## 1 Introduction
It is therefore crucial to improve the test precision by one or two orders of magnitude with respect to the state of the art in order to set significant physical constraints on acceptable gravity theories. Furthermore, it is worth stressing that we are referring here to the light deflection phenomenon due to the _local_ curvature of spacetime, namely the curvature generated by the mass-energy of individual physical bodies, rather than the one associated with the cosmological evolution of the Universe [12; 13; 14; 15]. It may be noted that, at this level of accuracy, the effects of the 2 PN order might be significant; in this case, the PPN formulation should be extended to cast the interpretation of measurements in terms of the parametrized post-post-Newtonian (PPPN) framework. Measurements of light deflection from the ground are affected by several shortcomings, such as short eclipse duration, high background flux from the Sun disk and inner solar corona, atmospheric disturbances and the limited number of bright sources in the field, thus limiting the achievable performance independently from near future technological improvements. We proposed a dedicated space experiment concept for high-precision  
Figure 1: Light deflection of stellar photons by the Sun. True star positions (red) appear displaced away (blue) from the Sun since the real curved trajectories (solid lines) are interpreted as the tangential (dotted) straight lines at the observer. determination of the deflection close to the Sun by means of a custom instrument optimized for both coronagraphy (to reject the overwhelming solar flux) and large field astrometry, in the framework of the ESA's calls for medium-class missions M3 ([PERSON] et al., 2016) and M4 ([PERSON] et al., 2016). Coronagraphy is a crucial tool of solar physics since the Sun's outer regions and their phenomena can only be observed by effectively masking the intense flux from the disk (and inner corona). In addition, energetic outbursts (e.g., solar flares) can induce severe damage to our orbital infrastructure ([PERSON] et al., 2016), relying on hundreds of satellites devoted to telecommunications, weather monitoring, Earth observation and so on. The 1859 event ([PERSON] et al., 2016) demonstrates that even the ground-based equipment of the time was liable to severe disruption, thus evidencing the criticality of present-day infrastructure with respect to space weather. PROBA 3 ([PERSON] et al., 2016) is the third mission of the European Space Agency (ESA) Project for On-Board Autonomy, planned for launch in 2024, which is aimed at demonstrating the feasibility of formation flying with the relative geometry of two independent spacecrafts at the sub-mm and arcsec level or better, respectively, on relative position and pointing accuracy over a 150 m distance. It will host the externally occulted solar coronagraph ASPIICS for observation of the solar corona at unprecedented spatial resolution and low altitude from the Sun disk, down to 1.1 \(R_{\sun}\). This will also demonstrate the technology of interest to our purposes. We propose the concept of a space mission devoted to fundamental physics and employing state-of-the-art astronomical techniques, in particular astrometry and coronagraphy, in order to assess the light deflection induced by the Sun's gravitational field. The Gravitational Light-bending Astrometry Dual-satellite Experiment (GLADE) takes advantage of the PROBA3 formation flying technology and the ASPIICS approach in splitting occulter and coronagraph units for astrometric measurements on stellar fields around the Sun, with the goal of high-precision determination of the light deflection predicted by general relativity and competing gravitation theories. This approach significantly mitigates the technical challenges involved in the previously proposed instrument concept ([PERSON] et al., 2016; [PERSON] et al., 2016). In this paper, we briefly outline the mission concept in Section 2, its expected performance in Section 3, and the roadmap toward implementation in Section 4.