C. Arduini

LINEARIZED TECHNIQUES FOR SOLVING THE INVERSE PROBLEM IN THE SATELLITE THERMAL CONTROL

Abstract The design of the passive temperature control on a spacecraft is generally based on the tentative assignment of the thermo-optical parameters, on the computation of the corresponding temperatures and, then, on a correction procedure based on a locally linearized “sensitivity”. In this paper, instead of this classical approach, we try a method that, still on a linearized basis, uses the pseudo-inverse of the temperature vector to realize the wished temperature in the structure. The solution of the problem uses the definition of null space and pseudo-inverse and the matrix of the unknown is determined in such a way that the physical characteristics of the coupling radiative and conductive matrix (that means symmetry and balance) are satisfied. Other constraints shall be imposed, and at the end, when the number of unknowns is equal to the equations, the problem is solved. Due to the arbitrariness in the additional constraints needed for closing the problem, the solution obtained with this method is not unique. It is possible in any case to choose optimized solutions as, for instance, the easiest to be realized in practice.

S. Marco V drag balance neutral density compared to the models

Abstract A main subset of the S. Marco V neutral densities measured by the Drag Balance Instrument (DBI) in the equatorial atmosphere at altitudes between 250 and 470 Km in the period April 21 – December 05 1988 has been subjected to systematic analysis at various phenomenological scales. This paper presents some results which can be relevant for global empirical modeling of the thermosphere. In particular the measured data are statistically compared with the MSIS86 model data. The comparison reveals non gaussian deviations with typical patterns which can be interpreted in terms of unmodeled seasonal variations; in particular we address in the paper the amplitude and the local time of the peak of the midnight secondary density maximum. The peak local time appears to be seasonally variable, consistently with published data on the temperature secondary peak and which explains the patterns of the observed deviations with respect to MSIS86.

Tidal analysis of the San Marco V and San Marco III: Density data in equatorial orbit

Abstract This paper presents an analysis of the diurnal density variations between ~200 km and 400 km as evidenced by a data set of unique characteristics, namely the density data collected by the Drag Balance Instrument (DBI, Broglio et al. , 1989; Arduini et al. , 1993) on board the San Marco III (SM3) satellite (launched 1971) and San Marco V (SM5) satellite (launched 1988). The measurements are covering similar seasonal periods and different solar cycle conditions. They have, in addition, very high space and time resolution and very good relative accuracy. These characteristics allow us to appreciate fine details of the diurnal variation. In the present paper, besides the data presentation, a few points will be addressed, namely: 1. —Midnight Density Maximum (MDM) feature as related to the diurnal tides; 2. —persistence and variability of the diurnal bulge on the long period; 3. —persistence and variability of the diurnal bulge on the short period; 4. —comparison with model results.

On radiative and conductive heat transfer in spacecraft

Abstract The “radiative” boundary condition in a heat conduction problem relates the heat flux in a point of the wall to the temperatures in all the other points of the surface system. That is, this condition is of a “functional” type. This “functional” can be solved in discrete terms by using the conventional technique of dividing the body in small discrete elements or “nodes”. This paper instead presents an approach on a continuous scheme, by which the functional is solved in terms of “spacewise harmonics” of the temperature inside the body; the heat conduction problem is thus reduced to an ordinary form differential system whose unknowns are these “harmonics”. An interative linearized procedure to solve this system is also suggested, by which “exact” solutions are obtained. The merits of these solutions, with respect to practical discrete element computations, are in the better spacewise resolution and in the consequent more accurate treatment of both radiation and conduction. The application of the procedure is, however, limited to particular geometries. It is relevant to note that among these possible geometries many are included of practical interest, like hollow cylinders and polygons, cubic boxes etc. A numerical example completes the work.

The Midnight Density Maximum in the S. Marco V and the S. Marco III equatorial density data sets

Abstract In a previous paper we showed some systematic deviations of the S. Marco V drag balance equatorial density data with respect to the MSIS86 model. We interpreted these deviations as due, at least in part, to the presence of a variable “Midnight Density Maximum” (MDM). In the data, there was in fact evidence of some altitude and seasonal variation of this pattern. In the present paper we consider, besides the S. Marco V data base (density measured during 1988), the S. Marco III data base, collected in 1971 almost in the same seasonal period and altitude range, with an instrument very similar to that of the S. Marco V. The use of both data sets is allowing a rather detailed description of the phenomenon as seen by the DBI instrument, for what concerns both the “seasonal” and altitude variations. In addition also some longitude effects are evidenced, for instance, by the MITS and QUITO data subsets of S. Marco III, taken respectively around 40° and 280° East longitude. Notice in addition that S. M. III data refer to the year 1971 (descending part of solar cycle 20) while SMV was launched in 1988 (ascending part of solar cycle 22); the comparison is thus allowing to evidence the persistence of the phenomenon and of its main characteristics. The observed data are consistent “at large” for both S. Marco III and V, while the differences in the details are providing hints on the mechanisms of the thermospheric dynamics (tidal theory and neutral-charged interactions). The paper presents the above said features together with a discussion on the characteristics of the two data bases and on their possible relevance for modeling the considered MDM feature.

