# Gravity Field Recovery Using High-Precision, High-Low Inter-Satellite Links
## 2 Mission Concept
### Instrumentation  
The main observable in the MOBILE mission are range measurements from the LEO to the MEOs, where the MEOs are alternating targets. The ranging accuracy is on the micrometer level in order to be sensitive for gravitational forces and its changes on Earth. For distances of several thousand kilometers, a laser-based distance measurement system can reach such an accuracy. The laser range interferometer is placed at the LEO satellite, while the MEOs are equipped with passive reflectors or transponders. In case of the GRACE Follow-On mission, the measurement of inter-satellite ranges by laser range interferometry (LRI) has been successfully established. The link between the two satellites was generated with an active laser on one satellite, and a phase-locked amplifying transponder on the second spacecraft [10]. For the MOBILE concept, the laser ranging instrument needs to be adapted due to the very large distance and the relative motion of the LEO and MEO satellites. In contrast to the GRACE Follow-On, the large distance and the relative speed lead to a range of Doppler shifts of several GHz compared to a few MHz, which causes the need of a reference laser source with a larger range of reference frequencies and a faster phase-tracking capability than implemented for the GRACE Follow-On. The required parameters (<10 GHz range, <10 MHz/s tracking) are within the range of existing, space qualified reference lasers (e.g., the one used for the ATmospheric LIDar (ATLID) instrument on the Earth Clouds Aerosols and Radiation Explorer (CARE) mission) [28], but their compatibility with the needs of an interferometric instrument has to be the subject of further studies. Due to the relative motion of the LEO and MEO satellites, pointing tracking capabilities are required, which requires a modified link implementation. The LEO satellite is selected to play the active part in the tracking mechanism, while the partner satellites (MEOs) are equipped with passive retroreflectors. This type of laser tracking and ranging has been successfully performed for decades with active laser systems on the ground and passive retroreflectors on satellites in orbit (e.g., Laser Geodynamics Satellite (LAGEOS), Ball Lens In The Space (BLITS)) [29; 30]. The scientific benefit of deploying a passive payload in space is the significantly increased mission duration when compared to complex active payloads. The main technological challenge in utilizing this setup for an LRI instrument is the need to achieve a sufficiently high level of retrieved power without the need for amplification between the two passes, ideally close to the 80 pW received by the GRACE Follow-On implementation, but at least to levels above \(\approx\)1 pW in order to allow phase tracking. The main design factors impacting the received power are the initial output power, the size of the retroreflector, and the size of the receiving telescope. Satellite on-board sensors play an important role in the gravity field retrieval by influencing satellite observations due to correlated noise. In our study, the error assumptions used for the laser ranging instrument in the MOBILE concept are based on the time-series provided by [PERSON] et al. 2013 [PERSON] et al. 2013 [PERSON] et al. The non-gravitational forces are typically sensed by the on-board accelerometers located in the center-of-mass of the satellite. In case of the LEO satellites, the implementation of an accelerometer is absolutely necessary due to air drag as the main contributor. For the low-low pair a GRACE-like electrostatic accelerometer is assumed with two highly sensitive axes oriented in the flight direction (largest signal) and in the radial direction, and one low-sensitive axis in the cross-track direction (see Figure 4, blue and red curves). The accuracy level in terms of accelerations is derived by [PERSON] et al.