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International audience; Tahiti is a unique geodetic site located in the south Pacific Ocean where few observatories exist nearby. The American mobile station MOBLAS-8 was installed in Tahiti in 1998, and GPS and DORIS systems were also deployed in its vicinity in order to develop this site into one of the fundamental colocated sites of the International Terrestrial Reference Frame. In order to make a new estimate of the colocation differences between the different techniques, a campaign of the French Transportable Laser Ranging System (FTLRS) was conducted in Tahiti between April and October 2011. The FTLRS was deployed close to the existing equipment. Observations for LAGEOS 1, LAGEOS 2 and Starlette were studied, and the solutions to the local ties between FTLRS, MOBLAS-8, DORIS and GPS were evaluated. Our results of the geodetic local-ties between laser stations and GPS agree well with the measurements made by the Institut National de l’Information Géographique et Forestière (IGN) during the campaign, with differences less than 2 mm in the vertical direction. The laser station range biases as a function of satellites are also presented, −3 (±2) mm for MOBLAS-8 and 3 (±3) mm for FTLRS, respectively. In addition, we investigated the role of time bias (ranging from a few hundreds of nanoseconds to one microsecond) given by the Time Transfer by Laser Link experiment, which shows a limited impact on the present SLR analysis. We also compared the coordinates of the three available techniques at Tahiti, i.e., laser, GPS and DORIS. We found the accuracy of laser solutions still needs to be improved, so that the SLR at Tahiti could contribute more effectively to the tracking of satellites and thus to the international reference frame. This study is useful in evaluating the SLR and other space techniques in order to prepare the deployment of new equipment in Tahiti in the near future.

International audience; DYNAMO is a small multi-instrument payload aimed at characterizing current atmospheric escape, which is still poorly constrained, and improving gravity and magnetic field representations, in order to better understand the magnetic, geologic and thermal history of Mars. The internal structure and evolution of Mars is thought to have influenced climate evolution. The collapse of the primitive magnetosphere early in Mars history could have enhanced atmospheric escape and favored transition to the present arid climate. These objectives are achieved by using a low periapsis orbit. DYNAMO has been proposed in response to the AO released in February 2002 for instruments to be flown as a complementary payload onboard the CNES Orbiter to Mars (MO-07), foreseen to be launched in 2007 in the framework of the French PREMIER Mars exploration program. MO-07 orbital phase 2b (with an elliptical orbit of periapsis 170 km), and in a lesser extent 2a, offers an unprecedented opportunity to investigate by in situ probing the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, and therefore the present atmospheric escape rate. Ultraviolet remote sensing is an essential complement to characterize high, tenuous, layers of the atmosphere. One Martian year of operation, with about 5,000 low passes, should allow DYNAMO to map in great detail the residual magnetic field, together with the gravity field. Additional data on the internal structure will be obtained by mapping the electric conductivity, sinergistically with the NETLANDER magnetic data. Three options have been recommended by the International Science and Technical Review Board (ISTRB), who met on July 1st and 2nd, 2002. One of them is centered on DYNAMO. The final choice, which should be made before the end of 2002, will depend on available funding resources at CNES.

