IPC-XIV

All participants of the IPC/FRC/IPgC are encouraged to give Symposium presentations. The purpose is to exchange experience and to share and build knowledge. Because the Symposium will take place when bad weather prevents measurements, the programme schedule can only be decided at short notice. Nevertheless, we plan to group the talks according to the following topics:

• Radiation Networks
• Instrumentation - New Developments
• Measurement and Calibration Methods
• Solar Energy Applications

Further details include:

- The Symposium will be held in parallel to the IPC/FRC/IPgC calibration campaigns.
- There will be no formal programme, as talks will be held at short notice if weather conditions prevent measurements from taking place.
- Colleagues who are not able to attend the IPC/FRC/IPgC in person can provide a recorded presentation.
- Talks can be up to 20 minutes long with 5 minutes for discussions at the end.

If you are not able to attend the Symposium in person, but would still like to give a presentation, then please send us a video recording. We will then show your presentation during the Symposium.

- We aim to record all live-presentations so they can be made available for the public on the WMO website.
- Colleagues who are not able to attend the IPC/FRC/IPgC in person are invited to provide a recorded presentation and upload it to vimeo.com. Uploading video presentations to Vimeo: i) register at vimeo.com, ii) on the side-menu click on “Videos", iii) select your video and click on “Upload video”.
- We invite all participants to prepare a proceedings paper which will be published as part of an WMO IOM report soon after the IPC (community.wmo.int/activity-areas/imop).

Please submit your abstract here: IPC/FRC/IPgC Symposium Abstract Form 🡆

Here is a draft sequence of the presentations. Date and time still TBD (depending on the weather conditions). Tentatively, we aim for 30 minutes per talk (20 min presentation plus 10 minutes questions).

Session 1: Measurement and Calibration Methods

Finsterle W. (Head of Solar Radiation Section, PMOD/WRC): The status of the World Standard Group (WSG) 

Doppler, L (Deutscher Wetterdienst (DWD), Germany): AOD retrievals from spectral DNI measured by spectroradiometers

van den Bos, C. J. (Hukseflux Thermal Sensors, Netherlands): Solar irradiance measurement; relevance to users of Solar photovoltaic energy systems

Piccioli, I. (Laboratorio de Energía Solar (LES ), Uruguay): Surface azimuth misalignments estimated from solar irradiance on tilted surfaces

Singh, Jaiprakash (National Institute of Solar Energy Gurgaon, India): Pyranometer sensitivity deviations: Results from calibration of 50 different field sensors

Muellejans, H. (European Commission, Joint Research Centre (JRC), Europe): Irradiance measurement uncertainty for cavity radiometers participating in IPC

Session 2: Radiation Networks

Nollas F., Carbajal Benitez G. (Servicio Meteorologico Nacional, Argentina): Solar Radiation measurements and calibrations by SMN in Argentina

Malan A., Singh J., Singh J. P., and Rihan M. (National Institute of Solar Energy, India): Quality Assessment of Long-Term Solar Radiation Data from India’s SRRA Network Using Standard QC Protocols

Sasaki, Shun (Japan Meteorological Agency, Japan): Recent Activities of Regional Radiation Center Tokyo

Meyer, A.J. (GeoSUN, South Africa): Solar Station Networks in Africa

Session 3: Solar Energy Applications

Kroeger, I., Pastuschek. M., Friedrich, D., Winter, S. (Physikalisch-Technische Bundesanstalt, Germany): Primary calibration of reference solar cells at PTB

Meyer, A.J. (GeoSUN, South Africa): Overview of Photovoltaic soiling measurement devices

Meyer, A.J. (GeoSUN, South Africa): Latest Trends in large scale Photovoltaics

Kilchhofer, K., Papachristopoulou, K. and Kazadzis, S. (PMOD/WRC, Davos, Switzerland):  Quantification of how solar radiation management affects photovoltaic energy production

