Radiometric Reference Research Articles

Novel use of an adapted ultraviolet double monochromator for measurements of global and direct irradiance, ozone, and aerosol

Abstract. A novel ultraviolet spectrometer has been developed and tested over 10 years at Lauder, New Zealand. The system, UV2, makes alternating measurements of the global and direct UV irradiance and can therefore be used to measure ozone and aerosol optical depth. After an analysis of the stability of UV2, these measurements, along with UV irradiance, are compared to relevant observations made by an additional UV spectrometer (UV4), a Dobson spectrophotometer (no. 072), and two radiometers measuring aerosol optical depth – a Prede sky radiometer and a Middleton Solar radiometer (SP02). UV2 irradiance is shown to be lower than UV4 by between 2.5 % and 3.5 %, with a standard deviation of a similar magnitude. Total column ozone values are shown to agree with Dobson spectrophotometer values with a mean bias of 2.57 Dobson units (DU) and standard deviation of 1.15 DU when using the direct sun measurements. Aerosol optical depth at 400–412 and 500 nm agrees to within 0.015 and is comparable to the difference between the reference radiometers. Further work is needed, particularly in the radiometric calibration at longer wavelengths, in order to determine if this instrument can supersede or enhance measurements made by the Dobson spectrophotometer or the aerosol radiometers.

Identifying the Optimal Radiometric Calibration Method for UAV-Based Multispectral Imaging

The development of UAVs and multispectral cameras has led to remote sensing applications with unprecedented spatial resolution. However, uncertainty remains on the radiometric calibration process for converting raw images to surface reflectance. Several calibration methods exist, but the advantages and disadvantages of each are not well understood. We performed an empirical analysis of five different methods for calibrating a 10-band multispectral camera, the MicaSense RedEdge MX Dual Camera System, by comparing multispectral images with spectrometer measurements taken in the field on the same day. Two datasets were collected, one in clear-sky and one in overcast conditions on the same field. We found that the empirical line method (ELM), using multiple radiometric reference targets imaged at mission altitude performed best in terms of bias and RMSE. However, two user-friendly commercial solutions relying on one single grey reference panel were only slightly less accurate and resulted in sufficiently accurate reflectance maps for most applications, particularly in clear-sky conditions. In overcast conditions, the increase in accuracy of more elaborate methods was higher. Incorporating measurements of an integrated downwelling light sensor (DLS2) did not improve the bias nor RMSE, even in overcast conditions. Ultimately, the choice of the calibration method depends on required accuracy, time constraints and flight conditions. When the more accurate ELM is not possible, commercial, user-friendly solutions like the ones offered by Agisoft Metashape and Pix4D can be good enough.

Activities to Promote the Moon as an Absolute Calibration Reference

The accuracy and consistency of Earth observation (EO) instrument radiometric calibration is a fundamental prerequisite for achieving accurate results and delivering reliable predictions. Frequent calibration and validation (Cal/Val) activities are needed during the instrument’s lifetime, and this procedure is often extended to historical archives. Numerous satellites in orbit and proposed future missions have incorporated lunar observation into their vicarious calibration components over recent years, facilitated by the extreme long-term photometric stability of the Moon. Since the birth of the first lunar calibration reference model, lunar-dependent calibration techniques have developed rapidly, and the application and refinement of the lunar radiometric model have become a welcome research focus in the calibration community. Within the context of the development of lunar observation activities and calibration systems globally, we provide a comprehensive review of the activities and results spawned by treating the Moon as a reference for instrument response and categorize them against the understanding of lunar radiometric reference. In general, this appears to be a process of moving from data to instruments, then back into data, working towards a stated goal. Here we highlight lunar radiometric models developed by different institutions or agencies over the last two decades while reporting on the known limitations of these solutions, with unresolved challenges remaining and multiple lunar observation plans and concepts attempting to address them from various perspectives, presenting a temporal development. We also observe that the methods seeking uncertainty reduction at this stage are rather homogeneous, lacking the combination of approaches or results from lunar surface studies conducted by many spacecraft missions, and joint deep learning methods to extract information. The factors that influence the accuracy of the measurement irradiance may be regulated when practical models arrive. As a central element in lunar calibration, the development of an absolute radiometric datum helps to better understand the Earth system.

