Space Geodesy We Mean, Then, Those

Home , Space geodesy

MOTION AND LENGTH OF DAY) dene this trans- Space Geo desy

formation.

Denitions and Principles

The role of the Earth's atmosphere

Geodesy is the science studying the size and the g-

ure of the Earth including the determination of the

In space geo desy the signals of the observed or ob-

Earth's gravity eld. Geodetic astronomy is that

serving celestial b o dies have to cross the Earth's

part of astronomy dealing with the denition and

atmosphere. This changes the path and the travel

realization of a terrestrial and a celestial reference

times of the signals. These are referred to as re -

frame (cf. TERRESTRIAL COORDINATE SYSTEMS &

fraction eects. Refraction is usually considered

FRAMES). By Space Geodesy we mean, then, those

a nuisance in astronomy, geo desy and geo dynam-

asp ects of geo desy and geo detic astronomy stud-

ics as a matter of fact it is the motivation for

ied by using natural or articial celestial b o dies as

many spaceb orne exp eriments related to this eld

observed ob jects or as observing platforms. In the

of science. In recent years refraction eects are

older literature the term Cosmic Geodesy is some-

more and more considered and understo o d as a pri-

times used as a synonym. Space geo desy is thus

mary source of information for atmosphere science

dened through the observation techniques, b elow

and are monitored through space geo detic meth-

referred to as space geo detic techniques, or meth-

o ds. Let us p oint out that the same signals and

o ds.

space geo detic analysis metho ds are used to study

Space geo desy evolved rapidly in the second

the Earth's atmosphere as for geo detic and geo-

half of the twentieth century. The space age wa s

dynamics purp oses. Interdisciplinary studies and

initiated by the launch of the rst articial satel-

pro jects have b ecome imp ortant asp ects in mo d-

lite, Sputnik I, on Octob er 4 of the International

ern space geo desy.

Geophysical Year 1957. In the space age it b e-

Whether the atmosphere related signal is use-

came p ossible to deploy and use articial satellites

ful dep ends on the wavelengths of the analyzed sig-

either to study size and gure of the Earth from

nals. If we measure, e.g., distances or distance dif-

space or to observe them as targets from the surface

ferences to satellites using optical signals, refrac-

of the Earth. The use of articial Earth satellites

tion eects may b e computed with sub-centimeter

for geo detic purp oses is also referred to as satel lite

accuracy using pressure, temp erature and humid-

geodesy. The second essential development consists

ity registrations at the observing sites. We may

of the Very Long Baseline Interferometry (VLBI)

therefore conclude, e.g., that laser ranging is not

technique as a new to ol to realize an extraordinarily

capable of contributing to atmosphere monitoring.

accurate and stable inertial reference system and to

This fact may also b e formulated in the p ositive

monitor Earth rotation using quasars (cf. EXTRA-

way: Laser observations are well suited for cali-

GALACTIC REFERENCE FRAMES).

brating other techniques, which are more prone to

Today, space geo detic techniques are the pri-

atmospheric eects.

mary to ols to study size, gure and deformation

For microwave techniques (Doppler, GPS,

of the Earth, and its motion as a nite b o dy in

VLBI) we have to distinguish b etween ionospheric

the inertial reference system (cf. SPACE&TIME REF-

refraction stemming from the ionized upp er part of

ERENCES: CONCEPTS). Space geo detic techniques

the atmosphere (extending up to ab out 1500km)

thus are the fundamental to ols for geo desy, geo de-

and trop ospheric refraction, stemming from the

tic astronomy, and geo dynamics.

lower, neutral layers of the atmosphere. Iono-

Space geo detic observations contain informa-

spheric refraction is wavelength-dependent and

tion ab out the p osition (and motion) of the ob-

may b e (almost completely) eliminated if coherent

served ob ject and the observer. Therefore, space

signals are sent through the atmosphere on dier-

geo detic observations also contain information con-

ent carrier wavelengths. In the VLBI technique

cerning the transformation b etween the terrestrial

this is achieved by observing the quasars in dier-

and the inertial systems. The Earth orientation pa-

ent wavelengths, in the Doppler or GPS technique

rameters, i.e., p olar motion, UT1, precession and

the same is achieved by using two dierent wave-

nutation (cf. EARTH ROTATION: THEORY, POLAR 1

vations from dierent sites was p ossible. lengths for signal transmission.