Session 4: Instrumentation - New Developements

van den Bos, C.J. (Hukseflux Thermal Sensors, Netherlands): Sensor design and calibration for 2000 x concentrated solar irradiance

Suter M., Buchli J. (Davos Instruments, Davos, Switzerland): Upgrade of the PMO5 WSG radiometer

Tatsiankou, V., Beal, R. (Spectrafy, Canada) : A New Sensor for Measuring Diffuse and Global Irradiance Using an Internal Automated Shadowband

Hülsen, Gregor (PMOD/WRC, Davos, Switzerland): Carl Dorno’s Apparat zur Dauerphotographie: Getting it back to live

2021 Programme and Presentations (IPC-XIII, FRC-V and IPgC-III , 27 Sept. - 15 Oct. 2021):

Proceedings of the Symposium on Radiation Measurement available at: WMO website
The presentations are also available as a zip-file (~700 MB).

Measurement and Calibration Methods

Authors: Doppler, L (Deutscher Wetterdienst (DWD), Germany)
Title: AOD retrievals from spectral DNI measured by spectroradiometers
Abstract: The reference method for retrieving aerosol optical depth (AOD) is based on the Beer-Lambert-Bouguer law, typically applied to sun photometers equipped with a limited number (usually 4 to 8) of discrete spectral channels. In this study, we explore the potential of retrieving AOD at high spectral resolution (1–2 nm) using measurements of the spectrum of direct normal irradiance (DNI) from array-type spectroradiometers. These instruments employ detectors with 1024 to 2048 spectral channels, covering the UVA–VIS–NIR range (300–1050 nm).
This work, supported by the COST Action HARMONIA, focuses on spectroradiometers including the array spectrometers BTS2048-VL, PSR, EKO-MS711, and the monochromator spectrometer QASUME, all currently participating in the FRC-VI intercomparison campaign at PMOD-WRC in Davos, held in parallel with IPC-XIV. We provide an overview of the current state of the technique, and present the various instruments and retrieval methodologies. Then, for earlier studies and for the current running campaign FRC-VI, we evaluate AOD retrievals from spectral DNI measurements against AOD retrievals from the reference photometer PFR. In the absence of PFR measurements in the comparison studies, we use the AOD retrieved from the Cimel photometer (AERONET) as reference.

Authors: van den Bos, C. J. (Hukseflux Thermal Sensors, Netherlands)
Title:Solar irradiance measurement; relevance to users of Solar photovoltaic energy systems
Abstract: As the energy transition progresses, one of the main applications of solar radiation measurements nowadays is the monitoring and performance evaluation of large scale photovoltaic solar renewable energy systems. The solar energy community works with the "IEC 61724 PV System Performance" standards. This presentation gives an overview of monitoring of PV systems and performance analysis as recommended by these standards, and the importance of pyranometers and their calibration to the solar industry. It also comments on the status of the IEC standards - they are now under revision.

Authors: Piccioli, I. (Laboratorio de Energía Solar (LES ), Uruguay)
Title: Surface azimuth misalignments estimated from solar irradiance on tilted surfaces
Abstract: Azimuth misalignment of receiver surfaces is a frequent source of uncertainty when working with solar irradiance measurements on tilted surfaces (GTI). Two complementary methods to estimate the true azimuth of a tilted surface from GTI data are implemented and evaluated; the first method being a new one, and the second a variation of a previously existing one. Ground data from an arrangement of tilted pyranometers with different azimuths, along with GHI, DNI and DHI data, are used to validate the methods. The results show good agreement with nominal azimuth values, with typical uncertainties of a few degrees that increase with the nominal azimuth. Both methods are complementary since they have different strengths and weaknesses..

