Dominic Dirkx

Space-time dynamics estimation from space mission tracking data

Many physical parameters that can be estimated from space mission tracking data influence both the translational dynamics and proper time rates of observers. These different proper time rates cause a variability of the time transfer observable beyond that caused by their translational (and rotational) dynamics. With the near-future implementation of (interplanetary) transponder laser ranging, these effects will become increasingly important, requiring a re-evaluation of the common data analysis practice of using a priori time ephemerides, which is the goal of this paper. We develop a framework for the simultaneous estimation of the initial translational state and the initial proper time of an observer, with the goal of facilitating robust tracking data analysis from next-generation space missions carrying highly accurate clocks and tracking equipment. Using our approach, the influence of physical parameters on both translational and time dynamics are considered at the same level in the analysis, and mutual correlations between the signatures of the two are automatically identified. We perform a covariance analysis using our proposed method with simulated laser data from Earth-based stations to both a Mars and Mercury lander. Using four years of tracking data for the Mars lander simulations, we find a difference between our results using the simultaneous space-time dynamics estimation and the classical analysis technique (with an ta priori time ephemeris) of around 0.1 % in formal errors and correlation coefficients. For a Mercury lander this rises to around 1% for a 1-month mission and 10 % for a 4-year mission. By means of Monte Carlo simulation, we find that using an a priori time ephemeris of representative accuracy will result in estimation errors that are orders of magnitude above the formal error when processing highly accurate laser time transfer data.

Continuous Aerodynamic Modelling of Entry Shapes

During the conceptual design phase of a re-entry vehicle, the vehicle shape can be varied and its impact on performance evaluated. To this end, the continuous modeling of the aerodynamic characteristics as a function of the shape is useful in exploring the full design space. Local inclination methods for aerodynamic analysis have proven sufficiently accurate for use at such a design stage, but manual selection of methods over the vehicle is inefficient for the exploration of a large number of design possibilities. This paper describes the model of an aerodynamic analysis code, written for use in conceptual vehicle shape optimization, which includes an automatic method selection algorithm. Panel shielding is also included in the analysis code to allow for the analysis of more complex geometries. The models used for the shape and aerodynamics are described and results for the Space Shuttle and Apollo are compared to wind tunnel data. They show an accuracy of better than 15% for most cases, which is sufficient for the use in conceptual design. Panel shielding is shown to be important in the prediction of control derivatives at low angle of attack, as well as the prediction of lateral stability derivatives. Finally, a simple guidance algorithm is used to assess the impact of the errors in the aerodynamic coefficients on the vehicle heat load and ground track length. Both show discrepancies of less than 10%.

On the contribution of PRIDE-JUICE to Jovian system ephemerides

Abstract The Jupiter Icy Moons Explorer (JUICE) mission will perform detailed measurements of the properties of the Galilean moons, with a nominal in-system science-mission duration of about 3.5 years. Using both the radio tracking data, and (Earth- and JUICE-based) optical astrometry, the dynamics of the Galilean moons will be measured to unprecedented accuracy. This will provide crucial input to the determination of the ephemerides and physical properties of the system, most notably the dissipation in Io and Jupiter. The data from Planetary Radio Interferometry and Doppler Experiment (PRIDE) will provide the lateral position of the spacecraft in the International Celestial Reference Frame (ICRF). In this article, we analyze the relative quantitative influence of the JUICE-PRIDE observables to the determination of the ephemerides of the Jovian system and the associated physical parameters. We perform a covariance analysis for a broad range of mission and system characteristics. We analyze the influence of VLBI data quality, observation planning, as well as the influence of JUICE orbit determination quality. This provides key input for the further development of the PRIDE observational planning and ground segment development. Our analysis indicates that the VLBI data are especially important for constraining the dynamics of Ganymede and Callisto perpendicular to their orbital planes. Also, the use of the VLBI data makes the uncertainty in the ephemerides less dependent on the error in the orbit determination of the JUICE spacecraft itself. Furthermore, we find that optical astrometry data of especially Io using the JANUS instrument will be crucial for stabilizing the solution of the normal equations. Knowledge of the dissipation in the Jupiter system cannot be improved using satellite dynamics obtained from JUICE data alone, the uncertainty in Ios dissipation obtained from our simulations is similar to the present level of uncertainty.

Shape Analysis - Winged Vehicle

Using the methods and models described in the previous chapters, we present the results of the shape optimization of the winged vehicle in this chapter. We start by discussing the results of a Monte Carlo analysis of the search space, from which we draw preliminary conclusions on the influence of the various constraint functions, and their correlation with the shape parameters. Subsequently, we show the optimization results for the following objective functions: vehicle mass, fuselage volume and ground track length. Both the double-objective results (for each combination) and the full triple-objective results are shown. We perform two additional alternative optimizations, one for which the pitch stability constrain is imposed for all angles of attach (instead of \(\alpha > 20^{\circ }\)) and one where the range objective is replaced with a maximum time at reference heat flux objectives. The results of these optimizations provide insight into the influence of a change in vehicle and mission requirements, respectively.

Shape Analysis - Capsule

Using the methods and models described in the previous chapters, we present the results of the shape optimization of the capsule vehicle in this chapter. We start by discussing the results of a Monte Carlo analysis of the search space, from which we draw preliminary conclusions on the influence of the various contraint functions. Subsequently, we show the optimization results for the following objective functions: optimal heat load, volumetric efficiency and ground track length. Both the double-objective results (for each combination) and the full triple-objective results are shown. By comparing these results, it is clear that full three-dimensional Pareto front could not have been reproduced or deduced from the three two-dimensional fronts, indicating the need for considering multiple competing objectives.