## 4 Discussion  
The process of defining and evaluating the performance of the future innovative gravity mission uses the Scientific Readiness Levels (SLR) as a guideline. The SLRs extend from SRL1, the consolidation of the Initial Scientific Idea through SRL5, the End to End Performance Simulations, to reach SRL 9, the Scientific Impact Quantification [31]. We collocate the work we have performed in the frame of the MOCAST+ project in relation to SRL4 (Proof of concept and Measurement concept validation). Our work is based on the Initial Scientific Idea (SRL1) that the determination of the gravity field with respect to GRACE and GOCE can be improved in order to observe a number of geophysical phenomena that are presently below the sensitivity of the actual missions. The hypothesis is that a larger impact can be achieved by observing a larger percentage of the overall existing phenomena and a at higher completeness level. In the example of hydrologic basins, it has been shown that the drawdown of large-scale hydrologic basins is observed by GRACE/FO, but it presents only a limited number of applications and coverage, and a small percentage of the total hydrologic volume. Therefore, a more complete identification including smaller basins is needed and is an observational innovation, since deep hydrologic reservoirs may not be accessible by terrestrial observations.  
Within the MOCAST+ study we assessed the value of this new mission concept by monitoring a wide range of geophysical phenomena that cause temporal variations of the mass on the Earth surface and underground. The whole study comprised the construction of a database of realistic mass models including the earthquake processes [11], the surface and subsurface hydrology, and mass changes related to glaciers. Herein, we discussed the lakes, hydrology, and glaciers in South America and the lakes in the Tibetan plateau. The database comprises phenomena at different spatial and temporal scales, including localized mass variations as deglaciation in Patagonia as well as widespread spatially continuous mass changes such as those due to the subsurface soil moisture over the entire Amazon basin. Seasonal and multi-year long-period trends were also considered.  
The database also includes the forward modelled gravity field; thus, we were able to create a link between mass variations involved in the process, their spatial extension, the temporal duration of the phenomenon, and the simulated gravity anomalies.  
With this database, we then quantitatively assessed the impact of the mission by firstly defining the detectability threshold of each phenomenon; this was performed by comparing the noise error curves of the satellite missions with the localized spectra of the geophysical phenomena.  
On the basis of this comparison, we then defined the improvement of the new mission with respect to the present state of the art, represented by GRACE and GRACE-FO observations. In particular, the improvement was defined in terms of:  
* The ability to observe a larger number of phenomena (i.e., smaller glaciers, smaller basins);
* An increase in the spatial resolution for the better characterization of a specific phenomenon;
* An increase in the sensitivity to lower mass changes (i.e., sensitivity to lower deglaciation rates).  
We summarize our main findings in Table 3.  
The comparison of the signal spectra with the satellite error curves has shown that a higher sensitivity regarding the deglaciation rates and height variations in the lakes was gained, both in terms of the ability to resolve smaller glaciers and lakes and the sensitivity to capture smaller mass changes. The localized spectrum depends on the cap radius, with a greater radius lowering the average spectral amplitudes and extending the spectra to lower harmonic degrees. Thus, there is a trade-off between the spectral amplitudes and the spectral interval over which the amplitudes are distributed.