GOES-R Series X-Ray Sensor (XRS): 1. Design and Pre-Flight Calibration

The X-Ray Sensor (XRS) has been making full-disk observations of the solar soft X-ray irradiance onboard National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellites since 1975. Critical information about solar activity for space weather operations is provided by XRS measurements, such as the classification of solar flare magnitude based on X-ray irradiance level. The GOES-R series of XRS sensors, with the first in the series launched in November 2016, has a completely different instrument design compared to its predecessors, GOES-1 through GOES-15. To provide continuity, the two GOES-R XRS spectral bands remain unchanged providing the solar X-ray irradiance in the 0.05-0.4 nm and 0.1-0.8 nm bands. The changes for the GOES-R XRS instrument included using Si photodiodes instead of ionization cells to improve performance, using multiple channels per X-ray band to allow for a wider dynamic range, and providing accurate radiometric calibrations using the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility in Gaithersburg, Maryland. In addition to the standard XRS data product of solar irradiances in the two X-ray bands, a new real-time flare location data product is also available from the GOES-R XRS instruments because two channels are quadrant photodiodes for position detection. The design and pre-flight calibration results for this next-generation XRS instrument are presented here in this XRS Paper-1, and in-flight solar X-ray measurements from GOES-16, GOES-17, and GOES-18 are provided in the XRS Paper-2.