About

Why Lunar Calibration?

Absolute calibration of Earth Observation sensors is key to ensuring long-term stability and interoperability, which is essential for long-term global climate records and forecasts. Sensors are rigorously characterised pre-flight, however, the harsh environment of space and the rigours of launch cause ground-to-orbit and in-orbit degradation and this means that in-flight calibration is essential to ensure satellite accuracy, stability and interoperability.

The Moon is an important vicarious calibration source. It provides a photometrically stable source, within the range of Earth’s radiometric levels and is free from atmospheric interference. However, to use this ideal calibration source one must model the variation of its disk-integrated irradiance, resulting from changes in Sun-Earth-Moon geometries. The cycle with the longest period is called the Saros cycle and its duration is 223 synodic months, which is 18 years, 11 days and 8 hours. After this cycle, Earth, Moon and Sun return to the same relative geometry. The shortest cycle is the variation in phase angle which takes about 28 days between two full moons.

Previous models of lunar irradiance, most notably the ROLO model (RObotic Lunar Observatory) have proven to be valuable tools for the monitoring of radiometric stability. However, with observed absolute differences of 5 % – 10 % so far it has not been possible to use the moon as a reference for absolute radiometric calibration.

With an uncertainty quantified lunar irradiance model, the Earth observation community would be provided with an invaluable tool to calibrate sensors, on-orbit, free from the constraints inherent in using terrestrial targets.

The LIME Project

As part of a previous ESA-funded project titled “Lunar Spectral Irradiance Measurement and Modelling for Absolute Calibration of EO Optical Sensors”, the Lunar Irradiance Model of ESA (LIME) was developed. This model was based on over two years of ground-based measurements collected from the Pico Teide and Izaña Observatory in Tenerife, Spain. The work was carried out by a consortium of scientists from the National Physical Laboratory (NPL, UK), the University of Valladolid (UVa, Spain), the Flemish Institute for Technological Research (VITO, Belgium), and the Spanish Meteorological Agency (AEMET, Spain).

The project aimed to determine an improved model based on the original ROLO lunar irradiance model by integrating new observational data and performing comparisons with both the ROLO/GIRO model and satellite-based measurements of the Moon.

By the project’s closure, a new lunar irradiance model, the Lunar Irradiance Model of ESA (LIME), had been developed using new ground-based multispectral observations from a high-altitude location at Teide Peak (Tenerife). While building on the ROLO model, this updated model introduced a rigorous uncertainty analysis, tracking uncertainties from instrument calibration through to model regression and achieving an expanded uncertainty of 2% (k=2) across the measured spectral bands. These measurements continue to expand LIME’s phase and libration angle coverage, with updated versions of the model released annually.

The lunar irradiance data were captured using the Cimel CE318-TP9, a multispectral sun photometer adapted to cover the necessary dynamic range to perform lunar and solar observations, hence referred to as a solar-lunar photometer. A multispectral instrument was chosen for its superior dynamic range compared to hyperspectral instruments. The Cimel 318-TP9 also features polarization measurement capability, allowing for characterization of how lunar polarization varies with phase angle, an area with limited prior observational data.

There are currently very few polarisation measurements of the Moon, and the lunar calibration community has shown great interest in the new degree of linear polarisation model that was derived using the Cimel photometer observations in the previous project for ESA. 

The lunar photometer is used to measure extra-terrestrial lunar irradiance using “night-Langley plots”. The Langley Plot method observes the Moon over a couple of hours as it rises or sets and therefore its light travels through varying thicknesses of the atmosphere. The resultant observations can be extrapolated to get the lunar irradiance with no atmosphere for one night. These measurements are made nightly for several years to get observations for different lunar phases and libration angles. The lunar observations are SI traceable through the calibration of the Cimel 318-TP9, which is directly traceable to the SI through the National Physical Laboratory’s primary radiometric scale, providing low-uncertainty observations. The uncertainty in the resulting model is estimated through Monte Carlo methods and has an expanded uncertainty of ~2% (k=2) at the Cimel bands.

Evolving the Model: Hyperspectral LIME and Spectral Interpolation

The follow-up ESA project, “Improving the Lunar Irradiance Model of ESA”, significantly extended LIME’s capabilities by incorporating hyperspectral lunar observations and developing a more advanced spectral interpolation algorithm with full uncertainty analysis.

While continuing to rely on Cimel photometer measurements, a total of 450 Langley plots were collected and fitted to derive LIME model coefficients.

A key aspect of this evolution was the creation of a comprehensive library of available spectral measurements of the lunar surface. This library combines:

• Laboratory studies of Moon rock samples
• Remote sensing observations from a wide range of international lunar missions and research groups
• New ground-based hyperspectral lunar observations with a well-characterized hyperspectral instrument from the same high-altitude location as the original CIMEL observations

Using this spectral library, the team developed a new interpolation method that enables the model to predict uncertainty-quantified lunar spectral irradiance across a continuous range from 400 nm to 2500 nm rather than just at the discrete Cimel bands. This marked a critical step in making LIME suitable for a broader range of EO sensors with varied spectral response functions (SRFs). The software was developed in a way that as new hyperspectral observations are made available, they can be added to the model, improving it further. This new LIME implementation was compared to several satellite sensors and other lunar models.

The updated model also improved the uncertainty treatment in key stages of the modeling process:

• The night-Langley method, used to derive extraterrestrial irradiance from ground measurements.
• The propagation of these uncertainties through the model regression.

The LIME Toolbox (TBX)

The “Improving the Lunar Irradiance Model of ESA” project developed the LIME Toolbox (LIME TBX): an open-source, Git-hosted Python module with a graphical user interface (GUI) designed to bring LIME directly to the EO calibration community.

LIME TBX enables users to:

• Simulate lunar spectral irradiance for any location and observation time
• Convolve outputs with user-defined sensor SRFs
• Access model outputs with quantified uncertainties

Designed with flexibility and extensibility in mind, LIME TBX supports the ongoing development of the LIME model and facilitates its use in on-orbit calibration, cross-sensor intercomparison, and radiometric performance validation of satellite instruments.

The code is available in the GitHub repository: https://github.com/LIME-ESA/lime_tbx

Team

Current Contributors

Former members

The LIME model owes its success to a diverse team of researchers, engineers, and field experts. Field campaigns were conducted at Mount Teide in Tenerife, where the solar-lunar photometer continues its long-term mission.