Fascinating results came out of this rst phase For microwave techniques trop ospheric refrac-

of satellite geo desy. The geo detic datums on dif- tion is the ultimate accuracy-limiting comp onent in

ferent continents could b e related to the geo center the error budget. As opp osed to range observations

and thus to each other with an accuracy of ab out in the optical band, we have to take into account

5m. First reliable co ecients of the gravity eld the so-called wet comp onent of trop ospheric re-

(spherical expansion up to degree and order 12-15) fraction, which is highly variable in time and space.

could b e also derived. This fact forces analysts using microwave observa-

The astrometric technique, when applied to ar- tions to introduce station and time sp ecic param-

ticial satellites in the 1960s and 1970s, had seri- eters (or to mo del the eect as a random pro cess).

ous disadvantages. The star catalogues were not It allows, on the other hand, analysts to determine

of suciently go o d quality and the pro cessing time the water vapor content ab ove an observatory with

(time b etween observation and availability of re- high accuracy and high temp oral resolution (Bevis

sults) was of the order of a few weeks in the b est et al., 1992).

case. This, and the advent of new observation tech-

niques promising higher accuracy, actually ruled

Optical p erio d

out astrometric techniques for a number of im-

For centuries optical (astrometric) observations

p ortant applications. The optical technique no

were the only observation type available in astron-

longer played a signicant role in space geo desy

omy. In the pre-space era a series of astromet-

after ab out 1975.

ric instruments was used for the purp ose of den-

In view of newly developed observation tech-

ing a terrestrial reference frame and for monitor-

niques (CCD, Charge Coupled Device techniques

ing Earth rotation. The photographic zenith tub e

(cf. OBSERVATION TECHNIQUES)) and much b et-

and the Danjon astrolabe were probably the most

ter star catalogues based on astrometry missions

advanced of these instruments. They were widely

(e.g., HIPPARCOS mission, (cf. HIPPARCOS)) it

used by observatories contributing to the Interna-

may well b e that optical observations will again

tional Polar Motion Service (IPMS) and the Bu-

play a role in space geo desy in the future.

reau International de l'Heure (BIH) to determine

the geographic latitude of a station with a precision

Doppler p erio d

of ab out 10-40mas (milliarcseconds) in one night.

The U.S. Navy Navigation Satellite System We refer to (Moritz and Mueller, 1988) for more

(NNSS), also called TRANSIT system after the information.

survey transit instrument, had a signicant impact Optical observations where already made of the

on the development of space geo desy. It proved rst generation of articial Earth satellites, like

that a system based on the measurement of the Sputnik 2 and Explorer 1. The ballo on satellites

Doppler shift of a signal generated by a stable oscil- Echo 1 and 2 and PAGEOS (passive geo detic satel-

lator on b oard a satellite could b e used for relative lite), which could even b e seen by naked eye, were

p ositioning with remarkably high accuracies (0.1- observed by a worldwide optical tracking network.

0.5m relative, ab out 1m geo centric). The satel- These satellites were (supp osedly) spherical, con-

lites sent information on two carrier frequencies sisted of layers of aluminized mylar foil, and, thanks

(400Mhz and 150MHz) near the microwave band. to their brightness, their tracks could easily b e pho-

The two frequencies allowed for a comp ensation tographed against the star background. It was not

of ionospheric refraction. Rather small receivers trivial to assign time-tags to sp ecic p oints of the

connected to omni-directional antennas made the track. Much b etter suited from this p oint of view,

technique well suited to establish regional or even although more dicult to track, were smaller satel-

global geo detic networks. Observation p erio ds of a lites like Geos 1 (Explorer 29) and Geos 2 (Ex-

few days were required to obtain the ab ove men- plorer 36) equipp ed with ash lamps allowing for

tioned accuracy. tens of thousands of high-precision optical observa-

The NNSS satellites were in p olar, almost cir- tions. Obviously, the quasi-simultaneity of obser- 2