The drag temperature model (DTM) is a semi-empirical model describing the temperature, density and composition of the Earth's thermosphere. Its first version (Ann. Geophys. 34 (1978) 9) used direct measurements of exospheric temperature and atmospheric densities derived from satellite drag data. It has later been refined (J. Geodesy 72 (1998) 161). However, both models have their lower boundaries at 120 km , which are not constrained by observations. Consequently in the lower thermosphere, the modelled temperature and density structure is uncertain. For predicting satellite orbits in the lower thermosphere, more realistic density models are required. We present a new DTM model having the following improvements: (a) Temperature and its gradient at 120 km are represented in agreement with theory and observation, using incoherent scatter radar and satellite-borne interferometer data. (b) Atmosphere explorer (AE) data, which have not been assimilated in DTM-94, are used as they cover a complete solar activity cycle. (c) The Mg II index is used whenever possible to represent the solar UV and EUV emission intensity instead of the solar decimetre radio flux, since it is more representative of solar instantaneous chromospheric activity than F10.7 is. The basic DTM mathematical representation of temperature and composition is used, with, however, some additions and modifications to take into account the variations at 120 km altitude. The temperature modelling accuracy has improved by 5–8%, and there is no model bias as a function of solar activity. The oxygen and helium modelling has improved as well, and this is demonstrated by the estimated drag scale coefficients issued from precise orbit computation. The scaling coefficients estimated using DTM-2000 are systematically closer to unity than those resulting from employing DTM-94 and MSIS-86 in the orbit computation. The minor constituents (O2 and H) modelling is unchanged. The molecular nitrogen modelling is not improved, but this is, at least partly, caused by the poor data quality. Despite these improvements, semi-empirical thermosphere models still suffer of weaknesses. First, they assume a steady-state equilibrium which is not necessarily reached in any circumstances. Second, the basic process of atmospheric heating by EUV is assumed to be represented by some indices (Mg II, F10.7), while particle precipitation is represented by an index associated to a latitude without a longitude effect (Kp), although winds that transport energy suggest this effect. Third, the data accuracy and their incomplete geographical and temporal coverage are significant sources of uncertainty. Assimilation of a long time series of data, with a complete geographical coverage, and using the Mg II index will probably increase model accuracy from the present-day RMS of 19% to 10–15%. The complete CHAMP accelerometer data set may allow the achievement of that goal after 5 years of operations in 2005.

The laser-based short-arc technique has been developed in order to avoid local errors which affect the dynamical orbit computation, such as those due to mismodeling in the geopotential. It is based on a geometric method and consists in fitting short arcs (about 4000 km), issued from a global orbit, with satellite laser ranging tracking measurements from a ground station network. Ninety-two TOPEX/Poseidon (T/P) cycles of laser-based short-arc orbits have then been compared to JGM-2 and JGM-3 T/P orbits computed by the Precise Orbit Determination (POD) teams (Service d'Orbitographie Doris/Centre National d'Etudes Spatiales and Goddard Space Flight Center/NASA) over two areas: (1) the Mediterranean area and (2) a part of the Pacific (including California and Hawaii) called hereafter the U.S. area. Geographically correlated orbit errors in these areas are clearly evidenced: for example, −2.6 cm and +0.7 cm for the Mediterranean and U.S. areas, respectively, relative to JGM-3 orbits. However, geographically correlated errors (GCE) which are commonly linked to errors in the gravity model, can also be due to systematic errors in the reference frame and/or to biases in the tracking measurements. The short-arc technique being very sensitive to such error sources, our analysis however demonstrates that the induced geographical systematic effects are at the level of 1–2 cm on the radial orbit component. Results are also compared with those obtained with the GPS-based reduced dynamic technique. The time-dependent part of GCE has also been studied. Over 6 years of T/P data, coherent signals in the radial component of T/P Precise Orbit Ephemeris (POE) are clearly evidenced with a time period of about 6 months. In addition, impact of time varying-error sources coming from the reference frame and the tracking data accuracy has been analyzed, showing a possible linear trend of about 0.5–1 mm/yr in the radial component of T/P POE.

The Drag Temperature Model (1978) has been improved to respond better to the actual requirements of space geodesy, especially under extreme solar and geomagnetic conditions. Extended data and an improved algorithm have been considered, leading to valuable improvements. Temporal variations of temperature of the thermopause, total density and major chemical constituent density are reviewed and compared to the DTM94, DTM78 and MSIS86 models. A comparison with data is performed, giving the mean ratios between observed and model values with their root mean squares for different physical and geometric conditions. This comparison is made for the three models and includes data used in the modelling as well as external data. The limits of the thermosphere modelling are discussed.

Averaging methods are convenient tools for studying long-periodic variations of the motion of artificial satellites. The main lines of a semi-analytical theory of the mean motion are given. We show how, when coupled with a careful reduction of the tracking data, this theory allows to determine parameters related to the temporal variations of the Earth gravity field (e.g. the amplitude of 18.6 years tide and the secular variation of even zonal harmonics). The theory is also very useful for other applications such as mission analysis.

