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CubeSats are currently gaining significant traction in Earth Observation, with increasingly advanced instrumentation such as hyperspectral imaging. However, the challenges of bringing such instrumentation to CubeSats are great; the platform suffers from severe physical, operational and budgetary constraints. Adopting a holistic design methodology may hold the key to allowing science-grade Earth Observation to be achieved from a CubeSat. Presented here is CHAFF (CubeSat Hyperspectral Application For Farming), a low-cost hyperspectral imager prototype, capable of taking 1024 spectral bands between 460 nm - 820 nm. CHAFF has been constructed using commercial off-the-shelf optics, in order to produce a design commensurate with the typical resources of a university CubeSat mission. CHAFF has been calibrated at the National Physical Laboratory, in order to assess the performance of the COTS optics. An impressive spectral resolution of 3.46 nm at 546 nm has been achieved, and 74.95% of CHAFF's pixels exhibit a linearity deviation of < 2%.

We present the results from Verification of Reference Irradiance and Radiance Sources Laboratory Calibration Experiment Campaign. Ten international laboratories took part in the measurements. The spectral irradiance comparison included the measurements of the 1000 W tungsten halogen filament lamps in the spectral range of 350 nm&ndash, 900 nm in the pilot laboratory. The radiance comparison took a form of round robin where each participant in turn received two transfer radiometers and did the radiance calibration in their own laboratory. The transfer radiometers have seven spectral bands covering the wavelength range from 400 nm&ndash, 700 nm. The irradiance comparison results showed an agreement between all lamps within &plusmn, 1.5%. The radiance comparison results presented higher than expected discrepancies at the level of &plusmn, 4%. Additional investigation to determine the causes for these discrepancies identified them as a combination of the size-of-source effect and instrument effective field of view that affected some of the results.

We study and analyse performance of a system for direct reflectance measurements from a drone. Key instruments of the system are upwards looking irradiance sensor and downwards looking imaging spectrometer. Requirement for both instruments is that they are radiometrically calibrated, the irradiance sensor has to be horizontally stabilized, and the sensors needs to be accurately synchronized. In our system, irradiance measurements are done with FGI Aerial Image Reference System (FGI AIRS), which uses novel optical levelling methodology and can compensate sensor tilting up to 15°. We performed SI-traceable spectral and radiance calibration of FPI hyperspectral camera at the National Physical Laboratory NPL (Teddington, UK). After the calibration, the radiance accuracy of different channels was between ±4 % when evaluated with independent test data. Sensors response to radiance proved to be highly linear and was on average 0.9994 for all channels. The spectral response calibration showed side peaks on several channels that were due to the multiple orders of interference of the FPI and highlighted the importance of accurate calibration. The drone-based direct reflectance measurement system showed promising results with imagery collected over Jokioinen agricultural grass test site, Finland. AIRS-based image- and band wise image adjustment provided homogenous and seamless image mosaics even under varying illumination conditions and under clouds.

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellites in the visible/near-IR has been set up in Gobabeb, Namibia in July 2017. The station location has been decided after a global analysis of satellite data assessing surface spatial homogeneity, cloud coverage, temporal variability, atmospheric turbidity etc. which has been compared with insitu data (surface reflectance, BRDF etc.) from a field campaign which took place in late 2015. The instrumentation automatically measures atmospheric (aerosol optical thickness etc.) and surface conditions (BRDF) from 414 nm to 1600 nm in 12 narrow spectral bands. The data processing is performed at CNES and includes the calibration of the photometer itself using in-situ measurements, and the simulation of the top-of-atmosphere radiance seen by any optical sensor over-passing the site, thus calibrating the sensor. This paper describes the instrument location, measurement protocol, calibration principle and calibration results of satellites Sentinel2A and Sentine12B.

Earth surfaces, such as deserts, salt lakes, and playas, have been widely used in the vicarious radiometric calibration of optical earth observation satellites. In 2009, the Infrared and Visible Optical Sensors (IVOS) sub-group of the Committee of Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) designated eight LANDNET reference sites to focus international efforts, facilitate traceability and enable the establishment of measurement "best practices." With support from the European Space Agency (ESA), one of the LANDNET sites, the Tuz Gölü salt lake located in central Turkey, was selected to host a cross-comparison of measurement instrumentation and methodologies conducted by 11 different ground teams across the globe. This paper provides an overview of the preliminary results of the cross-comparison of the ground-based spectral measurements made during the CEOS Land Comparison 13-27 August, 2010 with the simultaneous satellite image data acquisitions of the same site.

