Achieving High Precision for In-Orbit Instrument Calibration

Achieving High Precision for In-Orbit Instrument Calibration
by Clarence Oxford
Los Angeles CA (SPX) Jan 10, 2025

One of the most complex challenges in space-based remote sensing is attaining the high level of instrument calibration accuracy required while in orbit. The Moon, as an exoatmospheric calibration source, presents a promising solution. However, its current status as an absolute reference has accuracy limitations of 5-10%, which falls short of the rigorous requirements for Earth science observations.

To address this gap, ARCSTONE proposes a mission concept featuring an advanced orbiting spectrometer aboard a small satellite in low Earth orbit (LEO). This innovative approach aims to deliver lunar spectral reflectance with accuracy sufficient to establish a traceable calibration standard compatible with the International System of Units (SI) for a range of Earth weather and climate sensors, spanning past, current, and future missions.

Instrument calibration in orbit has been a persistent challenge for remote sensing. The Moon's utility as a calibration source has been demonstrated in missions like SeaWiFS, launched in the late 1990s, which was the first spaceborne sensor to use lunar imaging for radiometric performance calibration. However, the limited accuracy of absolute lunar irradiance measurements has often led mission operators to perceive the risks of using lunar maneuvers as outweighing their benefits.

ARCSTONE seeks to revolutionize this landscape. The mission will deploy an orbiting spectrometer that captures precise spectral measurements of lunar reflectance. By leveraging the Total and Spectral Irradiance Sensor (TSIS) observations, which provide accurate Spectral Solar Irradiance (SSI), ARCSTONE aims to achieve SI-traceable and spectral calibration on orbit, enabling enhanced accuracy and reliability.

Essential Observations
To achieve its objectives, ARCSTONE will conduct spectral measurements of both lunar and solar irradiance. The mission targets a spectral range of 350 nm to 2300 nm with a spectral sampling resolution of 4 nm. This rigorous approach is designed to retrieve lunar reflectance with an absolute accuracy of < 1.0% (k = 2). To ensure comprehensive coverage, daily observations will be conducted over a minimum period of three years, encompassing the required libration space.

Mission Benefits and Impacts
The ARCSTONE mission is set to collaborate with the Global Space-based Inter-Calibration System (GSICS) community to establish an international standard for absolute lunar calibration. This standard is critical for advancing Earth climate observations and is estimated to have an economic impact of approximately $12 trillion over the next 40 to 60 years.

ARCSTONE's highly accurate spectral measurements will enhance the performance of numerous Earth observation missions, including past programs like SeaWiFS, current initiatives such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS), and future Decadal Survey missions like Plankton, Aerosol, Cloud, ocean Ecosystem (PACE), Climate Absolute Radiance and Refractivity Earth Observatory (CLARREO), and the Advanced Composition Explorer (ACE).

By providing the data necessary for precise calibration, ARCSTONE will enable improved environmental monitoring and contribute significantly to scientific understanding and economic planning over the coming decades.