Spatial Variability of Albedo and Net Radiation at Local Scale Using UAV Equipped with Radiation Sensors

Energy balance closure is an important feature in studies of ecosystem exchanges of energy and greenhouse gases using the eddy covariance method. Previous analyses show that this is still a problem with imbalances in the order of 0.6–0.7 to full closure (for only a few sites). It has been suggested that mesoscale transport processes that are not captured by the eddy covariance measurements are the main reason behind the closure problem. So far, very little action has been taken to investigate another potential cause of the problem, namely, the role of spatial variation in net radiation at the scale of typical flux footprints. The reason for this knowledge gap is mainly due to the lack of suitable methods to perform such investigations. Here, we show that such measurements can be performed with an unmanned aerial vehicle equipped with radiation sensors. A comparison using a reference radiometer on a fixed mast with a hovering UAV equipped with pyranometers for incoming and outgoing shortwave radiation and an infrared thermometer for surface temperature measurements shows that incoming and outgoing shortwave radiation can be measured with a standard error of 7.4 Wm−2 and 1.8 Wm−2, respectively. An application of the system was made over a five-year-old forest flux site in Sweden. Here, the net longwave radiation was estimated from the measured surface temperature and the calculated incoming longwave radiation. The results show that during the mission around noon on a clear day, distinct ‘hotspots’ existed over the plantation with the albedo varying between 15.5 and 17.9%, the surface temperature varying between 22.2 and 25.5 °C and the net radiation varying between 330 and 380 Wm−2. These variations are large enough to have a significant impact on the energy balance closure problem. Our conclusion is that we now have the tools to investigate the spatial variability of the radiation regime over flux sites and that this should be given more attention in the future.

New Absolute Cavity Pyrgeometer equation by application of Kirchhoff's law and adding a convection term

Abstract. An equation for the Absolute Cavity Pyrgeometer (ACP) is derived from application of Kirchhoff's law and the addition of a convection term to account for the thermopile being open to the environment, unlike a domed radiometer. The equation is then used to investigate four methods to characterise key instrumental parameters using laboratory and field measurements. The first uses solar irradiance to estimate the thermopile responsivity, the second uses a minimisation method that solves for the thermopile responsivity and transmission of the cavity, and the third and fourth revisit the Reda et al. (2012) linear least squares calibration technique. Data were collected between January and November 2020, when the ACP96 and two IRIS radiometers monitoring terrestrial irradiances were available. The results indicate good agreement with IRIS irradiances using the new equation. The analysis also indicates that while the thermopile responsivity, concentrator transmission and emissivity of an ACP can be determined independently, as an open instrument, the impact of the convection term is minor in steady-state conditions but significant when the base of the instrument is being subjected to rapid artificial cooling or heating. Using laboratory characterisation of the transmission and emissivity, together with use of an estimated solar calibration of the thermopile, generated mean differences of less than 1.5 Wm−2 to the two IRIS radiometers. A minimisation method using each IRIS radiometer as the reference also provided similar results, and the derived thermopile responsivity was within 0.3 µV W−1 m2 of the solar-calibration-derived infrared responsivity estimate of 10.5 µV W−1 m2 estimated using a nominal solar calibration and provide irradiances within ±2 % of the terrestrial irradiance measured by the reference pyrgeometers traceable to the International System of Units (SI). The calibration method using linear least squares regression introduced by Reda et al. (2012) that relies on rapid cooling of the ACP base but utilising the new equation was found to produce consistent results but was dependent on the assumed temperature of the air above the thermopile. This study demonstrates the potential of the ACP as another independent reference radiometer for terrestrial irradiance once the magnitude of the convection coefficient and any potential variations in it have been resolved.