Drone-based remote sensing has evolved rapidly in recent years. Miniaturized hyperspectral imaging sensors are becoming more common as they provide more abundant information of the object compared to traditional cameras. Reflectance is a physically defined object property and therefore often preferred output of the remote sensing data capture to be used in the further processes. Absolute calibration of the sensor provides a possibility for physical modelling of the imaging process and enables efficient procedures for reflectance correction. Our objective is to develop a method for direct reflectance measurements for drone-based remote sensing. It is based on an imaging spectrometer and irradiance spectrometer. This approach is highly attractive for many practical applications as it does not require in situ reflectance panels for converting the sensor radiance to ground reflectance factors. We performed SI-traceable spectral and radiance calibration of a tuneable Fabry-Pérot Interferometer -based (FPI) hyperspectral camera at the National Physical Laboratory NPL (Teddington, UK). The camera represents novel technology by collecting 2D format hyperspectral image cubes using time sequential spectral scanning principle. The radiance accuracy of different channels varied between ±4% when evaluated using independent test data, and linearity of the camera response was on average 0.9994. The spectral response calibration showed side peaks on several channels that were due to the multiple orders of interference of the FPI. The drone-based direct reflectance measurement system showed promising results with imagery collected over Wytham Forest (Oxford, UK).

The Radiometric Calibration Network (RadCalNet, www.radcalnet.org) routinely provides top-of-atmosphere (TOA) reflectance data from instrumented ground sites. The data represents the nadir view of the ground for different sites that cover areas ranging from 50 m × 50 m to 1 km x 1 km. The smaller sites can only be used with high resolution sensors (≤ 30 m), but the larger sites, such as Railroad Valley (RRV) in Nevada can also be used for the validation or vicarious calibration of medium resolution sensors (> 250 m spatial resolution). Prior to utilising RadCalNet data in this manner, this paper describes the application of a high and a medium resolution sensor to assess potential biases between the RadCalNet data and satellite data at two different spatial resolutions. Results are shown for initial comparisons over RRV for the high resolution Sentinel-2 MultiSpectral Instrument (S2-MSI) and the medium resolution Sentinel-3 Ocean and Land Colour Instrument (S3-OLCI), and indicate the potential for RadCalNet to validate and vicariously calibrate sensors with differing spatial resolutions. The comparison analysis includes taking into account the temporal differences between the Sentinel-2 and Sentinel-3 overpasses and the time of RadCalNet data collection, as well as the spectral response functions (SRF) of the bands for both instruments. The comparison against the RRV site has shown there are significant biases between the RadCalNet data and S2-MSI and S3-OLCI for non-nadir viewing geometries that may be due to directional viewing and illumination effects and the non-Lambertian character of the RadCalNet RRV site.

Optical earth observation (EO) satellite sensors generally suffer from drifts and biases relative to their pre-launch calibration, caused by launch and/or time in the space environment. This places a severe limitation on the fundamental reliability and accuracy that can be assigned to satellite derived information, and is particularly critical for long time base studies for climate change and enabling interoperability and Analysis Ready Data. The proposed TRUTHS (Traceable Radiometry Underpinning Terrestrial and Helio-Studies) mission is explicitly designed to address this issue through re-calibrating itself directly to a primary standard of the international system of units (SI) in-orbit and then through the extension of this SI-traceability to other sensors through in-flight cross-calibration using a selection of Committee on Earth Observation Satellites (CEOS) recommended test sites. Where the characteristics of the sensor under test allows, this will result in a significant improvement in accuracy. This paper describes a set of tools, algorithms and methodologies that have been developed and used in order to estimate the radiometric uncertainty achievable for an indicative target sensor through in-flight cross-calibration using a well-calibrated hyperspectral SI-traceable reference sensor with observational characteristics such as TRUTHS. In this study, Multi-Spectral Imager (MSI) of Sentinel-2 and Landsat-8 Operational Land Imager (OLI) is evaluated as an example, however the analysis is readily translatable to larger-footprint sensors such as Sentinel-3 Ocean and Land Colour Instrument (OLCI) and Visible Infrared Imaging Radiometer Suite (VIIRS). This study considers the criticality of the instrumental and observational characteristics on pixel level reflectance factors, within a defined spatial region of interest (ROI) within the target site. It quantifies the main uncertainty contributors in the spectral, spatial, and temporal domains. The resultant tool will support existing sensor-to-sensor cross-calibration activities carried out under the auspices of CEOS, and is also being used to inform the design specifications for TRUTHS.

