The optimal design of co-orbiter space missions for Earth's gravity modeling

In recent years, strong scientific interest has been generated in a better understanding of the physical system of the Earth. It has been heightened the need for improving our knowledge of the gravity field of the Earth, both in terms of accuracy and spatial resolution. this could be globally and homogeneously possible only by means of space gravity missions. Nowadays, it is becoming increasingly difficult to ignore the widely used applications of the satellite gravity mission's information in studying the Earth system. For example, the application of the gravity information in geophysical and geotechnical research, is a new dimension for geodynamic research and seismology, Oceanography and determining ocean circulation, Hydrological research, Ice mass balance and sea level study.
There have been many motivations behind launching the space gravity missions. After the first gravity mission CHAllenging Minisatellite Payload (CHAMP) launched in 2000 for the gravity and atmosphere applications, Gravity Recovery And Climate Experiment (GRACE) mission was launched to improve the temporal and spatial resolution for hydrological and geophysical studies. As a new space-born gravity mission, Gravity field and steady-state Ocean Circulation Explorer (GOCE) was designed based on the gradiometry observation in height about 250km. Because of the short life-time of GOCE, this mission was designed to determine static gravity field and the temporal gravity field modeling was assigned to other space-born missions like GRACE. The GRACE was designed to determine and interpret the temporal gravity variations. By the help of GRACE monthly solutions, it is easily possible to extract the periodic and quasi-periodic signals of the gravity. It allows researchers to interpret the time gravity variation as the mass redistribution in the Earth dynamic system. The temporal gravity variation might be caused the global water cycle, ice mass loss in the poles, the glacial isostatic adjustment, the earthquake subsequences and geodynamic activities.
A follow-on mission to GRACE is desirable to bridge the gap in the time-series of the monthly gravity models. After the successful GRACE mission, in order to minimize the cost and technical risk, the same space mission has been proposed to measure the gravity field variations. Then, the GRACE follow-on mission will be a rebuild of the original GRACE with a few developments. Laser interferometry will be tested as a new experiment to improve the ranging precision whereas, the mission will be equipped with the microwave ranging system similar to the GRACE. As the first mission in the planetary science, the lunar GRAIL mission was proposed as a pair co-orbiting spacecraft similar GRACE. GRAIL that was launched in 2011 to improve our knowledge about the moon's gravity field.
In this study, we investigate the role of various parameters of the co-orbiter mission design to improve the gravity field modeling for post-GRACE missions. A proper definition of these parameters will have a large effect to improve the gravity field modeling. The redesign has been carried out based on the science and technology developments in recent years. Using laser ranging system instead of K-band ranging system, and decreasing the satellite height (assuming the use of active propulsion system) are new suggestions. The mission could not only improve the quality of the gravity modeling, but also bridge the gap in the time-series of the monthly solutions. In the co-orbit missions, the mission design commonly consists of designing the orbit and satellite separation. Assuming the mission is equipped with an active propulsion system, the height of the satellite pair will be reduced to 350 km. Laser interferometry will be tested as a new experiment to improve the ranging precision by considering its advantage and disadvantages. The results show that using the mission equipped with laser ranging system could not improve the quality of the gravity modeling, because of the limitation in average range