RESEARCH ARTICLE Global‐Scale Observations of the Limb and Disk 10.1029/2020JA027797 Mission Implementation: 1. Instrument Design Special Section: and Early Flight Performance Early results from the Global‐scale Observations of William E. McClintock1 , Richard W. Eastes1 , Alan C. Hoskins1, Oswald H. W. Siegmund2 , the Limb and Disk (GOLD) 2 3 4 4 mission Jason B. McPhate , Andrey Krywonos , Stanley C. Solomon , and Alan G. Burns 1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA, 2Space Sciences Laboratory, 3 This article is a companion to University of California, Berkeley, CA, USA, Florida Space Institute, University of Central Florida, Orlando, FL, USA, McClintock et al. (2020), https://doi. 4National Center for Atmospheric Research, High Altitude Observatory, Boulder, CO, USA org/10.1029/2020JA027809. The Global‐scale Observations of the Limb and Disk (GOLD) is a National Aeronautics and Key Points: Abstract • GOLD makes thermospheric images Space Administration mission of opportunity designed to study how the Earth's ionosphere‐thermosphere of OI and N2 LBH emissions, system responds to geomagnetic storms, solar radiation, and upward propagating atmospheric tides and ionospheric images of OI emission waves. GOLD employs an instrument with two identical ultraviolet spectrographs that make observations of and observes O2 absorption on the limb the Earth's thermosphere and ionosphere from a commercial communications satellite owned and • An overview of the instrument operated by Société Européenne des Satellites (SES) and located in geostationary orbit at 47.5° west design and performance based on longitude (near the mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N laboratory characterization is 2 provided Lyman‐Birge‐Hopfield (LBH) 137–162 nm radiances of the entire disk that is observable from geostationary • Imaging and spectroscopic orbit and on the near‐equatorial limb. They also observe occultations of stars to measure molecular oxygen fi performance con rm laboratory column densities on the limb. Here, we provide an overview of the instrument and compare its prelaunch results. Radiometric sensitivity using stars is ~20% less than ground and early flight measurement performance. Direct comparison of LBH spectra of an electron lamp taken measurement before launch with spectra on orbit provides evidence that both cascade and direct excitation are important sources of thermospheric LBH emission. Plain Language Summary The Global‐scale Observations of the Limb and Disk (GOLD) is a Correspondence to: W. E. McClintock, National Aeronautics and Space Administration mission of opportunity designed to study how the [email protected] Earth's ionosphere‐thermosphere system responds to geomagnetic storms, solar radiation, and upward propagating tides on time scales as short as 30 min. GOLD employs two identical ultraviolet spectrographs Citation: that make observations of the Earth's thermosphere and ionosphere from a commercial communications McClintock, W. E., Eastes, R. W., satellite owned and operated by SES and located in geostationary orbit at 47.5° west longitude (near the Hoskins, A. C., Siegmund, O. H. W., mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N2 LBH radiances of the McPhate, J. B., Krywonos, A., et al. ‐ (2020). Global‐scale observations of the entire disk that is observable from geostationary orbit and on the near equatorial limb. They also observe limb and disk Mission implementation: occultations of stars to measure molecular oxygen column densities on the limb. Here we describe the GOLD 1. Instrument design and early flight instrument including its optical system and detector. Its performance was characterized in the lab before performance. Journal of Geophysical Research: Space Physics, 125, launch. We compare measurements of laboratory sources made then to observations of the thermosphere e2020JA027797. https://doi.org/ after launch and find good agreement. 10.1029/2020JA027797 Received 11 JAN 2020 1. Introduction Accepted 13 APR 2020 ‐ Accepted article online 15 APR 2020 The Global scale Observations of the Limb and Disk (GOLD) is a National Aeronautics and Space Administration mission of opportunity designed to study how the Earth's ionosphere‐thermosphere system responds to geomagnetic storms, solar radiation, and upward propagating tides (Eastes et al., 2017). GOLD employs two identical ultraviolet spectrographs that make images of the Earth's thermosphere and ionosphere from a commercial communications satellite owned and operated by SES and located in geosta- tionary equatorial orbit at 47.5° west longitude (near the mouth of the Amazon River). From this vantage point, the instrument measures atomic oxygen 135.6 nm and N2 LBH emission intensities and performs stel- lar occultation observations. These support the specific measurement requirements of the GOLD Mission: Images on the disk: ©2020. American Geophysical Union. 