This paper describes a method to calibrate photodiode array spectrometers in the spectral radiance mode using a fixed-point blackbody as a reference source. Fixed-point blackbodies are characterized by their excellent emissivity, uniformity, and stability, which make them superior to both conventional standard lamps and variable temperature blackbodies. The temperature of these fixed points is accurately determined being traceable to either the International Temperature Scale (ITS-90) or thermodynamically through radiometric standards. The potential advantage of the fixed-point traceability chain is that it can be universally reproduced without recourse to any hierarchical calibrations or standards. The paper presents the calibration system and discusses the limitations of such an approach. The method used obtained an uncertainty of around 1.4 % ( $$k = 2$$ ) associated with radiance responsivity across the spectral region from 550 nm to 1050 nm, which is comparable to what is readily achieved with a lamp-tile or lamp-illuminated spherical source.

Vicarious calibration approaches using in situ measurements saw first use in the early 1980s and have since improved to keep pace with the evolution of the radiometric requirements of the sensors that are being calibrated. The advantage of in situ measurements for vicarious calibration is that they can be carried out with traceable and quantifiable accuracy, making them ideal for interconsistency studies of on-orbit sensors. The recent development of automated sites to collect the in situ data has led to an increase in the available number of datasets for sensor calibration. The current work describes the Radiometric Calibration Network (RadCalNet) that is an effort to provide automated surface and atmosphere in situ data as part of a network including multiple sites for the purpose of optical imager radiometric calibration in the visible to shortwave infrared spectral range. The key goals of RadCalNet are to standardize protocols for collecting data, process to top-of-atmosphere reflectance, and provide uncertainty budgets for automated sites traceable to the international system of units. RadCalNet is the result of efforts by the RadCalNet Working Group under the umbrella of the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) and the Infrared Visible Optical Sensors (IVOS). Four radiometric calibration instrumented sites located in the USA, France, China, and Namibia are presented here that were used as initial sites for prototyping and demonstrating RadCalNet. All four sites rely on collection of data for assessing the surface reflectance as well as atmospheric data over that site. The data are converted to top-of-atmosphere reflectance within RadCalNet and provided through a web portal to allow users to either radiometrically calibrate or verify the calibration of their sensors of interest. Top-of-atmosphere reflectance data with associated uncertainties are available at 10 nm intervals over the 400 nm to 1000 nm spectral range at 30 min intervals for a nadir-viewing geometry. An example is shown demonstrating how top-of-atmosphere data from RadCalNet can be used to determine the interconsistency between two sensors.

The response of a variety of Earth observation satellite sensors from multiple platforms, as well as different sensors on the same platform, were cross-compared over Dome C in Antarctica. A single unified methodology was employed to remove geometric, temporal, and atmospheric biases between different sensors with a range of spatial resolutions, bandwidths, and overpass times. The result assumes no “correct” baseline value, only relative responses. The resulting responses of the sensors, namely, AATSR, MERIS, and VEGETATION 2, are here presented and, where overlap occurs, compared with the results from other groups. In addition to applying the Quality Assurance for Earth Observation protocols for the quality assurance of environmental monitoring satellites, this paper discusses the intercalibration of several sensors used to establish the technical basis of the ESA GlobAlbedo project.

We report the fourth World Radiometric Reference (WRR)-to-SI comparison. At the National Physical Laboratory we compared three transfer pyrheliometer instruments in power mode with the SI radiometric scale. Compared with the three previous comparisons, we improved the experiment by operating the transfer instruments in vacuum. At the Total solar irradiance Radiometer Facility (TRF) located at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, we repeated the power comparison of one of the transfer instruments. The TRF also allowed the comparison and characterization of this instrument in irradiance mode. Using the WRR comparisons performed in Davos, we find that the WRR is 0.34% higher than the SI scale. Comparing irradiance mode calibrations with power mode calibrations reveals that previous estimates of stray light of PMO6-type radiometers were very low. The instrument calibrated at TRF was integrated in the space experiment PREMOS on the French satellite PICARD and carries the first vacuum irradiance calibration to space.

