The Delfi-n3Xt was the second nanosatellite developed at the Delft University of Technology, and launched in 2013. Its successor Delfi-PQ is expected to be launched in first half of 2019. The Attitude of a satellite can be referred to as its orientation in space with respect to inertial reference frame. The Delfi-n3Xt was the first satellite from Delft University of Technology, to include three-axis Attitude Determination and Control System/Subsystem (ADCS). It was designed with 5 modes of operation. Four of these were advanced modes. In addition, the Delfi-PQ is not intended to include advanced modes of operation. Hence, this thesis considers using the Delfi-n3Xt ADCS software. This software is extended as a baseline implementation on MSP-EXP432E401Y launchpad. Nearly, 32 % of total nominal power is assigned to ADCS. Hence, energy efficient design alternatives could be considered for future satellite missions. In addition, ADCS is a critical subsystem, failure of ADCS means failure of satellite mission. This thesis aims to improve performance and energy consumption of ADCS. This thesis considers study of three different Digital Signal Processing (DSP) alternatives: Double Precision (DP), Single Precision (SP) and Fixed Point (FxP) arithmetic. Study in this thesis concludes that FxP alternative provides approximately 6.7 times better performance, and approximately 7 times better energy efficiency over baseline. Hence, this thesis proposes the use of FxP DSP alternative. It was also concluded that, the SP arithmetic has equivalent accuracy compared with DP. Moreover, SP provides approximately 3 times better performance, and approximately 2.7 times better energy efficiency over baseline. Therfore, future implementations could benefit from an SP alternative. A major part of the ADCS power is allocated to sensor and actuator. This leaves only 10 % of the total nominal power assigned to ADCS software. Hence, the proposed alternative might not provide considerable improvements on total nominal power. However, for future satellite missions, if there exists a computationally intensive algorithm assigned with more significant amount of total nominal power, then, the proposed alternative could serve as an initial study. However, this does not guarantee that the suggested alternative could satisfy more accurate requirements. In such case, FxP implementation might result in accuracy violation. And use of SP alternative is proposed. In such case, a new study is suggested in order to benefit from FxP alternative.

The Jupiter Icy moons Explorer (JUICE) will generate ephemerides of the Galilean moons to assess their ability to sustain live. However, determining the ephemerides only using radiometric tracking (acquired using PRIDE and 3GM) results in an unstable solution which is related to the absence of flybys of especially Io, such that its dynamics can only be estimated indirectly (through the Laplace resonance). This problem is partially mitigated in this thesis by optimizing the observation schedule for optical astrometry of Io (provided by JANUS) using the NSGA-II algorithm. The optimal epochs for space-based astrometry are found to be preferentially distributed around the closest approaches of JUICE with respect to Io. During these events, the observations show a high sensitivity with respect to the dynamics of Io and have a low positional uncertainty. Using the optimized astrometry, both the condition number and formal errors can be substantially reduced.