5. Project CASSINI
6. Project ROSETTA
7. Project EUVN
8. The Astrogeodynamical Observatory
9. The tidal observatories
10. Interpretation and theoretical research
10.1. Studies of motions of short-period comets
10.2. Modelling of backscatter radiation of neutral sodium in the lunar atmosphere
10.3. Earth rotation studies
10.4. Geodetic studies

5. Project CASSINI
The CASSINI spacecraft was launched on October 15, 1997 and it will reach Saturn in 2004. In December 1998, a successful check of the Huygens probe, which is carrying a SRC-built thermal pro-be, was performed. The next check is scheduled for September 1999. Modelling of Titan ionosphere: Modelling of coupling between the neutral atmosphere and the ionosphere of Titan was performed (a one dimension, photochemical model). The results suggest that many neutral components are relatively strongly affected by coupling with ionosphere (from 10 to 100%). Height profiles of density of 60 kinds of ions were computed. Photoionisation, ionisation by magnetospheric electrons, recombination and two and three component reactions with neutral atoms and molecules were taken into acco-unt. According to the model, the maximum concentration of electrons, equal to ~6000 cm-3, is expec-ted at the height of ~1000 km. The dominant ion at this height is the HCNH+ molecule, at lower he-ights the most important are massive hydrocarbons and nitrils. It was demonstrated that three-component reactions can be significant at about 700 km (in lower ionosphere) if one assumes that the reaction rates are very high, that is similar to the rates of reactions with N2 as the third reaction sub-strate in lab experiments.(Principal Investigator: M. Banaszkiewicz)

6. Project ROSETTA
The structural-Thermal Model (STM) of the body of the MUPUS instrument was manufactured. It is a hammering device with a penetrator and an extension system. In December 1998, the instrument was successfully integrated with Rosetta's lander in the facilities of the Max-Planck Institut für Aero-nomie (Katlenburg-Lindau, Germany).

A prototype of the extension system for the MUPUS penetrator for the ROSETTA lander.

For the Institute of Planetology (Munster, Germany), the following pieces of equipment were manu-factured: an electronics block to control the temperature- and heat conductivity sensors (PEN-EL); a test system for them from thin-layer titanium (the work performed in cooperation with the Institute of Telecommunications PIT and the Institute of Radio Technology ITR); and an interface to integrate the sensors with the penetrator. The design and documentation of systems of a lab model of an isotope soil densitometer to study the nucleus of the Comet Wirtanen were developed. (Principal Investigator: M. Banaszkiewicz)

The Structural Thermal model of the MUPUS penetrator for the ROSETTA mission.

Development of the support system for the MUPUS instrument continued. The system involves a test and control hardware with necessary software. The system will be used for electrical tests of MU-PUS. Autonomous tests of the instrument will be performed with the use of an Electric Ground Support System MUPUS-EGSE based on a PC computer connected to a simulator of the telemetry system On-Board Data Handling ODBH and to a simulator of the on-board power supply system of the ROLAND lander. A test software and data processing and visualisation package for the Microsoft Windows 98 operating system is under development in the Delphi 4 programming language. Curren-tly, the telemetry simulator is ready, as well as the main part of the control software. Development of the power supply simulator nears completion. (M. Hlond) Work on the VIRTIS instruments (Visible-Infrared Thermal Imaging Spectrometer) was initiated. The experiment involves modelling of optical effects from gas, dust and surface of the Comet Wirta-nen. In cooperation with the Institute of Physical Chemistry PAS, the first model of a cuvette with methane for calibration of the spectrometer was completed. The spectrometer it self is developed by the Meudon Astronomical Observatory (Meudon, France) (M. Blecka)

7. Project EUVN
The international project European Vertical Reference Network (EUVN) continued. The aim of the project is to establish a unified height system for Europe and to link it to the mean sea level around the continent. The project is run under auspices of the International Association of Geodesy but the re-sponsible institutions for the national parts are the national geodetic surveys. SRC participates in the effort within the framework of a solicited project from the SCSR. In 1998, a solution of the Polish segment of the EUVN network, involving territories of Poland, Ukraine, the Baltic Republics, Slovakia, and Hungary was worked on. (Project Head: A. Lyszkowicz)

8. The Astrogeodynamical Observatory
The Astrogeodynamical Observatory in Borowiec near Poznań continued activity for national and international geodynamic and geodetic programs, carrying out on regular basis GPS and laser measurements supported by a high accuracy time service. The use of the two techniques to track the Earth rota-tion makes possible to maintain a very high accuracy of coordinates and to support the continuity of observations since the beginning of operation of the Observatory, more than 40 years ago. The research in Borowiec is performed with the use of four technologies: laser, GPS, and DGPS observations and the time service. GPS observations involve systematic measurements of GPS satellites in the IGS network to obtain coordinates in the International Earth Rotation Service system. Measurements at other observing sites are also conducted for other projects, including EUREF, CEGRN, SA-GET, and Baltic Sea Level. The results of observations are processed on site with the use of the BERNESE 3.4 software package.