This paper reviews developments in the science of field spectroscopy, focusing on the last twenty years in particular. During this period field spectroscopy has become established as an important technique for characterising the reflectance of natural surfaces in situ, for supporting the vicarious calibration of airborne and satellite sensors, and for providing a means of scaling-up measurements from small areas (e.g. leaves, rocks) to composite scenes (e.g. vegetation canopies), and ultimately to pixels. This paper describes the physical basis of the subject and evaluates the different methods and instruments which have been employed across a range of studies. The development and use of field goniometers is described, and related to methods for estimating the bidirectional reflectance distribution function (BRDF) from directional reflectance measurements in the field. The paper also considers the practical aspects of field spectroscopy, and identifies a number of factors affecting the useability of field spectroradiometers, including the weight and cost of the instruments, limitations of some commonly used methodologies and practical issues such as the legibility of displays and limited battery life. The prospects for the future of field spectroscopy are considered in relation to the increasingly important contribution that field spectral data will make to EO-based global measurement and monitoring systems, specifically through their assimilation into numerical models. However, for this to be achieved it is essential that the data are of high quality, with stated levels of accuracy and uncertainty, and that common protocols are developed and maintained to ensure the long-term value of field spectroscopic data. The importance of employing a precise terminology for describing the geometric configuration of measurements is highlighted in relation to issues of repeatability and reproducibility. Through such refinements in methodology, field spectroscopy will establish its credentials as a reliable method of environmental measurement, underpinning quantitative Earth observation and its applications in the environmental and Earth sciences

Below the freezing point of silver, radiation thermometers are generally calibrated by implementing the multi-point interpolation method using blackbody measurements at three or more calibration points, rather than the ITS-90 extrapolation technique. The interpolation method eliminates the need to measure the spectral responsivity and provides greater accuracy at the longer wavelengths required below the silver point. This article identifies all the sources of uncertainty associated with the interpolation method, in particular, those related to the reference blackbody temperatures (either variable-temperature or fixed-point blackbodies) and to the measured thermometer signals at these points. Estimates are given of the ‘normal’ and ‘best’ uncertainties currently achievable. A model of the thermometer response is used to propagate all the uncertainties at the reference points and provide a total uncertainty at any temperature within the calibration range. The multi-point method has the effect of constraining the total uncertainty over this range, unlike the ITS-90 technique for which the uncertainties propagate as T 2. This article is a joint effort of the working group on radiation thermometry of the Consultative Committee for Thermometry (CCT), summarizing the knowledge and experience of all experts in this field.

A small palm-sized, reference spectrometer, mounted on a remote-controlled model helicopter is being developed and tested by the National Physical Laboratory (NPL) in conjunction with City University, London. The developed system will be used as a key element for field vicarious calibration of optical earth observation systems in the visible-near infrared (VNIR) region. The spectrometer is hand held, low weight, and uses a photodiode array. It has good stray light rejection and wide spectral coverage, allowing simultaneous measurements from 400 to 900 nm. The spectrometer is traceable to NPL’s primary standard cryogenic radiometer via a high-temperature metal-carbon eutectic fixed-point blackbody. Once the fixed-point temperature has been determined (using filter radiometry), the eutectic provides a high emissivity and high stability source of known spectral radiance over the emitted spectral range. All wavelength channels of the spectrometer can be calibrated simultaneously using the eutectic transition without the need for additional instrumentation. The spectrometer itself has been characterized for stray light performance and wavelength accuracy. Its long-term and transportation stability has been proven in an experiment that determined the “World’s Bluest Sky”—a process that involved 56 flights, covering 100,000 km in 72 days. This vicarious calibration methodology using a eutectic standard is presented alongside the preliminary results of an evaluation study of the spectrometer characteristics.

