## 4 Instrument Sensitivity, Accelerometer and Drag Compensation Assessment  
The aim of NGGM as part of the MAGIC constellation is to obtain relatively high-resolution measurements in space and time, including the capability to determine and separate the contributions in the variations of the gravity field due to mass change in terrestrial water storage (i.e., for hydrology thematic field), cryosphere, oceans, solid Earth and climate change signals. This capability enables the mission to serve science and operational applications, including services. The user requirements in [29; 34] have been established based on an exhaustive list of mass change signals for each of the thematic fields of interest listed above and are expressed as cumulative equivalent water height (EWH) error thresholds/goals per spherical harmonic (SH) degree of expansion of the Earth gravity field model. Such requirements form the basis for deriving observation requirements. We describe here a tool that helps in trading off between technical feasibility and fulfilling user requirements for the accuracy and spatiotemporal resolution of the gravity field solutions taking into account the heritage from previous missions and engineering knowledge of specific technology to be embarked on the satellites. Thus, the scope is to search for the acceptable noise level of the ranging and acceleration observations in dependence on altitude and intersatellite distance. These two mission parameters have an unavoidable impact on the mission design and performance: a lower altitude implies a (multiaxis) drag compensation, and a larger intersatellite distance reduces the impact of accelerometer noise but changes the sensitivity to small-scale gravity signals. The search space for the space segment design is rather vast. Thus, in order to assess which user requirements can be fulfilled, according to their prioritization, semianalytical simulations of error propagation for different sensor systems have been performed for a single, polar satellite pair, disregarding temporal aliasing (obviously tackled at constellation level) and focusing on the estimation of covariance matrices of the SH coefficients [43; 44]. Such simulations test the impact of nominal intersatellite distances between, e.g., \(d_{min}=50\) km and \(d_{max}=300\) km, and different mission altitudes between \(h_{min}=300\) km and \(h_{max}=500\) km, taking into account current and future instrument technology and mission limiting factors, such as limited onboard resources (if orbiting too low) or limited \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{1}{c}{**ID**} & \multicolumn{2}{c}{\(h_{l}\)**Variation (km\({}^{-1}\))**} & \multicolumn{2}{c}{\(\Delta\lambda_{shift}\)**Variation (deg km\({}^{-1}\))**} \\ \hline  & 3.days & 5.days & 7 days & 3.days & 5.days & 7 days \\ \hline
3d\_M & \(-0.05\) & (0.31) & - & \(-0.23\) & \((-0.39)\) & - \\
3d\_H & **0.04** & - & \(=0.21\) & \(=0.23\) & \(=0.54\) \\
5d\_Ma & \(-0.07\) & \(0.13\) & - & \(-0.23\) & \(-0.39\) & - \\
5d\_Mb & \((-0.10)\) & \(-0.10\) & - & \((-0.23)\) & \(-0.39\) & - \\
5d\_H & \((0.08)\) & \(-0.11\) & - & \((-0.23)\) & \(-0.39\) & - \\
7d\_M & - & \((-0.19)\) & \(-0.19\) & - & \((-0.39)\) & \(-0.55\) \\
7d\_H & 0.04 & - & \(-0.21\) & \(-0.23\) & - & \(-0.54\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sensitivity of the near homogeneity and longitude shift of orbit candidates to altitude changes. The values are estimated with a linear fit at the scenarios’ altitudes for the represented range of altitudes (\(\pm 2\) km). The values between brackets are associated with subcycles with \(2<h_{l}<3\) (less homogeneous subcycles). instrument sensitivity (if orbiting too high).