The MICADO instrument has been built to measure temperature and wind in the E - and F -regions. It employs a thermally stable field-compensated Michelson interferometer to allow wind measurements. During the winter of 1988–1989, the MICADO instrument was operated at Sodankyla (67°22′N, Finland). Measurements were made by observing the O 1 S (low thermosphere) and the O 1 D lines (high thermosphere) emission. Two co-ordinated campaigns were organized with the EISCAT radar, which operated in special modes. Neutral wind and temperature are derived from EISCAT data. Results of the two instruments are shown. The differences between the two sets of results are discussed and show that most of the discrepancy is due to the presence of vertical winds during the observations where the magnetic activity was high.

Twelve years after the discovery of the orbit shrinking of the geodynamic satellite Lageos, we do not know for sure why this spacecraft is falling down to earth. We review the main explanations that have been advocated to explain an average drag of −3.1×10 −12 ms −2 and periodic oscillations of the same order of magnitude. The charged particle drag, the tehrmal thrust induced by the earth infrared and reflected solar radiation are likely to be the main causes of the secular effect. Also these radiation forces and thermal effects are thought to be responsible for the variable components.

International audience; DYNAMO is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/ CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for DYNAMO of 120–130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Additional data on the internal structure will be obtained by mapping the electric conductivity. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere and water from the planet.

Secular and long periodic perturbing forces acting on the orbit of an artificial satellite are modelled and included in a semi-analytical theory of the mean orbital motion. Gravitational and nongravitational forces are averaged using analytical transformations and numerical quadratures, respectively. As non-gravitational modelling is the first source of problems arising in the determination of geodynamical parameters, special attention is given to this question. Resulting averaged equations of motion are integrated numerically allowing the visualization and deconelarion of long periodic signals in the classical mean orbital elements. Observed mean orbital elements of LAGEOS (18 years) and Starlette (14 years) have been computed from Satellite Laser Ranging (SLR) measurements which had fast been reduced by classical orbit fits on short arcs. The results of these long arc solutions exhibit variations of geodynamical coefficients at different periods ranging from semi-annual and annual to 9.3 and 18.6 years. Values of coefficients are compared to other results obtained recently in this field.

Averaging methods are convenient tools for studying long-periodic variations of the motion of artificial satellites. The main lines of a semi-analytical theory of the mean motion are given. We show how, when coupled with a careful reduction of the tracking data, this theory allows to determine parameters related to the temporal variations of the Earth gravity field (e.g. the amplitude of 18.6 years tide and the secular variation of even zonal harmonics). The theory is also very useful for other applications such as mission analysis.

The fourth generation long-wave global Earth gravity field models, GRIM4, accomplished by a joint German/French effort, undergo continuous improvement with respect to accuracy and resolution by the incorporation of more satellite observations and the consideration of the most recent Earth surface data sets.

The orbital analysis over a very long period of time (10 years and beyond) is of great interest for the determination of specific coefficients of the gravity field (e.g., zonal harmonics, tides) including their temporal variations. Until the development of Earth gravity field models which are based on global tracking data, the determination of the long-wavelength geoid has been realized through “independent” low-degree solutions. Because only terms of order zero in the geopotential expansion give long-period perturbations and secular effects (even zonals) in satellite orbits (in the absence of resonance effects), analyses have been conducted without solving for tesseral harmonic coefficients. The most familiar method for studying the long-periodic and secular perturbations of an artificial satellite orbit is to compare the variations of the mean orbital elements (observed) with the changes predicted by theory. This method has been used with success by many investigators to determine conditional equations for geopotential coefficients (e.g., King-Hele, 1981).