For accurate determinations of thermodynamic temperature, NPL has developed its absolute radiation thermometer (ART), which is calibrated traceably against a cryogenic radiometer. This article reviews some of the potential sources of systematic uncertainty present in the calibration and use of ART. In particular, this article is concerned with the evaluation of the size-of-source effect and the lens transmittance, as well as potential differences in the responsivity of a transfer trap detector when calibrated in terms of radiant power and used in irradiance mode.

We have developed a multi-band ground viewing radiometer based on light-emitting diodes (LEDs) to create a self-calibrating sensing system for land-based measurements. The system is intended for in-situ data collection needed for vicarious calibration, using the reflectance-based method, for earth observing satellites. An autonomous prototype radiometer, which is part of a sensor web of 5 radiometers which could be geographically distributed over hundreds of meters, has been realized. Temporally continuous in-situ measurement of the land reflectance in 4 spectral bands, from 350 nm to 900 nm, are acquired. The system has the ability to regularly re-calibrate autonomously and in the field, communicating results to a remote home base.

The design of a new detector-stabilized spectral irradiance source is presented, together with an evaluation of its performance. This new source consists of a tungsten-halogen lamp that is current-controlled using feedback from an array of filtered photodiodes. The bandpass and central wavelength of these filtered detectors may be selected specifically to meet the required application of the source. The lamp is fully enclosed (except for the exit aperture) in an air-cooled, light-tight housing that minimizes unwanted light pollution. The lamp current is stabilized using feedback from one photodiode; the recording of the non-stabilization channels allows a real-time check on the effective colour temperature of the lamp, and provides an instant diagnosis of the state of calibration of the lamp. The calibration data for each lamp (in its associated housing) are stored on a microchip within the lamp housing, which allows the lamp to be transported independently of its associated control electronics for calibration purposes. The performance of the lamp indicates ageing rates of better than 2 × 10−5 h−1, across the ultraviolet-A (UV-A) and visible spectral regions.

Improving the accuracy of the measurement of ultraviolet (UV) radiation is one of the most challenging problems facing radiometrists today. The difficulties arise in part from the stability and performance of the optical components required to make the measurements, such as sources, detectors, filters and diffusers, as well as from the relative intensity of the UV radiation compared with the visible background. A particular application of note is the need to measure changes in the level of terrestrial solar UVB radiation. This is required to assess the effects of ozone depletion and the potentially damaging impact on the biological ecosystem. To have a meaningful global measurement programme, uncertainties of around 1 % to 2 % are required. However, this is less than or close to the quoted uncertainties of primary UV spectral irradiance scales maintained by national standards laboratories. This paper describes work being undertaken at the National Physical Laboratory (NPL) to improve the measurement of UV radiation, in terms of establishing both primary spectral emission scales and improved transfer standards. Most of this work is centred around improving the design and characterization of filter radiometers. The paper presents details on the performance of the components used to construct the filter radiometers, as well as that of the complete devices. Calibration of the complete filter radiometers has been carried out using radiation from both monochromators and tunable lasers, and a comparison of the results obtained by the two methods is presented.

Near-ambient-temperature black-body sources are routinely used for calibration in terms of radiance of a variety of infrared instruments such as those used in remote sensing and thermal imaging. The black-body radiance is usually determined by reference to a measured temperature and a calculated effective emissivity. The temperature is measured with one or more contact thermometers positioned close to the emitting black-body surface. In this case traceability to the International System of Units (SI) is to the kelvin through the ITS-90. This paper describes an alternative, more direct method based on the use of absolutely calibrated filter radiometers. These filter radiometers form part of a new facility called AMBER (Absolute Measurements of Black-body Emitted Radiance) which has been designed to determine the radiance of an ambient-temperature black body with an uncertainty of about 0.1% (which corresponds to a radiance temperature difference of 25 mK at 4 mum) and a resolution of 0.001% (0.3 mK). The facility obtains its traceability to the SI directly through radiometric standards in the form of a cryogenic radiometer rather than through the ITS-90.

Pre-flight radiometric calibration is critical to ensure that sensors can demonstrate that they meet their design performance specification. However, following launch it is critical that this performance is verified and in particular that the radiometric accuracy is validated or if necessary corrected. As Earth observation data becomes widely used it becomes increasingly important that data from each satellite sensor can be relied upon in the short and longer term and any potential biases between sensors can be removed. This post-launch calibration/validation is thus recognized as a key activity by all satellite operators. There are various methods available for carrying out this post-launch vicarious calibration but one of the most common and generic approaches is to use a dedicated and characterised “test site”. In such a method, ground based measurements of surface reflectance/radiance using similar solar illumination angles and sensor view angles (or at least corrected for these) is propagated to the top of the atmosphere (TOA) using a radiative transfer code. For the highest accuracy, surface measurements should be made within a few minutes of the satellite overpass and the characteristics of the atmosphere at that time also measured, particularly its optical depth.

