NASA Goddard Space Flight Center (GSFC), in collaboration with the Korea Astronomy and Space Science Institute (KASI) plans to develop a coronagraph and to install it on the International Space Station (ISS). The primary goal of CODEX is to Understand the physical conditions in the solar wind acceleration region. Solar wind source and acceleration models are highly under constrained due to current data limitations. CODEX will provide the first global, comprehensive data sets allowing us to provide these crucial constraints on a daily basis, and answer targeted essential questions such as: Are there signatures of hot plasma released into the solar wind from previously closed fields? What are the velocities and temperatures of the density structures that are observed so ubiquitously within streamers and coronal holes? The coronagraph is externally occulted with a field of view from ~3-10 solar radii (R⊙), simultaneously covering all latitudes. The observation wavelength is ~400 nm. As demonstrated at total solar eclipse opportunities, photometric filter ratio observations around this band enable the estimation of the 2D electron temperature and electron velocity distribution in the corona. This will be the first time electron density, temperature, and velocity have been measured simultaneously for this field-of-view, and we will do it globally for 6 months! This regular, systematic, comprehensive dataset will test all theories of solar wind acceleration and source region as well as serve to validate and improve space-weather/operational models in the crucial source region of the solar wind. The coronagraph consists of an optical assembly, an external and internal occulter, a polarization camera, command and data handling electronics and a pointing system (gimbals, sun sensor, gyroscopes, drive electronics). The coronagraph itself will have only three mechanisms: an aperture door, filter-wheel rotator, and a launch latch. The polarization camera utilizes a grid directly in front of the detector that eliminates the need for a polarization wheel mechanism and enables obtaining images in close temporal proximity as required for the filter-ratio technique. This proposal will exploit the expertise at GSFC, Wallops Flight Facility, and KASI. The proposal team has extensive experience in coronagraphy, ISS payloads, and solar wind studies and hence is well suited to successfully complete the proposed work. CODEX directly builds upon the Balloon-borne Investigation of the Temperature and Speed of Electrons in the Corona (BITSE), a NASA scientific balloon technology demonstration mission currently under development by the same proposal team and expected to fly for ~8 hours from Ft. Sumner, NM in September 2019. Although BITSE and ground-based eclipses provide some insight, such investigations are too short in duration and limited by sky brightness to obtain all the data needed to achieve CODEX's science objectives. The remote-sensing data from the CODEX mission will augment the in situ and remote sensing measurements from the Parker Solar Probe and Solar Orbiter. The CODEX mission will also exploit the synergy with the currently operating NASA missions such as SOHO, STEREO, and SDO to enhance the science return by combining coronal imagery in overlapping fields of view. Data returned from CODEX will be deposited in the Solar Data Analysis Center web site, a publicly accessible NASA repository. CODEX is highly relevant to the HTIDeS program. The proposed mission is a complete investigation including payload development, instrument calibration, launch, and data analysis. The proposed project offers well-defined technology and science breakthroughs. CODEX's origins of the solar wind investigation will help achieve Strategic Objective 1.1 of the NASA Strategic Plan 2018: "Understand the Sun, Earth, Solar System, and Universe," in particular, the core context of "Discovering the Secrets of the Universe."