The main effects of tesserai harmonics of a gravity potential expansion on the motion of a satellite, are short period variations as well as long period variations due to resonances. However, other smaller long period and secular variations can arise from interactions between tesserai terms of the same order. The analytical integration of these effects is developed, using numerical evaluation of Kaula eccentricity and inclination functions. Examples for some Earth’s geodetic satellites show that secular effects can reach a few decameters per year. The secular variations can even reach several hundred of meters per year for the Mars natural satellite Phobos.

A short-arc orbit technique was developed, and a very precise and efficient tool for regional orbit determination with a rigorous error budget is proposed for analyzing the TOPEX/POSEIDON (T/P) and ERS 1 missions. The purpose is to be able to determine very accurately, in some conditions, the local orbit, with a very small error in the radial direction, using satellite laser ranging data. A precision at the level of 2 cm is obtained. With this it was possible to cross validate all the different orbit determination techniques as well as the results of the altimeter calibration, showing the great progress made with the TOPEX/POSEIDON mission. After that the T/P mean sea profiles were computed and improved with high accuracy, thanks also to the short-arc orbit technique. As a result, a combined mean sea surface using TOPEX/POSEIDON and ERS 1 data has been computed with crossover difference techniques, the T/P mean sea profiles serving as reference. Fine structures of the mean sea surface have been evidenced. Using this combined mean sea surface, the ERS 1 altimeter calibration was performed over the Mediterranean, again using the short-arc orbit technique. The result is in good agreement with the previous ones (−41.6±7 cm). Finally, the T/P mean sea profiles were computed for different seasons and compared with a yearly averaged mean sea surface. Variations of about 15 cm are evidenced, associated notably with the thermal expansion between fall and winter. The short-arc orbit technique appears to be well suited for monitoring very accurately the temporal variability of the Mediterranean Sea at a subcentimetric level.

A tous les âges de l'histoire, l'homme a irrigue les cultures, plus ou moins suivant les climats. C'est l'evolution rapide des techniques et notamment l'automatisation qui a favorise l'extension de l'irrigation (on est passe de la rigole au goutte a goutte, en passant par les asperseurs plus ou moins automatises). On est passe de l'irrigation du potager familial au verger, puis aux grandes cultures, satisfaisant ainsi les exigences en quantite et en qualite des produits de l'industrie agroalimentaire, ainsi que celles de l'environnement. La reforme de la PAC, ayant un objectif de reduction de production base sur la desintensification (aides a l'extensification et a la diminution des intrants), remet en cause l'irrigation des grandes cultures cerealieres qui sont au cœur de cette reforme. Les consequences economiques, pour les exploitations et les industries agro-alimentaires (investissements realises reorientation des productions) sont considerables. En l'etat actuel des reglements d'application de la reforme, c'est un coup d'arret aux investissements nouveaux en cereales a paille et en mais, car les aides compensatoires a l'hectare sont les memes pour les surfaces irriguees que pour les cultures seches, sauf pour les departements ayant choisi un plafond de surfaces irriguees. Meme dans ce cas, ne seraient prises en compte que les surfaces deja irriguees et celles resultant des investissements engages au 1 aout 1992. Seules les cultures de printemps sont concernees, a l'exclusion du tournesol. Ceci veut dire en clair, que les quintaux produits audessus du rendement de reference departemental sont payes au prix mondial. Ce reglement est donc dissuasif a tout investissement d'irrigation, par absence de rentabilite. C'est regrettable, car l'irrigation n'est pas synonyme que de surproduction. Elle permet de realiser les objectifs de recolte en qualite et quantite, et contrairement a ce que l'on entend trop souvent dire, l'irrigation bien conduite concourt positivement a la reduction des nitrates dans les eaux en permettant la bonne utilisation de la fertilisation azotee par la plante, et donc moins de reliquats apres recolte. de plus en plus d'irrigants tiennent compte de la quantite de nitrates apportee par l'eau d'irrigation dans le calcul de leur fertilisation azotee. Ils contribuent ainsi a denitrifier. Monsieur l'ingenieur general Cothenet disait tout a l'heure que « les conflits font avancer ». C'est en partie vrai. J'espere que de ces conflits jaillira la lumiere, faute de faire jaillir l'eau.

