## 5 Single Satellite Pair System Engineering Requirements
The instrument and service equipment boxes (instrument electronics, laser stabilization unit, power control and distribution units, etc.) are accommodated on either side of the central bay, accommodated according to function [51]. The main driving spacecraft system requirements are associated with the assumptions on DFAOCS and propulsion system. The two systems are fully explained and assessed in the next section. Enabling Technologies for the NGGM Mission and Derived Control Requirements  
The basic requirements of time-variable gravity measurements from space can be realized with a space segment having (a) orbit altitude as low as possible to maximize signal strength, (b) retrieval periods as short as possible to maximize the time resolution of the gravity field solutions and (c) near-homogeneous ground track coverage as dense as possible for each retrieval period (such as a week and a month) for maximum spatial resolution. The lowest possible altitude is dictated by satellite engineering constraints related to the satellite configuration, the cross-section area exposed to air drag, the propulsion type and the amount of propellant. Here, we discuss specific orbit and attitude control solutions that enable formation and ground track maintenance and relative attitude control required by the LTI in the altitude range around 350 km. The attitude and the environmental disturbing accelerations will be controlled within the frequency range from 1 to 100 mHz according to the derived control requirements in Table 7. The formation of the two satellites will tend to drift apart under the action of differential air drag and differential accelerometer biases, which can be corrected by commanding a thrust bias as in the case of the GOCE mission [52]. Dedicated control acting below 1 mHz can ensure that the mean semimajor axis remains within \(\pm\)100 m of the nominal value and the relative distance is remained within 10% of the nominal intersatellite distance. \begin{table}
\begin{tabular}{c c c} \hline
**Control Domain** & **Item** & **Requirement** \\ \hline Formation & Altitude control range & \(\pm\)100 m \\ \hline \multicolumn{3}{c}{Satellite-to-satellite distance} & 100 km + 0–10\% \\ \hline Drag Compensation & Measurement frequency & 1–100 mHz \\ Control & bandwidth & \\ \hline \multirow{3}{*}{\
\begin{tabular}{c}  Linear acceleration \\ Linear acceleration ASD \\ Angular acceleration ASD \\ \end{tabular} \(\leq\)5 \(\times\) 10\({}^{-9}\) m/s\({}^{2}\)/vHz \\ \cline{2-3}  & Angular acceleration ASD & \(\leq\)10\({}^{-6}\) rad/s\({}^{2}\) \\ \hline Attitude Control & Satellite-to-satellite pointing & \(\leq\)2 \(\times\) 10\({}^{-5}\) rad \\ \hline \multirow{3}{*}{
\begin{tabular}{c}  Satellite-to-satellite pointing \\ error ASD \\ \end{tabular} \(\leq\)2 \(\times\) 10\({}^{-6}\) rad/vHz \\ \cline{2-3}  & (10 \(\leq\) f \(\leq\) 100 mHz) \\ \end{tabular} } \\ \cline{1-1} \end{tabular}
\end{table} 
Table 7: Drag-free formation, attitude and orbit control system (DFAOCS) requirements. The spacecraft propulsion enabling the DFAOCS and orbit control functions is the main challenge of the spacecraft design. For the selected low orbit in the worst-case scenario of average solar maximum conditions, the thrust range and modulation capability imposed by the mission profile, coupled with the lifetime requirement, can likely only be achieved with electric propulsion, which trades propellant mass for electric power. Generating the requested system power of approximately 1 kW is a challenging task for a mission that needs to keep the drag cross-section small (below 1 m\({}^{2}\)) with high seasonal and orbital variation of the solar aspect angle. Moreover, a high specific impulse is only available over a limited thrust range, and the thrust demand varies by a factor of 3 to 5 during one orbital revolution and even more between the highest and lowest solar activity encountered during the mission lifetime.