Since the late 1970s, successive satellite missions have been monitoring solar activity and recording Total Solar Irradiance (TSI) data. The Digital Absolute Radiometer (DARA) on board the Chinese FY3E spacecraft was launched on July 4, 2021, and has since been recording TSI observations. Here, we analyze these observations and assess the performance of DARA, including sensor degradation of 5 ppm after 2 years in orbit, resulting from exposure to ultraviolet and extreme ultraviolet radiation. Comparing the new dataset’s mean values with observations from active instruments on other spacecraft (i.e., PMO6 on board the VIRGO/SOHO and the TIM/TSIS), along with the Solar Irradiance Absolute Radiometer (SIAR) also on board FY3E/JTSIM, we find that DARA observations closely align with TIM/TSIS, with a difference of approximately 0.07 W/m2. Based on these findings, we generate a new TSI dataset (JTSIM-DARA product) at a 6-hour sampling interval. Finally, we have incorporated this new dataset into the TSI composite time series released by the PMOD/WRC. The results indicate that the inclusion of DARA-recorded observations does not alter the consistency, reliability, and stability of the time series.

Since the late 1970s, successive satellite missions have been monitoring solar activity and recording Total Solar Irradiance (TSI) data. The Digital Absolute Radiometer (DARA) on board the Chinese FY3E spacecraft was launched on July 4, 2021, and  has since been recording TSI observations. Here, we analyze these observations and assess the performance of DARA, including sensor degradation of 5 ppm after 2 years in orbit, resulting from exposure to ultraviolet and extreme ultraviolet radiation. Comparing the new dataset’s mean values with observations from active  instruments on other spacecraft (i.e., PMO6 on board the VIRGO/SOHO and the TIM/TSIS), along with the Solar Irradiance Absolute Radiometer (SIAR) also on board  FY3E/JTSIM, we find that DARA observations closely align with TIM/TSIS, with a difference of approximately 0.07 W/m2. Based on these findings, we generate a new TSI dataset (JTSIM-DARA product) at a 6-hour sampling interval. Finally, we have incorporated this new dataset into the TSI composite time series released by the PMOD/WRC. The results indicate that the inclusion of DARA-recorded observations does not alter the consistency, reliability, and stability of the time series.

The Project for On-Board Autonomy-3 (PROBA-3) is the fourth satellite technology development and demonstration precursor mission within ESA's GSTP (General Support Technology Program) series. The primary mission objective is to demonstrate the technologies required for formation flying of multiple spacecrafts. The PROBA-3 mission concept comprises two independent minisatellites in highly-elliptical Earth orbits in precise formation flying, close to one another with the ability to accurately control the attitude and separation of the two satellites. The mission launch is scheduled for end of 2023.PROBA-3 mission consists of a coronograph spacecraft, hosting the coronograph APIICS, and the occulter spacecraft with the Digital Absolute RAdiometer (DARA). The radiometer to record total solar irradiance is mounted on the front satellite pointing to the Sun. DARA is developed and manufactured in Switzerland by the PMOD/WRC. We have done two pre-flight calibration campaigns: one at the World Radiation Center in Davos, Switzerland, and one at the Total Solar Irradiance (TSI) Radiometer Facility of the Laboratory for Atmospheric and Space Physics in Boulder Colorado, USA. We report on the results of the laboratory comparisons and discuss uncertainties of several instrument parameters, which are used to transform the raw measurements, which are voltage and current, into solar irradiance values.

The Project for On-Board Autonomy-3 (PROBA-3) is the fourth satellite technology development and demonstration precursor mission within ESA's GSTP (General Support Technology Program) series. The primary mission objective is to demonstrate the technologies required for formation flying of multiple spacecrafts. The PROBA-3 mission concept comprises two independent minisatellites in highly-elliptical Earth orbits in precise formation flying, close to one another with the ability to accurately control the attitude and separation of the two satellites. The mission launch is scheduled for end of 2023.PROBA-3 mission consists of a coronograph spacecraft, hosting the coronograph APIICS, and the occulter spacecraft with the Digital Absolute RAdiometer (DARA).  The radiometer to record total solar irradiance is mounted on the front satellite pointing to the Sun. DARA is  developed and manufactured in Switzerland by the PMOD/WRC. We have done two pre-flight calibration campaigns: one at the World Radiation Center in Davos, Switzerland, and one at the Total Solar Irradiance (TSI) Radiometer Facility of the Laboratory for Atmospheric and Space Physics in Boulder Colorado, USA. We report on the results of the laboratory comparisons and discuss uncertainties of several instrument parameters, which are used to transform the raw measurements, which are voltage and current, into solar irradiance values.

