The Uranus Multi-Experiment Radiometer for Haze and Clouds Characterization

Space Science Reviews (2024) 220:6 https://doi.org/10.1007/s11214-023-01040-3 The Uranus Multi-Experiment Radiometer for Haze and Clouds Characterization V. Apéstigue1·D. Toledo1·P.G.J. Irwin2·P. Rannou3·A. Gonzalo1· J. Martínez-Oter1·J. Ceballos-Cáceres4·J. Azcue1·J.J. Jiménez1·E. Sebastian1,5 · M. Yela1·M. Sorribas1·J.R. de Mingo1·A. Martín-Ortega1·T. Belenger1·M. Alvarez1· D. Vázquez-García de la Vega4·S. Espejo4·I. Arruego1 Received: 23 September 2023 / Accepted: 22 December 2023 © The Author(s) 2024 Abstract The aerosols (clouds and hazes) on Uranus are one of the main elements for understanding the thermal structure and dynamics of its atmosphere. Aerosol particles absorb and scatter the solar radiation, directly affecting the energy balance that drives the atmospheric dynamics of the planet. In this sense, aerosol information such as the vertical distribution or optical properties is essential for characterizing the interactions between sunlight and aerosol particles at each altitude in the atmosphere and for understanding the energy balance of the planet’s atmosphere. Moreover, the distribution of aerosols in the atmosphere provides key information on the global circulation of the planet (e.g., regions of upwelling or subsidence). To address this challenge, we propose the Uranus Multi-experiment Radiometer (UMR), a lightweight instrument designed to characterize the aerosols in Uranus’ atmosphere as part of the upcoming Uranus Flagship mission’s descending probe payload. The scientific goals of UMR are: (1) to study the variation of the solar radiation in the ultra-violet (UV) with altitude and characterize the energy deposition in the atmosphere; (2) to study the vertical distribution of the hazes and clouds and characterize their scattering and optical properties; (3) to investigate the heating rates of the atmosphere by directly measuring the upward and downward fluxes; and (4) to study the cloud vertical distribution and composition at pressures where sunlight is practically negligible (p >4-5 bars). The instrument includes a set of photodetectors, field-of-view masks, a light infrared lamp, and interference filters. It draws on the heritage of previous instruments developed at the Instituto Nacional de Técnica Aeroespacial (INTA) that participated in the exploration of Mars, where similar technology has demonstrated its endurance in extreme environments while utilizing limited resources regarding power consumption, mass and volume footprints, and data budget. The radiometer’s design and characteristics make it a valuable complementary payload for studying Uranus’ atmosphere with a high scientific return. Keywords Planetary radiometer ·Ice Giants ·Uranus ·Clouds and haze 1Introduction Uranus and Neptune, known as the Ice Giants, stand out as the only planets in the Solar System that have not been the focus of dedicated missions. However, delving into the study Extended author information available on the last page of the article 6 Page 2 of 30 V. Apéstigue et al. of these planets is essential for us to grasp how our planetary system formed (Mandt et al. 2015) and changed over time (Mousis et al. 2022), especially since ice planet systems are quite common in other planetary systems (Borucki et al. 2011; Fulton et al. 2017). Our current understanding mainly comes from “limited” observations made by Earth and space telescopes and a brief visit by the Voyager 2 spacecraft over thirty years ago. According to the recent Decadal Survey Report (National Academies 2022), NASA has placed a priority on sending a significant mission to Uranus, in addition to their ongoing missions to Mars and Europa. Similarly, the Voyage 2050 study (ESA Senior Committee, 2021) by the European Space Agency (ESA) aligns with this direction, suggesting that ESA should join forces in a future mission through collaboration, following the successful model of partnerships in Cassini-Huygens. During the last two decades, several reference missions have been proposed to reach the Ice Giants system (Hubbard et al. 2010; Arridge et al. 2012; Hofstadter et al. 2017,2019; Bayon et al. 2019). These mission proposals aim to investigate the optimal arrangement for accomplishing a number of scientific goals using the available technology at the time. The most recent and comprehensive study conducted to achieve the scientific objectives of the last Decadal report is the Uranus Orbiter and Probe (UOP) mission (Simon et al. 2021). This Flagship-class mission concept consists of an orbiter and a descent probe to be launched preferably by 2031. For the orbiter, the scientific priorities extracted from the Decadal Survey are: to study the planet’s bulk composition and internal structure, the magnetic field, the atmosphere circulation, the rings, and the satellite system. To this end, the spacecraft includes as referenced payload package of six instruments. In the case of the descent probe, its primary mission is to obtain the atmospheric noble gas abundances, noble gas isotope ratios, and the thermal structure of the atmosphere and winds, using four reference instruments: an Atmospheric Structure Instrument, a Mass Spectrometer, an Ultra Stable Oscillator, and an Ortho-Para Hydrogen Sensor. However, these probes could also be capable of studying the hazes composed of small photochemically produced particles (hydrocarbon ices) and the abundances of species that can condense in the atmosphere and form clouds, such as NH3,CH 4,H 2S, and H2O. These aerosol particles are crucial in the radiative heat budget of the planet, and measuring their optical properties and spatial distribution can provide useful information about the chemistry and dynamics of the Uranus stratosphere and troposphere, constraining planetary formation models (e.g., Atreya et al. 2020; Mousis et al. 2018). Orbital observations could give partial information about these atmospheric structured layers but to give a “ground truth” of the vertical distribution and composition of the aerosols we need the use of a descent probe with dedicated instrumentation and in-situ observations compatible with the resources of the probe’s primary mission. 