processing time (time between
observation and availability of results) was of the order of a few weeks
in the best case. This, and the advent of new observation techniques
promising higher accuracy, actually ruled out astrometric techniques for a
number of important applications. The optical technique no longer played a
significant role in space geodesy after about 1975.

In view of newly developed observation techniques (CCD, Charge Coupled
Device techniques ({\footnotesize cf. OBSERVATION TECHNIQUES})) and much
better star catalogues based on astrometry missions ({\it e.g.}, HIPPARCOS
mission, ({\footnotesize cf. HIPPARCOS})) it may well be that optical
observations will again play a role in space geodesy in the future.

\subsection*{Doppler period}

The U.S. Navy Navigation Satellite System (NNSS), also called TRANSIT
system after the survey transit instrument, had a significant impact on
the development of space geodesy. It proved that a system based on the
measurement of the Doppler shift of a signal generated by a stable
oscillator on board a satellite could be used for relative positioning
with remarkably high accuracies (0.1-0.5m relative, about 1m geocentric).
The satellites sent information on two carrier frequencies (400Mhz and
150MHz) near the microwave band.

The two frequencies allowed for a compensation of ionospheric refraction.
Rather small receivers connected to omni-directional antennas made the
technique well suited to establish regional or even global geodetic
networks. Observation periods of a few days were required to obtain the
above mentioned accuracy.

The NNSS satellites were in polar, almost circular, orbits about 1100km
above the Earth's surface. Only one satellite at a time could be observed
by one receiver. As opposed to astrometry the Doppler technique was
weather-independent. Until a significant part of the Global Positioning
System (GPS) was deployed (around 1990) the NNSS played a significant role
in space geodesy. Many Doppler campaigns were organized to establish
local, regional or global networks. With the full deployment of the GPS in
the 1990s the geodetic community eventually lost interest in the Doppler
system. The Transit system was shut down as as a positioning system in
December 1996 but continued operating as an ionospheric monitoring tool.
For more information concerning the Doppler system we refer to (Kouba,
1983).

\subsection*{Satellite and Lunar Laser Ranging (SLR and LLR)}

Laser stands for {\it Light Amplification through Stimulated Emission of
Radiation}. The laser technique, developed in the 1950s, is able to
generate high energetic short light pulses (of a few tens of picoseconds
(ps) (1 ps=10$^{-12}$s)). These pulses are sent out by a conventional
astronomical telescope, travel to the satellite (or the Moon), are
reflected by special corner cubes (comparable to the rear reflectors of
bicycles) on the satellite (Moon) back to the telescope, where they are
detected. The measurement is the travel time $\Delta t$ of the laser pulse
from the telescope to the satellite and back to the telescope. Apart from
refraction this light travel time, after multiplication with the speed of
light $c$ in vacuum, equals twice the distance $\rho^s_r$ between
satellite and telescope at the time the light pulse is reflected from the
satellite $\rho_r^s \approx \Delta t \cdot c / 2$. Today's Satellite Laser
Ranging (SLR) technique is used to determine the ``true'' distances
between observatories and satellites with an accuracy of a few millimeters
and, if required, with a high repetition rate (several times per second).

{\it SLR} techniques may be used for every satellite equipped with corner
cubes. Fig.\ \ref{lageos2} shows Lageos II, a typical SLR-dedicated
satellite which was launched in 1992. Lageos II is a spherical satellite
with a diameter of 0.6m, a weight of 405kg. 426 corner cubes are inlaid in
its surface. Lageos II is a close relative of Lageos I, which was launched
in 1976. The two Lageos satellites are in stable, almost circular orbits
about 6000km above the surface of the Earth.

The two Lageos satellites are primary scientific tracking targets for the
{\it International Laser Ranging Service (ILRS)}. The two satellites have
contributed in a significant way to the determination of the Earth's
gravity field. Many more targets are regularly observed by the ILRS. Some,
like the French low orbiting satellite {\it Starlette}, with a diameter of
24cm, are similar in design to the Lageos satellites and serve a similar
purpose. For others SLR is just the primary or backup technique for
precise orbit determination.

\begin{figure}[H]
\begin{center}
\mbox{\epsfig{file=d:/txt/slides/leos/lageos2.eps,angle=0,width=0.49\textwidth}}
\caption{The Lageos II Spacecraft} \label{lageos2}
\end{center}
\end{figure}
With the exception of UT1, the SLR technique is able of determining all
parameters of geodetic interest (station coordinates and motion, Earth
rotation parameters, gravity field). The unique and most valuable
contributions lie in the determination of the Earth's (variable) gravity
field, in the determination of the geocenter ({\it i.e.}, the location of
the polyhedron formed by connecting the SLR stations with respect to the
geocenter), and in calibrating geodetic microwave techniques.

