## 5 Single Satellite Pair System Engineering Requirements  
The specific NGGM mission and system design will build on the experience of GOCE, in particular for the design of the attitude and orbit control, GRACE, for the concept of SST via precise metrology in low-Earth orbit, and GRACE-FO for the LTI. Whereas Section 4 focused on the technology selection for the space segment to fulfill user requirements, we derive here the detailed engineering requirements. The NGGM mission requirements were consolidated through a series of system studies and are summarized as follows:  
First of all, the scientific instrument of each satellite of the NGGM shall comprise a laser interferometer (or a functional part of it), one accelerometer positioned at the center of mass of the satellite or multiple accelerometers around it, GNSS receivers and a passive retroreflector for laser ranging from the ground. The operational lifetime for the NGGM satellite pair shall be 7 years as a goal, after a commissioning phase of 6 months for both satellites. Orbiting at a constant mean altitude during the entire mission lifetime is preferred over a variable altitude profile. Phase 0 studies identified that the lowest viable altitude is 340-350 km, which is compatible with the sensitivity requirements (cf. Table 5) and with the resources needed for orbit maintenance and drag compensation over the complete lifetime. The current baseline of the intersatellite distance is set around 100 km as stated in Section 4. The altitude shall be maintained as for the GOCE mission, within a range around a specified value that will be selected to realize a controlled longitude shift of the ground track as described in Section 3. The engineering requirements for the satellite design derive from the top-level system requirements formulated in Section 4. Figure 14 shows the ASD of the system measurement requirements (threshold and goal) of the fundamental observables of the mission: the intersatellite relative distance variation and the projection of the differential nongravitational acceleration along the satellite-to-satellite direction, based on the ultraprecise accelerometers such as the ones of the GOCE mission (cf. Table 6). The system performances for the NGGM pair of satellites are summarized in Figure 15.  
The primary objective of the accelerometer sensor suite on NGGM is to measure the satellite nongravitational acceleration in the satellite-to-satellite tracking direction, with a low-frequency noise (below 1 mHz, where it becomes the dominant error source) possibly better than in GOCE. Several options for accelerometer accommodation are under consideration: one accelerometer is installed in the center of mass, or two (or more) accelerometers can be arranged symmetrically around the center of mass. Moreover, the accelerometers shall provide the measurements on board for the drag-free formation, attitude and orbit control system (DFAOCS) ensuring orbit and formation maintenance, drag compensation, control of the satellite angular accelerations and rates and a high stability pointing of the laser beam. The internal layout is dictated by the requirement that the optical reference for the intersatellite distance measurement shall be placed in the center of mass and the accelerom  
Figure 14: System measurement performance requirements. Left: intersatellite relative distance variation measurement error. Right: relative nongravitational acceleration measurement error. Figure 15: NGGM overall system performances. eters close to and symmetrically accommodated with respect to each satellite center of mass. A stringent temperature stability requirement of  
\[\mathrm{T(f)}<2\cdot\sqrt{\left[1+\left(\frac{5\cdot 10^{-3}}{f}\right)^{2} \right]}^{2}\mathrm{mK/Hz} \tag{19}\]  
applies in the compartment enveloping the optical bench, where T is the temperature as a function of frequency f, on which all temperature-sensitive items are mounted, including parts of the laser equipment (optical bench assembly and retroreflector) and the accelerometer sensor heads.