Authors: Singh, Jaiprakash (National Institute of Solar Energy Gurgaon, India)
Title: Pyranometer sensitivity deviations: Results from calibration of 50 different field sensors
Abstract: Sensitivity of a pyranometer is used for the conversion of measured voltage (or current) signal to its equivalent irradiance value. The value of the sensitivity although specified by the original equipment manufacturer, varies after exposure to field conditions. Calibrating these sensors, with valid traceability, ensures the righteousness of the value conversions. Solar radiometric calibration lab at National institute of solar energy is engaged in calibration of field pyranometers submitted by various solar developers/installers. The change in sensitivity value, over a period of at least one year, of 50 different pyranometer samples is discussed in this paper. These pyranometers were operating in different power plants / field conditions across India. Analysis indicate, the deviations were both positive and negative, within 3 %. Study on these variations is significant, as irradiance value is one of the critical parameter in evaluating the performance checks as per the project contracts.

Authors: J.P. Montillet, W. Finsterle, M. Haberreiter, D. Pfiffner (PMOD/WRC, Davos, Switzerland)
Title: Presentation of the various total solar irradiance products released by PMOD/WRC
Abstract: Since 1978, various space missions have measured total solar irradiance (TSI). The Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC) has developed precision radiometers for this purpose, including the PMO6 instrument on SOHO/VIRGO, operational since 1996.
PMOD/WRC has released several TSI data products, the most recent being PMO6v8 (2021). Unlike earlier versions, which relied on manual degradation corrections based on instrument knowledge, PMO6v8 introduces a data-driven approach using machine learning and data fusion to correct sensor degradation.
In parallel, the Joint Total Solar Irradiance Monitor (JTSIM) aboard Fengyun-3E (launched July 2021) carries the new Digital Absolute Radiometer (DARA). More recently, PROBA-3, launched in December 2024, includes the latest DARA version, for which we have already evaluated the first light. Together, these missions pave the way for future high-resolution (minute-level) TSI data releases.

Authors: Moustaka, A., Kazadzis, S., Logothetis, S.-A., Ades, M., Inness, A., Flemming, J., Huijnen, V., Rémy, S., Tourpali, K., Zerefos, C., Amiridis, V., Gkikas, A.
Title: Two Decades of Aerosol Trends: Insights from CAMS Reanalysis
Abstract: Aerosol optical depth (AOD) is a key indicator of global climate change, and its long-term trends reflect the evolving climatic impact of atmospheric aerosols. In this study, we use the 2003–2024 Copernicus Atmosphere Monitoring Service Reanalysis (CAMSRA) dataset to analyze global and regional AOD trends over multiple sub-periods. Eighteen regions of interest (ROIs) were selected for regional analysis, representing dust-, biomass-burning-, anthropogenic-, and sea-salt-dominated areas. Trends were estimated for total AOD as well as for the five aerosol species represented in CAMSRA: dust, sea salt, black carbon, organic matter, and sulfates. Comparison with AERONET stations characterized by significant trends (95% confidence) shows high correlation (R = 0.89), low error (RMSE = 0.0021), and correct reproduction of trend sign at 96.7% of stations, although positive trends are occasionally underestimated. Over the 22-year period, China exhibited strong decreases in AOD (exceeding –0.014 yr⁻¹), while India showed consistent increases (exceeding +0.012 yr⁻¹); the trend in eastern China reversed from positive to negative during the last decade. Most regions showed trend reversals between the two decades (2003–2013 and 2014–2024), except Southern Asia and the Middle East, which maintained notable positive trends (exceeding +0.014 yr⁻¹ over Iran and Afghanistan). Regional analysis also revealed several reversed aerosol-speciated trends, highlighting that dominant aerosol types do not necessarily control total AOD trends, as long-range transport and mixing of aerosols also play a key role, highlighting the complex interplay of multiple aerosol species.

Radiation Networks

Authors: Nollas F., Carbajal Benitez G. (Servicio Meteorologico Nacional, Argentina)
Title:  Solar Radiation measurements and calibrations by SMN in Argentina
Abstract: The National Meteorological Service of Argentina (SMN) operates a solarimetric network covering diverse regions, including the Antarctic Marambio Base. This network provides high-quality solar radiation measurements through manual and automated quality control procedures. Calibration is a key component, using a standard group of pyrheliometers (PMO8, AHF, TMI-Kendall) and campaigns for UV radiometers (2018, 2022). Regional collaboration and integration with global data centers strengthen data reliability. Future plans include pyranometer and pyrheliometer calibration campaigns (2025–2027), infrastructure development aligned with ISO 9846, and closer cooperation with academic institutions to enhance solar radiation monitoring capacity.