1976. The two Lageos satellites are in stable, al- cular, orbits ab out 1100km ab ove the Earth's sur-

most circular orbits ab out 6000km ab ove the surface. Only one satellite at a time could b e observed

face of the Earth. by one receiver. As opp osed to astrometry the

The two Lageos satellites are primary scientic Doppler technique was weather-independent. Un-

tracking targets for the International Laser Rang- til a signicant part of the Global Positioning Sys-

ing Service (ILRS). The two satellites have con- tem (GPS) was deployed (around 1990) the NNSS

tributed in a signicant way to the determination played a signicant role in space geo desy. Many

of the Earth's gravity eld. Many more targets Doppler campaigns were organized to establish lo-

are regularly observed by the ILRS. Some, like the cal, regional or global networks. With the full de-

French low orbiting satellite Starlette, with a di- ployment of the GPS in the 1990s the geo detic com-

ameter of 24cm, are similar in design to the Lageos munity eventually lost interest in the Doppler sys-

satellites and serve a similar purp ose. For others tem. The Transit system was shut down as as a

SLR is just the primary or backup technique for p ositioning system in December 1996 but contin-

precise orbit determination. ued op erating as an ionospheric monitoring to ol.

For more information concerning the Doppler sys-

tem we refer to (Kouba, 1983).

Satellite and Lunar Laser Ranging (SLR

and LLR)

Laser stands for Light Amplication through Stim-

ulated Emission of Radiation. The laser technique,

developed in the 1950s, is able to generate high

energetic short light pulses (of a few tens of pi-

coseconds (ps) (1 ps=10 s)). These pulses are

sent out by a conventional astronomical telescop e,

travel to the satellite (or the Mo on), are reected

by sp ecial corner cub es (comparable to the rear re-

ectors of bicycles) on the satellite (Mo on) back to

Figure 1: The Lageos I I Spacecraft

the telescop e, where they are detected. The mea-

surement is the travel time t of the laser pulse

With the exception of UT1, the SLR technique is

from the telescop e to the satellite and back to the

able of determining all parameters of geo detic in-

telescop e. Apart from refraction this light travel

terest (station co ordinates and motion, Earth ro-

time, after multiplication with the sp eed of light

tation parameters, gravity eld). The unique and

c in vacuum, equals twice the distance b etween

most valuable contributions lie in the determina-

satellite and telescop e at the time the light pulse

tion of the Earth's (variable) gravity eld, in the

is reected from the satellite t c=2.To-

determination of the geo center (i.e., the lo cation

day's Satellite Laser Ranging (SLR) technique is

of the p olyhedron formed by connecting the SLR

used to determine the true distances b etween ob-

stations with resp ect to the geo center), and in cal-

servatories and satellites with an accuracy of a few

ibrating geo detic microwave techniques.

millimeters and, if required, with a high rep etition

From the technique p oint of view there is no

rate (several times p er second).

principal dierence b etween SLR and LLR (Lunar

SLR techniques may b e used for every satel-

Laser Ranging): Light travel times are measured

lite equipp ed with corner cub es. Fig. 1 shows La-

from the observatory to one of the laser reectors

geos I I, a typical SLR-dedicated satellite which was

deployed on the Mo on by the Ap ollo space mis-

launched in 1992. Lageos I I is a spherical satellite

sions or the Russian unmanned Lunokho d missions.

with a diameter of 0.6m, a weight of 405kg. 426

The scientic impact of LLR is signicant. LLR

corner cub es are inlaid in its surface. Lageos I I is

was, e.g., capable of measuring directly the secu-

a close relative of Lageos I, which was launched in

lar increase of the Earth-Mo on distance (3.8cm p er 3

of the GPS have a deep impact on geo desy and year), an eect which is in turn coupled with the de-

atmosphere sciences. celeration of the angular velocity of Earth rotation.