In this paper, the response of 7 Earth observation satellite sensors from multiple platforms, as well as different sensors on the same platform, are cross-compared over the Dome C plateau, Antarctica, during the austral summer of 2008–2009. A single, unified methodology is employed to remove geometric, temporal and atmospheric biases between different sensors with a range of spatial resolutions, bandwidths and overpass times. The result assumes no “correct” baseline value, only relative responses. The method is derived from an in-depth sensitivity analysis, presented elsewhere [1], [2], that takes into account: BRDF effect, cloud cover, macro-scale hoar-frost change and variations in atmospheric pressure, temperature and composition, including water vapour, ozone and aerosol content variation with altitude. The resulting responses of the sensors: AATSR, AVNIR-2, ETM+ (Landsat 7), MERIS, NigeriaSat-1, VEGETATION 2 and UK-DMC-1 are presented, and where overlap occurs, compared with the results from other groups [3], [4], [5]. In addition to introducing the QA4EO protocols for the quality assurance of environmental monitoring satellites, this paper also discusses several sensors used to establish the technical basis of the ESA GlobAlbedo project: an open access, validated, 15 year long record of broadband land surface albedo (both snow and snow-free) from European sensors alone [6].

The Tuz Golu site does demonstrate some challenges for being used as an absolute calibration site. These include: spatial and temporal variability on the order of +/− 0.02, atmospheric transmittance variability on the order of between 0.52 and 0.86 at 380 nm, lower spectral reflectance levels of less than 0.10 in the longer wavelengths, and limitations on the site availability (only two month period in summer). But, with each successful campaign made at this location, these challenges are being overcome and site characteristics have become better understood and techniques for compensation are being devised. With proper techniques, instrumentation, and coincident data collection this site remains a viable candidate as a vicarious calibration site for optical imaging sensors.

This paper presents the results of the laboratory and field based reflectance panel and radiometer comparisons that took place as part of the CEOS 2010 Key comparison of “techniques and instruments used for the vicarious calibration of Land surface imaging through a ground reference standard test site”. The results of the comparisons are presented which shows that the different ways in which reflectance panels are calibrated can give different results for the reflectance attributed to a test site and that changing illumination and environmental conditions can effect the measured target reflectance.

This paper summarizes lessons learned from the 2009 and 2010 joint field campaigns to Tuz Golu, Turkey. Emphasis is placed on the 2010 campaign related to understanding the equipment and measurement protocols, processing schemes, and traceability to SI quantities. Participants in both 2009 and 2010 used an array of measurement approaches to determine surface reflectance. One lesson learned is that even with all of the differences in collection between groups, the differences in reflectance are currently dominated by instrumental artifacts including knowledge of the white reference. Processing methodology plays a limited role once the bi-directional reflectance of the white reference is used rather than a hemispheric-directional value. The lack of a basic set of measurement protocols, or best practices, limits a group's ability to ensure SI traceability and the development of proper error budgets. Finally, rigorous attention to sampling methodology and its impact on instrument behavior is needed. The results of the 2009 and 2010 joint campaigns clearly demonstrate both the need and utility of such campaigns and such comparisons must continue in the future to ensure a coherent set of data that can span multiple sensor types and multiple decades.

Temperature and nonlinearity effects are two important factors that limit the use of photodiode array spectrometers. Usually the spectrometer is calibrated at a known temperature against a reference source of a particular spectral radiance, and then it is used at different temperatures to measure sources of different spectral radiances. These factors are expected to be problematic for nontemperature-stabilized instruments used for in-the-field experiments, where the radiant power of the site changes continuously with the sun tilt. This paper describes the effect of ambient temperature on a nontemperature-stabilized linear photodiode array spectrometer over the temperature range from 5 °C to 40 °C. The nonlinearity effects on both signal amplification and different levels of radiant power have also been studied and are presented in this paper.