Thermal analytical model of bidimensional and unidimensional structural elements in a space environment

Abstract When affording interactive thermo-structural problems, a first difficulty is the different modeling approach in the thermal and in the structural field. While in fact bidimensional and unidimensional structural theories are common practice (plates, shells, beams) the same is not true in the thermal field, where the three-dimensional general heat conduction equations are commonly used. In this paper the global analytical thermal conductive-radiative modeling of one- and two-dimensional structural elements is proposed, based on approximations of the temperature field analogous to those made on the displacements in the elastic problems. The resulting models and their use and limitations are presented and discussed.

An approach to the dynamics of modular repetitive structures

Abstract The dynamics of modular structures is approached in this paper by means of the discrete Fourier transform. This method, applied to a structure with N bays and ring type boundary conditions, leads to N uncoupled systems of the size of a single bay. For other boundary conditions, it leads to a “spectrally resolved” eigenproblem, that is a form whose dominant terms in each field of frequency are evidenced. Approximate reduced models in narrow frequency fields can therefore be generated by using the “spectral condensation” technique. The method can be applied with general boundary conditions, but the present paper deals mostly with the “clamped edges” boundary condition and shows that numerical advantages can be obtained, particularly for the large space structures.

Solar activity effects on the equatorial thermosphere temperature profile

Abstract In this paper we present the effects of solar activity on the temperature profiles of the equatorial thermosphere as derived from the neutral density data collected by the San Marco 5 (SM5) satellite. This satellite flew during the increasing part of the solar cycle 22 (1988). It had a quasi-equatorial orbit, with inclination lower than 3°. The range of measurements, from April to December, allows the inference of seasonal and diurnal effects on the temperature profiles. The density data are collected every second along arcs of orbit lasting up to 50 minutes. The analysis of these densities has been already partially presented and provided evidence for several interesting features, in particular the vertical structure of the diurnal harmonic content and its seasonal variations. The temperatures derived from the same data set provide a useful complement to this picture. The SM5 satellite carried on board 5 instruments for studying the equatorial ionosphere and thermosphere, among them, the Drag Balance Instrument (DBI) for measuring the neutral density and the Ion Drift Meter and Potential Retarding Analyzer (IVI) that allow the evaluation of ions concentration, velocity and temperature. It is possible, therefore, to compare, to compare the neutral temperature derived from the neutral density data with the ion temperature given by the IVI.

New insight on the internal waves in the equatorial thermosphere by the S. Marco 5 spacecraft data

Abstract Waves with both acoustic and internal gravity characteristics have been recorded by the S. Marco 5 spacecraft during its operation in 1988. The data set evidenced the dependence of the number of wave events upon the local time (with maxima during nighttime) and a correlation with simultaneously measured ionospheric irregularities. Although an accurate analysis of the possible influence of the experimental environment has been already made during the first analysis which indicated the physical nature and the reliability of these data, a supplementary investigation has been added by way of demonstration and is discussed in this paper. Since the gravity waves are recognized as a possible mechanism for the insemination of the the ionospheric bubbles phenomena, the correlation between waves and bubbles events detected on the ion density (IVI) and in the zonal electric field component (EFI) is not surprising. The previously presented analysis proved the correlation on the basis of the occurrence frequency of the bubbles and wave events. In this paper an attempt will be made to analyse the contemporary behaviour of ionospheric, electric and neutral density data searching for point by point possible correspondence and related interaction mechanisms.

MICROSATELLITES AND SPACE STATION FOR SCIENCE AND TECHNOLOGY UTILISATION

The main results of an ESTEC study on microsatellites and the International Space Station (ISS) are shown herein. One of the original objectives of the space station has been to be a staging post for assembly, check-out and deployment of satellites and deep-space missions. ISS will not be equipped to process and launch large spacecraft requiring complex assembly and propellant operations, but deployment of a microsatellite might still be feasible and advantageous, as other international partners show similar interests. The assembly, launch and retrieval of small satellites with ISS (e.g. up to 225 kg, 100 W of power from body mounted solar cells or 225 W from solar panels, size less than 1.6 × 0.8 × 1 m, 250 b s−1 to 2 kb s−1 of up and down link data) is a future but a technically viable option to raise the attractiveness of ISS for those payloads whose requirements would not be satisfied by internal or external accommodation on ISS. Two classes of payloads were identified: (1) Space Station investigation (SSI) type; (2) Space Station exploitation (SSE) type. SSI payloads conduct investigations of direct interest to ISS as electromagnetic, radiation and particle near-environment (ISS sniffers). Instead, SSE investigations are totally unrelated to ISS and they make solely advantage of its infrastructure and facilities. For the latter it will be necessary to compare the same experiment with and without ISS in the mission scenario, as a proof of effectiveness (in costs and/or scientific return) with respect to satellites launched by traditional means. Such effectiveness may be represented by (i) simplicity of design, e.g. the proximity of ISS eases the TT&C subsystem, (ii) servicing and re-utilisation for successive experiments, (iii) readiness to launch when a specific and unpredictable event occurs (e.g. supernovae). Different scenarios and implementations have been studied for launch and operations. Safe orbital trajectories have been identified also without a propulsion system. In case of a on-board propulsion system, about 50 kg for cold gas fuel and another 50 kg for dry propulsion system, were found necessary for a mission lifetime in ISS neighbourhood (within 50 km) up to about 6 months. The study has conceived 13 proposals in disciplines ranging from metrology and general physics, to space science and technology, as examples of potential applications but in no manner constituting an exhaustive picture of user interests. An SSI experiment for the analysis of ISS environment was selected as potential first mission.