Total density data were obtained from the accelerometer CACTUS on board of CASTOR-D5B 1975-39A. Numerous and precise data were obtained between 250km and 600km altitude in the equatorial region (±30° latitude) for a period extending from May 1975 (minimum of solar activity) to February 1979 (already important solar activity). Since CACTUS data have not yet been used for the construction of empirical thermospheric models, a significant part of the data file is compared with several thermospheric models in order to provide an external test of the reliability of such models. Standard deviations of the order of 20% are apparent. The most significant differences extend over a few weeks and cannot be represented by the geophysical indices as they are presently used in the empirical models. Such an experimental fact suggests that the mathematical and physical aspects of the empirical models should be refined in order to achieve a better representation of physical reality.

During the last solar activity minimum, a great deal of very precise total density data was obtained in the equatorial regions from the CACTUS accelerometer experiment. Due to the eccentricity of the orbit, it is also possible to determine a density scale height by considering that the density profiles between the perigee (270 km) and 400 km are quasi-vertical. Densities and density scale heights are analysed during magnetic storms and their variations are compared with their behaviour during quiet periods. For densities as well as for scale heights, an asymmetrical structure in latitude and longitude is exhibited with respect to the magnetic equator. Their values are relatively higher in the northern hemisphere than in the southern one. The hypothesis (previously suggested) of a greater energy input in the southern hemisphere inducing asymmetrical winds, explains the results well.

A great deal of accurate total density data from the CACTUS accelerometer experiment has been collected in equatorial regions during the last minimum of the solar cycle. Using these data, an analysis of the magnetically disturbed thermosphere has been performed with an improved resolution. Time delay of the response, latitudinal and longitudinal variations for solstice, day-time and night-time conditions have been studied. An attempt is made to interpret the behaviour of the thermosphere in the light of existing theories. An asymmetrical heating in latitude and a heating depending on longitude are needed in order to interpret the results.

The availability of various global thermospheric models allows intercomparisons to be made for identical geophysical conditions. Solar activity effect, geomagnetic effect, diurnal variation, annual variation and latitudinal-seasonal dependence are analyzed in four semi-empirical models, viz. DTM, MSIS, ESRO 4 and J77. Emphasis is given to the discrepancies between the models, and some direct comparisons are made with satellite drag data and micro-accelerometer results. None of the semiempirical models can represent correctly all geophysical aspects of the terrestrial thermosphere.

International audience; A large volume of data (temperatures, densities, concentrations, winds, etc.) has been accumulated showing that in addition to seasonal changes in the thermosphere, annual variations are present and have a component that is a function of latitude. It appears that the helium concentrations have much larger variations in the southern hemisphere than in the northern hemisphere; the same holds true for the exospheric temperatures deduced from Ogo 6 data. Similarly, satellite drag data in the 250- to 400-km range indicate that the bulge of density tends to stay over the southern hemisphere, whereas winds deduced from Ogo 4 and 6 data show a tendency to blow northward across the equator. If part of the explanation of these asymmetries can be found in a latitude independent component induced by the changing sun-earth distance between solstices (Volland et al., 1972; Ching and Chiu, 1972, 1973), the fact that an asymmetry is still present at the equinoxes suggests that this is not the sole cause: more energy seems to be available for the thermosphere in the southern hemisphere during the equinoxes; this may be the result of an asymmetry in the geomagnetic field or an asymmetrical dissipation of tidal waves induced by an asymmetrical worldwide ozone distribution.

Une des techniques de determination fine et globale du champ de gravitation terrestre U est la gradiometrie spatiale, dans laquelle on mesure a bord d'un satellite sur orbite basse certaines combinations lineaires des composantes du tenseur ∂2 U/∂xi ∂xj dans des axes {x i } lies au satellite. Un tel projet, appele GRADIO, est actuellement a l'etude en France et pourrait aboutir a partir de 1990. Apres avoir rappele les objectifs scientifiques d'une telle mission, nous en donnons les specifications—etayees par une serie d'etudes analytiques; nous definissons ensuite le satellite porteur et ses caracteres techniques, en insistant sur les points delicats de la faisabilite (facteurs d'echelle des micro-accelerometres constituant l'appareil, connaissance de l'attitude...) et en presentant des idees de solution en cours d'approfondissement.