2 Descent Probes Aerosol Instrumentation Precedents Historically, descent probes have incorporated instrumentation for the study of clouds based on the optical properties of their aerosols, and the deposition of solar energy as a function of altitude. The Russian Venera program, in its descent probe of mission number 8, employed a visible-light photometer. Its purpose was to obtain the optimal exposure value for future surface photographs of Venus, which would be sought in the subsequent missions. However, this instrument also conducted the initial measurements of solar radiation variation in Venus’ atmosphere (Avduevsky et al. 1973) during the descent. The Venera 9, The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 3 of 30 6 10, and 11 descent probes included nephelometers (back and multi-scattering angles). They helped identify three cloud layers and the underlying haze. As a result, they could determine the microphysical properties of the particles in these layers (Marov et al. 1980). In 1978, the Pioneer mission, also dedicated to the study of Venus, deployed a set of atmospheric probes alongside an orbiter. The Venus Sounder Probe was the largest and most complete descent module, equipped with 11 instruments, out of which 4 were focused on the analysis of aerosols and thermal balance a backscattering nephelometer (Ragent et al. 1980), a solar flux radiometer (Tomasko et al. 1980), an infrared radiometer (Boese et al. 1980), and a cloud particle size spectrometer (Knollenberg and Gilland 1980). On the other hand, the other three smaller twin probes were outfitted with 7 instruments, of which 2 were also dedicated to cloud studies: the same nephelometer as in the main probe (Ragent et al. 1980) and a miniaturized net flux radiometer (Sromovsky et al. 1980). In 1995, the Galileo probe conducted the first in-situ exploration of the atmosphere of a giant planet. During its descent of over 150 km, it studied Jupiter’s aerosols using a nephelometer (Ragent et al. 1992) capable of measuring simultaneously the scattering properties of particles at 5 different angles, drawing inspiration from the Venera nephelometers. Additionally, the probe included Galileo’s Net Flux Radiometer (Sromovsky et al. 1992)forthe atmosphere energy balance determination. Finally, there is the precedent of the Huygens probe for the Cassini mission, developed jointly by NASA and ESA, with the objective of studying the atmosphere of Titan. In this case, the Descent Imager/Spectral Radiometer (DISR) was the key instrument for the characterization of the aerosols (Tomasko et al. 2002). It encompassed the measurement of total solar radiation flux as well as measurements of scattering in two spectral bands and across two polarization planes, along with wavelength-dependent extinction measurements across different layers of the atmosphere. Moreover, it incorporated a camera system to capture winds and details of the Titan’s surface. Although the spatial coverage of the probe observations is very limited compared with the orbital observations, the characterization of the atmosphere (e.g., the aerosols) with in-situ measurements is carried out with a level of detail that is not achievable from orbiter observations. Consequently, various instruments have been proposed in recent years for the descent probe of a prospective Ice Giant Mission. One such instrument is the Nef Flux Radiometer (Aslam et al. 2020), which builds upon the legacy of the aforementioned NFR instrument from Galileo’s mission. Its objective is to measure the atmospheric heat balance and structure, the three-dimensional flow in the troposphere, and the compositions and opacities of the cloud layers. Another notable instrument is the LONSCAPE nephelometer and particle counter (Renard et al. 2020). This instrument was proposed to retrieve both the concentrations and phase functions of aerosols in the range of 0.2 to 50 µm, thereby constraining their composition and formation process. 3InstrumentConcept The characterization of aerosols in the various planetary atmospheres visited by descent probe missions has been crucial for comprehending their global atmospheric processes. Nevertheless, due to the limited resources of the future mission, primarily resulting from the considerable distance to Uranus as well as the stringent constraints imposed on the descent probe in terms of mass, volume, and power availability, the study of aerosols could be compromised. 6 Page 4 of 30 V. Apéstigue et al. Fig. 1 The Uranus Multi-Experiment Radiometer and its accomodation in the UOP descent probe. The Optical Head is attached to the external wall of the vessel (in blue) and the Processing Electronic is included in the internals of the probe (green). Some of the Fields of View of different optical channels are shown as examples. Credits: Ahmad Alromeadheen for the artistic view of Uranus’s clouds background; descent probe inspired by Simon et al. 2021; UMR and final picture composition, INTA To prevent this significant loss of atmospheric information from Uranus, considering the challenges in developing other future missions to the Ice Giants, and understanding that this kind of science could only be done in situ, we propose the Uranus Multi-experiment Radiometer (UMR) to be included in descent probe of the next mission (see Fig. 1). As will be explained in this work, this instrument concept is simple, compact, flexible, and lowpower, with limited controlled mass and volume, offering high scientific yield with minimal resource demand (see a summary in Table 1). Much of its design is based on previous INTA miniaturized instrument developments for Mars devoted to dust and cloud characterization: the Solar Irradiance Sensor (SIS) for the MetNet lander mission (Harri et al. 2017); the Solar Irradiance Sensor (SIS-16), which flew on ExoMars 2016 as part of the DREAMS payload (Arruego et al. 2017; Esposito et al. 2018); the Radiation and Dust Sensor (RDS), which is currently on-board Perseverance as part of the MEDA weather station (Apestigue et al. 2022; Rodriguez-Manfredi et al. 2021); and the Solar Irradiance Sensor 2020 (SIS-20) for the ExoMars 2018 mission (Arruego et al. 2016), integrated into the Meteo Package of the surface platform Kazachock (see Zelenyi et al. 2015). This approach allows us to use well-established technologies with a high Technological Readiness Level (TRL). The UMR incorporates a suite of photodetectors, interference filters, light sources, and optomechanical masks to effectively configure its optical channels, which enables the instrument to capture a range of measurements at different heights, geometries of observation, and wavelength bands. It takes miniaturization as the main design driver, minimizing its impact on the probe accommodation by dividing the instrument into two units. An optical head (UMR-OH), located on the external side of the probe, includes the sensors and the necessary proximity electronics. It can amplify, digitalize, and send the resulting signals to the other unit for electronic processing (UMR-PE), which is located inside. UMR-PE has the ability to control and process the signals. The communication between the two units is carried out using a differential-type serial interface, which utilizes a sealed pass-through between the exterior and interior of the vessel. Then redundant digital serial and power interfaces are used to connect the radiometer to the probe’s onboard computer and power subsystems. The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 5 of 30 6 Table 1 UMR main characteristics Parameter Value Dimensions OH: Ø150 mm x 35 mm PE: 102 mm x 64 mm x 24 mm Mass OH: 350 g PE: 220 g (Harness not included) Temperature Environment OH: 55 K to 110 K PE: ∼293 K Radiation Environment 100 krad Power Consumption UV-Vis-NIR: 0.75 W (avg.) Mid-IR: 1.25 W Heater: 4 W Data rate 33 bytes/s (avg.) Interface Redundant RS-422 @ 57.6 kbps Structure >20 bar Design lifetime 14 years 4 Scientific Objectives The scientific objectives of the instrument fall into four areas including (1) to study the ultraviolet (UV) energy absorption by aerosols and gases, and determine its penetration in the atmosphere; (2) to study the size, vertical distribution, and optical properties, sources and sinks of aerosols (hazes and clouds) by measuring the scattered sunlight profiles at different wavelength bands, from the visible to the near infra-red (IR); (3) to study the thermal balance of the atmosphere by measuring the up-and-down fluxes from the UV to NIR; and (4) to study the cloud vertical distribution, density and composition at pressures with negligible Sunlight (p >4-5 bars) by using flash-light emission and detection in the IR. Table 2offers an overview of the distinct channel sets assigned to fulfilling particular scientific goals, in addition to those designated for crucial auxiliary measurements. These auxiliary measurements encompass determining the sensor’s orientation in relation to the Sun and assessing the potential degradation of channels, for example, due to radiation exposure. The specific placement of each channel within the UMR-OH is visualized in Fig. 2. 