GPS is a navigation system allowing for in- Also, LLR is well suited to evaluate gravitational

stantaneous, real-time, absolute p ositioning on or theories (cf. REFERENCE FRAMES & TIMESCALES

near the surface of the Earth with an accuracy of IN GENERAL RELATIVITY).

a few meters. An unlimited number of users may

use the system simultaneously. Absolute means

Very Long Baseline Interferometry

that the estimated co ordinates may b e established

Very Long Baseline Interferometry (VLBI) is the

using only one receiver and that they refer to a geo-

only non-satellite geo detic technique contributing

centric Earth-xed co ordinate system. This co or-

data to the International Earth Rotation service

dinate system, the WGS-84 (World Geo detic Sys-

(IERS). Its main features are discussed in (cf. EX-

tem), is to day aligned with sub-meter accuracy to

TRAGALACTIC REFERENCE FRAMES).

the ITRF, the International Terrestrial Reference

Its unique and fundamental contribution to

Frame maintained by the IERS (cf. TERRESTRIAL

geo desy and astronomy consists of the realization

COORDINATE SYSTEMS & FRAMES)).

of the inertial reference system and in the mainte-

The space segment of GPS nominally consists of

nance of the long-term stability of the transforma-

24 satellites (21 op erational satellites plus 3 active

tion b etween the celestial and terrestrial reference

spares). The satellites are in almost circular orbits

frame.

distributed in six planes approximately 20,000km

The ICRS, International Celestial Reference

ab ove the Earth's surface. These planes are sep-

System, was dened by the International Earth Ro-

arated by 60 deg on the equator and inclined by

tation Service (Arias et. al., 1995). It was adopted

55 deg with resp ect to the equator. The revolution

by the International Astronomical Union (IAU) as

h m

p erio d is half a sidereal day (11 58 ), which means

the primary celestial reference system replacing the

that for a given lo cation on the Earth's surface the

optical predecessors.

satellite constellation ab ove horizon rep eats itself

An accurate and stable celestial reference frame

after one sidereal day (solar day minus four min-

is a prerequisite for a terrestrial reference system.

utes). Fig. 2 shows the Blo ck I I satellite.

In this sense VLBI plays a decisive role in the de-

nition of the terrestrial reference system, and in es-

tablishing the transformation b etween the two sys-

tems. In particular, VLBI is the only technique

providing the dierence UT1-UTC, i.e., the dif-

ference b etween Earth rotation time and atomic

time with state-of-the-art accuracy and excellent

long-term stability. Also, VLBI is the only tech-

nique capable of determining precession and nuta-

tion with an angular resolution b elow the milliarc-

second level.

The observation and analysis asp ects are to day

co ordinated by the IVS, the International VLBI

Service (Table 1).

The Global Positioning System (GPS)

GPS is probably the b est known space geo detic

technique, to day. The system has an impact on sci-

ence and so ciety reaching far b eyond space geo desy.

GPS revolutionized surveying, timing, car and air-

Figure 2: Blo ck I I Satellite

craft navigation. Virtually millions of hand-held

receivers are in use to day. Spaceb orne applications

This is the satellite with which the rst full 4

GPS positioning and navigation: A GPS receiver GPS generation was built. We distinguish the main

is simultaneously observing several satellites (ide- b o dy of the satellite with the antenna array p oint-

ally all that are ab ove the horizon). Using broad- ing to the center of the Earth and the solar panels.

cast orbits to compute the satellite p ositions in the The attitude is maintained by momentum wheels,

Earth-xed system, standard atmosphere mo dels which have to guarantee that the antenna array is

to account for refraction, and the broadcast clo ck always p ointing to the center of the Earth and that

information to adjust the satellite clo cks to system the solar panel axes are p erp endicular to the Sun-

time, we are left with only four unknowns in eqn. satellite direction. The satellite is then capable of

(1), namely the three co ordinates of the receiver p o- rotating the solar panels into a p osition p erp endic-

sition and the receiver clo ck error t provided ular to the same direction.

we consider only simultaneous observations. It is Each satellite broadcasts on two carrier fre-

thus necessary and sucient that a receiver tracks quencies L1 and L2 of 1.57542GHz and 1.22760GHz

simultaneously four satellites in order to compute resp ectively, corresp onding to wavelengths of

an instantaneous p osition (and a receiver clo ck er- 19 cm and 24 cm. Two types of co des are sent

ror) with an accuracy of a few meters or ab out out, allowing the users to reconstruct the so-called

100m for users having no access to P-co de, resp ec- pseudorange p (denition in eqn. (1)).