A GLONASS-GPS antennae system.

In 1998, two radio sources jamming the GPS receiver of the IGS = BOR1 permanent station were installed near the facility. The destructive influence of these sources was largely eliminated by perfor-ming a number of statistical tests and applyingsmart management. Owing to the high quality of the GPS observations performed in Borowiec, the station was promoted to the reper station status, with the regional IAG network (Commission -X - Global and Regional Geodetic Network, Subcommission for Europe - EUREF), effective of 1 March, 1998. The laser group conducted regular laser observations of selected satellites for several international programs, including: NASA Dynamics of the Solid Earth, Working Group of European Geoscientists for the Establishment of Networks for Earth-science Research (WEGENER), European Space Agency Remote Sensing Satellite, International GLONASS Experiment (IGEX), and others. The laser station is part of the EUROLAS consortium of European laser stations. The basic goal of the observation programme is to determine the coordinates of the Borowiec station in the frame of the International Earth Rotation Service. The activity of the station staff involves also maintaining of and modernising the laser station to increase the accuracy of measurements. In 1998, refurbishment of the laser system was completed and from July 1 it was possible to reduce the operational crew of the station to one person. The refurbishment resulted also in a significant in-crease of the accuracy of observations owing to elimination of several systematic errors (the increase of accuracy of the reading of pressure and timing of the laser firing). The time service activity involves maintaining of accurate time and providing the GPS and laser observation programs as well as third parties with highly accurate reference frequency. Two caesium standards and receivers of the GPS time provide linking to the universal time with the accuracy better than 2 ns. In 1998, a system of comparison of the atomic clocks reading was launched. It is based on a system of multi channel GPS observations composed of a Motorola GPS receiver, a counter, and an appropriate software package developed in cooperation with the Bureau des Poids et Mésures (BIPM, Sevres, France). Systems like that are currently operated in France (BIPM and the Astronomical Observatory, Besancon), in Switzerland (Bureau des Poids et Mésures, Bern), and in Poland (Main Bureau of Measures and Weighs, Warsaw; Communications Institute, Warsaw; Polish Telecomm Main Laboratory, Warsaw). (Head of the Observatory: S. Schillak)

9. The tidal observatories
The SRC tidal observatories continued observations of terrestrial tides. The horizontal components were observed in the Ksiaz facilities, an the vertical component in the Warsaw lab. Long time series, 24-years and 17-years, respectively, are obtained. The analysis of observations to determine parameters of tidal waves has been performed in annual cycles. It was completed for the data till 1995. Apart from on-site analysis, the data are submitted to the ICET database in Brussels, Belgium. In cooperation with the Institute of Physics of the Earth in Paris, France, the state-of-the-art compact quartz Blum clinometers at the Ksiaz facilities were adapted for tidal observations. The observatory developed recently a unique instrument to measure horizontal components of terrestrial tides a long water tube. The instrument is performing test observations; it is based on the state-of-the-art technology for instruments of that kind, including solid-state lasers, TV cameras and digital processing of interference images. (Head: T. Chojnicki)