From the technique point of view there is no principal difference between
SLR and LLR (Lunar Laser Ranging): Light travel times are measured from
the observatory to one of the laser reflectors deployed on the Moon by the
Apollo space missions or the Russian unmanned Lunokhod missions. The
scientific impact of LLR is significant. LLR was, {\it e.g.}, capable of
measuring directly the secular increase of the Earth-Moon distance (3.8cm
per year), an effect which is in turn coupled with the deceleration of the
angular velocity of Earth rotation. Also, LLR is well suited to evaluate
gravitational theories ({\footnotesize cf. REFERENCE FRAMES \& TIMESCALES
IN GENERAL RELATIVITY}).

\subsection*{Very Long Baseline Interferometry}

Very Long Baseline Interferometry (VLBI) is the only non-satellite
geodetic technique contributing data to the International Earth Rotation
service (IERS). Its main features are discussed in ({\footnotesize cf.
EXTRAGALACTIC REFERENCE FRAMES}).

Its unique and fundamental contribution to geodesy and astronomy consists
of the realization of the inertial reference system and in the maintenance
of the long-term stability of the transformation between the celestial and
terrestrial reference frame.

The ICRS, International Celestial Reference System, was defined by the
International Earth Rotation Service (Arias et. al., 1995). It was adopted
by the International Astronomical Union (IAU) as the primary celestial
reference system replacing the optical predecessors.

An accurate and stable celestial reference frame is a prerequisite for a
terrestrial reference system. In this sense VLBI plays a decisive role in
the definition of the terrestrial reference system, and in establishing
the transformation between the two systems. In particular, VLBI is the
only technique providing the difference UT1-UTC, {\it i.e.}, the
difference between Earth rotation time and atomic time with
state-of-the-art accuracy and excellent long-term stability. Also, VLBI is
the only technique capable of determining precession and nutation with an
angular resolution below the milliarcsecond level.

The observation and analysis aspects are today coordinated by the IVS, the
International VLBI Service (Table \ref{services}).

\subsection*{The Global Positioning System (GPS)}
GPS is probably the best known space geodetic technique, today. The system
has an impact on science and society reaching far beyond space geodesy.
GPS revolutionized surveying, timing, car and aircraft navigation.
Virtually millions of hand-held receivers are in use today. Spaceborne
applications of the GPS have a deep impact on geodesy and atmosphere
sciences.

GPS is a navigation system allowing for instantaneous, real-time,
``absolute'' positioning on or near the surface of the Earth with an
accuracy of a few meters. An unlimited number of users may use the system
simultaneously. ``Absolute'' means that the estimated coordinates may be
established using only one receiver and that they refer to a geocentric
Earth-fixed coordinate system. This coordinate system, the WGS-84 (World
Geodetic System), is today aligned with sub-meter accuracy to the ITRF,
the International Terrestrial Reference Frame maintained by the IERS
({\footnotesize cf. TERRESTRIAL COORDINATE SYSTEMS \& FRAMES})).

The space segment of GPS nominally consists of 24 satellites (21
operational satellites plus 3 active spares). The satellites are in almost
circular orbits distributed in six planes approximately 20,000km above the
Earth's surface. These planes are separated by 60 deg on the equator and
inclined by 55 deg with respect to the equator. The revolution period is
half a sidereal day (11$^h$58$^m$), which means that for a given location
on the Earth's surface the satellite constellation above horizon repeats
itself after one sidereal day (solar day minus four minutes). Fig.
\ref{gpssat} shows the Block II satellite.
\begin{figure}[H]
\begin{center}
\mbox{\epsfig{file=d:/eps/gpsbl2.eps,angle=0,width=0.49\textwidth}}
\end{center}
\caption{Block II Satellite} \label{gpssat}
\end{figure}
This is the satellite with which the first full GPS generation was built.
We distinguish the main body of the satellite with the antenna array
pointing to the center of the Earth and the solar panels. The attitude is
maintained by {\it momentum wheels}, which have to guarantee that the
antenna array is always pointing to the center of the Earth and that the
solar panel axes are perpendicular to the Sun-satellite direction. The
satellite is then capable of rotating the solar panels into a position
perpendicular to the same direction.