tively. The GPS constellation was designed, as a The pseudorange p and the geometric distance

matter of fact, to ensure that (to the extent p os- b etween the satellite at signal emission time and

sible) four or more satellites are available all the the receiver at signal reception time are related

time at each lo cation on the surface of the Earth. through

For scientic purp oses the phase observation

s s s

p = + c (t t )+ ; (1)

r atm

plays a decisive role. It is in essence identical with r r

the so-called accumulated Doppler observation of

where t is the error of the satellite clo ck with

the NNSS and it is closely related to the GPS co de

resp ect to the true (or system) time, t is the

observation, as well. From the mathematical p oint

receiver clo ck error. is the correction of the

atm

of view, there are only two essential dierences

light travel time due to the atmosphere (sum of

b etween phase and co de, namely the much higher

ionospheric and trop ospheric refraction).

measurement accuracy of phase (millimeters rather

In addition, and among other imp ortant infor-

than meters), and an additional unknown, the ini-

mation, broadcast orbits allowing computation of

tial phase ambiguity parameter N , per satel lite

the satellite p osition at emission time and satel lite

pass. All GPS receivers used for high accuracy

clock corrections mitigating the satellite synchro-

geo detic applications record the phase observations

nization error t w.r.t. the true or system time

on b oth carriers L1 and L2, in addition to the co de.

are sent out continuously using the phase mo dula-

The phase observations yields lo cal GPS net-

tion technique on L1 and L2.

works with mm-accuracy, regional and global net-

Two kinds of co de have to b e distinguished, the

works with ab out cm-accuracy. This is only p ossi-

so-called C/A-co de (coarse acquisition co de) allow-

ble, if precise satellite orbit and clo ck information,

ing an accuracy of ab out 3m, and the P-co de (pre-

such as generated by the International GPS Service

cision co de) allowing an accuracy of ab out 0.3m.

(IGS), is available. Fig. 3 shows the IGS network

Mo dern digital receivers show even a much b etter

as of Octob er 1998.

p erformance. The P-co de currently is only avail-

Over 200 IGS sites, distributed all over the

able to privileged (i.e., U.S. Department of Defense

glob e p ermanently observe all satellites in view,

authorized) user. The C/A co de is transmitted on

transmit their observations (at least) on a daily ba-

L1, an encrypted version of the P-co de on b oth

sis to IGS Data Centers.

carriers. The broadcast orbits usually have an ac-

The data are then analyzed by IGS Analysis

curacy of ab out 3m. The satellite clo ck information

Centers, which deliver rapid and nal pro ducts.

is made available to the non-authorized user only

Rapid IGS pro ducts are available with a delay of

with mo derate accuracy, which limits real-time ab-

ab out one day, nal pro ducts with a delay of ab out

solute p ositioning accuracy to ab out 100m.

eleven days. Daily pro ducts include satellite or-

Let us briey address the principles of absolute 5

bits with an accuracy of ab out 0.05-0.1m, satellite include time and frequency transfer and that it is

clo cks with an accuracy of ab out 0.3ns (nanosec- able to monitor the ionosphere. For more informa-

onds), daily values of p olar motion comp onents tion concerning the IGS and its interdisciplinary

accurate to ab out 0.1 mas (milliarcseconds), cor- impact we refer to (Beutler et al., 1999).

resp onding to 3mm on the Earth's surface), and From the p oint of view of space geo desy GPS

length of day (lo d) estimates with an accuracy of is a work horse with imp ortant contributions to

ab out 30s/day. These pro ducts are essential con- the establishment and maintenance of a dense ter-

tributions to the monitoring of Earth rotation (cf restrial reference frame, and which provides Earth

POLAR MOTION AND LENGTH OF DAY). rotation parameters with a high time resolution. It

should not b e forgotten, that the GPS like every satellite geo detic metho d is not able to maintain

nyal

UT 1 or of precession and thu1 a long-term stability of

trom

kely kiru nutation. Moreover, despite the fact that GPS is a fair hofn yell reyk yakz whit mets mag0 60 chur onsa mdvo kstu flin sch2 wsrt bor1 lama zwen will kosg joze irkt petr satellite geo detic technique, it is not well suited to holb prds hers dubo wtzr glsv uclu drao brus penc albh algo nrc1 stjo zimm wes2 graz sofi zeck pol2 sele nlib vill toul medi quin gode usna gras ankr nssp casa amct madr mate kit3 usud gol2 usno sol1 cagl suwn determine the Earth's gravity eld or the motion pie1 sfer noto xian taej tskb iavh nico cice brmu ramo 30 mdo1 mas1 lhas wuhn shao rcm6 bahr

kokb aoml kunm taiw

w.r.t. the geo center. The mkea Southern cro1 of its station p olyhedron California Integrated barb iisc guam GPS Network moin (45 sites) kwj1 gala kour bogt ntus

