The test bodies, their mechanical coupling and the capacitance read-out are the core of the GG mission. Once the experimental design has been conceived, the features of the required spacecraft, its attitude and orbit are also identified. In the first place, the cylindrical symmetry of test bodies and PGB and the request to spin (in order to provide high frequency signal modulation), suggest a spacecraft of cylindrical symmetry too, stabilized by one-axis rotation along the symmetry axis. The nature of the signal (Figure 2.1 and^{26, Eq. 2.1} ) requires the spin axis to be as close as possible to the orbit normal; the need to reduce non gravitational perturbations on the spacecraft surface (Sec. 2.3) suggest that it should be small and compact (which in addition helps reducing its cost); the need to reduce perturbations on the test bodies from nearby moving masses suggests to use electric minithrusters of high specific impulse (FEEP) in order to reduce the amount of propellant required for drag compensation during the mission; indeed, the need is reduced to only a few grams of Caesium for the entire 6 months duration of the GG mission (6 FEEP thrusters are needed; see^{26, Fig. 5.14}) as opposed to a few hundred liters of He for the mechanical He-thrusters of STEP). The drag-free control analysis and simulations are given in^{26, Ch.6.1.15}.

A section of the GG satellite through its spin/symmetry axis showing how the PGB and the experimental apparatus is accommodated, in a nested arrangement inside it, is shown in Figure 2.4; a 3D view is given in Figure 2.3 (details in^{26, Ch. 5}). The spacecraft is 1m wide and 1.3m high. The area of the external (cylindrical) surfaces covered by solar cells is dictated by the power needs of the mission (112W, of which 66W for the payload; see^{26, Table 5.18}); the compactness of the spacecraft (similar to a spinning top in shape) is for maximizing the moment of inertia with respect to the symmetry axis whereby providing passive spin stabilization around it. The current nominal spin rate is 2Hz (120rpm), yielding a peripheral acceleration of about 8-g, which is well doable. A careful analysis of the perturbing torques which could tilt the spin axis of the GG spacecraft shows that no active control of the direction of the spin axis in space is needed, the simple physical reason behind this fact being that the kinetic spin energy once at the nominal rate of 2Hz is so high compared to all torques that they would need a very long time to even slightly displace the spin axis^{26, Ch. 2.1.2}. As for phase differences due to a different rotation rate between the s/c outer shell and the PGB, they are mostly compensated passively, residuals are sensed and corrected with FEEP thrusters (details in^{26, Ch. 2.1.2}).

The total mass amounts to 231.7Kg (262.7kg with system margins), as shown in Table 4.1.

The case for a low, almost circular orbit is apparent; the preference for it to be almost equatorial is explained in^{26, Ch. 2.1.2} as a trade off between stronger signal and passive attitude control vs larger thermal perturbations (as long as they are compatible with pure passive thermal control, which is the case). None of the requirements on orbit and attitude is strict. Three candidates launch vehicles have been identified as suitable^{26, Ch. 5.1}: the Orbital Sciences Corporation Pegasus launcher; the Indian Space Research Organization PSLV launcher; the future European small launcher VEGA.

The characteristics of Pegasus (fairing envelope, limit loads, payload mass) are by far the most constraining of the three. Therefore, our design exercise has been focused on Pegasus. As for orbit injection performance, no capability for correction of injection errors is to be provided on board of GG. Instead, the satellite is designed to have an autonomous system for spin-up to the final nominal spin rate of 120rpm as well as for correction of angular momentum depointing and damping of angular rates. See^{26, Ch. 5} for details (to Phase A level) of the GG satellite and specific analysis, in particular, on: structural design, materials selection, finite element model, structural analysis and thermal control.