Radiometric Traceability for Pyranometer Calibration Based on High Flux LEDs and Reference Detector

This article aims to establish a new system for pyranometer calibration traceable to the radiometric chain. Different spectrum high flux LEDs have been used as tunable irradiance sources. The calibration procedure, uncertainty budget, and traceability chain of the solar irradiance measurements are presented. Additionally, the results were evaluated using a reference pyranometer, calibrated traceable to radiometric traceability and World Radiometric Reference (WRR). The uncertainty values are also estimated to be around 1.19 % (k = 2) for LEDs-based pyranometer-pyranometer calibration (WRR traceability), and about 0.68 % ( k = 2 ) for reference standard radiometer based (Radiometric traceability). The deviation between the reference pyranometer and the pyranometer under test was about 0.8% using LED at 655nm for irradiance of 500 W/m2. The sensitivity results have been evaluated by reference standard pyranometer traceable to WRR using a xenon lamp as a solar source, with a deviation of about 1 %.

Characterization of absolute cavity radiometers for traceability to SI of solar irradiance

Solar-type cavity radiometers are instruments of the highest metrological level for measuring solar direct normal irradiance. To ensure their traceability and performance, they are periodically compared to the World Group of Standards, which realizes the World Radiometric Reference (WRR), in the International Pyrheliometer Comparisons (IPCs). Additionally, they can be characterized in an absolute way, with direct traceability to SI units and with their measurement uncertainty calculated. This paper describes the different techniques and procedures applied for the characterization and calibration of solar cavity radiometers, with the main results obtained to date in the case of an Automatic Hickey–Frieden (AHF) radiometer. Voltmeters, resistors, temperature sensors and the area of the precision apertures have been calibrated, while the effective absorptance, temperature coefficients, optical scattering and non-equivalence factor have been evaluated. The temperature dependence of the electrical current in the cavity heater has also been analysed. The resulting corrections obtained for the AHF by characterization are compatible with the WRR factors obtained by this instrument in the past IPCs. An uncertainty of 0.42% (k = 1) has been obtained, and this paper discusses further improvements that may be able to reduce this figure to the desired expanded uncertainty of U = 0.1% (k = 2).

New traceability chain for spectral irradiance measurement at LNE-Cnam

LNE-Cnam has changed its traceability chain for measuring the spectral irradiance of a light source. This facility allows calibration of spectral irradiance of a standard lamp based on a comparison with a high temperature blackbody (HTBB). The reference spectral irradiance is determined by measuring the temperature of the HTBB with a filter radiometer calibrated against our radiant flux reference. This traceability scheme differs from the one used in our former setup, which was mainly based on the International Temperature Scale, ITS-90. Both principles are used in National Measurement Institutes. Our facility uses well known methods adapted to our best capabilities as well as a particular development in optical arrangement and filter radiometer calibration. Thanks to this new measurement setup, our spectral irradiance reference is now traceable to radiometric reference, i.e. our cryogenic radiometer. We have extended our measurement capability to cover the spectral range from 250 nm to 2500 nm. We have simplified the process by reducing the number of benches (three in one) and the number of operations, and we have designed a compact measuring setup through the use of a rotating integrative sphere. This allows us to reduce by at least a factor two our measurement uncertainties over almost the entire spectral range. With this new measurement facility, France participates in the ongoing CCPR k1.a key comparison. This is a key comparison of the Consultative Committee of Photometry and Radiometry for Spectral irradiance from 250 nm to 2500 nm of tungsten halogen lamps. This communication shows the method used, and its validation.

Research on a Partial Aperture Factor Measurement Method for the AGRI Onboard Calibration Assembly.

A partial aperture onboard calibration method can solve the onboard calibration problems of some large aperture remote sensors, which is of great significance for the development trend of increasingly large apertures in optical remote sensors. In this paper, the solar diffuser reflectance degradation monitor (SDRDM) in the onboard calibration assembly (CA) of the FengYun-4 (FY-4) advanced geostationary radiance imager (AGRI) was used as the reference radiometer. It was designed for measuring the partial aperture factor (PAF) for the AGRI onboard calibration. First, the linear response count variation relationship between the two was established under the same radiance source input. Then, according to the known bidirectional reflection distribution function (BRDF) of the solar diffuser (SD) in the CA, the relative reflectance ratio coefficient between the AGRI observation direction and the SDRDM observation direction was calculated. On this basis, the response count value of the AGRI and the SDRDM was used to realize the high-precision measurement of the PAF of the AGRI B1~B3 bands by simulating the AGRI onboard calibration measurement under the illumination of a solar simulator in the laboratory. According to the determination process of the relevant parameters of the PAF, the measurement uncertainty of the PAF was analyzed; this uncertainty was greater than 2.04% and provided an important reference for the evaluation of the onboard absolute radiometric calibration uncertainty after launch.