0 height of the GPS satellites is one of the limiting fort mali sey1 asc1 dgar coco thti areq braz

noum factors. eisl hark hrao yar1 30 suth sant pert auck lpgs tidb

goug For more information concerning GPS as a to ol hob2 chat kerg

mac1 for geo desy and geo dynamics we refer to (Teunissen 60 ohig cas1

dav1 and Kleusb erg, 1998). vesl

mcm4

120 60 0 60 120 180

October 1998

Other Satellite microwave techniques

The Russian GLONASS (Global Navigation Satel-

Figure 3: The International GPS Service Network

lite System) is so closely related to the GPS

that there are a number of combined GPS and

In addition the IGS Analysis centers p erform

GLONASS receivers available. These receivers

weekly global co ordinate solutions of their p ortion

were used in the rst global GLONASS track-

of the IGS network. These results are used, to-

ing and analysis campaign, the IGEX-98 (Interna-

gether with the results of the other space tech-

tional GLONASS Exp eriment 1998). The exp er-

niques, for the establishment of the International

iment revealed that a combined analysis of GPS

Terrestrial Reference Frame (cf. TERRESTRIAL CO-

and GLONASS is very promising for science and

ORDINATE SYSTEMS & FRAMES).

navigation.

The IGS pro ducts (orbits, Earth rotation

The French DORIS system (Doppler Orbitog-

parameters, co ordinates and velocities of IGS

raphy by Radiop ositioning Integrated on Satellite)

stations) are used as known a priori in-

proved to b e a very p owerful to ol for orbit deter-

formation to establish regional networks for

mination. It is, e.g., one of the orbit determina-

crustal deformation studies (e.g., the Califor-

tion system used in the TOPEX/Poseidon mission

nian SCIGN (http://www.scign.org), the Japanese

(see b elow). Also, DORIS p ossesses a very well

1000-receiver network for Earthquake monitor-

designed ground tracking network. This is one rea-

ing) or for regional reference frame establish-

son why DORIS was accepted as an ocial space

ment and maintenance (e.g., the EUREF network

technique by the IERS (see Table 1).

http://www. oma.b e:/KSB-ORB/EUREF/ eu-

The German PRARE (Precise Range and

refhome.html, or the South American SIRGAS net-

Range-rate Equipment) system may b e viewed as

work http://www.dg.badw-muenchen.de /gps/sir-

the German counterpart of the DORIS system. It

gas.html).

is used as an orbit determination to ol, e.g., on the

More and more, the IGS network is used for

Europ ean Space Agency's ERS-2 (Earth Remote

purp oses other than space geo desy. Let us men-

Sensing) spacecraft.

tion that the IGS network has b een enhanced to 6

Satellite Missions GRACE (Gravity Recovery and Climate Exp eri-

ment, U.S./German mission ), and GOCE (Grav-

There were many satellite missions in the past and

ity eld and Ocean Current Explorer, ESA mis-

there will b e more in the future in which the satel-

sion) are fascinating. It is exp ected that our knowl-

lite is used as an observing platform to study as-

edge of the gravity eld (using spaceb orne GPS re-

p ects of the Earth relevant to geo desy and geo dy-

ceivers, accelerometers, or gradiometers) to mea-

namics. Let us mention in particular that altime-

sure the non-gravitational forces resp. gravity gra-

try missions signicantly improved our knowledge

dients will signicantly increase through such mis-

of the sea surface top ography, o cean currents, tidal

sions.

motions of the o ceans, etc.

Also, CHAMP, GRACE and GOCE are able to

Fig. 4 shows the TOPEX/Poseidon spacecraft.

pro duce atmosphere proles using the occultation

The mission is a combined U.S. and French al-

method: the signal (phase and co de) of a GPS satel-

timetry mission. It is actually the rst mission

lite is monitored by a spaceb orne GPS receiver on a

which was sp ecially designed to investigate o cean

low Earth orbiter (LEO) during the time p erio d the

currents. One entire volume of the Journal of

line of sight LEO-GPS satellite scans through the

Geophysical Research was devoted to this mission

Earth's atmosphere. These developments supp ort

(JGR, 1994).

our initial statement that interdisciplinary asp ects

are b ecoming more and more imp ortant in Space

Geo desy.

