## 4 Instrument Sensitivity, Accelerometer and Drag Compensation Assessment
The preliminary requirements of the satellite-to-satellite tracking (SST) instrument and of the differential accelerometer performance (i.e., the difference between each satellite's accelerometer measurements projected along the virtual line connecting the centers of mass of the two satellites) are shown for all combinations of altitude and intersatellite distance in terms of the maximum noise level at which the requirements mentioned before are still fulfilled. Thus, the instrument requirements are derived directly from the fulfillment of the selected user requirements in terms of errors at the different SH degrees and at different temporal scales (from daily to monthly solutions, up to long-term trends). To meet the user requirements, the noise level of the instruments shall therefore be lower than the values listed in the matrices of Table 5. The mission scenario is consequently given by the combination of the boxes of the ranging instrument and accelerometer at the selected altitude and intersatellite distance. Every possible mission scenario has a color code according to the needed level of sensitivity (grey = technologically not feasible, orange = major technological improvements needed, yellow = minor technological improvements needed, green = achievable with existing technology), with different granularity within the same level. In the top part of Table 5, we can observe that shorter distances are beneficial for the ranging, where distances of 50-100 km at an altitude of 300 km fulfill the requirements with existing technology. Further, we find that altitudes of 500 km and higher cannot meet all the user requirements with existing ranging technology or minor improvements thereof. Orbiting at such a low altitude for years is challenging and may result in a complex mission design. But even if orbiting for years at such low altitude is not feasible, it does not mean that the mission cannot be done. In fact, a subset of the initial user requirements--or a different set of them--can be fulfilled when orbiting higher and with different intersatellite separations, also enabling trade-offs for a relaxation of the instrument and/or sensor requirements. Then, the exercise can be repeated starting from different sets of orbits in Table 3, up to a preliminary verification of the user requirements with new or updated ranging instrument and accelerometer performances. The design methodology based on the system-sizing parameters in Table 5 has allowed identifying the mission scenarios studied during the feasibility assessment of the NGGM mission. The preliminary design of the NGGM satellite pair has targeted the most challenging scenario in a low (300 to 350 km) and generic (not sun-synchronous) circular orbit, embarking a high-accuracy laser ranging instrument (named laser tracking instrument) and ultrafine accelerometers, implementing a drag compensation system combined with an attitude, pointing and \"loose\" formation orbiting control. This corresponds to an intrinsic higher sensitivity of the instrument at low altitude where the gravity signal is stronger. The best compromise for the intersatellite separation among mission performances, acceleration sensitivity (where a large separation is preferable) and laser ranging instrument performance (where a medium/short separation is desirable) has been found around a baseline value of 100 km at these low altitudes. This intersatellite distance may need to be re-evaluated for higher altitude ranges.