Each satellite broadcasts on two carrier frequencies L1 and L2 of
1.57542GHz and 1.22760GHz respectively, corresponding to wavelengths of
$\lambda_1 \approx 19$~cm and $\lambda_2 \approx 24$~cm. Two types of
codes are sent out, allowing the users to reconstruct the so-called {\it
pseudorange} $p^s_r$ (definition in eqn.\ (\ref{eqn1})).

The pseudorange $p_r^s$ and the geometric distance $\rho^s_r$ between the
satellite at signal emission time and the receiver at signal reception
time are related through
\begin{equation} \label{eqn1}
p_r^s = \rho_r^s + c \cdot (\Delta t_r - \Delta t^s) + \Delta \rho_{atm},
\end{equation}
where $\Delta t^s$ is the error of the satellite clock with respect to the
``true'' (or system) time, $\Delta t_r$ is the receiver clock error.
$\Delta \rho_{atm}$ is the correction of the light travel time due to the
atmosphere (sum of ionospheric and tropospheric refraction).

In addition, and among other important information, {\it broadcast orbits}
allowing computation of the satellite position at emission time and {\it
satellite clock corrections} mitigating the satellite synchronization
error $\Delta t^s$ {\it w.r.t.} the ``true'' or system time are sent out
continuously using the phase modulation technique on L1 and L2.

Two kinds of code have to be distinguished, the so-called C/A-code (coarse
acquisition code) allowing an accuracy of about 3m, and the P-code
(precision code) allowing an accuracy of about 0.3m. Modern digital
receivers show even a much better performance. The P-code currently is
only available to privileged ({\it i.e., U.S. Department of Defense
authorized}) user. The C/A code is transmitted on L1, an encrypted version
of the P-code on both carriers. The broadcast orbits usually have an
accuracy of about 3m. The satellite clock information is made available to
the non-authorized user only with moderate accuracy, which limits
real-time ``absolute'' positioning accuracy to about 100m.

Let us briefly address the principles of {\it absolute GPS positioning and
navigation}: A GPS receiver is simultaneously observing several satellites
(ideally all that are above the horizon). Using broadcast orbits to
compute the satellite positions in the Earth-fixed system, standard
atmosphere models to account for refraction, and the broadcast clock
information to adjust the satellite clocks to system time, we are left
with only four unknowns in eqn.\ (\ref{eqn1}), namely the three
coordinates of the receiver position and the receiver clock error $\Delta
t_r$ -- provided we consider only simultaneous observations. It is thus
necessary and sufficient that a receiver tracks simultaneously four
satellites in order to compute an instantaneous position (and a receiver
clock error) with an accuracy of a few meters or about 100m for users
having no access to P-code, respectively. The GPS constellation was
designed, as a matter of fact, to ensure that (to the extent possible)
four or more satellites are available all the time at each location on the
surface of the Earth.

For scientific purposes the {\it phase observation} plays a decisive role.
It is in essence identical with the so-called accumulated Doppler
observation of the NNSS and it is closely related to the GPS code
observation, as well. From the mathematical point of view, there are
``only'' two essential differences between phase and code, namely the much
higher measurement accuracy of phase (millimeters rather than meters), and
an additional unknown, the initial phase ambiguity parameter $N_r^s$, {\it
per satellite pass}. All GPS receivers used for high accuracy geodetic
applications record the phase observations on both carriers L1 and L2, in
addition to the code.

The phase observations yields local GPS networks with mm-accuracy,
regional and global networks with about cm-accuracy. This is only
possible, if precise satellite orbit and clock information, such as
generated by the {\it International GPS Service (IGS)}, is available.
Fig.\ \ref{igsmap} shows the IGS network as of October 1998.

Over 200 IGS sites, distributed all over the globe permanently observe all
satellites in view, transmit their observations (at least) on a daily
basis to IGS Data Centers.

