# Hybrid Electrostatic-Atomic Accelerometer for Future Space Gravity Missions
## 4 Conclusions  
In this article, a hybrid accelerometer concept, combining an atom interferometer and an electrostatic accelerometer, was studied for use in future space gravity missions, especially in a satellite-to-satellite tracking scenario similar to GRACE, GRACE-FO, or the future NGGM. To assess the potential of such an instrument, numerical simulations were conducted to evaluate the associated benefits in terms of gravity retrieval. These  
Figure 31: Temporal sequence describing the intermittent mode. The standard mode (up) allows acceleration measurements each \(T_{c}\). In the intermittent mode (bottom), the measurements are now only each \(n\cdot T_{c}\). In this example, we take \(n=4\). In between, the CAI is put in low power consumption mode (standby mode) allowing lowering the average electrical power consumption. Figure 32: Evolution of the CAI noise (QPN) due to intermittent operation for an interrogation time \(T=2\) s and a cycling time \(T_{c}=6\) s in standard mode. The orange, purple, and green curves correspond, respectively, to an effective measurement cycling time of 6 s (\(n=1\)), 42 s (\(n=7\)), and 402 s (\(n=67\)). simulations showed that improved gravity retrieval could be achieved for some threshold low-frequency atomic instrument performance, but that this beneficial impact vanishes once temporal variations of the gravity field are taken into account, because of temporal aliasing effects. Improved de-aliasing models and/or more sophisticated parametrization schemes that allow for a reduction of temporal aliasing effects (or, preferably, a Bender-type constellation employing both) are then required in order to maximize the impact of accelerometer performance gain. The hybrid instrument, allowing a better knowledge of the scale factor accelerometer, should also lead to relaxation of the drag-free requirements for the satellite formation. Some assumptions of the acceleration performance levels were made concerning the hybrid instrument. To evaluate their validity, a simulator modeling the hybrid sensor was developed with the objective to estimate ultimately the achievable acceleration performance of such instrument in-orbit. The hybrid instrument simulator was tested successfully by reproducing noise characteristics of a hybrid accelerometer prototype operating on the ground. The detrimental impact of satellite rotation on the CAI contrast was also studied. While the contrast was completely lost in-orbit when a fixed CAI reference mirror was used, it was possible to compensate for the orbital rotation with the proof-mass of the electrostatic accelerometer acting as the reference mirror. In this case, we observed a total recovery of the interferometric contrast. Although the simulator could give a quite good overview of the hybrid instrument behavior, it should be further tested and developed to take into account more effects that could have a significant impact on the final performance of the accelerometer. For instance, concerning the EA, simulations showing the effects of the EA transfer function uncertainties and scale factor errors on the acceleration measurements still need to be performed. A study of a full calibration (bias + scale factor) procedure of the EA should also be performed and validated with the simulator. Concerning the CAI, the impact of the position and velocity stability of the atomic cloud should be looked at and some improvements concerning the CAI model should be made to take into account, for instance, the double-diffraction processes, the beamsplitter and mirror velocity selection, the laser beams intensity inhomogeneity, the detection system response, the wavefront aberrations of the Raman laser, etc. The development of this simulator will be of prime importance in the future to extrapolate on-ground performance towards space environment.