Recalibration and Assessment of the SNPP CrIS Instrument: A Successful History of Restoration After Midwave Infrared Band Anomaly

The Suomi National Polar-orbiting Partnership (SNPP) cross-track infrared sounder (CrIS) has provided critical observations for environmental applications for nearly ten years. However, on 26 March 2019, the Joint Polar Satellite System (JPSS) interface data processing segment (IDPS) stopped producing the operational SNPP CrIS sensor data record (SDR) product due to a failure of the midwave infrared (MWIR) band. Following a comprehensive risk assessment, the switch from primary Side-1 to redundant Side-2 electronics was made on 24 June 2019, successfully recovering the full capabilities of the sensor. Comprehensive assessment results demonstrate the high quality of the CrIS SDR product resulting from the sensor recalibration, thus meeting the JPSS Level-1 requirements with margin. The spectral calibration prioritized consistency with the CrIS SDR product prior to the side switch to minimize the impact on users. The results show that the radiometric impact on the CrIS SDR product resulting from the side switch is not significant and is within the calibration radiometric uncertainty. It is demonstrated that after instrument restoration, the SNPP CrIS SDR product recovers the quality needed to be used as radiometric reference for calibration and validation of infrared remote sensing instruments. The recovery of the SNPP CrIS MWIR band is expected to support improvements in numerical weather forecasting by restoring the MWIR band channels sensitive to tropospheric water vapor. This should also help maintain continuity and redundancy of one of the backbone observations of the global observing system.

Calibration of solar radiometers with traceability to the world radiometric reference using an absolute cavity radiometer

This study presents the actual field results from a calibration procedure of reference solar radiometers using an Absolute Cavity Radiometers (ACR), which is directly traceable to the World Radiometric Reference (WRR) scale maintained by the World Radiation Center (WRC). The pyranometer and pyrheliometer traceability is established using Eppley’s AHF-AWX ACR as per the prescribed standards. The Eppley solar radiometers were exposed to solar irradiance for 3.5 years and the Hukseflux solar radiometers were exposed to solar irradiance for 1.5 years. The deviation of the new sensitivity values from the old sensitivity values is reported to be 5.42% (pyranometer, Eppley PSP), −0.21% (pyranometer, Hukseflux SR25), 0.23% (pyrheliometer, Eppley NIP) and 0.4% (pyrheliometer, Hukseflux DR03). The uncertainty values are found to be ±0.04 µV/Wm−2, ±0.05 µV/Wm−2, ±0.03 µV/Wm−2 and ±0.03 µV/Wm−2 for Eppely PSP, Hukseflux SR25, Eppley NIP and Hukseflux DR03 solar radiometers, respectively.

Hyperspectral Empirical Absolute Calibration Model Using Libya 4 Pseudo Invariant Calibration Site

The objective of this paper is to find an empirical hyperspectral absolute calibration model using Libya 4 pseudo invariant calibration site (PICS). The approach involves using the Landsat 8 (L8) Operational Land Imager (OLI) as the reference radiometer and using Earth Observing One (EO-1) Hyperion, with a spectral resolution of 10 nm as a hyperspectral source. This model utilizes data from a region of interest (ROI) in an “optimal region” of 3% temporal, spatial, and spectral stability within the Libya 4 PICS. It uses an improved, simple, empirical, hyperspectral Bidirectional Reflectance Distribution function (BRDF) model accounting for four angles: solar zenith and azimuth, and view zenith and azimuth angles. This model can perform absolute calibration in 1 nm spectral resolution by predicting TOA reflectance in all existing spectral bands of the sensors. The resultant model was validated with image data acquired from satellite sensors such as Landsat 7, Sentinel 2A, and Sentinel 2B, Terra MODIS, Aqua MODIS, from their launch date to 2020. These satellite sensors differ in terms of the width of their spectral bandpass, overpass time, off-nadir viewing capabilities, spatial resolution, and temporal revisit time, etc. The result demonstrates the efficacy of the proposed model has an accuracy of the order of 3% with a precision of about 3% for the nadir viewing sensors (with view zenith angle up to 5°) used in the study. For the off-nadir viewing satellites with view zenith angle up to 20°, it can have an estimated accuracy of 6% and precision of 4%.