1. Images of the disk of the Earth, over a latitude range of ±60° and a longitude range of ±70° relative to All Rights Reserved. spacecraft nadir, with a cadence of 30 min MCCLINTOCK ET AL. 1of18 Journal of Geophysical Research: Space Physics 10.1029/2020JA027797 2. Measurements of emission from atomic oxygen (O) at 135.6 nm and from the Lyman‐Birge‐Hopfield (LBH) bands of molecular nitrogen (N2) with a cadence of 30 min and a spatial resolution of 250 × 250 km (at nadir) and with a spectral resolution of 0.2 nm full width half maximum 3. Measurements of the atomic oxygen (O) 135.6 nm emission from the nighttime equatorial arcs (northern and southern) at a 1‐hr cadence and a spatial resolution of 2° in latitude and a longitudinal resolution of 100‐km (at nadir) Profiles on the limb: 1. Measurements of N2 LBH emission up to 350 km above the surface at the equatorial limbs with an alti- tude resolution of 50 km 2. Stellar occultation measurements in the 135–155 nm wavelength range and 140–200 km altitude range. In each channel, a single‐mirror telescope images the Earth onto the 11.2° tall entrance slit of an imaging spectrograph. The spectrograph disperses the light and images it onto a two‐dimensional detector that pro- vides an independent spectrum for every position along the slit resulting in a single one‐dimensional spatial by one‐dimensional spectral image. Only a narrow slice of the image made by the telescope enters the spec- trograph at a given time and the second spatial dimension of a three‐dimensional image cube is built up “scanning” the Earth's image across the slit in small steps from east to west (Figure 5). This is accomplished within the GOLD instrument by two scan mirrors, one located in front of each telescope. The GOLD instrument has two operating states: science and safe. During science, it operates from 03:00 spacecraft local time (SLT) until 21:30 SLT each day. From 03:00 SLT until 20:00 SLT, it acquires complete spatial‐spectral image cubes of the disk and sunlit limb with a 30‐min cadence. Beginning at 17:00 SLT and continuing until 21:30 SLT, it also scans the nightside disk beginning at longitudes approximately 15° east of the terminator. Limb scans and occultations are interleaved with the disk scans to maintain a 30‐min cadence as described by McClintock et al. (2020). In order to protect itself from incursion by the Sun, GOLD enters its safe state at 21:30 SLT and remains there until 03:00 SLT the following day. 2. Instrument Design The GOLD instrument is a hosted payload aboard the SES‐14 spacecraft, which is based on the Eurostar E3000EOR bus manufactured by Airbus Defense and Space (ADS). GOLD was integrated to the spacecraft at the ADS facility in Toulouse France and is operated by SES in Luxemburg through a contract to SES Government Solutions. GOLD was launched aboard Ariane flight VA241 from Ariane Launch Complex No. 3 in Kourou, French Guiana on 25 January 2018. After orbit insertion and spacecraft commissioning, GOLD instrument checkout started in early September. Nominal operations and observing began 9 October 2018. During the day, GOLD measures O atomic (135.6 nm) and N2 LBH (137–162 nm) thermospheric radiances above the disk and on the limb. It also observes stellar occultations to measure O2 densities by absorption on the limb. At night, it measures 135.6 nm radiances emitted by O+ as it recombines in the ionosphere. Data collected by the instrument are packetized and routed to the spacecraft for immediate downlink to an SES ground station in Woodbine Maryland where they are recorded and then transmitted to the GOLD Science Operations Center (SOC) at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado. There they are formatted before transmission to the GOLD Science Data Center (SDC) at the University of Central Florida. The SOC also prepares weekly instrument command loads that are transmitted to the SES satellite operations center for upload to the spacecraft where they are routed to the instrument over a 1553 communications bus. 2.1. Instrument Description The instrument, illustrated in Figure 1 and summarized in Table 1, is mounted to the spacecraft nadir‐facing deck and views the entire ~18° observable disk and limb of the earth (~±80° in longitude and latitude about the subspacecraft point). It consists of two identical, independent telescope‐imaging spectrograph channels, referred to as Channels A and B. These are mounted on either side of the GOLD Electronics Box (GOLD E‐box) that controls the channels and provides the instrument electrical interface to the spacecraft. The channels are equipped with large sunshades and protective covers that were opened after the spacecraft MCCLINTOCK ET AL. 2of18 Journal of Geophysical Research: Space Physics 10.1029/2020JA027797 Figure 1. (a) The GOLD instrument consists of two independent telescope‐spectrograph channels, referred to as A and B, and an electronics box located between the two channels.