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 sur- face. 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).