4.1 Ultra-Violet Profiles A basic objective of the instrument investigation is to measure solar ultra-violet (UV) energy deposition in Uranus’ atmosphere and its variation with altitude. This will be done using two photodetectors pointing at 90° with respect to the instrument plane (see Fig. 2) and with different UV interference filters (technical description is given in Sect. 5.5). Each photodetector has a field of view (FOV) of 80°, which allows measuring at each altitude the UV radiance coming from almost all directions. To define the dynamic range of the sensors, we will make use of radiative transfer simulations as follows: (1) simulate the sky brightness (radiance field) as a function of the zenith and azimuth angles at different wavelengths 6 Page 6 of 30 V. Apéstigue et al. Table 2 UMR channel definition: scientific rationale, wavelength, field of view, pointing, and dynamic range Channel Type Scientific Rationale Channel Number Wavelength (nm) Elevation* (°) Field of View (°) Dyn. Range (mW/m2) A Sun position and trajectory reconstruction 1-4 200-1100 0 120 2775 B.1 Aerosol phase function characterization 5-10 (3x) 560 ±10 5-10 5 97.62 (3x) 937 ±10 45.28 B.2 Spectral Aerosol Optical Depth (AOD) and UV levels 11-14 200-210 0 80 0.144 285-305 21.8 690-710 79.5 990-1010 39.3 C Energy Balance 15-16 (2x) 190-1100 15-30 ∼180 ×90 3100 D Ice species determination and height distribution 17-18 3200-3370 0 80 6.8 3850-3930 7.2 IR Lamp 0-4000 40 E Displacement Damage 19-20 200-1100 N/A N/A N/A Blind-Detectors 3300 *With regard to the relative average sun position calculated for the mission Fig. 2 Schematic design of the Uranus Multi-Experiment Radiometer (UMR). The different channels are identified based on the scientific goals (see Table 2) and operational modes (see Table 5) and pressures; (2) integrate the radiance field over the sensor FOV and wavelength band to compute the irradiance (units of W m−2); and (3) convert the irradiance into detector photocurrent based on the sensor filters and responsivity (this part is treated in Sect. 6). Figure 3a shows the variation of the sky brightness at p =0.73 bar as a function of the zenith and azimuth angles simulated using the NEMESIS correlated-k radiative-transfer and retrieval code (Irwin et al. 2008). For these simulations, we used the aerosol/gas model described in Irwin et al. (2022) (referred to as IRW22 thereafter), where the sun is at a zenith angle of 79.1°. By integrating the radiance field over the sensor’s FOV and corresponding spectral band at each pressure level, we derive the irradiance expected during the experiment. Fig- The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 7 of 30 6 Fig. 3 Simulations of the solar irradiance in Uranus’ atmosphere using the NEMESIS code and the IRW22 model. In all the simulations, the solar zenith angle (SZA) is 79.1°. (a) Variation of the sky brightness with zenith and azimuth angles at 0.11 bar level and in the 700 ±10 nm wavelength range. The transparent-grey circles are added to show the different field of view (FOV) of channels B.1, B.2, and C. (b) Irradiance vertical profiles at 205 ±5 nm, 295 ±10 nm, 700 ±10 nm, and 1000 ±10 nm for the FOV of B.2 channels and a pointing direction of 70° in zenith angle and 90° in azimuth (concerning the sun azimuth). Corresponding color vertical lines indicate the minimum irradiance level detectable for each channel (see Sect. 0). (c) same as (b) but for channels B.1 560 nm and B.1 937 nm. (d) integrated upward and downward flux for channel C ure 3b shows, as an example, the irradiance profiles for different spectral bands (specific UV bands have not yet been chosen) and the FOV of B.2 sensors (red circles in a radiance field map Fig. 3a). Thus, by doing this analysis for different sensor orientations (since the probe is likely to tilt and spin during the descent), we can define the dynamic range of each sensor. Methane dissociation by solar UV and energetic particles leads to a network of chemical reactions that trigger the formation of more complex hydrocarbons such as C2H6,C 2H2, or C4H2(Moses et al. 2018). These hydrocarbons can condense into their respective ices, forming the planet’s stratospheric haze. Although methane photolysis and the subsequent haze formation take place in the higher parts of the stratosphere, and thus at altitudes where the probe is not expected to be in measurement mode, questions remain as to whether such processes (formation of photochemical haze) could also occur at deeper levels in the atmosphere. For instance, Irwin et al. (2023) showed that Neptune’s dark spots are caused by darkening at short wavelengths of a deep ∼5 bar aerosol layer, and one hypothesis is that this dark material is a photochemical product produced by the photolysis of H2Sby UV radiation. Therefore, from these measurements, we will determine the penetration of UV light in the troposphere and establish the altitudes at which the production of photochemical aerosols is possible. Also, by combining these measurements with those made by the channels dedicated to the aerosol (haze and clouds) characterizations (next section), we will determine whether, in a given layer, the decrease in UV radiance is mostly due to the absorption of gases or aerosols. In the following section, it will be explained how these measurements will also be used to constrain the aerosol’s optical properties in the UV. 6 Page 8 of 30 V. Apéstigue et al. Table 3 Pre-selected detectors for the UMR channels. Manufacturer data for 293 K operational temperature Detector UMR Channel Package Sensitive Area Wavelength Photosensitivity Dark current*Shunt resistance Rsh* Noise equivalent power NEP (mm2) (nm) (A/W) (pA) (M)(W/Hz 1/2) Hamamatsu S5980 A8.8×10.6 mm 5 ×5/4 segments 320-1100 (960 peak) 0.72 0.002 – 1.4 ×10-14 Ceramic Surface Mount Hamamatsu S1337-1010 B.1, B.2, C&E 16.5 ×15 mm 100 190-1100* (960 peak) 0.5 200 200 1.8 ×10-14 Ceramic 2 pin terminals 12.5 mm pitch Hamamatsu P16112- 033MF D-E TO-46 0.7 ×0.7 3270–3330 (3300 peak) 0.0028 – 0.18 1.1 ×10−10 Hamamatsu P16112- 039MF D TO-46 0.7 ×0.7 3820–3980 (3900 peak) 0.0032 – 0.18 9.5 ×10−11 *For reverse voltage VR=10 mV 4.2 Distribution and Properties of Haze and Cloud Particles Haze and cloud particles can directly affect the energy balance of Uranus’ atmosphere, and