The Fengyun 3E (FY3E) spacecraft was launched on the 4th of July 2021 at 23h 28min UTC according to CASC (China Aerospace Science and Technology Corp.) on a Long March 4C vehicle from JSLC (Jiuquan Space Launch Center) in China. The orbit is a sun-synchronous near-circular with an altitude of 800 km, and an inclination of 98.7 degrees. The nominal lifetime of the satellite is eight years. The JTSIM experiments belong to the solar activities monitoring package. The solar radiation is absorbed by the black-coated cavity and the induced different heat-flux between the primary and reference cavity is measured, and the electrically calibrated differential heat-flux is used to compute the solar irradiance. SIAR has three identical channels A, B, and C, and each channel has a different solar exposure time to study the instrument’s nonlinear drift due to degradation. DARA also has three cavity radiometers and electrical substitution radiometers (Channel A, Channel B, and Channel C). The difference is that they are aligned in a triangle. Compared to VIRGO/PMO6, DARA inverts the aperture geometry to eliminate stray light. DARA and SIAR absolute radiometers are not operating at the same time due to the different designs and measurement sequences. On August 18, 2021, both instruments successfully passed the first commission phase, and they started to observe the total solar irradiance since then.

The Joint Total Solar Irradiance Monitor (JTSIM) onboard the Fengyun-3E spacecraft has been launched successfully the 4th of July 2021. It aims at measuring 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). The JTSIM experiment will use the two different types of TSI radiometers to track the stability of TSI measurements, and to better understand instrumental degradation in space. We will present results from this new experiment at first light. We will compare the measurements from DARA and SIAR over the first few months and relate them to other active missions (SOHO/VIRGO/PMO6v, SORCE/TSIS).

The angular response setup of PMOD/WRC was modified to facilitate measurements of the narrow field of view of radiometers for direct solar irradiance. First, The pointing of the JTSIM-DARA radiometer was measured four times during its construction in the optic laboratory of PMOD/WRC. The final offset of the pointing before shipping relative to the optical axis, defined by a removable alignment cube, is 1.07° / 0.67° (β/γ-axis) for the four-quadrant sensor and 0.095°/-0.017° for the radiometer cavity A. Next to JTSIM-DARA the DARA for the occulter of the satellite Proba-3 was characterised at PMOD/WRC. First tests of the pointing have been carried out and the final pointing characterization will be carried out in summer 2021. Finally, the angular response setup was also used the angular responsivity of solar direct irradiance filter radiometers. The first test was carried out using Precision Filter Radiometer (PFR) F-064.

The Joint Total Solar Irradiance Monitor (JTSIM) onboard the Fengyun-3E spacecraft has been launched successfully the 4th of July 2021. It aims at measuring 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). The JTSIM experiment will use the two different types of TSI radiometers to track the stability of TSI measurements, and to better understand instrumental degradation in space. We will present results from this new experiment at first light. We will compare the measurements from DARA and SIAR over the first few months and relate them to other active missions (SOHO/VIRGO/PMO6v, SORCE/TSIS).

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.

AbstractThe Compact Lightweight Absolute Radiometer (CLARA) is orbiting Earth on-board the Norwegian NorSat-1 micro-satellite since 14th of July 2017. The first light total solar irradiance (TSI) measurement result of CLARA is 1360.18 W m−2 for the so far single reliable Channel B. Channel A and C measured significantly lower (higher) TSI values and were found being sensitive to satellite pointing instabilities. These channels most likely suffer from electrical interference between satellite components and CLARA, an effect that is currently under investigation. Problems with the satellite attitude control currently inhibit stable pointing of CLARA to the Sun.

The compact lightweight absolute radiometer (CLARA) experiment aims at measuring the total solar irradiance (TSI) in space and is scheduled to fly on the Norwegian NORSAT-1 micro satellite. The CLARA experiment will contribute to the long term monitoring of the TSI variability to support the analysis of potential long term trends in the Sun's variability. CLARA is traceable to the National Institute of Standards and Technology radiometric scale and will provide further evidence for the TSI value on an absolute scale. In this paper we present the design, characterization, and calibration details of the CLARA instrument. The combined measurement uncertainty for the calibrated SI-traceable CLARA flight instrument is 567-912 ppm (k = 1) depending on the measuring channel.

Continuous and precise Total Solar Irradiance (TSI) measurements are indispensable to evaluate the influence of shortand long-term solar variability on the Earth’s energy budget. The existence of a potential long-term trend in the suns activity and whether or not such a trend could be climate effective is still a matter of debate. The Compact Lightweight Absolute Radiometer (CLARA) is one of PMOD/WRC’s future contributions to the almost seamless series of space borne TSI measurements since 1978. CLARA was designed and built by PMOD/WRC and characterized and calibrated by PMOD/WRC as part of the “European Metrology Research Program” (EMRP) project “European Metrology for Earth Observation and Climate” (MetEOC-2) funded by the European Commission. The main goals of the CLARA experiment are to continue the TSI data record with high accuracy and precision and to facilitate monitoring with its compact and adaptable design. CLARA will be one of three payloads of the Norwegian micro satellite NORSAT-1, along with Langmuir probes for space plasma research and an Automatic Identification System (AIS) receiver to monitor maritime traffic in Norwegian waters. The launch of NORSAT-1 is planned for March 2017.

