## 3 Results
### Photon Limited Performance: Astrometric Noise Level  
The nominal S/N (blue line) and related astrometric uncertainty (black lines), respectively, for one minute (elementary exposure, dotted line), one hour (dashed line) and six hours (full eclipse observation, solid line), as a function of magnitude are shown in Figure 6.  
Each source in the Gaia catalog may be considered a materialization of the celestial reference system; therefore, the set of relative positions \((\alpha_{M},\delta_{M})\), as measured by GLADE observing around the Sun, may be compared with the expected values \((\alpha_{E},\delta_{E})\) associated to the undeflected (catalog) coordinates, the current pointing (nominally centered on the Sun) and the ideal (GR) value \(\gamma=1\). The comparison allows an assessment of the light deflection discrepancy with respect to GR over the field.  
Each star is located with a precision (Equation (2)) basically related to its magnitude and S/N, whereas the overall reference system for differential astrometry [30] is defined by the whole set of field stars. Reference values of S/N (blue dashed line) and location uncertainty for near solar spectral-type stars are shown in Figure 6, respectively, for a one-minute elementary exposure (dotted black line), an intermediate one-hour period (dashed black line) and the whole six-hour observing time allowed by good alignment with the occulter during one orbit (solid black line). Over the selected magnitude range, the curves do not depart significantly from the photon limit, i.e., the effects of background and readout noise are negligible; detector pixel saturation on one-minute exposures occurs for stars brighter than \(G\simeq 11.5\) mag, which will require shorter elementary integration.  
Figure 6: S/N (blue) and astrometric noise level (black) as a function of magnitude. The full orbit (solid line), intermediate (dashed black line) and elementary exposure (dotted line) precision are shown to demonstrate the cumulative measurement contributions.  
The cumulative precision of light deflection increases with the number of measurements throughout the mission's lifetime. The limited duty cycle of observation (6 h in each 19.6 h orbit, \(\sim\)30%) requires a three-year mission lifetime to actually cover the full ecliptic strip (minor orbital adjustments may be required to properly phase the observations). The relative cumulative precision in light deflection, or equivalently, in the PPN \(\gamma\) parameter, is shown in Figure 7, respectively, setting a limiting magnitude of \(G=12\,\)mag (dotted line), \(G=14\,\)mag (dashed line) and \(G=16\,\)mag (solid line). Also, the values achieved after relevant time periods are listed, in \(10^{-7}\) units, in Table 2 for the same magnitude range.  
We remark that the actual improvement in precision provided by a large number (Table 1) of comparably faint stars is marginal in spite of their significant impact on system resources (e.g., memory and telemetry). However, their relevance is to be taken into account with respect to accuracy since they allow better calibration and averaging of individual catalog errors. The mission may be further extended (e.g., to five years) to achieve higher precision and, above all, better accuracy, i.e., reliability of the results.