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

Abstract

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.

Keywords: X‐ray photometer; solar X‐Ray irradiance; solar flares; space weather instrumentation.

Figure 1

A cutout slice of the Geostationary Operational Environmental Satellites XRS instrument showing the low‐Z Al housing (green) on the outside of the optical cavity and the high‐Z W/Cu alloy (orange) that provides the inner walls. The Be filters, photodiodes, detector electronics, field‐of‐view baffles/collimator, and the magnet assembly (gray) are also shown.

Figure 2

The PORD‐required minimum and maximum spectral responsivity (gray) for XRS‐A (left panel) and XRS‐B (right panel) and the GOES‐R XRS responsivity (black), showing the required spectral responsivity for each channel is met.

Figure 3

The Astrophysical Plasma Emission Code estimated solar irradiance at solar cycle maximum (blue) and minimum (red) conditions. The nominal passbands are also shown for XRS‐A and XRS‐B as the gray horizontal bars.

Figure 4

The modeled signals using the Astrophysical Plasma Emission Code Quiet and Active reference solar spectra and also a flat spectrum are accumulated from lower wavelength and from upper wavelength and normalized to the total signal to obtain the 5% edges to derive the ideal passband for 90% of the signal. The green, light‐blue, and red lines are the model signals accumulated from lower wavelength for the Quiet spectrum, Active spectrum, and flat spectrum, respectively. The gold, dark‐blue, and purple lines are the model signals accumulated from upper wavelength for the Quiet spectrum, Active spectrum, and flat spectrum, respectively. Left panel is for XRS‐A, and right panel is for XRS‐B. The thick green bar is the ideal passband for the solar signals.

Figure 5

The GOES‐16 X‐Ray Sensor (XRS) modeled responsivities, as based on Synchrotron Ultraviolet Radiation Facility calibrations, are plotted as a function of wavelength. These spectral responsivities are convolved with the reference spectrum over the XRS bands (Equation 1), and those passband responsitivites are provided in Table 3 and used in XRS data processing.

Figure 6

Field of view (FOV) maps for GOES‐18 XRS‐B1 are in panels A and C, and FOV maps for GOES‐18 XRS‐B2 are in panels B and D. Top panels are maps from prelaunch calibration, and the bottom panels were from an inflight FOV map obtained on 28 November 2023. The bottom FOV maps are over ±15 arcmin (0.25°), and the flight map range of ±15 arcmin is represented as a solid‐black box in the top FOV maps that are over ±45 arcmin (0.75°).

Figure 7

Model‐fit results for all X‐Ray Sensor (XRS) diodes background signal for the GOES‐U (GOES‐19) XRS unit. These results are typical for the other XRS flight models (FM). The Dark diodes are D1 and D2. AXUV‐100 diodes are used for XRS A1 and B1. AXUV‐PS6 quadrant diodes are used for A2 and B2, with labels for its four diodes as X21, X22, X23, and X24 (where X is A or B).

Figure 8

The GOES‐18 XRS optical gain versus temperature for the EXIS A‐side and B‐side electronics. The red and orange lines are linear fits to the gains for the Synchrotron Ultraviolet Radiation Facility optical gain calibrations for the A‐side and B‐side, respectively. The dark diode gains are not shown because their apertures are blocked off so they can not see light. The diode names used in this figure are the same as defined for Figure 7.

Figure 9

The GOES‐17 XRS‐B2 linearity calibration at Synchrotron Ultraviolet Radiation Facility (SURF). (left) Beam current (BC)‐normalized signal (summed over all quadrants) versus SURF BC reveals a flat level independent of SURF BC. Blue diamonds represent individual data points, and red squares show the average values for measurements taken at different BC levels. The rollover at high BC is due to electrometer saturation. (right) Deviations from the mean normalized signal is also very flat, with ±1% dotted lines and ±2% dashed lines shown for reference.

Figure 10

The relative statistical uncertainty, σ _E /E, plotted in the top panel as a function of irradiance (in W m⁻²) for both XRS‐A1 and ‐B1 aboard GOES‐18. Bottom panel shows the statistical uncertainties for XRS‐B1 on GOES‐16, ‐17, and ‐18. These results are for in‐flight solar measurements on 3 May 2022 (day of year 123).

Figure 11

Example of in‐flight X‐Ray Sensor (XRS) signals for GOES‐16 XRS A2 and B2 on day 2017/249 (6 September 2017) when there was a large X10 flare near 12 UT. These XRS signals are first corrected for the background signal, which is a bigger effect during non‐flaring times for A2 than B2. The estimated signals for the Astrophysical Plasma Emission Code solar minimum and maximum reference spectra are the horizontal dashed lines (blue for A2 and red for B2).

Figure 12

GSFC e‐beam facility results with an incident electron beam energy of 300 keV (top panel) and Geant4 model results (bottom panel) for an identical experimental configuration and conditions show similar incidence‐angle dependence for the XRS magnet assembly. The Geant4 model also indicates that electrons dominate the background much more than the electron‐excited photons.