# Drag and Attitude Control for the Next Generation Gravity Mission
## 2 NGGM Objectives, Measurement Technique, and Fundamental Observables  
### User's Need and Objectives of the Next Generation Gravity Mission  
The results of the GRACE, GOCE, and GFO missions induced the user's community to invoke continuity and evolutions of gravity missions to observe the mass changes and transport processes in the Earth system. The main thematic fields and phenomena which shall be investigated by NGGM are based on users and scientific needs collected in the reference documents [1,18,19]. They are summarized in Table 1 together with the required measurement accuracy (threshold and goal levels) of the Earth's gravity signal produced by the phenomenon/event to be monitored, expressed in terms of equivalent water height (i.e., the height of a water layer over a region with the size equal to the required spatial resolution that produces the gravity signal corresponding to that phenomenon/event). Note: only the measurement accuracy associated to the minimum time scales plus long-term trends and to the minimum spatial resolutions of the phenomenon/event to be monitored are reported in Table 1 by way of example; the complete set of requirements at all resolutions can be found in [1,18,19]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Thematic Field**} & \multirow{2}{*}{ \begin{tabular}{c} **Geophysical Phenomena/** \\ **Events/Quantities** \\ \end{tabular}  } & \multirow{2}{*}{**Time Scale 1**} & & \multicolumn{2}{c}{**Resolution**} & \multicolumn{2}{c}{**Gravity Signal**} \\  & & & & & **Measurement Accuracy** & **(cm of EWH 3)** \\ \cline{5-6}  & & & **km** & **SH 2 Max. Degree** & **Threshold** & **Goal** \\ \hline \multirow{6}{*}{ \begin{tabular}{c} Hydrology \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **\(\bullet\)** \\ **Groupd-water storage** \\ \end{tabular}  } & D & 280 & \(\mathrm{SHD}_{\mathrm{max}}=71\) & 6 cm & 0.6 cm \\ \cline{3-6}  & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \cline{3-6}  & & & M & 260 & \(\mathrm{SHD}_{\mathrm{max}}=77\) & 4.8 cm & 0.48 cm \\ \cline{3-6}  & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \cline{3-6}  & & & D & 280 & \(\mathrm{SHD}_{\mathrm{max}}=71\) & 6 cm & 0.6 cm \\ \cline{3-6}  & & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \cline{3-6}  & & & M & 260 & \(\mathrm{SHD}_{\mathrm{max}}=77\) & 4.8 cm & 0.48 cm \\ \cline{3-6}  & & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \cline{3-6}  & & & M & 260 & \(\mathrm{SHD}_{\mathrm{max}}=77\) & 4.8 cm & 0.48 cm \\ \cline{3-6}  & & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \cline{3-6}  & & & M & 260 & \(\mathrm{SHD}_{\mathrm{max}}=77\) & 4.8 cm & 0.48 cm \\ \cline{3-6}  & & & M & 260 & \(\mathrm{SHD}_{\mathrm{max}}=77\) & 4.8 cm & 0.48 cm \\ \cline{3-6}  & & & L & 150 & \(\mathrm{SHD}_{\mathrm{max}}=133\) & 5 cm/yr & 0.5 cm/yr \\ \hline \multirow{6}{*}{\ \begin{tabular}{c} Cryosphere \\ **\end{tabular}  } & \multirow{2}{*}{
\begin{tabular}{c} **\(\bullet\)** \\ **Groupd-plane** \\ **GroupThe scientific objectives for NGGM can be synthetized in the measurement of the geoid with an accuracy of 1 mm at a spatial resolution of 500 km every 3 days and 150 km every 10 days [9]. The achievement of these objectives will constitute a significant improvement in the monitoring of Earth's gravity field and of the underlying geophysical phenomena listed in Table 1 by one order of magnitude or more with respect to the current state-of-the-art represented by GRACE and GFO. In fact, a single pair of satellites, flying on a near-polar orbit with an uncontrolled altitude cannot meet the operational and global user's community needs as identified by IUGG [1] and support key applications like the careful management of the fresh water and other natural resources.