Analysis of formation flying satellite missions for mapping the time-variable gravity field of the Earth

PhD student: Joao de Teixeira da Encarnacao

Scientific objectives for Earth observation programs have become more and more ambitious. The classical approach of using large satellites with multiple scientific instruments onboard has reached its limits in terms of costs, spatial and temporal resolution and coverage. Formation flying possesses the potential to enable order of magnitudes improvements compared with classical earth observation platforms.

The concept of formation flying satellites involves two or more spacecrafts that use an active control scheme to maintain their relative positions and velocities.

Formation flying of satellites has opened a new path for space science and technology and quickly revolutionizes the way the space community conducts science missions around the Earth and in deep space. A large number of future planned space missions will be based on the use of highly coordinated micro- and nano-satellite formations to increase the overall efficiency and performance. In particular, GPS positioning of a satellite in a formation may be done with a much higher accuracy than positioning of a stand-alone satellite. Furthermore, formation flying will improve the mission survivability and will reduce costs of launchers and maneuvers. It enables the separation of scientific payloads, which often present competing and conflicting requirements on one satellite platform and its operation. Finally, the sensitivity of scientific instruments can often be increased by expanding the effective observation baselines.

Among the broad range of potential applications of formation flying satellites is measuring the gravity field of a planet. The GRACE mission launched in 2002 can be seen as the first and most simple realization of such a concept. Nevertheless, the results obtained are revolutionizing our knowledge and understanding of the static and time-varying gravity field of the Earth and related processes. A broad variety of applications are profiting from these results, including hydrology, oceanography, geophysics and others. The success of the GRACE mission encourages further development of gravity field observation techniques based on satellite formations. We expect, in particular, that future missions will be both more advanced technically and more specialized. New sensors and observation systems will increase the inter-satellite ranging accuracy as well as the accuracy of GNSS positioning. Furthermore, the setup of future missions is to be tuned in order to reduce the influence of such effects as aliasing of high-frequency temporal variations in the Earths gravity field (they cannot be modeled on the basis of satellite data due to insufficient temporal resolution of the latter) and the limited sensitivity of the line-of-sight relative distance to fine structures in the gravity field. It looks reasonable to consider different mission setups depending on the type of processes of interest: short-term processes (e.g. tides), versus middle-term processes (e.g. water transport) and long-term processes (e.g. ice balance, isostatic adjustment); as well as regional versus global processes. Furthermore, we believe that gravity field models obtained with new formation flying missions will open up new areas of applications.

A break-through is expected when combining new measurements sensors and technology with formation flying satellites. Studies on next-generation gravity field missions focus on low-low satellite-to-satellite tracking using laser interferometry, advanced gradiometry technology and clusters of GNSS-tracked small satellites.

The objective of the PhD project is to analyze the performance of different satellite formations and future sensor technology to map the time-varying gravity fields of the Earth. Special emphasis will be on the problem of aliasing, which has shown to be a major limitation of the on-going GRACE satellite mission. The results and conclusions of the study will be verified against the GRACE mission, which can be used as benchmark.