Micro Satellite Systems and Modeling Methods

A unique phenomenon observed in FHD experiments is the spin-up flow by magnetic surface stress. It occurs at the interaction of alternating magnetic fields with the free surface of the paramagnetic liquid, a ferrofluid. The liquid phase in an open channel has a higher magnetic susceptibility than the gaseous phase above. This leads to an unsymmetrical magnetic stress which pulls the free surface into the direction of the rotating magnetic field while the stress on the walls is compensated by friction. This results in a Couette flow pattern. The effect shows a high potential for an achievable flow rate of litres/min and a lifetime above 15 years.

To function under microgravity conditions and to provide high reliability, the alternating magnetic field of the pump needs to fulfil two challenging objectives during the duty cycle. These are keeping the correct positioning of the liquid and gas phase and at the same time pumping the liquid in the desired direction.

We apply a recursive engineering approach with tests in Droptower and Gravitower followed by updating the theory and repeat.

The density of the thermosphere is of high relevance for the propagation and determination of satellite trajectories in Low Earth Orbit (LEO). Hence, it is important for a lot of satellite operations in LEO. The drag acceleration, resulting from the gas-surface interaction of the rarefied atmosphere with satellites and other bodies, results in a change of trajectory and an orbital decay. This has implications for active satellites, where a precise orbit or position knowledge might be crucial for the missions success (e.g. altimetry, gravimetry, remote sensing), for maneuver planning and satellite lifetime prediction, as well as for space debris, where position determination is important for collision avoidance and reentry calculations. Furthermore, in atmospheric sciences, thermospheric density data can help to restrict, develop and validate complete physics-based thermosphere-ionosphere models.

Atmospheric density models used in satellite dynamics calculations are usually semi-analytical models, which are heavily based on measurement data. Thus the quality of such models relies also on the quality of the underlying density data. Prominent models are e.g. NRLMSISE, JB08 or DTM.

Due to the lack of direct measurements of thermospheric density, derived densities from satellite orbit data are currently the best data source. The most accurate atmospheric density estimates are computed from data of geodetic satellite missions, with onboard accelerometers. Nevertheless, differences in published datasets are rather high. Depending on the temporal resolution and space weather conditions (solar and geomagnetic activity), differences between those datasets might be 100% and 25% on short time scales below orbit period (>1.5h) and longer time scales around some orbit periods, respectively.

The density retrieval approach from accelerometer data is based on three separate competences, which are: (1) Precise radiative non-gravitational force modeling, (2) Modeling of the interaction between the rarefied atmospheric gases and the satellite, i.e. modeling of drag coefficients, and (3) the calibration of the accelerometer data by POD.

The modeling of the drag coefficient is the area with the greatest uncertainty. The gas particle-surface interaction in the an extremely thin atmosphere with very high relative velocities is not yet fully understood. The improved understanding of these interactions and the description by suitable models and parameters is one of the objectives of this project.

The ultra-sensitive accelerometers onboard geodetic satellites like GRACE, CHAMP, GOCE, Swarm and GRACE-FO offer the possibility for density retrieval at different altitudes, epochs and at different solar activities. Thus make up a valuable data set. Furthermore different operation conditions allow for the possibility to disentangle parameters in the drag modeling.

Besides the very high temporal resolution of thermometric densities retrieved from satellites with   accelerometer, an approach based on fitting parameterized density models or functions in a POD process with lower temporal resolution is developed and compared. This offers to use a wider data base of satellites.

To meet these requirements, we have developed a coarse sun sensor based on thin film cadmium telluride solar cell technology. The goal was to design a sun sensor, which offers good performance without the need for a thermal control system, comes at reduced mass and has a high radiation hardness to be compatible with new electric thrusters.

Simulated space environment campaigns were carried out to test the degradation and loss of accuracy of the sun sensor over mission lifetime. We could show that the sensor keeps an accuracy of ±3° after 10 000 cycles of thermal shock between -60°C and 95°C and 1000h at an elevated temperature of 95°C. The radiation hardness was tested with an irradiation of 1 MeV electrons.

The project was funded by the BMWK under grant number 50RK1909