The Fengyun-3E/Joint Total Solar Irradiance Absolute Radiometer: Instrument Design, Characterization, and Calibration

The Joint Total Solar Irradiance Monitor (JTSIM) is due to fly onboard the Fengyun-3E spacecraft and aims to measure the Total Solar Irradiance (TSI) in orbit. The instruments on the Fengyun-3E/JTSIM include the Digital Absolute Radiometer (DARA) from the Physikalisch Meteorologisches Observatorium, Davos and World Radiation Center (PMOD/WRC) and the Solar Irradiance Absolute Radiometer (SIAR) from the Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences (CIOMP/CAS). Radiometers from Switzerland and China will monitor the TSI variability on the same pointing system for eight years. The scientific data from JTSIM will support the analysis of potential long-term trends in the Sun’s variability. In this article, we describe the sensor box and the electronics box of JTSIM, the measurement principle, and the operation mode of SIAR. Before launch, we accomplished some primary calibrations of SIAR in the CIOMP laboratory, including the aperture area, cavity absorption, non-equivalence, diffraction, etc. Other parameters will be calibrated on orbit. The combined uncertainty of SIAR for characterization is 231 – 233 ppm depending on the measurement channel. The characterization of SIAR is an International System of Units (SI)-native scale calibration. An end-to-end calibration against the World Radiometric Reference (WRR) standard or the Total Irradiance Radiometer Facility (TRF) is a procedure where SIAR is directly calibrated with the WRR reference radiometers. The WRR factor for SIAR is 0.99939 – 1.00092 and the combined measurement uncertainty is 0.074% – 0.099%, depending on the measurement channel.

Development of the Chinese Space-Based Radiometric Benchmark Mission LIBRA

Climate observations and their applications require measurements with high stability and low uncertainty in order to detect and assess climate variability and trends. The difficulty with space-based observations is that it is generally not possible to trace them to standard calibration references when in orbit. In order to overcome this problem, it has been proposed to deploy space-based radiometric reference systems which intercalibrate measurements from multiple satellite platforms. Such reference systems have been strongly recommended by international expert teams. This paper describes the Chinese Space-based Radiometric Benchmark (CSRB) project which has been under development since 2014. The goal of CSRB is to launch a reference-type satellite named LIBRA in around 2025. We present the roadmap for CSRB as well as requirements and specifications for LIBRA. Key technologies of the system include miniature phase-change cells providing fixed-temperature points, a cryogenic absolute radiometer, and a spontaneous parametric down-conversion detector. LIBRA will offer measurements with SI traceability for the outgoing radiation from the Earth and the incoming radiation from the Sun with high spectral resolution. The system will be realized with four payloads, i.e., the Infrared Spectrometer (IRS), the Earth-Moon Imaging Spectrometer (EMIS), the Total Solar Irradiance (TSI), and the Solar spectral Irradiance Traceable to Quantum benchmark (SITQ). An on-orbit mode for radiometric calibration traceability and a balloon-based demonstration system for LIBRA are introduced as well in the last part of this paper. As a complementary project to the Climate Absolute Radiance and Refractivity Observatory (CLARREO) and the Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS), LIBRA is expected to join the Earth observation satellite constellation and intends to contribute to space-based climate studies via publicly available data.