From a large number of accelerometer measurements, total density, density scale height, temperature and O/N2 ratio can be deduced simultaneously. By taking advantage of these data, the amplitude of thermospheric parameters changes is reexamined in equatorial regions for altitudes between 290-390 km. The behaviour of the different responses is described and evaluated according to the value of the geomagnetic index Kp and the local solar time, which appear to be the major parameters. The results are compared to different models.

The recent development of laser tracking has allowed to determine satellite's distance to the cm-level. Observations of the geodetic satellite LAGEOS have shown the existence of unmodeled forces producing an average along-track acceleration of −3×10 −12 ms −2 . Among the various explanations proposed the interaction of the satellite with the near-Earth plasma is the most likely. We present and discuss the physics of the model. Eventually we would like to show how the capabilities of the laser tracking on a specifically designed satellite could be used to improve the knowledge of the plasmasphere.

Different types of models of the thermosphere, either with experimental characteristics or with theoretical characteristics, have been elaborated since the Space Age. These models describe the variation of different parameters (temperature, density, pressure, concentrations,…). A brief description of these models is given. As far as theoretical models are concerned, the conservation equations of energy, of momentum or mass and physico-chemical processes are taken into account. Experimental models are based upon empirical representation of variations of parameters at a reference altitude, assuming diffuse equilibrium for each component. Because this field of research is too large, this study deals mainly with the comparison between the experimental models. This comparison generally gives satisfactory agreement. However significant disagreements also clearly appear and typical examples are given. Several reasons can be put forward on the origin of such disagreements: indicators of sinks and sources of energy, distribution of data with respect to space-time and geophysical parameters, knowledge of some parameters such as the eddy diffusion coefficient, knowledge of interactions with other zones of the atmosphere (e.g. mesosphere, ionosphere, magnetosphere). This list is not exhaustive. Of course, new programs are developed (MAP, EISCAT, UARS, SPACELAB experiments), but this will take time to bear fruit and to gather data. Therefore, all existing data constitute a priceless bank of data over two solar cycles, even if some have been only partly recorded and prevent us from making a perfect model. On the other hand, a model can be very useful without being perfect. It has to be a clear conventional reference which gives a satisfactory mean of the behaviour of the thermosphere. It must be easy to use and also be a practical tool to test data or be compared with theoretical models to be developed. Finally, it has to permit one to solve partly some physical problems in assuming some parameters as known. Our opinion is that a new international experimental reference model based upon simplified physical concepts and upon new measured parameters should be very useful if an easily-accessible data bank is created at the same time.

Using unique optical measurements obtained by the Fabry-Perot interferometer on board the OGO-6 satellite, a model of the exospheric temperature has been built. This model gives the exospheric temperature for any given conditions in day, local time, latitude and solar and magnetic activity. Due to the nature of the data, this model presents in general two differences with observed temperature by incoherent scatter radar: at mid-latitude, a lower day to night ratio and a lower temperature in December than the observed ones. Therefore, keeping the same optical data and adding incoherent scatter data obtained at Millstone Hill, St Santin and Arecibo stations, a new model is presented. Comparisons with the most recent model are made.

The Cactus accelerometer, developed at ONERA and making up the payload of the Castor satellite, has been operating continuously and normally for 40 months. Thanks to the great quantity of data acquired and to a well adapted processing technique, acceleration measurements were in fact obtained with a better sensitivity than that expected before launch. This paper presents a synthesis of the work carried out in the past year and concerning the measurement of very low accelerations. These are due, for the most part, to the Earth IR radiation pressure when the satellite is in the Earth's shadow, and to the thrust it is subjected to under the influence of the anisotropy of its own radiation.