thus the atmospheric dynamics, by absorbing and scattering the solar radiation. Different properties of the haze and cloud particles are important for understanding their interactions with solar radiation. The particle size, shape, refractive index, and concentration determine the following aerosol parameters: (1) the phase function (angular distribution of the light scattered by an aerosol); (2) the single scattering albedo (ratio of scattering efficiency to total extinction efficiency); and (3) the optical depth (measure of the extinction of the radiation by the aerosols) of the aerosols. UMR will derive many of these properties using combinations of spectral irradiance measurements from the UV to the near IR. Six sensors with narrow FOVs (±5°) and at two spectral bands (channels B.1), from 550 nm to 570 nm and from 927 nm to 947 nm, are used to study the aerosol phase function and particle size. To accomplish a wide range of observable scattering angles, each pair of spectral sensors has different pointing angles: 0°, −30°, and 30° relative to the normal of the instrument plane (see Fig. 2) to have zenithal coverage. For azimuthal measurements, the instrument capitalizes on the anticipated tilt and spin of the probe during descent. It incorporates a four-sector sun sensor (refer to Table 2 and Table 3Channel A) to reconstruct the instrument’s attitude. A dynamic model of the descent probe will be developed for this purpose, integrating, at a minimum, data from the sun sensor and the accelerometer included in the selected Atmospheric Structure Instrument (i.e., Ferri et al. 2020). This will ascertain the Sun’s relative position to the UMR at all sampling points. This type of sun sensor is frequently employed for cost-effective attitude determination in low-earth-orbit (LEO) satellites (De Boom et al. 2006), with a precision of up to 0.15° (1σ). It utilizes a quadruple photodetector device (e.g., Hamamatsu S5981), where sunlight reaches the device’s center through an aperture or pupil. The section of the aperture and its The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 9 of 30 6 distance to the surface of the photodetectors define the solar sensor’s field of view (FoV). The signals measured on each quadrant correlate with the Sun’s azimuth and elevation (Sun et al. 2023) relative to the sensor’s normal. Therefore, this instrument configuration ensures measurements at both small and large scattering angles. At small scattering angles, the phase function is very sensitive to the particle radius (the larger the particles, the greater their dispersion in the forward direction) and structure (e.g., for aggregates, the slope of the phase function depends on the fractal dimension). Thus, the observations made by B.1 sensors at small scattering angles in the solar aureole will be used to constrain the aerosol particle radius and structure. At backscattering angles (>90°), the phase function is very sensitive to the particle shape (e.g., spheres, fractal aggregates, or irregular ice crystals), and this information is key for understanding under which conditions the aerosols are formed at a given altitude. For the retrieval of the phase function, different aerosol shapes and sizes will be employed to study which aerosol model provides the best fit between the observations and simulations. These measurements require a wide sensor dynamic range to cover the range that goes from the low intensities in the backscatter angles to the direct light intensity when the sun is within the sensor FOV. This will be done by using a number of channel gains (see Sect. 0). Figure 3c shows irradiance profiles simulated for channels B.1. As for the UV sensors, the definition of the dynamic range will come from the radiative transfer simulations. In addition to B.1 channels, two more sensors, pointed normally to the sensor, with 80° FOVs, and spectral bands of 700 ±10 nm and 1000 ±10 nm will be used for the aerosol retrievals (last B.2 channels of Table 2). The analysis of these sensors’ measurements, along with those from the UV sensors, will provide information on the optical depth and single scattering albedo at four spectral bands. Because of their wide FOV, their observations are less affected by the phase function than the B.1 channels, being the aerosol total opacity and single scattering albedo as the main parameters controlling their measurements. The spectral aerosol opacity and single scattering albedo depend on the particle radius and refractive index. Thus, the aerosol properties and vertical distribution will be retrieved by using combinations of the B.1 and B.2 channel measurements. This implies that both B.1 and B.2 channels must operate simultaneously during the descent and up to 1-2 bar level. 4.3 Solar Net Flux As one of the objectives of the UMR is to directly estimate the heating rates in Uranus’ atmosphere, the instrument has two photodetectors looking upward and downward to measure the solar flux in the spectral band from 190 nm to 1100 nm. The detectors are pointing upwards and downwards with a FOV that covers about 180° in azimuth angles and about 90° in the zenith (channels C, see Fig. 2) using a masked diffuser. Due to the parachute’s blockage and the fact that the instrument is designed to be on one side of the probe, these measurements cannot cover all the zenith and azimuth angles. However, they are captured through successive samples as the probe rotates. The net flux is computed from the difference between the downward flux and the upward flux, and the difference in the net flux at the up and down boundaries of an atmospheric layer gives the solar energy absorbed by it (heating rate). Thus, from these measurements, the planet’s heating profile can be directly estimated. This information is key for understanding the role of hazes and clouds in the stratosphere and troposphere energy budgets. Before using these measurements, three corrections need to be made: (1) extending the range from 190 nm to 1100 nm to 190 nm to 2500 nm; (2) combining different observations to cover the whole range of azimuth angles and compute by RT simulations the intensity at 6 Page 16 of 30 V. Apéstigue et al. Since the irradiance reaching the planet is almost 160 times less than that reaching Mars, a detector model with nearly 100 times more active area has been chosen to capture a greater amount of light (Hamamatsu S1337-1010bq) for B and C sets. However, this increases the device’s dark current could potentially compromise the dynamic range, but this effect is nearly insignificant when operating at such low temperatures (50 K-100 K). In the case of A set, we have selected a segmented device with 4 photodiodes in a 2 ×2 configuration. The orifice of its FoV mask collects the light as a beam in the middle of the device if the Sun’s path is normal to its surface. The four sensors receive the same light in this configuration. When the instrument turns, the light spot over the detector will move and modify its shape. The relationship between the 4 sensors signals will be used to calculate the relative position between the instrument and the Sun. The diffuse light could introduce some error in this attitude determination but it is estimated that the