Heather Filippini | Searcharxiv

arXiv2026

Numerical Model Simulation of the Carruthers GCI Images

The Carruthers Geocorona Observatory, launched in September 2025, is NASA's first mission devoted to investigating the fundamental nature of Earth's exosphere from its distant vantage in halo orbit around the Earth-Sun Lagrange (L1) point. Its primary payload, the GeoCoronal Imager, consists of two coaligned photometric imagers that measure the radiance of ultraviolet emission at 121.6 nm (Lyman-$\alpha$, or Ly-$\alpha$) from exospheric hydrogen atoms simultaneously at wide and narrow fields of view. In order to validate the calibration and hydrogen density retrieval algorithms used in the Carruthers data processing pipeline, we developed a comprehensive numerical simulator to produce realistic images similar to those collected by the actual imagers on orbit. This paper details the algorithms used to simulate the exospheric emissions, background scene components, and instrument measurement model necessary to produce synthetic raw images.

astro-ph.IM↗

arXiv2026

Carruthers Data Processing Pipeline: Photon Background Removal

The Carruthers Geocorona Observatory, launched in September 2025, is NASA's first mission devoted to investigating the fundamental nature of Earth's exosphere from its distant vantage in halo orbit around the Earth-Sun Lagrange 1 (L1) point. Its primary payload, the GeoCoronal Imager, consists of two coaligned photometric imagers that measure ultraviolet Lyman-alpha emission radiance from exospheric hydrogen simultaneously at wide- and narrow- fields of view. These observations will map the exosphere's global spatial structure and observe its temporal variability in response to geomagnetic storms. However, a critical step in that analysis is isolating the in-band exospheric hydrogen Lyman-alpha signal from any other source of photons, including in-band InterPlanetary Hydrogen photon background and out-of-band photon backgrounds, which are emitted from Earth's limb. This paper details the algorithms used to retrieve and remove photon backgrounds from the GCI science images acquired on-orbit. Finally, the science data processing pipeline that transforms instrument-effect corrected images (L1B science data product) into absolutely-calibrated exospheric H measurements in physical units (L1C science data product) is detailed. Evaluation of algorithm performance based on a realistic pre-flight case study using synthetic data demonstrates that these photon background removal algorithms achieve high accuracy, leaving a residual systematic bias in isolated exospheric Lyman-alpha of only 3% under beginning-of-life conditions.

astro-ph.IM↗

arXiv2026

Carruthers Data Processing Pipeline: Radiometric Responsivity Calibration

The Carruthers Geocorona Observatory is NASA's first mission dedicated to investigating the fundamental nature of Earth's exosphere and its dynamic response to space weather. Its primary payload, the GeoCoronal Imager, consists of two co-aligned broadband photometric imagers that support simultaneous, common-volume sensing of ultraviolet emission at 121.6 nm (Lyman-alpha) by exospheric hydrogen atoms. Accurate exospheric parameter retrieval from these images requires accurate knowledge of the instrument's optical responsivity, which enables conversion of measured exospheric signal rates into the scientifically relevant quantity of emission radiance. The Carruthers mission achieves absolute sensitivity calibration through the acquisition of photometric measurements of stars and subsequent inversion of the observed fluxes to retrieve the wavelength-dependent responsivity across the passband for each imaging configuration. The retrieval algorithm performance is enhanced by its incorporation of an objective, algorithm-driven ranking criterion to systematically select target calibration stars from a stellar spectral library. The end-to-end workflow, from the stellar selection criterion to the passband inversion, is validated using synthetically generated stellar measurements. These validation tests establish that our optical responsivity retrieval approach has high recovery fidelity, achieving error rates of <5% at the Lyman-alpha wavelength for all primary science imaging modes.

astro-ph.IM↗

arXiv2026

On-orbit Calibration of the Carruthers GCI: Instrument Effect Correction

The Carruthers Geocorona Observatory -- launched in September 2025 -- is NASA's first mission devoted to investigating the fundamental nature of Earth's exosphere from its distant vantage in halo orbit around the Earth-Sun L1 Lagrange point. Its primary payload, the GeoCoronal Imager, consists of two coaligned photometric imagers that measure ultraviolet Lyman-alpha emission radiance from exospheric hydrogen simultaneously at wide- and narrow- fields of view. The imagers use Micro Channel Plate intensified Complimentary Metal Oxide Semiconductor detectors, which are known to add various artifacts to the final image telemetered from the spacecraft, hereby known as instrument effects. This paper details the algorithms used to retrieve and remove instrument effects from raw telemetry on-orbit, including detector voltage bias, thermal dark current, particle radiation, flat-field, and distortion. Finally, the science data processing pipeline from raw telemetry to instrument-effect corrected images is detailed. Algorithm performance is measured via a synthetic numerical image generator or validated on pre-launch experiments.

astro-ph.IM↗