Organizations

Table 1 gives an overview of the institutions rele-

vant for space geo desy.

They all are IAG (International Asso ciation of

Geo desy) 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 Geo desical

Data Analysis Services).

IGS, ILRS and IVS are technique-specic ser-

vices. The IERS is a multi-technique service. It

was established in 1988 as successor of the In-

Figure 4: The TOPEX/Poseidon Spacecraft

ternational Polar Motion Service (IPMS) and the

Earth rotation branch of the Bureau International

For space geo desy the TOPEX/Poseidon mission

de l'Heure (BIH). The IERS pro ducts are based on

was a kind of rosetta stone mission b ecause its

the pro ducts of the technique-specic services.

orbit was determined using three indep endent sys-

CSTG is a commission of IAG and a sub com-

tems, the French DORIS system, SLR tracking,

mission of COSPAR (Commission on Space Re-

and the GPS. All three systems proved their capa-

search). It has a co ordinating function within space

bility. The radial comp onent of the orbit (which is

geo desy. In the time p erio d 1995-1999 it was, e.g.,

of crucial imp ortance for altimetry missions) could

resp onsible for creating the ILRS and the IVS, and

b e established with an accuracy of a few centime-

it organized the rst global GLONASS exp eriment

ters. Let us mention that TOPEX/Poseidon was

IGEX-98.

neither the rst, nor will it b e the last altimetry

More information ab out these services may b e

mission.

found at the internet addresses in Table 1.

For geo desy, geo dynamics, and atmosphere

physics the up coming missions CHAMP (Chal-

lenging Mini-Satellite Payload for Geophysical

Research and application, German mission), 7

Acronym Name, Mission, Internet

and geo dynamic satellite Doppler p ositioning

CSTG Commission on International Co ordi-

Rev.Geophys.Space Phys., 21 27-40

nation of Space Techniques. Co ordina-

Moritz, H and Mueller I I 1988. Earth Rotation,

tion b etween space geo detic organiza-

Theory and Observation (The Ungar Publish-

tions, organize pro jects.

ing Company)

IERS International Earth Rotation Service.

Teunissen, P J G and Kleusb erg A (editors) 1998

Establish and maintain celestial and

GPS for Geodesy, 2nd Edition (Springer Ver-

terrestrial reference frame, generate

lag)

combined Earth orientation parameter

series

http://hpiers.obspm.fr

IGS International GPS Service. Make avail-

able GPS data from its global network,

pro ducing and disseminating high accu-

racy GPS orbits, Earth rotation param-

eters, station co ordinates, atmospheric

information, etc.

http://igscb.jpl.nasa.gov

ILRS International Laser Ranging Service.

Collects, archives, and distributes SLR

and LLR datasets. Generates scientic

and op erational pro ducts

http://ilrs.gsfc.nasa.gov

IVS International VLBI Service for Geo desy

and Astrometry. Op erate or supp ort

VLBI programs. Organize geo detic, as-

trometric, geophysical research and op-

erational activities.

http://ivscc.gsfc.nasa.gov

Table 1: Space Geo detic Services

References

Arias, E F, Charlot, P, Feissel, M, Lestrade, J-F

1995 The Extragalctic Reference System of the

International Earth Rotation Service, ICRS,

Astron. Astrophys., 119 604-608.

Beutler, G, Rothacher M, Schaer S, Springer T A,

Kouba J, and Neilan R E 1999 The Interna-

tional GPS Service (IGS): An interdisciplinary

service for Earth sciences, Advances for Space

Research, ?? ????-????.

Bevis, M Businger S, Herring T A, Ro cken C., An-

thes R A, and Ware R H 1992. GPS Mete-

orology: Remote Sensing of Atmospheric Wa-

ter Vapor using the Global Positioning System,

Journal of Geophysical Research, 97 75-94

JGR1994 Journal of Geophysical Research, 99.

Kouba, J 1983. A review of geo detic and 8