10. Interpretation and theoretical research

10.1. Studies of motions of short-period comets
Studies of nongravitational component of perturbations in cometary motions continued. The orbital motion was coupled with a forced precession of rotation axis of the cometary nucleus. A model of nongravitational effects for the following comets were developed: 7P/Pons-Winnecke, 8P/Tuttle, 31P/Schwassmann-Wachmann 2, 21P/Giacobini-Zinner, 16P/Brooks 2, and a revised model for the comet 32P/Comas - Sola. The first two comets show only mild nongravitational effect but nevertheless from the nucleus precession model of one of them (7P/Pons-Winnecke) the shape of the nucleus (ob-lateness) was determined to be equal to 0.3. The subsequent four (31P/Schwassmann-Wachmann 2, 21P/Giacobini-Zinner, 16P/Brooks 2) reveal strongly variable nongravitational effects and thus are referred to as erratic. Their motion was modelled with the assumptions that (1) the cometary activity may vary during subsequent apparitions, and (2) a parameter characterising the time shift of the maxi-mum of activity with respect to the time of perihelion passage can rapidly change. The orbital models developed, including strong and rapidly variable non-gravitational effects, reproduce the observed motion of the comets during the whole observations time, which in some cases cover almost a century. The successful reproduction of observations with the models suggests that the method taking into ac-count the nucleus precession can be successfully used to study short-period comets and can bring valu-able information on their nuclei, enabling a cross-check of theories of their nuclei. The orbital models of comets 31P/Schwassmann-Wachsmann 2 and 32P/Comas-Sola and the models for other comets with strongly variable non-gravitational effects suggest that not only variations in the cometary activi-ty, as it had been believed before, but first of all the nucleus precession is largely responsible for the variations of non-gravitational effects. Furthermore, within the framework of studies of non-gravitational effects in the orbits of short-period comets, a refined model of nucleus of the comet 46P/Wirtanen was worked up. In that research, observations from recent apparition (June 1995-December 1997) were taken into account. The model suggests that the nucleus is slightly oblate (the ratio of the equatorial radius to polar radius equal to 1.1) and that the ratio of the rotation period to the equatorial radius is equal to 4.9 h/km. Assuming that the radius is in the range from 0.5 to 2.0 km one can thus infer that the rotation period is in the range of 2.5-10 hours which is in agreement with the period assessed from observations. This is another, inde-pendent confirmation of the validity of the model. (Head: G. Sitarski).

10.2. Modelling of backscatter radiation of neutral sodium in the lunar atmosphere
In connection with a project of construction of an optical detector of sodium for the planned ESA Lunarsat mission, whose task would be investigation of lunar exosphere, a 3D modelling of the lunar exosphere was performed. It was assumed that the sources of sodium atoms in the exosphere are (a) thermal adsorption, (b) photoadsorption, (c) sputtering by solar wind protons, (d) evaporation after meteorite impacts. Relative contributions of the four processes and their dependence on height and angular distance from the local noon were computed. With the instrument response to resonant fluore-scence taken into account it was demonstrated that in suitable observing conditions (at ~100 km above the lunar surface) the signal-to-noise ratio is very high (more than 100). A possibility to observe the OH line 3085 A was analysed. The conclusion is that for the assumed OH production rate the signal should be slightly weaker than for sodium. (M. Banaszkiewicz)