Authors: van den Bos, C.J. (Hukseflux Thermal Sensors, Netherlands)
Title: Sensor design and calibration for 2000 x concentrated solar irradiance
Abstract: The measurement of concentrated solar radiation has its own set of challenges. In the past 4 years Sandia National Laboratory (Concentrating Solar Technologies Department & National Solar Thermal Test Facility) and Hukseflux jointly developed and tested a sensor and calibration method for highly - over 2000 times - concentrated solar radiation. The presentation highlights the main steps in the process of design and testing.

Authors: Malan A., Singh J., Singh J. P., and Rihan M. (National Institute of Solar Energy, India)
Title: Quality Assessment of Long-Term Solar Radiation Data from India’s SRRA Network Using Standard QC Protocols
Abstract: This study evaluates the quality of minute-level solar radiation data recorded from 115 Solar Radiation Resource Assessment (SRRA) stations across India during 2010–2020, using internationally recognised quality control (QC) procedures. The assessment framework is based on Baseline Surface Radiation Network (BSRN) guidelines and other best practices, involving physical limit tests, inter-variable consistency checks, and temporal continuity assessments. Key quality indicators include data availability, flagged data ratio, component closure error, and sensor drift pattern. Outputs were evaluated for Global Horizontal Irradiance (GHI), Direct Normal Irradiance (DNI), Diffuse Horizontal Irradiance (DHI), temperature, wind, and humidity. The analysis identified significant variations in data completeness and quality across sites, with common issues such as unrealistic spikes, shading effects, and calibration drift. The analysis contributes to strengthening India’s ground-based solarimetric infrastructure and aligns it with international best practices, offering insights relevant for both national deployment and global climate datasets.

Authors: Sasaki, Shun (Japan Meteorological Agency, Japan)
Title: Recent Activities of Regional Radiation Center Tokyo
Abstract: Japan Meteorological Agency (JMA) has been conducting solar radiation measurements traceable to the international standard scales since the 1930s, and serving as one of Regional Radiation Centers in the WMO Regional Association II (RA II, Asia), RRC-Tokyo, since 1965. RRC-Tokyo has participated in the International Pyrheliometer Comparison (IPC) continuously since IPC-III and has held the Regional Pyrheliometer Comparison (RPC) once every 5 years in the next fiscal year of IPC, in principle. RRC-Tokyo has contributed to maintaining the traceability of the World Radiometric Reference (WRR) in the surface radiation network of RA II for a long time through these activities. The last RPC (RPC-V) was held as a joint event of RA II and RA V (South-West Pacific) at Mt. Tsukuba, Japan in January 2023, and 19 pyrheliometers from 5 National Radiation Centers (NRCs) (China, Hong Kong, Indonesia, Korea and New Zealand), WRC, RRC-Melbourne and one manufacturer, Davos Instruments AG, participated in RPC-V. The results of RPC-V are reported in WMO IOM report No.145. RRC-Tokyo has also prepared to operate the Absolute Cavity Pyrgeometer (ACP) for maintaining a reference of infrared radiation. In addition, JMA has been operating four domestic and an Antarctic BSRN stations to contribute to climate monitoring. Next RPC is planned in January 2027, and the traceability of WRR will be disseminated to NRCs in RA II by our regional standard pyrheliometers which are participating in IPC-XIV.