The data are then analyzed by IGS Analysis Centers, which deliver {\it
rapid} and {\it final} products. Rapid IGS products are available with a
delay of about one day, final products with a delay of about eleven days.
Daily products include satellite orbits with an accuracy of about
0.05-0.1m, satellite clocks with an accuracy of about 0.3ns (nanoseconds),
daily values of polar motion components accurate to about 0.1 mas
(milliarcseconds), corresponding to 3mm on the Earth's surface), and
length of day (lod) estimates with an accuracy of about 30$\mu$s/day.
These products are essential contributions to the monitoring of Earth
rotation ({\footnotesize cf POLAR MOTION AND LENGTH OF DAY}).
\begin{figure}[H]
\begin{center}
\mbox{\epsfig{file=d:/eps/map9810.eps,angle=0,width=0.49\textwidth}}
\caption{The International GPS Service Network} \label{igsmap}
\end{center}
\end{figure}

In addition the IGS Analysis centers perform weekly global coordinate
solutions of their portion of the IGS network. These results are used,
together with the results of the other space techniques, for the
establishment of the International Terrestrial Reference Frame
({\footnotesize cf. TERRESTRIAL COORDINATE SYSTEMS \& FRAMES}).

The IGS products (orbits, Earth rotation parameters, coordinates and
velocities of IGS stations) are used as known {\it a priori} information
to establish regional networks for crustal deformation studies ({\it
e.g.}, the Californian SCIGN (http://www.scign.org), the Japanese
1000-receiver network for Earthquake monitoring) or for regional reference
frame establishment and maintenance ({\it e.g.}, the EUREF network
http://www. oma.be:/KSB-ORB/EUREF/ eurefhome.html, or the South American
SIRGAS network
http://www.\-dgfi.\-badw-muen\-chen.de/\-gps/\-sir\-gas.html).

More and more, the IGS network is used for purposes other than space
geodesy. Let us mention that the IGS network has been enhanced to include
time and frequency transfer and that it is able to monitor the ionosphere.
For more information concerning the IGS and its interdisciplinary impact
we refer to (Beutler et al., 1999).

From the point of view of space geodesy GPS is a ``work horse'' with
important contributions to the establishment and maintenance of a dense
terrestrial reference frame, and which provides Earth rotation parameters
with a high time resolution. It should not be forgotten, that the GPS ---
like every satellite geodetic method --- is not able to maintain a
long-term stability of $UT1$ or of precession and nutation. Moreover,
despite the fact that GPS is a satellite geodetic technique, it is not
well suited to determine the Earth's gravity field or the motion of its
station polyhedron {\it w.r.t.} the geocenter. The height of the GPS
satellites is one of the limiting factors.

For more information concerning GPS as a tool for geodesy and geodynamics
we refer to (Teunissen and Kleusberg, 1998).


\subsection*{Other Satellite microwave techniques}

The Russian GLONASS (Global Navigation Satellite System) is so closely
related to the GPS that there are a number of combined GPS and GLONASS
receivers available. These receivers were used in the first global GLONASS
tracking and analysis campaign, the {\it IGEX-98} (International GLONASS
Experiment 1998). The experiment revealed that a combined analysis of GPS
and GLONASS is very promising for science and navigation.

The French DORIS system (Doppler Orbitography by Radiopositioning
Integrated on Satellite) proved to be a very powerful tool for orbit
determination. It is, {\it e.g.}, one of the orbit determination system
used in the TOPEX/Poseidon mission (see below). Also, DORIS possesses a
very well designed ground tracking network. This is one reason why DORIS
was accepted as an official space technique by the IERS (see Table
\ref{services}).

The German PRARE (Precise Range and Range-rate Equipment) system may be
viewed as the German counterpart of the DORIS system. It is used as an
orbit determination tool, {\it e.g.}, on the European Space Agency's ERS-2
(Earth Remote Sensing) spacecraft.

\subsection*{Satellite Missions}

There were many satellite missions in the past and there will be more in
the future in which the satellite is used as an observing platform to
study aspects of the Earth relevant to geodesy and geodynamics. Let us
mention in particular that {\it altimetry missions} significantly improved
our knowledge of the sea surface topography, ocean currents, tidal motions
of the oceans, {\it etc}.

Fig.\ \ref{topex} shows the TOPEX/Poseidon spacecraft. The mission is a
combined U.S. and French altimetry mission. It is actually the first
mission which was specially designed to investigate ocean currents. One
entire volume of the Journal of Geophysical Research was devoted to this
mission (JGR, 1994).
\begin{figure}[H]
\begin{center}
\mbox{\epsfig{file=d:/eps/topex.eps,angle=0,width=0.49\textwidth}}
\caption{The TOPEX/Poseidon Spacecraft} \label{topex}
\end{center}
\end{figure}
For space geodesy the TOPEX/Poseidon mission was a kind of ``rosetta stone
mission'' because its orbit was determined using three independent
systems, the French DORIS system, SLR tracking, and the GPS. All three
systems proved their capability. The radial component of the orbit (which
is of crucial importance for altimetry missions) could be established with
an accuracy of a few centimeters. Let us mention that TOPEX/Poseidon was
neither the first, nor will it be the last altimetry mission.