The Infrared Absolute Radiance Interferometer (ARI) for CLARREO

The Absolute Radiance Interferometer (ARI) is an infrared spectrometer designed to serve as an on-orbit radiometric reference with the ultra-high accuracy (better than 0.1 K 3‑σ or k = 3 brightness temperature at scene brightness temperature) needed to optimize measurement of the long-term changes of Earth’s atmosphere and surface. If flown in an orbit that frequently crosses sun-synchronous orbits, ARI could be used to inter-calibrate the international fleet of infrared (IR) hyperspectral sounders to similar measurement accuracy, thereby establishing an observing system capable of achieving sampling biases on high-information-content spectral radiance products that are also < 0.1 K 3‑σ. It has been shown that such a climate observing system with <0.1 K 2‑σ overall accuracy would make it possible to realize times to detect subtle trends of temperature and water vapor distributions that closely match those of an ideal system, given the limit set by the natural variability of the atmosphere. This paper presents the ARI sensor's overall design, the new technologies developed to allow on-orbit verification and test of its accuracy, and the laboratory results that demonstrate its capability. In addition, we describe the techniques and uncertainty estimates for transferring ARI accuracy to operational sounders, providing economical global coverage. Societal challenges posed by climate change suggest that a Pathfinder ARI should be deployed as soon as possible.

The Moon as a Climate-Quality Radiometric Calibration Reference

On-orbit calibration requirements for a space-based climate observing system include long-term sensor response stability and reliable inter-calibration of multiple sensors, both contemporaneous and in succession. The difficulties with achieving these for reflected solar wavelength instruments are well known. The Moon can be considered a diffuse reflector of sunlight, and its exceptional photometric stability has enabled development of a lunar radiometric reference, manifest as a model that is queried for the specific conditions of Moon observations. The lunar irradiance model developed by the Robotic Lunar Observatory (ROLO) project has adequate precision for sensor response temporal trending, but a climate-quality lunar reference will require at least an order of magnitude improvement in absolute accuracy. To redevelop the lunar calibration reference with sub-percent uncertainty and SI traceability requires collecting new, high-accuracy Moon characterization measurements. This paper describes specifications for such measurements, along with a conceptual framework for reconstructing the lunar reference using them. Three currently active NASA-sponsored projects have objectives to acquire measurements that can support a climate-quality lunar reference: air-LUSI, dedicated lunar spectral irradiance measurements from the NASA ER-2 high altitude aircraft; ARCSTONE, dedicated lunar spectral reflectance measurements from a small satellite; and Moon viewing opportunities by CLARREO Pathfinder from the International Space Station.

The synthetic double wavelength technique: a simple robust method for thermodynamic temperature determination

This paper describes a new relative technique developed at LNE-Cnam, for the determination of the thermodynamic temperature of blackbodies without recourse to a radiometric reference. This technique is referred to as the ‘synthetic double wavelength technique’ (SDWT) as it is considered to be a particular case of the ‘double wavelength technique’ (DWT). It offers a new experimental technique for the determination of the thermodynamic temperature at high temperature and as such a new means for the mise-en-pratique of the new definition of the kelvin achievable by any national metrology institute provided a multi-wavelength radiation thermometer combining large and narrow bandwidths is available. In this work, a first experimental implementation of this technique based on a wavelength-tuneable spectroradiometer providing both narrowband and broadband signals with the particularity of the broadband signal being virtually synthesised from the spectral distribution of the narrowband signals sampled over a wide spectral range. SDWT determination of the thermodynamic temperature of a blackbody at 2760 K was performed with a level of uncertainty that confirms the promising capabilities of this technique.

Intercalibration of FY-3C MWRI Against GMI Using the Ocean Microwave Radiative Transfer Model

This work addresses the intercalibration of the Microwave Radiation Imager (MWRI) on the Chinese second-generation polar-orbiting meteorological satellite Fengyun 3C (FY-3C) against the Microwave Imager (GMI) on the Global Precipitation Measurement (GPM) Core Observatory , in which a modified Double Difference (DD) method is developed and the Brightness Temperatures (TBs) in FY-3C MWRI and GMI channels are simulated using the ocean microwave Radiative Transfer Model (RTM) fed with the fifth generation of European Centre for Medium-range Weather Forecast (ECMWF) atmospheric reanalysis (ERA5) data. With the modified DD method, the intercalibration of FY-3C MWRI in 2017 are obtained. The results show that the MWRI observations are underestimated, especially for the low frequency channels. The calibration biases (mean of DDs) in FY-3C MWRI channels are temperature dependent, and decrease with the frequency increment. The in-orbit calibration of the MWRI descending (MWRID) data is 1~2 K worse than that of the MWRI ascending (MWRIA) data. At the TBs of standard scene defined by the Global Space-based Inter-Calibration System (GSICS), the calibration errors (mean of DDs ± standard deviation at the mean) of MWRIA data in January 2017 are −6.7±0.4 K, −8.3±0.8 K, −3.0±0.7 K, −1.9±1.0 K, −2.5±1.1 K, −3.9±0.8 K, −2.1±1.5 K, −1.5±1.0 K and −0.4±2.3 K in the 10V/H, 18V/H, 23V, 36V/H and 89V/H channels, respectively, while the calibration errors of MWRID data in January 2017 are −7.6±0.8 K, −9.1±1.2 K, −4.4±0.8 K, −2.9±1.3 K, −3.6±1.2 K, −5.1±0.8 K, −2.6±1.4 K, −2.5±1.1 K and −1.2±2.5 K in the nine channels, respectively. Although calibration biases exist, the in-orbit calibration of FY-3C MWRI is generally stable in 2017. The results of the modified DD method are consistent with that of the DD method. The modified DD method is promising to be applied to the intercalibration with both target and reference radiometers on polar-orbiting satellites.

