Global-scale observations of the limb and disk mission implementation: 2. Observations, data pipeline, and level 1 data products

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 atmospheric tides and waves. GOLD employs two identical ultraviolet spectrographs that make observations of the Earth's thermosphere and ionosphere from a commercial communications satellite owned and operated by Societe Europeenne des Satellites (SES) and located in geostationary orbit at 47.5 degrees west longitude (near the mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N-2 Lyman-Birge-Hopfield radiances from the entire disk that is observable from geostationary orbit and on the near-equatorial limb. They also observe occultations of stars to measure molecular oxygen column densities on the limb. Here we describe the algorithms and science data processing that convert downlinked data to spectral-spatial image cubes and occultation spectra, calibrated in geophysical units. These Level 1 data products include disk and near-equatorial limb images of spectra acquired on a 30-min cadence beginning at 06:10 Coordinated Universal Time and ending at 23:10. Nighttime images of the disk, covering regions east of the terminator, begin at 20:10 Coordinated Universal Time and also continue until 00:40 the next day. During the day, some limb images are replaced by time series of stellar spectra that exhibit absorption by molecular oxygen as the star is occulted by the Earth's atmosphere. Instrumental artifacts that occasionally appear in the released data are discussed.