## 4 Instrument Sensitivity, Accelerometer and Drag Compensation Assessment
### Accelerometer Selection and Drag Compensation Assessment
In fact, its performance can be customized to the mission needs by optimizing the following parameters:  
* Shape and mass of the proof mass: a heavier and cubic proof mass can potentially bring the performance along all three axes closer to the \(10^{-13}\) m/s\({}^{2}\)/\(\sqrt{\text{Hz}}\) noise floor (as opposed to the GRACE and GOCE accelerometers, which offer only two ultraprecise axes);
* Increasing the gap between proof mass and electrodes (at the cost of a smaller dynamic range);
* Changing the material and the stiffness of the proof-mass grounding wire, which keeps the proof mass at the polarization voltage avoiding discharges and parasitic electrostatic forces;
* Redesign of the read-out and proof-mass control electronics, for decoupling the measured translational and rotational motion of the proof mass. Such new sensors will enable NGGM satellites to orbit at higher altitudes with a fine-tuned intersatellite distance and with a higher sensitivity over a wider range of frequencies, especially at the low end of the science measurement bandwidth. Accelerometer selection and orbit altitude have to be traded off against the need for a drag compensation system. For NGGM, the \"drag\" experienced by each satellite due to residual atmosphere in the altitude range between 300 and 400 km can be modeled as in Figure 10, where the in-track drag forces are reverted to accelerations for a GOCE-like satellite of 1 t mass. Figure 10: Drag acceleration profiles as function of the altitude. The curves are drawn for min/mean/max solar activity confidence level at 95%. Courtesy TAS Turin (IT). Drag accelerations have to be counteracted to ensure a good accelerometer performance and to avoid saturation of the accelerometer measurements. For instance, accelerations of 1-6 \(\times\) 10\({}^{-6}\) m/s\({}^{2}\) need to be compensated by thrusters on average in case of maximum solar activity. To guarantee the best accelerometer performance, also the cross-track (Y-axis) and radial accelerations (Z-axis), which are one order of magnitude smaller than the in-track drag acceleration, as well as the angular accelerations need to be compensated by the drag-free formation, attitude and orbit control system (DFAOCS). Consequently, all the engineering requirements of Section 5 have been derived for this worst-case scenario. One of the current subjects of study is the impact on the system and mission performance when the system design deviates from this scenario, namely when one or more of the engineering requirements are relaxed. As an example, since the errors in the Y- and Z-axis acceleration components only enter the main measurement as projections on the acceleration along the line of sight between the satellites, the corresponding specifications could therefore be relaxed. Some further relaxation of the thrust requirements will result from the concurrent actuation of the magnetic torquers, which are included in the design, as implemented in the GOCE control. Yet another area of investigation is the attitude and pointing control requirements. All these aspects will drive the final design, which will anyway retain, as distinguishing features, at least the along-track drag compensation and the laser beam steering by orienting the satellites. This avoids continuously moving mirrors, which degrade the performance of the measurement of the intersatellite distance (changing the measured optical path and introducing noise in the measurements) and potentially worsen the correction provided by the nongravitational accelerations.