Assessing the Quality of Shortwave and Longwave Irradiance Observations over the Ocean: One Year of High-Time-Resolution Measurements at the Lampedusa Oceanographic Observatory

AbstractMeasurements of downwelling shortwave (SW) and longwave (LW) irradiance were carried out on an oceanographic buoy close to the island of Lampedusa (Italy), in the central Mediterranean Sea. Irradiance measurements on the buoy were acquired at high time resolution together with the radiometer pitch and roll angles. The measurements carried out during 2016 have been compared with ground-based observations made at the Lampedusa Atmospheric Observatory, about 15 km northeast of the buoy. The radiometers were compared before and after deployment on the buoy and are traceable to the World Radiometric Reference scale. The SW measurements were corrected for the thermal offset. A small bias (measurements over the sea are smaller than on land) of about −2 W m−2 is found in the daily mean SW, and a moderate bias of +6.2 W m−2 (irradiance over the sea is larger than on land) is found in the LW. Similar biases are found when instantaneous measurements obtained with horizontal radiometers, clean domes, and cloud-free conditions are selected, suggesting that impacts of the moving platform and poor dome cleaning are minor at this site. The effect of the mean tilt angle was also investigated. Deviations in the hourly mean SW irradiance are on the order of 20% for a mean offset of 4° with respect to the solar zenith angle; the effect of tilt angle on LW irradiance appears to be negligible. Radiative transfer calculations show that the observed biases may be ascribed to the differences in the instrument altitude (through radiation absorption, scattering, and emission by the atmospheric constituents in the lowest atmospheric layers) and in the SW surface albedo.

Empirical Absolute Calibration Model for Multiple Pseudo-Invariant Calibration Sites

This work extends an empirical absolute calibration model initially developed for the Libya 4 Pseudo-Invariant Calibration Site (PICS) to five additional Saharan Desert PICS (Egypt 1, Libya 1, Niger 1, Niger 2, and Sudan 1), and demonstrates the efficacy of the resulting models at predicting sensor top-of-atmosphere (TOA) reflectance. It attempts to generate absolute calibration models for these PICS that have an accuracy and precision comparable to or better than the current Libya 4 model, with the intent of providing additional opportunities for sensor calibration. In addition, this work attempts to validate the general applicability of the model to other sites. The method uses Terra Moderate Resolution Imaging Spectroradiometer (MODIS) as the reference radiometer and Earth Observing-1 (EO-1) Hyperion image data to provide a representative hyperspectral reflectance profile of the PICS. Data from a region of interest (ROI) in an “optimal region” of 3% temporal, spatial, and spectral stability within the PICS are used for developing the model. The developed models were used to simulate observations of the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+), Landsat 8 (L8) Operational Land Imager (OLI), Sentinel 2A (S2A) MultiSpectral Instrument (MSI) and Sentinel 2B (S2B) MultiSpectral Instrument (MSI) from their respective launch date through 2018. The models developed for the Egypt 1, Libya 1 and Sudan 1 PICS have an estimated accuracy of approximately 3% and precision of approximately 2% for the sensors used in the study, comparable to the current Libya 4 model. The models developed for the Niger 1 and Niger 2 sites are significantly less accurate with similar precision.