precision could be around 1° both azimuth and zenith. Similarly to what occurred in the Mars mission design, these components may experience degradation due to exposure to radiation during the lengthy journey to Uranus, stemming from the damage caused by energetic particles and cosmic rays. Consequently, a shielded photodiode has been incorporated, and its dark current is measured to assess potential degradations across the various channels. On the other hand, the spectral transmittance of these devices will need to be thoroughly characterized, as low temperatures alter the properties of silicon, modifying its internal absorption and refractive index (Nagakubo et al. 2015). Additionally, carrier mobility is also reduced under these conditions, and the combination of these two factors results in a decrease in responsivity with temperature (Camin and Grassi 2006). In the study by Glebov et al. (2018), they analyze this effect in pre-selected photodiodes by UMR (S1337-1010BQ), obtaining responsivity losses of less than 6% for spectral bands at 900, 700, and 500 nm. However, they indicate that at 1000 nm, losses can reach up to 60% at 90 K. For the IR channels (Group D), we have pre-selected new photodetector devices, Indium Arsenide Antimonide (InAsSb), from the Hamamatsu family P16112-XXMF (see Table 3). This detection technology is uncooled and highly sensitive. The devices in this family include band-pass filters centered, among others, at 3.3 um and 3.9 um, which align perfectly with the scientific requirements for detecting and characterizing clouds (see Fig. 9a). Different types of InAsSb structures have been tested for both ionizing and non-ionizing radiation effects. Superlattice structures appear to be quite robust (Steenbergen et al. 2013), but barrier architectures exhibit degradation in quantum efficiency (Morath et al. 2019). As a tentative selection, a corresponding qualification campaign for radiation and cryogenic temperatures will be conducted on these devices (among others equivalent candidates) in the subsequent stages of our technological UMR development program. For the case of the IR emitter, we have pre-selected a nichrome (NiCr)-filament-based lamp (INFRASOLID HIS2000R-A300-6), that offers a high-optical-power emission (up to 620 mW, see Fig. 9(b&c)) in a very compact device (Table 4). It includes a gold-plated reflector that offers enough narrow angular radiation emission and a sapphire window that closes the hermetic TO-8 package. All sapphire windows used for protecting detectors and emitters should include a heater (see Fig. 6) to prevent potential condensation of species that reach their dew point (e.g., CH4, H2S) during descent. The aim is to slightly heat the external surface of the windows above the atmospheric temperature, as suggested in previous instrumentation for descent probes (e.g., Sromovsky et al. 1992; Ragent et al. 1992). To achieve this, we propose using an Indium tin oxide-based coating (ITO) on or below them. These materials are commercially used as transparent heaters in the UVA-NIR region of the light spectrum. The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 17 of 30 6 Table 4 Pre-selected IR lamp for the UMR channels. Manufacturer data at operation temperature of 293 K IR Lamp UMR Channel Package Radiating element area Wavelength Optical output power Electrical power Electrical resistance Modulation frequency (mm2) (µm) (mW) (W) ()(Hz) Infrasolid HIS2000R- A300-6 A TO-8 40 2-6 620 2.5 5-6 4-10* *50%-20% modulation depth Fig. 9 (a) IR selected sensors family wavelengths. (b) IR lamp emission spectrum and (c) optical power emission. Credits: component datasheets INTA has a solid track record in using this coating for space applications. The institution developed the liquid-crystal variable retarders included in the Solar Orbiter SO/PHI (Solanki et al. 2020) and METIS/COR (Antonucci et al. 2020) instruments. These devices use ITO-deposited films for liquid-crystal active control (Alvarez-Herrero et al. 2011)and have consequently been qualified for the space environment (Alvarez-Herrero et al. 2017). For mid-IR detectors, we plan to explore the use of ITO coatings doped with other compounds (e.g., ZnO or Zn) that could enhance Mid-IR transmittance (Genty et al. 2011), where traditional ITO coatings exhibit absorption behavior. Preliminary calculations indicate that we will need 25 mW to prevent condensation on each window, totaling 400 mW of power during the 1 h descent phase. Another potential cause of channels degradation could be the deposition of particles on the protection windows (which are expected to be predominantly ice crystals). In this regard, a fluid dynamic study will be conducted to evaluate different designs of the OH shape and windows integration, or even to include deflection surfaces in order to minimize this effect, which will need to be characterized on Earth by means of wind tunnel facilities. 5.6 Electronic Design 5.6.1 Optical Head – UMR-OH INTA has maintained a collaborative partnership with the Institute of Microelectronics of Seville (IMSE) for over 15 years, focusing on the development of mixed-signal ASICs for space use and extreme temperature conditions. During these years, IMSE has developed its own radiation-hardened libraries for the standard AMS CMOS 0.35 µm technology (Ramos-Martos et al. 2012 and 2013). These libraries employ design hardening techniques 6 Page 18 of 30 V. Apéstigue et al. Fig. 10 (a) SIS20 ASIC physical dimensions and layout. (b) SIS20 ASIC Functional Blocks. (c) SIS20 ASIC Digital Block in detail (Sordo-Ibañez et al. 2014) both to mitigate the effects of radiation and to use it at low temperatures (323 K −148 K). IMSE has also qualified the encapsulations for these developments, compatible with MIL-STD-883 standards and with extreme thermal cycling through long-duration PQV (Package and Qualification Verification) tests. This effort has allowed the institute to develop the ASIC for the wind sensors (WS) of the MEDA meteorological station, which currently operates in the harsh conditions of the planet Mars. For the ExoMars 2018 mission, a mixed-signal ASIC (Vázquez et al. 2018)wasdeveloped for INTA’s radiometer, the Solar Irradiance Sensor 20 (SIS-20). This ASIC could not undergo radiation testing, but it shares the same building block that was previously tested. Therefore, it can be inferred that it exhibits the same radiation-hardened behavior (Ramos-Martos et al. 2012 and 2013): (i) the technology and library were tested up to 318 krad (Si) for total ionizing dose (TID); (ii) it is single event latch-up (SEL) free up to 81.8 MeV·cm2/mg (heavy ions); (iii) it is single event transient (SET) free up to 40 MeV·cm2/mg; and (iv) it is single event upset (SEU) free up to 22.5 MeV·cm2/mg. This latter development has been taken as a starting point for the electronic design of the Optical Head unit of the UMR (UMR-OH). The main components of the UMR-OH electronics are a pair of mixed ASICs previously developed for the SIS-20 radiometer (Vázquez et al. 2018). This device, Fig. 10 (a), comprises four major subsystems (Fig. 10 b) summarized in: i) an analog front-end that includes 10 trans-impedance amplifiers (TIAs) with two programmable