For geodesy, geodynamics, and atmosphere physics the upcoming missions
CHAMP (Challenging Mini-Satellite Payload for Geophysical Research and
application, German mission), GRACE (Gravity Recovery and Climate
Experiment, U.S./German mission ), and GOCE (Gravity field and Ocean
Current Explorer, ESA mission) are fascinating. It is expected that our
knowledge of the gravity field (using spaceborne GPS receivers,
accelerometers, or gradiometers) to measure the non-gravitational forces
{\it resp.} gravity gradients will significantly increase through such
missions.

Also, CHAMP, GRACE and GOCE are able to produce atmosphere profiles using
the {\it occultation method}: the signal (phase and code) of a GPS
satellite is monitored by a spaceborne GPS receiver on a low Earth orbiter
(LEO) during the time period the line of sight LEO-GPS satellite scans
through the Earth's atmosphere. These developments support our initial
statement that interdisciplinary aspects are becoming more and more
important in Space Geodesy.

\subsection*{Organizations}

Table \ref{services} gives an overview of the institutions relevant for
space geodesy.

They all are IAG (International Association of Geodesy) services. The IERS
and the IVS are in addition IAU services (International Astronomical
Union). The IERS and the IGS are members of FAGS (Federation of
Astronomical and Geodesical Data Analysis Services).


IGS, ILRS and IVS are technique-specific services. The IERS is a
multi-technique service. It was established in 1988 as successor of the
International Polar Motion Service (IPMS) and the Earth rotation branch of
the Bureau International de l'Heure (BIH). The IERS products are based on
the products of the technique-specific services.

CSTG is a commission of IAG and a subcommission of COSPAR (Commission on
Space Research). It has a coordinating function within space geodesy. In
the time period 1995-1999 it was, {\it e.g.}, responsible for creating the
ILRS and the IVS, and it organized the first global GLONASS experiment
IGEX-98.

More information about these services may be found at the internet
addresses in Table \ref{services}.
\begin{center}
\begin{table}[H]
\begin{tabular}{|l|p{6.5cm}|}
\hline

Acronym & Name, Mission, Internet  \\

\hline

CSTG & Commission on International Coordination of Space Techniques.
Coordination between space geodetic organizations, organize projects. \\
IERS & International Earth Rotation Service. Establish and maintain
       celestial and terrestrial reference frame, generate combined Earth
       orientation parameter series \\
     & http://hpiers.obspm.fr \\
IGS  & International GPS Service. Make available GPS data from its global
network, producing and disseminating high accuracy GPS orbits, Earth
rotation parameters, station coordinates, atmospheric information, etc. \\
     & http://igscb.jpl.nasa.gov \\

ILRS & International Laser Ranging Service. Collects, archives, and
distributes SLR and LLR datasets. Generates scientific and operational
products \\
     & http://ilrs.gsfc.nasa.gov \\

IVS & International VLBI Service for Geodesy and Astrometry. Operate or
      support VLBI programs. Organize geodetic, astrometric, geophysical
      research and operational activities. \\
     & http://ivscc.gsfc.nasa.gov \\

\hline
\end{tabular}
\caption{Space Geodetic Services} \label{services}
\end{table}
\end{center}

\subsection*{References}


\begin{list}{--}{\itemsep0pt\parsep0pt\labelwidth0.7cm\labelsep0pt
              \leftmargin\labelwidth
              \topsep4ex\parskip0pt\partopsep0pt}

\item[Arias, ] E F, Charlot, P, Feissel, M, Lestrade, J-F 1995 The
Extragalctic Reference System of the International Earth Rotation Service,
ICRS, {\it Astron. Astrophys.}, {\bf 119} 604-608.

\item [Beutler, ] G, Rothacher M, Schaer S, Springer T A, Kouba J, and
    Neilan R E 1999 The International GPS Service (IGS): An
    interdisciplinary service for Earth sciences,
    {\it Advances for Space Research}, {\bf ??} ????-????.


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\end{document}