Authors: Meyer, A.J. (GeoSUN, South Africa)
Title: Solar Station Networks in Africa
Abstract: A number of solar monitoring station networks have been established across Africa, varying in scope, activity, and accessibility. This paper provides an overview of these networks, discussing both currently active and non-active initiatives. The emphasis is placed on networks that provide freely accessible data collected with high-accuracy, Class A sensors. Key examples include the Baseline Surface Radiation Network (BSRN), the Southern African Universities Radiometric Network (SAURAN), as well as stations supported through programs funded by the World Bank and the International Finance Corporation (IFC). These networks represent valuable resources for solar resource assessment related research and large scale solar energy project development in the region.

Solar Energy Applications

Authors: Kroeger, I., Pastuschek. M., Friedrich, D., Winter, S. (Physikalisch-Technische Bundesanstalt, Germany)
Title: Primary calibration of reference solar cells at PTB
Abstract: "The Physikalisch-Technische Bundesanstalt (PTB) is Germany's national metrology institute (NMI). Working Group 4.52, 'Solar cells', provides ""primary"" calibration of reference solar cells for photovoltaic calibration and testing laboratories worldwide using the differential spectral responsivity (DSR) method. This method is recognised as ""primary"" calibration according to IEC 60904-4. This article provides details of the DSR method, insight into specific measurement uncertainties, and intercomparison results.
In addition to the DSR method, PTB has recently implemented another ""primary"" calibration method according to IEC 60904-4: the direct sunlight method (DSM). We will present its technical implementation at PTB, alongside a comparative measurement of several reference solar cells using both methods."

Authors: Meyer, A.J. (GeoSUN, South Africa)
Title: Overview of Photovoltaic soiling measurement devices
Abstract: Soiling-related production losses can be substantial in operating photovoltaic (PV) projects. To accurately quantify these losses and optimize cleaning schedules, dedicated soiling measurement devices are essential. This paper reviews and compares several commercially available PV soiling loss measurement technologies, including optical sensors and two-panel systems. Particular attention is given to compare the differences between manually cleaned and automatically cleaned sensors, highlighting their respective maintenance requirements.

Authors: Meyer, A.J. (GeoSUN, South Africa)
Title: Latest Trends in large scale Photovoltaics
Abstract: In recent years, the deployment of photovoltaic (PV) modules has expanded at an unprecedented pace worldwide. This presentation reviews the latest technology trends shaping the industry, including glass-glass configurations, bifacial designs, half-cell architectures, and large-format modules. Emerging applications are also explored, such as agrivoltaics, building-integrated photovoltaics (BIPV), floating PV systems, and vertically mounted modules. Beyond module design and deployment, the integration of PV with complementary technologies—particularly hybrid PV–wind systems and large-scale battery energy storage (BESS)—is highlighted.

Authors: Kilchhofer, K., Papachristopoulou, K. and Kazadzis, S. (PMOD/WRC, Davos, Switzerland)
Title: Quantification of how solar radiation management affects photovoltaic energy production
Abstract: Solar radiation modification (SRM), also called climate intervention, is increasingly viewed as a potential backstop against global warming. SRM encompasses several techniques, including stratospheric aerosol injection (SAI), marine cloud brightening (MCB), cirrus cloud thinning (CCT), and surface albedo enhancement. Here, we investigate the photovoltaic (PV) potential under different SAI scenarios as part of a first ESA project related to SRM (STATISTICS). The libRadtran radiative transfer model (Mayer & Kylling, 2005, Emde et al., 2016) is used to calculate downwelling surface shortwave irradiance as a function of stratospheric aerosol layers, i.e. liquid homogeneous spherical particles made of sulphuric acid at a height of 19 km and for different solar geometries. The refractive index as a parameter to describe the optical properties of the sulphuric acid particles was taken from Hummel et al., 1988. In addition, we derived the direct and diffusive irradiance components as a function of tilting angles of the PV panels and will compare our results with a recent global climate modelling study conducted by Baur et al., 2024. The results provide insights into the influence of possible SRM applications on the PV power output and Earth’s radiation budget.
Baur, S., Sanderson, B. M., Séférian, R., & Terray, L. (2024). Solar radiation modification challenges decarbonization with renewable solar energy. Earth System Dynamics, 15(2), 307–322. https://doi.org/10.5194/ESD-15-307-2024
Emde, C., Buras-Schnell, R., Kylling, A., Mayer, B., Gasteiger, J., Hamann, U., Kylling, J., Richter, B., Pause, C., Dowling, T., & Bugliaro, L. (2016). The libRadtran software package for radiative transfer calculations (version 2.0.1). Geoscientific Model Development, 9(5), 1647–1672. https://doi.org/10.5194/gmd-9-1647-2016
Hummel, J. R., Shettle, E. P., & Longtin, D. R. (1988). A New Background Stratospheric Aerosol Model for Use in Atmospheric Radiation Models.