Space weather and space climate studies require accurate Solar Spectral Irradiance (SSI) observations. The PREcision Monitoring Sensor (PREMOS) instrument aboard the PICARD satellite acquired solar irradiance measurements in specific spectral windows in the UV, visible and near infrared from October 2010 to March 2014. This contribution aims at presenting the Level 3 data, corrected for non solar features as well as for degradation. These level 3 data has been tested over different scientific cases, such as observations during the Venus transit and the presence of the p-mode signature within high-cadence data. The PREMOS Level 3 data have also been compared to others data sets, namely the SOLSTICE and SIM instruments aboard SORCE, for nearly 3 and half years. An excellent correlation has been found for the UV spectral ranges. We have also found a rather good correlation for visible and near-infrared observations for short-term variations, for which an error of about 200 ppm has been estimated within PREMOS visible and near-infrared observations. The PREMOS data could also be used to address several scientific topics, i.e. for validating semi-empirical models of the solar irradiance. We will emphasize about our new irradiance model, COSIR for Code of Solar Irradiance Reconstruction, which is successful at reproducing the solar modulation as seen in the PREMOS, SoHO/Virgo and SORCE data.

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(a) to contribute to the long term monitoring of the total solar irradiance,(b) to use irradiance observations for 'nowcasting' the state of the terrestrial middle atmosphere and(c) 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.

Within the Space Situational Awareness Programme (SSA), ESA has initiated the assessment of two missions currently considered for implementation to enable enhanced space weather monitoring capabilities of ESA. These missions will position satellites at the Lagrangian points L1 and L5. Phases A and B1 with a focus on the L5 mission are now completed and the Bridging Phase studies are now initiated. The Lagrange mission to L5 considers four remote sensing optical instruments and several in-situ instruments to analyze the Sun, the energetic particle streams, the magnetic field and solar wind conditions, in order to provide early warnings of increased solar activity. Within this frame, an Extreme UltraViolet Imager (EUVI) instrument is under study to image the full solar corona. The overall Remote Sensing Instruments study will be led by RAL (GB). In particular, the EUVI study is carried out by three institutes - CSL (BE), ROB (BE) and PMOD/WRC (CH) - and is built upon the heritage of previous projects such as PROBA2/SWAP, ESIO and SolO/EUI to which CSL contributed. The work is shared between CSL, ROB and PMOD with the following responsibilities: - CSL: overall management, system study, optical engineering, thermal engineering, - PMOD: electrical engineering, mechanisms and mechanical engineering - ROB: instrument requirements, instrument operation, ground segments Several EUV imaging telescopes have been developed for scientific solar spacecraft, e.g. SOHO/EIT, TRACE, Proba-2/SWAP, STEREO/SECCHI-EUVI, SOLAR ORBITER/EUI. Another study called ESIO, based on an ESA General Support Technology Programme activity (GSTP), was also performed until 2015 jointly by the Centre Spatial de Liege (CSL) and the Royal Observatory of Belgium (ROB). The name of the instrument designed in that frame was the EUV Solar Imager for Operations (ESIO). The ESIO instrument was intended to provide solar EUV images for operational use as part of the ESA SSA (Space Situational Awareness) programme. The ESIO design is a low resolution, low data rate, compact solar EUV imager associated to a flux monitor for operation use in space weather monitoring and forecasting. The EUVI instrument intends to image the full solar disc, in a single spectral line as the requirement and multiple lines as a goal. The idea for this instrument is to combine heritages from SWAP, ESIO and SolO/EUI to benefit from the experience CSL gained during those projects. The optical design of its telescope originates from PROBA-2/SWAP: it consists of a two-mirror telescope imaging the Sun to a focal plane assembly. The actual structural solution also benefits from the SWAP heritage: a simple optical bench onto which the optical elements will be attached. Within Lagrange, the global philosophy is indeed to re-use as much as possible heritage from previous missions and limit the amount of new development. The previous accommodation of SWAP has however been updated to fit with the new optical design characteristics and benefits from some updates coming from SolO/EUI. The spectral selection is achieved with a set of aluminum foil filters, together with EUV reflective multilayer coatings deposited on the mirrors. The overall stack is specifically designed to provide reflectivity in the extreme ultraviolet range, and to achieve the spectral selection in the selected narrow band pass (1.5 nm at full width half maximum). The accuracy on central wavelength adjustment is within +/- 0.2 nm. The focal plane assembly comprises an EUV CMOS detector coming from the spare list of SolO/EUI. A decontamination heater located on the sensor will be used periodically to outgas the condensed layer that builds up on the cold sensor. Cleanliness of such EUV instrument is indeed critical as molecular deposits can absorb EUV radiation and dramatically decrease photon throughput. The focal plane assembly also includes the proximity electronics (FEE) required to readout the sensor and a thermal link to the cold space for the detector cooling down. An overview on the technical development and decisions taken will be presented during the presentation to highlight the tight relation between the design selection, user requirements, interfaces and cost/schedules aspects.