The accurate on orbit radiometric calibration of optical sensors has become a challenge for space agencies which have developed different technics involving on-board calibration systems, ground targets or extra-terrestrial targets. The combination of different approaches and targets is recommended whenever possible and necessary to reach or demonstrate a high accuracy. Among these calibration targets, the moon is widely used through the well-known ROLO (RObotic Lunar Observatory) model developed by USGS. A great and worldwide recognized work was done to characterize the moon albedo which is very stable. However the more and more demanding needs for calibration accuracy have reached the limitations of the model. This paper deals with two mains limitations: the residual error when modelling the phase angle dependency and the absolute accuracy of the model which is no more acceptable for the on orbit calibration of radiometers. Thanks to PLEIADES high resolution satellites agility, a significant data base of moon and stars images was acquired, allowing to show the limitations of ROLO model and to characterize the errors. The phase angle residual dependency is modelled using PLEIADES 1B images acquired for different quasi-complete moon cycles with a phase angle varying by less than 1°. The absolute albedo residual error is modelled using PLEIADES 1A images taken over stars and the moon. The accurate knowledge of the stars spectral irradiance is transferred to the moon spectral albedo using the satellite as a transfer radiometer. This paper describes the data set used, the ROLO model residual errors and their modelling, the quality of the proposed correction and show some calibration results using this improved model.

Improving ROLO lunar albedo model using PLEIADES-HR satellites extra-terrestrial observations

The accurate on-orbit radiometric calibration of optical sensors has become a challenge for space agencies who combine their efforts through international working groups such as CEOS/WGCV or WMO/GSICS with the objective of ensuring the consistency of space measurements and to reach the absolute accuracy required by increasingly demanding scientific requirements. Different targets are generally used for calibration, depending on the specific features of the sensor or spacecraft: from on-board calibration systems to ground targets, they can all profitably be characterized and modeled. Thanks to their agility, some satellites have the capability to view extra-terrestrial targets such as stars. Although the spectral irradiance of some stars is known with very high accuracy, very little work has been done on their use as an absolute reference for the calibration of remote sensors. The reason is their low irradiance, requiring a small instantaneous field of view to observe them. Moreover, their use as a calibration reference also requires a good knowledge of the instrument’s modulation transfer function (MTF). The MTF which drives the spatial quality is a major parameter for high-resolution (HR) optical remote sensors as it indicates how spatial frequencies are transmitted and weakened by the imaging system. On the other hand, stars provide ideal targets for MTF measurement as they can be assimilated to point sources. This paper proposes a method for simultaneously computing the absolute radiometric calibration of PLEIADES’ HR optical sensors and their MTF, using stars. The radiometric model is solved in Fourier space for point sources whose irradiance can be easily controlled. Results are provided for both PLEIADES satellites and compared to the official radiometric calibration and MTF. The quality of the results and the main error contributors are discussed.

On-Orbit Star-Based Calibration and Modulation Transfer Function Measurements for PLEIADES High-Resolution Optical Sensors

The accurate on-orbit radiometric calibration of optical sensors has become a challenge for both ensuring the consistency of space measurements and reaching the absolute accuracy required by increasingly demanding scientific requirements. Different targets are traditionally used for calibration depending on the sensor or spacecraft specificities: from on-board calibration systems to ground targets, they all take advantage of our capacity to characterize and model them. Thanks to their agility, some satellites have the capability to view extra-terrestrial targets such as the moon or stars taking advantage of the absence of atmosphere. The moon is widely used for calibration and its albedo is known through ROLO (RObotic Lunar Observatory) USGS model but its limited accuracy constrains its use to sensor drift monitoring or cross-calibration. The spectral irradiance of some stars is known with a very high accuracy, providing an absolute reference for remote sensors calibration. But the low irradiance of stars requires an instrument with a small Instantaneous Field Of View to observe them. A good knowledge of the instrument’s Modulation Transfer Function (MTF) is also necessary to perform an accurate radiometric calibration. This paper describes a method based on stars for simultaneously computing the radiometric calibration of PLEIADES 1B’ high resolution optical sensor and its MTF. The radiometric model is solved in Fourier space for point sources whose irradiance is controlled. Results are compared to the official MTF and radiometric calibration based on Pseudo Invariant Calibration Sites (PICS) and the moon. The quality of long time series of measurements is discussed as well as their accuracy.