Instrumentation - New Developments

Authors: Hülsen, Gregor (PMOD/WRC, Davos, Switzerland)
Title: Carl Dorno’s Apparat zur Dauerphotographie: Getting it back to live
Abstract: Carl Dorno’s "Apparat zur Dauerphotographie" was the first UV Spectrometer built by Zeiss, Jena (Germany) in 1907 from the ideas developed by Dorno. It was used for the first spectral solar UV measurements in the year 1907 till 1911. The spectral range covers the whole UV range. The "detector" was a photographic film mounted every day. The apparatus includes a manual clock driven sun tracker, enabling automatic recording for the whole day. The processed films show the typical diurnal shift of the solar spectra of sharp edge between the UV-A and the UV-B wavelength range. Dorno analyzed the measurements for the integral UV intensity change during the measurement period. After 1911 the instrument was stored in the archive of PMOD.
Now the historic spectroradiometer was investigated for its mechanical and optical functionality. First, the sun tracking clock drive was repaired and test. It shows a few minutes delay in 24 h of operation. Secondly, the optic was tested using an Ekspla tunable laser. The laser lines were scanned at the output slit of the apparatus using an Acton SP2500 (Princeton Instruments) double monochromator. The lines starting from 290 nm till 410nm could be recorded. The slit width of Dorno's spectroradiometer is around 1.5 nm. The project will continue with the wavelength distribution at the output slit. Photographic films could be used for potential solar UV measurements. Alternatively, a UV detection camera is needed for easy operation of the instrument.

Authors: Suter M., Buchli J. (Davos Instruments, Davos, Switzerland)
Title: Upgrade of the PMO5 WSG radiometer
Abstract: The PMO5 radiometer, a member of the World Standard Group (WSG), has been upgraded with a new control unit in 2024. As this radiometer is a unique specimen built in the 1970’s, several adaptions to a Davos Instruments standard control Unit “Linard” have been necessary in order to achieve this upgrade. An overview on history and design of the PMO5 as well as the upgrade process will be presented.

Authors: Tatsiankou, V., Beal, R (Spectrafy, Canada)
Title: A New Sensor for Measuring Diffuse and Global Irradiance Using an Internal Automated Shadowband
Abstract: Accurate and reliable solar irradiance data are crucial for assessing the technical performance and financial suitability of every PV technology. Recently there has been a renewed push in the solar industry for routine measurements of diffuse horizontal irradiance (DHI), which is a key parameter in irradiance transposition and forecasting models. Furthermore, DHI measurements have broader applications in climate research and meteorology. However, obtaining cost-effective and reliable in situ measurements of the DHI can be difficult, due the cost and size of the equipment required, sensor performance issues, and/or the reliability concerns associated with external moving parts. In this talk, we will propose a new way of simultaneously measuring global and diffuse horizontal irradiances from a single, compact sensor with no external moving parts. The device uses several photodiodes with quartz diffusers and an automated, internal shadowband that slowly rotates over the course of each day, constantly shading one of the photodiodes. We will assess the theoretical performance of the device by simulating spectral radiances for each element of the hemispherical sky grid under different atmospheric conditions and sun angles. Key parameters such as the spectral error and shading losses are quantified. Finally, we will discuss the on-sun performance of several prototypes at our test site against reference instruments.