gains per channel, 4 PT1000 platinum resistance channels, an 8-bit current-output DAC for driving light calibration sources like LEDs, and 3 general-purpose voltage amplifiers with configurable gain; ii) The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 19 of 30 6 Fig. 11 Electrical functional blocks detail of the two units that comprise the UMR a 16-bit ADC with multiplexed input and a preceding instrumentation amplifier with configurable gain; iii) a 10-bit DAC used as an input to the previous instrumentation amplifier, providing sub-ranging capabilities; and iv) a digital layer, as depicted in Fig. 10 (c), that encompasses digital signal processing blocks enabling various automatic functions such as oversampling up to 256 samples averaging, standard deviation calculations, a 100 Kbits/sec slave SPI interface, configuration/control registers, a Power-on-reset (POR) circuitry, and a CLK signal generator. As discussed in Sect. 4, to achieve the scientific objectives, the UMR employs 20 optical channels integrated into the OH. To maximize integration, we use two of these ASICs (see Fig. 11). This approach allows for a high degree of integration by automatically conditioning and digitizing the data with a power consumption of 250 mW each. The conditioning of both the Si photodiodes and the InAsSb photodetectors is based on a solution in the designs of both RDS and SIS-16 (see Fig. 12 (a)). In this scheme, the first trans-impedance stage contains the main amplification that provides the dynamic range for the optical channels. This first stage allows for configuring two gains, adapting the range to measure low values of scattered diffuse light intensity or, conversely, for situations where direct sunlight may enter the FoV, causing possible saturation. A second stage is added to the analog chain, including an instrumentation operational amplifier in combination with a DAC. This configuration allows the application of sub-ranging techniques to maintain high levels of accuracy when changing the main gain. This stage also provides a final gain to the result of the subtraction configurable with 1 or 50 amplification factors. Due to the fact that measurements are performed at low frequency, the required amplification levels can be achieved with the first transimpedance stage without bandwidth limitation. The dynamic range is calculated based on simulations obtained with the radiative transfer model (last column Table 2), for worst cases with the sun inside the FoV. This amplification levels necessitate the use of feedback resistances (Fig. 12(a) Rf1 and Rf2) with very high values (from mega to giga ohms), making thermal noise in these resistances dominate over the electronic noise of the amplifier. In Fig. 12(b), we present an example of calculating the signal-to-noise ratio (SNR) as the ratio of the current required by the photodiode to the current noise calculated. The last is calculated as the root mean square sum of three terms: (i) the thermal noise from the feedback resistance Rf; (ii) the thermal noise of the intrinsic resistance of the photodiode Rshunt; and (iii) the shot noise resulting from the photocurrent received by the photodetector and its dark current (at worst case mission operational temperature of 120 K). 6 Page 20 of 30 V. Apéstigue et al. Fig. 12 (a) UMR photodetectors acquisition chain shematics. (b) Example of noise levels calculations for each detector. In this case, we analyze channel 11 (B.2 UV) with corresponds to the limit of detection (vertical line) associated with this channel in Fig. 3(b) To infer the minimum irradiance level for each channel, limited by the noise, we have established SNR =100 as a lower limit (Fig. 12 (b)). With the corresponding received power at the receiver, considering its effective active area (accounting for losses due to filter transmittance and FoV masks), we have calculated the minimum irradiance levels for each of the channels. These can be observed as vertical lines in Fig. 3(b), (c), and (d). In the case of mid-infrared (mid-IR) channels, a scheme identical to that of Si photodiodes will be employed. In this context, the emphasis is on the faint signal derived from the backscattering of aerosol particles when illuminated by the lamp. Noise calculations analogous to those for the Si photodiodes have been conducted for these devices, but in this instance, the minimum and maximum received light power from the scattering of the lamp light (Fig. 4b) are taken as input. For Channel 17, we obtain a signal-to-noise ratio (SNR) ranging from 29 to 200 for the minimum and maximum expected received power (which depends on the species present during the measurement). For Channel 18, we have obtained an SNR ranging from 20 to 271 in a similar manner. Both results could be enhanced using the ASIC oversampling capability, which could rise theoretically the SNR up to a x16 factor (for 256 samples) with a conversion time of 377.5 ms. To mitigate background noise that may arise from the atmosphere and even from the channel housing, and to filter the desired signal, a digital lock-in filter will be implemented. Due to the high levels of amplification required, the source will be excited at the order of tens of hertz, and the ASIC will be responsible for generating the carrier by driving a power driver that controls the lamp. Additionally, the ASIC will digitize the signals and provide them to the microcontroller, which will handle the processing tasks. Besides eliminating background noise, this technique can also be employed to enhance potential secondary amplification. Once the signal is converted to voltage through the initial transimpedance stage, the second amplification is conducted in voltage, and thus, the 1/f noise from the operational amplifier becomes significant. Utilizing a filter with a 10 Hz carrier can mitigate this noise by a factor of 5, depending on the operational amplifier’s characteristics. However, the final frequency selected shall have a compromise between the IR-lamp operational frequency capability, the oversampling selected in the ASIC and the final acceptable SNR figure. To accommodate all these electronics, the UMR-OH has four boards linked together forming a unique rigid-flex polyamide PCB. Two of them are close to the upper structure that supports the three lateral photodetectors (B.1 and C sets). The central rigid PCB supports the B.1 and B.2 photodiodes, the solar sensor (A set), and the IR lamp and photodetectors (D set). Finally, at the bottom of the set, we have the main PCB that contains the ASICs and the driver for the lamp. The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 21 of 30 6 5.6.2 Processing Electronic Unit – UMR-PE The UMR-PE is equipped with two electronic boards that serve for two primary functions: (i) to adapt the power levels provided by the platform and (ii) to accommodate a small payload computer. This design is based on the COMPACT architecture (Configurable Miniaturized Processing Architecture) developed by INTA (Martin-Ortega et al. 2017). The basic idea of the COMPACT approach is the recursive employment of several components with predefined configurations, allowing for the customization of an optimal computer for each