10.3. Earth rotation studies
Investigation of evolution of Chandler nutation and free core nutation of the influence of El Nino on changes in polar motion were continued. In the studies of variations of amplitude of Chandler nutation, oscillations with periods of 75, 40, 30, and 20 years were discovered. The strongest is the 40-year oscillation. This suggests a coupling with variations of the terrestrial magnetic field, which has oscillations with a similar period. Predictions of Chandler amplitude variations, computed with the use of the least squares and autoregresion methods, indicate a deep minimum of the Chandler amplitude at the beginning of the 21 century, similar to the minimum from the turn of the twenties and thirties of the 20 century. The amplitude variations of the Free Core Nutation was determined by analysis of nutation residua of the EOP series with the use of a narrow-band Fourier Transform Filter, developed by W. Kosek. (B. Kolaczek, W. Kosek) The influence of El Niňo on variations of polar motion was analysed by means of analysis of the correlation coefficient between the atmospheric and geodetic excitation functions of the polar motion. It was demonstrated that during El Niňo events the correlation coefficient is disturbed and the disturbances correlate with El Niňo data. El Niňo is also responsible for irregular disturbances of the polar motion during that period. While the influence of El Nino on variations of the Earth's rotation is strong and has been studied by many researchers, its influence on the polar motion has not been known till now. (B. Kolaczek) Analysis of variations of sea level anomalies based on altimetric data from the Topex/Poseidon satellite was performed. Spectra of these variations of selected ocean regions, and particularly of the Baltic Sea, were calculated. Spectral analysis and sea level topography determination of the global ocean were initiated with the use of a 2D Fourier Transform Filter. (W. Kosek) It was discovered that the exchange of angular momentum between the solid Earth and the ocean and between the solid Earth and the atmosphere above land is one of the main causes of short-term variations of the polar motion. The influence of the atmosphere on the polar motion is eliminated due to changes of the sea level according to the inverted barometer effect. The next step of the research should be analysis of physics of the angular momentum transfer between different regions of the atmosphere and of the solid Earth to find out what climatic phenomena are responsible for the dominance of the Eurasian region in the excitation of short-term variations of the polar motion. Another noteworthy result is determination of the oceanic angular momentum excitation functions. The addition of the oceanic excitation function to the atmospheric excitation function improved the agreement between the polar motion observed and computed from the excitation functions. Employment of more advanced oceanic models should further increase this agreement. The direct influence of El Niňo on short-term variations of the polar motion is mainly limited by strong transfer of these variations through the Eurasian region. An indirect influence of El Nino on polar motion variations is seen as disturbances of the correlation coefficients between the variation of polar motion and of atmospheric angular momentum. The time delay of the occurrence of the maximum value of the correlation coefficient with respect to El Nino epoch is different for different El Nino events. The next step in analysis of these phenomena should be investigation of the relations be-tween El Nino events and regional variations of the oceanic angular momentum. (J. Nastula) Research of high frequency atmospheric effects on Earth rotation continued on the basis of on the 6 hourly homogeneous time series of the atmospheric angular momentum (AAM), spanning the period of 1968.0 - 1997.3. An earlier estimate of the atmospheric contribution to the nutation amplitudes was extended on the whole diurnal and semidiurnal bands in polar motion and length of day. A first estima-tion of the nontidal oceanic influence on nutation was completed, where "nontidal" means fluctuations of the oceanic angular momentum driven by the observed variations of the surface atmospheric pressure and winds. Comparison with the VLBI nutation data shows that the global ocean circulation model used in the analysis is not realistic within the diurnal band, nevertheless such a comparison provides important information how to improve this model. (A. Brzezinski) The free core nutation (FCN) mode in Earth rotation was studied with the use of VLBI observations of the nutation angles. One new result is an explanation of the difference between the FCN period derived from the indirect effect on nutation, that is from the resonant enhancement of the nutation amplitudes (slightly more than 430 days), and the period obtained from spectral analysis of the freely excited FCN signal in nutation (between 415 and 425 days). (A. Brzezinski).

10.4. Geodetic studies
In cooperation with Dr. R. Noomen from the Delft University of Technology, faculty of Aerospace and Engineering, Delft, the Netherlands, the stability of the global network of laser stations, and particularly of the Borowiec station, was studied. The goal was to determine the optimum network to minimise errors in the variables determined from observations. The influence from other sources, like the kind of observations of the LAGEOS-1 and 2 satellites, the orbital arclengths, and a priori assumptions on the accuracy of determination of the laser network, was studied. (M. Rutkowska) Free-fall gradiometer an analytic theory of the use of gradiometric data measured at stratospheric heights was developed. The data that could be obtained at a probe in free vertical fall or onboard of balloons and aircraft flying at altitudes between 20 and 40 km. From the formulae derived one can determine gravimetric anomalies, potential coefficients, and geoid undulations. (J.B. Zielinski) A study on definitions and description of geodetic reference systems as well as of positions of po-ints and objects determined with the use of geodetic methods for research and modelling of geody-namic phenomena, was performed. The methods specified in the European Standard CEN/TC 287 and the formal language EXPRESS-G conforming with the ISO 10303-11 standard were used. Geodetic data are presently in use for many purposes, including the global change, environmental monitoring and local applications measurements, to name only a few, as well as in a broader scope, in basic science, economy, biology, social studies etc. Geodetic technologies used in these applications return precise determination of positions and gravitational potential parameters and their variations in time. They are registered in various projections and reference and coordinate systems, with various accuracies, and are used by different users. Therefore a system of data acquisition and processing is needed that would be systematic, well defined, unambiguous and well understood by the users. Such system must be suitable for computer data processing and storing in geodynamic databases. The study performed involves a formulation of the problem and specification of the formal tools used, including EXPRESS. (W. Pachelski) A study of round-off error cumulation in the reduction of the Polish height warp was carried out. The warp (jointly of 1-st and 2-nd classes) involves about 43000 parameters determined during the reduction. Due to ill-conditioning of the system the solution is numerically unstable. By theoretical research and numerical modelling it was demonstrated that in order to obtain a homogeneous solution without the dominant influence of roundoff errors one must not divide the warp into groups and classes to be reduced separately. Instead, the reduction should be performed jointly. An algorithm for such reduction, based on sparse matrix0 technology, was proposed. (W. Pachelski)