International audience; PREMOS is a space experiment scheduled to fly on the French solar mission PICARD. The experiment comprises filter radiometers and absolute radiometers to measure the spectral and total solar irradiance. The aim of PREMOS is 1. to contribute to the long term monitoring of the total solar irradiance, 2. to use irradiance observations for 'nowcasting' the state of the terrestrial middle atmosphere and 3. to provide long term sensitivity calibration for the solar imaging instrument SODISM on PICARD. In this paper we describe the calibration of the instruments. The filter radiometer channels in the visible and near IR were characterized at PMOD/WRC and the UV channels were calibrated at PTB Berlin. The absolute radiometers were compared with the World Radiometric Reference at PMOD/WRC and a power calibration relative to a primary cryogenic radiometer standard was performed in vacuum and air at NPL.

Satellite sensors can measure the solar energy reflected from the Earth's atmosphere and surface, and provide the only feasible method of obtaining global scale data on the state of the planet. The accuracy and reliability of data from satellite sensors is therefore of vital importance, and much effort goes into characterizing such instruments before launch. However, the stress of launch, and the extreme environment of space is known to affect the calibration of sensors, introducing uncertainty into the data they provide, especially the long time-series data necessary to investigate and monitor climate change. It is therefore vitally important that methods for the post-launch calibration and validation of data from satellite sensors are developed and tested. In order to do this it is necessary to have highly accurate data on the angular reflectance of natural surfaces, and this project aims to develop and test a new instrument to achieve this. To improve the accuracy of field measurements, a new instrument is being designed, in conjunction with the Natural Environmental Research Council Field Spectroscopy Facility (NERC FSF). The Gonio RAdiometric Spectrometer System (GRASS) is being developed at the National Physical Laboratory (NPL), in its Optical Radiation Measurement Team. It is intended to provide quasi-simultaneous, multi-angle, multi- spectral measurements of Earth surface reflected sunlight to support vicarious calibration of satellite sensors operating in the optical region and the validation of their products. The instrument is required to be easily and quickly assembled in remote situations, be robust and transportable to remote locations and its measurements fully traceable to SI units. The GRASS instrument is currently being assembled at the NPL and will be deployed in summer 2006 as part of an experiment to validate data from a range of airborne and satellite sensors. The paper will review existing methods of measuring BRDF in the field, highlight the improvements that GRASS will provide to users, and describe the experiments performed to test its performance.

The Optical Radiation Measurement (ORM) group of the National Physical Laboratory is responsible for the development of the UK's primary radiometric quantities. One of its principle aims in recent years has been to improve the accuracy of "user" measurements through improved traceability routes to the SI system of units. The Earth Observation (EO) community is arguably the most demanding, requiring specialist techniques and transfer standards. To improve accuracy, NPL has developed a range of lamp illuminated integrating spheres, called Transfer Standard Absolute Radiance Source (TSARS). The TSARS is calibrated directly against the NPL primary blackbody source with uncertainties

The demand for improved radiometric accuracy of the remote sensing instrumentation used for diagnostic applications involving hot gas emission spectroscopy requires regular “in-field” recalibration. The most convenient calibration source for such applications is a large emitting area blackbody capable of operating at temperatures approaching 1000 K which is also compact and portable. NPL in collaboration with the University of Reading have designed and assembled a large emitting area blackbody that meets these requirements. The blackbody design is based on a grooved base that is electrically heated to temperatures up to 1000 K. The base is coated with a high emissivity coating, which does not deteriorate during prolonged heating under atmospheric conditions. This base is enclosed by a specularly reflecting cavity that is water-cooled. Monte Carlo calculations were used to design the shapes of the base and reflective cavity to ensure that despite a cavity depth of 203 mm and a black body aperture diameter of 102 mm, the spectral radiance of the blackbody is known with a 1% uncertainty in the 2.5 μm to 14 μm wavelength range. The presentation will describe the design of the blackbody and the processes used for selecting the black coating of the base and the reflective coating of the specularly reflecting cavity.

The technological innovations embodied in the instrument described in this paper can significantly improve the performance of black bodies and their associated thermometry and electronics as used for on-board calibration in many space-borne infrared radiometer instruments, whilst simultaneously reducing mass and power requirements. The same innovations can provide similar advantages for terrestrial based calibrations for both pre-flight and validation activities.