application based on its specific requirements, including computational capacity, performance, power consumption, and footprint. Consequently, this architecture employs various devices for power conversion, such as isolated DC-DC converters and linear regulators, as well as different components for signal processing and coprocessing, including microcontrollers and FPGAs. COMPACT components are chosen based on their radiation resistance and undergo testing to ensure their performance at low temperatures avoiding the use of heaters. In the framework of COMPACT, various base configurations have been predefined depending on the system to be controlled, with the simplest configuration for the UMR consisting of a microcontroller paired with an external program and massive data storage memory (Fig. 11). The selected microcontroller for UMR is the SAMRH707. It is a RadHard device (100 krad; LET >78 MeV·cm2/mg) based on the ARM Cortex M7 architecture developed by Microchip, capable of reaching up to 100 DMips of processing power. It comes equipped with its internal memories, including 128 kbytes of Flash, 320 kbytes of SRAM, and 128 kbytes of ROM. It offers a wide range of communication peripherals, including high-speed (SpaceWire), and advanced analog features: a 12-bit ADC and DAC. Additionally, it includes numerous low-power modes, allowing for fine-tuning of its computational capabilities in response to energy demands. For the UMR, using a 10 MHz clock, the maximum nominal power consumption would be 400 mW with a sleeping mode of ∼1mW. The use of a microcontroller instead of low-resource anti-fuse FPGAs, as in our previous radiometers (SIS, SIS-16, RDS, and SIS-20) represents a significant advance in flexibility and capabilities for the UMR. This transition offers advantages ranging from the ease of hardware development and testing during the design phases to the expanded capabilities that can be imparted to the instrument. These capabilities include independence from its operation concerning the central computer, the ability to perform in-flight floating-point calculations, and the application of compression algorithms to telemetry data, among others. 6 UMR Operational Modes The UMR has been designed to meet various scientific objectives, and as a result, it incorporates different sets of sensors to achieve them. Consequently, these sensor sets operate in distinct ways throughout the mission, depending on the specific phase. We refer to these distinct operational states as Operational Modes (OPM). For each OPM, it is crucial to determine which channels need to be activated, identify the required housekeeping signals, and establish the specific sampling frequency necessary to meet the scientific objectives, including factors like vertical resolution and azimuthal data. Armed with this information, we can then estimate the amount of data needed throughout the mission and the corresponding energy consumption for each scenario. Table 5summarizes these estimations for the UMR and permits comparison with the UOP mission (Simon et al. 2021). 6 Page 22 of 30 V. Apéstigue et al. Fig. 13 Mission Operational Modes by mission stages: (a) interplanetary trip, (b) during the probe descent over Uranus atmosphere For the 1-hour whole mission, the UMR generates 161.9 kilobytes of raw data (Table 5), which represents 7.5% of the total data budget for the entire probe (2.3 megabytes or 17 megabits as per Simon et al. 2021). Using, for example, the same delta-encoding compression algorithm used in MEDA for the RDS (Rodriguez-Manfredi et al. 2021), these figures could be reduced by 55%. Regarding energy consumption, only 2.3% (2.75 watt-hours) of the 121 watt-hour probe’s energy budget would be used by our instrument, which is a negligible amount in total. The different operational modes are described below: OPM 1: This operational mode encompasses four checkouts and calibrations throughout the mission trajectory: the first one post-launch (labeled “OPM 1.1” in Fig. 13 (a)), the second one before the Jupiter flyby (“OPM 1.2”), the third shortly after the flyby (“OPM 1.3”), and the final one preceding the separation of the orbiter capsule (“OPM 1.4”). These checkouts are conducted to assess the instrument’s condition and monitor potential degradation of sensor elements during the mission, with the expectation that the most significant deterioration may occur during the Jupiter gravity assist maneuver. This assessment involves measuring the dark currents of various sensors as their values increase with displacement damage (performed in previous instruments Jimenez et al. 2012; Arruego et al. 2017;Apestigue et al. 2022), measure the temperatures of the sensors during all the scientific operation due its relationship with dark current values, along with monitoring leakage currents from ASICs’ operational amplifiers, which could subsequently affect absolute channel measurements. Utilizing this information helps mitigate radiation-induced effects on channel signals. Additionally, the lamp will be employed for in-situ calibrations. This lamp will produce light that will reflect off reflectors strategically placed inside the aeroshell (following the approach used in previous missions, as described by Ragent et al. 1992). The light will be detected by different channels, facilitating validation of the various sensor chains. Despite being designed to minimize data usage and power consumption, this mode is not considered critical in this regard since it is utilized while the probe is still attached to the orbiter, and these data fall outside the total data budget allocated for the descent probe. OPM 2: This operational mode serves as a minimal calibration step just before the aeroshell is discarded. At this point, the probe has already begun its descent through the planet’s atmosphere (labelled as “OPM 2” in Fig. 13 (b)), and the objective is to acquire the ultimate offset values for each channel, which will serve as references during the descent phase. Consequently, all UMR channels must be measured under dark conditions during OPM 2. The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 23 of 30 6 OPM 3: The third operational mode begins when the probe’s onboard computer triggers the UMR, signifying the successful deployment of the second parachute (0.1 bar; +55 km height). Then the UMR-OH is exposed to Uranus’ atmosphere and initiates the scientific phase (identified as “OPM 3” in Fig. 13 (b)) that will be manage by a sequence timer. During this phase, spanning from this point until crossing the cloud belt at 1-2 bars, precise knowledge of the relative position of the Sun becomes critical for achieving specific scientific objectives. The probe uses a swivel mechanism to decouple its spin from the parachute’s and is also equipped with 3 spin vanes to control the rotational rate. The experience of Galileo, described in Lanzerotti et al. 1998, is that the initial spin rate was around 40-35 rpm and was modulated by the aerodynamical system down to 15 rpm during the descent. This implies that at the beginning of the UMR mission, the probe could rotate more than 180° per second and consequently, the sun sensor channels should be sampled at a high rate to be able