Cette experience montre que la Geodesie locale, par methode Doppler, est possible, c’est-a-dire que ne disposant que de peu de stations, il est possible de les relier geodesiquement entre elles; dans le cas de deux stations assez eloignees, la precision parait toutefois un peu faible du point de vue geodesique (40 m); par contre, comme l’ont montre les experiences “Diademes”, avec trois stations correctement disposees, la precision est bien meilleure, proche du decametre. (P. CHASSAING, 1969 [6]).

The exospheric temperature can be deduced from incoherent-scatter measurements as well as from an analysis of the orbital decay of satellites. However, it appears that there are systematic differences that vary with season in the results obtained by the two methods. In this paper an interpretation of these differences is attempted by using static-diffusion models where the values of the atmospheric parameters are changed at 120 km. A seasonal model characterized by a maximum of oxygen concentration in winter is found to give a better agreement between the two types of observation and to confirm results found by several authors.

Ultrasonographic images were correlated with radiological and pathological findings in a case of generalized adenomyomatosis of the gallbladder. Emphasis is placed on the importance of the methods employed for filling the biliary tracts with opaque medium, to confirm a suspected diagnosis suggested by the heterogeneous irregular thickening, with a fringed internal limit, of the gallbladder wall.

Using satellite drag data a three-dimensional thermospheric model is developed in terms of spherical harmonics. The model gives total density, partial densities (He, O, N 2 ) and thermopause temperature as a function of solar and geomagnetic activity, local time, day of the year, altitude and latitude.

The first nine months of total density data measured by the low-g accelerometer CACTUS have been analysed as a function of classical neutral thermospheric parameters and also as a function of longitude. Some interesting features are shown, in particular near the south Atlantic anomaly. Neutral density waves are exhibited during magnetic storms and also in other circumstances. Profiles of total densities in the 300-600 km altitude range are analysed and give information on neutral gas temperature and concentration.

Dans la determination de la trajectoire d’un satellite artificiel, l’ecart quadratique moyen des observations est en general superieur a la valeur prevue. Les principales causes en sont enoncees. Pour interessante que soit une telle solution, elle ne permet pas d’extraire toute l’information contenue dans les observations. Parmi toutes les methodes possibles pour permettre d’analyser au mieux cette information, nous nous sommes attaches a determiner les parametres orbitaux sur des arcs tres courts en n’utilisant necessairement qu’un nombre restreint d’observations. Une solution tres satisfaisante peut etre obtenue mais osus des conditions restrictives precises qui sont etudiees.

Apres un rappel des notions fondamentales utilisees en geodesie terrestre et en geodesie spatiale, les auteurs decrivent les methodes de construction d'un systeme geodesique, soit par la triangulation et le nivellement astro-geodesique, soit par l'observation des satellites sur fond d'etoiles. Puis, on discute la determination des trois constantes fondamentales relatives a la terre: rayon equatorial, constante geocentrique de la gravitation, et facteur d'ellipticite geopotentielle. Les diverses determinations du champ de gravitation terrestre (par la dynamique des satellites, la gravimetrie ou par des methodes mixtes incluant des determinations de direction) sont decrites et comparees en utilisant en particulier les cartes generales du geoide. Puis toutes les methodes geometriques ou dynamiques sont comparees a l'aide de cartes locales du geoide.

Within recent years, a new satellite technique has been successfully developed for studying the oceans: altimetry. The SEASAT experiment presents a particular interest because of the great accuracy of the altimeter data, though its capabilities are bounded by the accuracy of the trajectory restitution. The altimetric signal variations result from both marine and tectonic effects, the discrimination of which may be difficult. In the present study on the western Mediterranean Sea, the altimeter data processing, the satellite trajectory, and the geoid features are considered. The satellite topography of the Mediterranean Sea surface shows irregularities which are 1 order of magnitude greater than the expected oceanic effects and so are mainly connected to the earth structure. In fact, in spite of neglecting the oceanic effects, correlation with emerged land (islands) or immerged land (bathymetry) and also with main tectonic features can be seen. In particular, evidence exists for isostatic compensation of Airy type in the continental margins and oceanic areas.