In its role as the national standards laboratory for the UK, the National Physical Laboratory (NPL) maintains, develops and disseminates, amongst others, the UK's detector spectral responsivity scale and material spectrometric scales (regular, hemispherical and angular reflectance and transmittance). In order to carry this work out detectors, materials, methods and facilities are continually under development at NPL. This paper will present the latest measurement techniques used at NPL that are applicable for the characterisation of infrared detectors and materials. NPL has extensive calibration capabilities, making use of grating and FT spectrometers and tuneable lasers, covering a wide spectral range, catering for single element, array, sub-pixel resolution and photon counting devices. As well spectral responsivity, detector spatial uniformity and linearity measurements are available. The UK spectrometric scales are maintained from 200 nm to 56 μm and include regular, hemispherical and angular reflectance and transmittance scales, and artefacts for the wavenumber and ordinate calibration of mid-infrared spectrometers.

NPL, in conjunction with many NMIs, has been seeking to improve the accuracy of its primary scales of spectral irradiance and radiance. In common with other laboratories this has been done through the use of an ultra-high temperature blackbody characterized using filter radiometers calibrated against a cryogenic raediometer. While such work is of importance to the Earth Observation community, it is also recognized that of at least equal importance is an improvement in the quality of the scales that are provided to the end user. This paper will describe new transfer standard sources of both spectral radiance and iradiance that have been developed not only to improve accuracy to the end user, but also to provide it in a form that is both robust and convenient to use. For example the radiance source has a spatial non-uniformity of

This paper describes the properties of two transfer standard sources developed to improve the accuracy of dissemination of spectral irradiance and radiance. The sources both make use of a novel concept, detector stabilization and monitoring, which allow the sources to provide high accuracy calibrations over a period of time more than four times that of conventional sources. Such sources have applications in many technology sectors but they are of particular relevance to the Earth Observation community as they offer high reliability in different environmental conditions and are less susceptible to change on transportation. The radiance source has the additional property of providing an extremely spatially uniform output

A knowledge of the absolute efficiency of the photomultiplier tubes (PMTs) used in the Sudbury Neutrino Observatory (SNO) is important for modelling the behaviour of the detector. While the quantum efficiencies for the PMTs are known, the efficiency in single-photon counting mode (which includes collection efficiency and counting efficiency) is of more relevance. A calibration of the 8 in. Hamamatsu R1408 photomultipliers in this mode requires a known flux of incident light at the very low level of 10 −16 W cm −2 (250 photons cm−2 s−1). A method of performing such measurements, utilising the extreme linearity obtainable with silicon photodiodes, is presented in this paper. The results of efficiency measurements on four PMTs are given and a comparison is made with previous results obtained using a Cherenkov source of known intensity.

A study has been carried out for the European Space Agency (ESA) on strategies for radiometric and spectral calibration of data produced by optical sensors observing Earth surface from Earth orbit. The study relates specifically to data produced by high-resolution imaging spectrometers and imaging radiometers, such as the PRISM instrument which is currently under development by ESA. The typical instrument specification includes 50m spatial resolution on ground, with 10nm spectral resolution through the visible/near-IR and short-wave IR spectral wavebands, plus imaging in a few selected wavebands in the thermal IR. This paper is limited to discussion of the first step in the radiometric calibration process, in which raw digitized data from the space instrument are converted to accurate values for at- sensor radiances. Relative spectral response information, for each resolved spectral band, must accompany the calibrated radiance data. The calibration process, for at- sensor radiances, requires accurate characterization of the space-instrument response characteristics. Part of the specification for the PRISM instrument is given as an example of performance requirements, and a possible design form for PRISM is briefly described as an introduction to discussion of the relevant response characteristics. Critical pre-flight and in-flight characterization requirements are noted, and methods for in-flight characterization are discussed briefly, including use of on- board hardware and space-views, and vicarious calibration using ground target areas.

The paper describes the establishment of an infrared spectral responsivity scale at NPL with an uncertainty of 1.6% and the development of facilities to characterize the optical properties of infrared detector, arrays and cameras. The paper will illustrate the use of the calibration facilities with examples of uniformity, linearity and spectral responsivity measurements on a range of detectors.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.