to capture enough scattering angles. Depending on the characteristics of the inertial sensors integrated into the probe, a subsequent study will be conducted to fine-tune the minimum sampling rate for these channels, ensuring the capability to reconstruct the probe’s attitude by combining data from both sensors, but for this first definition, we have considered 5 Hz. The UMR’s B.1 set of detectors should also be sampled at the same rate as the sun position to record scattering angles as comprehensively as possible. The B.2 and C sets of channels will also be measured during this phase at a lower rate of 0.2 Hz which corresponds to 500 meters of vertical resolution. OPM 4: The last mode is dedicated to characterize the clouds and therefore it starts at a depth of approximately 2 bars (∼768 seconds after the beginning of OPM 3 see Table 5). Sets B.2 and C continue to be sampled as the probe descends further (up to 5-6 bar), but at lower sampling rates (0.07 Hz) due to a reduction in the probe’s speed. The D photodetector sets will be utilized in conjunction with the IR lamp in a flashing sequence, with each flash lasting 0.5 seconds. This sequence will be repeated approximately every 14 seconds, resulting in a vertical resolution close to 500 meters. 7 Calibration and Validation Activities 7.1 UV-Visible-NIR Channels Calibration (A, B and C Types) The calibration method for the Si photodiodes will be an adaptation of the previous work done for the calibration of RDS, that is described in Jiménez et al. 2018. It is based on the spectro-radiometric transfer from a standard lamp to a standard detector in well-controlled laboratory conditions (e.g., Wyatt 1978,1991;Hülsenetal.2008; Datla et al. 2016). In this calibration model it is assumed that wavelength (λ), temperature (T) and incidence angle (θelevation, ϕazimuth sensor coordinates) can be considered as independent variables (Wyatt 1978 and 1991) and thus, the photodiode output signal I (A), could be expressed as: I(T,ϕ,θ,ESun)=ARF (θ,ϕ)·TRF(T)·Rλ2 λ1Sun ·Eλ2 λ1Sun +off s e t (T ) (1) where ARF(θ,ϕ) is the angular response function of the channel; TRF(T) is the temperature response function or channel dependence with the temperature; Rλ2 λ1Sun is the mean responsivity for a channel with λ1−λ2 wavelength, if the calibration is performed with a solar simulator with an Eλ2 λ1Sun irradiance level. To calculate equation (1), the following steps will be undertaken during the calibration procedure: (i) Offset calibration, step that involves utilizing a vacuum cryostat in which the 6 Page 24 of 30 V. Apéstigue et al. Table 5 Summary of Operational Modes (OPM) of the UMR during the whole mission. For each, information on the different channels used, the sampling frequency, and the data generated is presented Operational Mode (OPM) Mode Rationale Channel Type used*Sampling sequence duration Sampling Rate Vertical resolution Power Required Energy Generated Data (s) (Hz) (m/sample) (W) (Wh) (kbytes) 1 Health Checkouts Heating 1800 n/a n/a 4 2** 0.1** All in darkness 100 1 0.8 0.022** 2** All light ramp 100 1 5** 2 On-board calibration Heating 1800 n/a n/a 4 2 0.1 All in darkness 100 1 0.8 0.022 2 3 Sun position A 768 5 19 0.9 0.192 45 UV B.1 5 19 102 Aerosols B.2 0.2 470 0.6 0.128 3 Net Flux C 0.2 470 1.5 Transparent Heaters All 0.4 0.008 4 Aerosols B.2 2291 0.07 470 0.6 0.381 3 Net Flux C 0.07 470 1.5 Clouds D 0.07 470 1.25*** 2.7E-2 3.8 Transparent Heaters All 0.4 0.25 TOTAL 3.0 161.9 *Corresponding with Table 1classification **Not accounted for final figures due to the mission phase ***20 Hz Flash of 0.5 s duration, at 50% duty cycle (2.5 W maximum) The Uranus Multi-Experiment Radiometer for Haze and Clouds... Page 25 of 30 6 OH is exposed to darkness and subjected to a full temperature profile range (100 K−170 K); (ii) Temperature response function calibration using the vacuum cryostat with a constant light level exposure through a sun simulator, covering the complete temperature profile range (100 K – 170 K); (iii) average responsivity calibration, using different steps of light levels of the sun simulator at constant temperatures (it should be repeated for different temperatures); (iv) angular response function calibration, using a collimated and constant level light and a gimbal robotic structure to map the angular response of each channel. 7.2 Mid-IR Channels Calibration (D Type) The basic equation relating detector output current Iout to the external radiation flux is essentially (1). This equation is the results of adapting equations in (Sebastian et al. 2020) (Sebastian et al. 2021) for a photodetector: Iout =S(Td)·As·Fd−t·Lt+iSd (2) where S(Td)is the detector responsivity including dependence on temperature, Tdis detector temperature, Asis the detector area, Fd−tis the view factor between the detector and the target (considering in their calculus detector relative angular response), Ltis the irradiance form the target. Consider a flat radiation field, as an extended area blackbody, then Ltis given by the Planck radiance function Bλ(T ) and can be expressed as: Lt=∞ 0 sλe ·Bλ(Tt)dλ (3) where sλe is the detector relative spectral response at wavelength λand Ttis the target temperature. The term iSd is the photodetector current generated by background light, and it accounts for the flux exchange with the inner surfaces of detector package and the external mechanical assemblies conforming the external FoV. Since there is no filter limiting the bandpass of the inner fluxes, they represent an important source of uncertainty (Sebastián et al. 2011; Foote 1999). During operations and descent in Uranus, these fluxes will take a non-zero value because of the thermally challenging environment and their effect will be compensated using the electronics chopping of the IR lamp. To compute detector output according to the above equation (1), the following set of calibration tests will be performed: (i) Relative angular response. It will use a blackbody source operating above 500 K, a mechanical chopper to modulate blackbody signal, and a pan-and-tilt system as an angular scanning system. (ii) Relative spectral response. It will be performed using a 1000 K SiC global lamp, a monochromator to select wavelengths, and a reference detector to measure the monochromator output for comparison with detector output. (iii) Relative response versus temperature for each channel. The optical head will be mounted on a thermoelectric cooler within a sealed chamber to recreate thermal-vacuum conditions in which the instrument’s operating temperatures will be swept. The detector will be exposed to an extended area of blackbody as the radiation source. For each detector operating temperature, two or more blackbody temperatures will be programmed. In this way, a differential process to avoid uncertainties due to background light is implemented (Sebastian et al. 2020). (iv) Absolute responsivity measurements. To implement this calibration, test a blackbody source that fills uniformly the detector’s field of view will be used. The use of an extended area or cavity blackbody will be studied, taking into account parameters such as FoV, uniformity of the radiating surface and apparent emissivity. Once again, differential