About

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.

History

In the frame of a previous ESA-funded project “Lunar spectral irradiance measurement and modelling for absolute calibration of EO optical sensors” the Lunar Irradiance Model of ESA (LIME) was developed based on ground measurements collected over a period spanning more than 2 years from the Pico Teide and Izaña Observatory (Spain) by the National Physical Laboratory of UK (NPL), the University of Valladolid (UVa, Spain), the Vlaamse Instelling voor Technologisch Onderzoek (VITO, Belgium) and the Spanish Meteorological Agency (AEMET, Spain). The project aimed to determine an improved model based on the ROLO model, but with new lunar observations and to compare this improved model with the original ROLO/GIRO model and with satellite observations of the Moon.

At the closure of LIME project, a new Lunar irradiance model (LIME) was developed using new ground-based multispectral observations from a high-altitude location in Tenerife (Teide Peak). The model is based on the original ROLO model and importantly provided a rigorous uncertainty analysis from instrument calibration through to model regression achieving 2 % expanded uncertainty (k=2) in the measured spectral bands. These observations are continuing to increase phase and libration coverage, with new versions of LIME due to be released each year. 

The instrument used for the lunar irradiance measurements is the Cimel 318-TP9, a multispectral “Sun photometer”, modified to cover the necessary dynamic range to observe the Moon as well as the Sun and is therefore known as a “solar-lunar photometer”. A multispectral instrument was chosen because it had an improved dynamic range compared to a hyperspectral instrument. The Cimel 318-TP9 also has polarization capability, which ensures we can measure the change in lunar polarization with phase angle. 

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 phase 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 primary radiometric scale, providing low uncertainty observations. The uncertainty in the resulting model is estimated through Monte Carlo methods and has expanded uncertainty of ~2% (k=2) at the Cimel bands 

This project “Improving the Lunar Irradiance Model of ESA” further developed LIME, acquired a set of hyperspectral Lunar measurements providing an improved spectral interpolation algorithm and uncertainty analysis, and provide users in the lunar calibration community a Git-hosted Python module and graphical user interface (GUI) to perform lunar calibration using LIME called LIME toolbox (TBX). While maintaining the Cimel photometer measurements in total of 450 Langley plots collected and fitted to derive LIME model coefficients.

This project improved LIME spectral interpolation by compiling a library of available spectral measurements of the lunar surface, from both laboratory studies of Moon rocks, and from remote sensing observations by all research groups active in this field. This project aimed to contribute to this library of hyperspectral measurements by acquiring new lunar observations with a well-characterized hyperspectral instrument from the same high-altitude location as the CIMEL observations. 

A new spectral interpolation algorithm was developed using these spectra, to develop an uncertainty quantified spectral model, making LIME a valuable tool for sensor calibration across the range of 400 nm to 2500 nm. The software was developed in a way that as new hyperspectral observations are made available, they can be added to, and further improve, the model. The new implementation of LIME was compared to several satellite sensors and other lunar models. 

The new model also includes improvements to the uncertainty analysis in the night-Langley plots and propagation of these uncertainties through the model regression to the LIME model. The model is also developed into a Git-hosted Python module and GUI, called LIME TBX, to allow users to simulate LIME outputs at user-specified observation location and time, and for specified sensor SRF.

Team

Former members