PLEIADES high resolution optical sensors radiometric and spatial calibration based on stars

&lt;p&gt;This work develops a framework for the development of an SI-traceable model of lunar irradiance over the visible to near-infrared spectral region. Based on revisions to the Robotic Lunar Observatory (ROLO) model of lunar reflectance developed by the United States Geological Survey, it is called the Common Lunar Model-ROLO (CLM-R). Inputs from several different instruments are included in its development: SI-traceable lunar measurements by the airborne-LUnar Spectral Irradiance (air-LUSI) instrument are used for an estimate of the phase correction to ROLO while residual corrections to the ROLO model for lunar phase and libration angles are developed using long-term lunar data sets from Moderate Resolution Imaging Spectrometers (MODIS) on NASA’s Terra and Aqua satellites and Visible Infrared Imaging Radiometers (VIIRS) on NOAA’s Suomi National Polar-orbiting Partnership and NOAA-20 satellites. Additionally, this work examines the potential use of the Moon as an absolute, SI-traceable, celestial calibration target by providing an estimate of the uncertainty in a sensor calibration using the Moon. Based on lunar measurements by MODIS and VIIRS, a combined standard uncertainty less than 1 % is achievable for the calibrations of satellite sensors using the Moon. &lt;/p&gt; 

Development of an uncertainty budget for a disseminated lunar irradiance scale over the visible to near-infrared spectral region

This work develops a framework for the development of an SI-traceable model of lunar irradiance over the visible-to-near-infrared (VNIR) spectral region and provides an estimate of the uncertainties in the disseminated scale. The lunar model, called the common lunar model—robotic lunar observatory (ROLO) (CLM-R), is based on revisions to the ROLO model of lunar reflectance developed by the United States Geological Survey. Inputs from several different instruments are included in its development; SI-traceable lunar measurements by the airborne-lunar spectral irradiance (air-LUSI) instrument are used to establish the absolute scale and give an estimate of the phase correction to ROLO and residual corrections to the ROLO model for lunar libration angles. Uncertainties in a disseminated lunar irradiance scale are developed using long-term lunar datasets from Moderate Resolution Imaging Spectrometers (MODISs) on NASA’s Terra and Aqua satellites. Based on lunar measurements by MODIS and the visible and infrared imaging radiometer (VIIRS) on NOAA’s Suomi National Polar-orbiting Partnership and NOAA-20 satellites, a combined standard uncertainty of less than 1% for lunar-based calibrations of satellite sensors may be achievable.

The accurate on-orbit radiometric calibration of optical sensors has become a challenge for space agencies who gather their effort through international working groups such as CEOS/WGCV or GSICS with the objective to insure the consistency of space measurements and to reach an absolute accuracy compatible with more and more demanding scientific needs. Different targets are traditionally used for calibration depending on the sensor or spacecraft specificities: from on-board calibration systems to ground targets, they all take advantage of our capacity to characterize and model them. But achieving the in-flight stability of a diffuser panel is always a challenge while the calibration over ground targets is often limited by their BDRF characterization and the atmosphere variability. Thanks to their agility, some satellites have the capability to view extra-terrestrial targets such as the moon or stars. The moon is widely used for calibration and its albedo is known through ROLO (RObotic Lunar Observatory) USGS model but with a poor absolute accuracy limiting its use to sensor drift monitoring or cross-calibration. Although the spectral irradiance of some stars is known with a very high accuracy, it was not really shown that they could provide an absolute reference for remote sensors calibration. This paper shows that high resolution optical sensors can be calibrated with a high absolute accuracy using stars. The agile-body PLEIADES 1A satellite is used for this demonstration. The star based calibration principle is described and the results are provided for different stars, each one being acquired several times. These results are compared to the official calibration provided by ground targets and the main error contributors are discussed.

The stars: an absolute radiometric reference for the on-orbit calibration of PLEIADES-HR satellites.

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The stars: an absolute radiometric reference for the on-orbit calibration of PLEIADES-HR satellites.