Pico-Light H2O (GSMA/DT-INSU) is the successor of the Pico-SDLA H2O hygrometer which has been operated since 2007 and allows in-situ monitoring of atmospheric water vapor in the UTLS. The direct absorption technique allows to unambiguously probe the atmosphere. The hygrometer has been widely tested and validated through simulation chamber campaigns (AQUAVIT-1  and 2) and compared in-flight with the ELHYSA frost-point hygrometer  and the FLASH-B Lyman-α hygrometer . The agreements scale from 0.5 to 2% in the tropical tropopause layer (TTL), within the GRUAN requirements.
Based on the same components, Pico-Light (weight: 2.7 kg) demonstrates the same performances in-flight. The precision is determined from the standard deviation of the measurements in-flight and is found to be of about 130 ppbv (30 ppbv away from the GCOS goal threshold) for a 1s integration time. The hygrometer has been compared to the NOAA FPH instrument during the AsA 2022 campaign. Preliminary results from the comparison shows that, in the stratosphere (above the cold point tropopause, CPT), the mean difference between Pico-Light and the NOAA FPH is of about (4.70 ± 2.48)%. In the tropopause layer (between the lapse rate tropopause and the CPT), the mean difference is of about (-0.17 ± 6.39)%. Indeed, these values have been obtained without considering the origin of the observed difference (i.e., temperature, atmospheric variability). Considering the temperature-induced bias and excluding differences induced by atmospheric fine-scale processes, the differences trim down by a large amount (analysis ongoing).
It has also been compared to other existing hygrometers within the AQUAVIT-4 campaign. In the frame of the HYGROMATMO project, funded by CNES since 2021, the hygrometer is intended to be calibrated against humidity standards in the frame of a collaboration with LNE-Cnam and LNE-CETIAT (national metrological institutes).
NOAA frost-point hygrometer (CIRES/NOAA ESRL GML) is based on the chilled mirror technique in which the mirror temperature is rapidly adjusted to maintain a stable layer of frost on the mirror. When the frost layer is stable, the frost layer is in equilibrium with the water vapor in the air flowing over the mirror, and the mirror temperature is equal to the frost point temperature. Well-established empirical relationships exist between the temperature of (water) ice and the vapor pressure above it. The partial pressure of water vapor in the air flowing over the mirror is directly determined from the frost point temperature, and with simultaneous measurements of the atmospheric pressure, the water vapor mixing ratio (mole fraction) is easily calculated. The NOAA FPH has been used for the last 40 years to monitor upper atmospheric water vapor over Boulder, Colorado [26,27]. Improvements were made as technologies advanced (analogue to digital circuits for frost control, advanced LEDs and photodiodes, etc.) Other monitoring sites, Lauder, New Zealand, and Hilo, Hawaii, were added to our network in 2004 and 2010, respectively (see : https://www.esrl.noaa.gov/gmd/ozwv/wvap/).
Micro-hygrometer (LPC2E) is a frost point hygrometer coupled with the air-temperature measurement newly developed by the LPC2E for the study of the stratosphere that can be deployed on radiosounding balloons. The instrument is lightweight (<3 kg) and the manufacturing cost will be low (≤12 k€) allowing for occasional instrument loss unavoidable in difficult to access terrain (Island, Amazonian forest). The cost of the instrument will also be attractive for future large-scale production for observation networks. The principle of measurement by frost point (or dew point) is the reference method in the stratosphere because it is well adapted to low concentrations and has a great dynamic range. A high miniaturization is obtained by replacing the mirror with a piezoelectric element, a quartz, with a temperature controlled by a thermoelectric cooler. Additionally, atmospheric temperature is measured by thermocouples with an accuracy of 0.5 K and a precision of 0.1 K. The instrument is currently at TRL 5 stage. The hygrometer has realized its first flight in Orléans in Mai 2022 and 3 flights have occurred within the AsA 2022 campaign. The flights realized during AsA 2022 have allowed to identify the weaknesses of the original version of the instrument, thereby opening the room for improvements. Between 9 and 12 km, the differences between the Micro hygrometer and Pico-Light H2O is within 12%. Following, an international patent application has been granted in 2022 by the INPI, managed by CNES.
M20 sondes from MeteoModem: The M20 is one of the radiosondes commercialized by Meteomodem. The M20 was released in 2021, and it is fully compatible with the SR10 receiver system and the Meteomodem software. Its dimension is 98mm*63mm*42 mm for 36 g with a lithium battery. The M20 is composed of a capacitive humidity sensor covered by an innovative metal coated shield that allows good ventilation while protecting it from direct radiation and from water droplet freezing on the sensor. A temperature sensor measures the air temperature and is positioned at the very end of the sensor boom. A GNSS system provides measurements of the position, from which the pressure, the vertical velocity, the wind speed and direction are derived. The capacitive humidity sensor is composed of three primary components: a basic layer that acts as an electrode; a dielectric material, whose characteristics are a function of relative humidity; and a fast response porous electrode that acts as the second electrode of the capacitor. A second thermistor is located under the protective shield close to the humidity sensor in order to have an approximative measurement of the temperature of the capacitive humidity
sensor. The UPSI France Company is the subcontractor for this capacitive humidity sensor and these sensors are made speciﬁcally and exclusively for Meteomodem.
Meteomodem radiosonde technology is used in 28 countries. All Meteomodem stations installed since 2011 use the M10 and M20 technology. Before 2011, sites used the former version of M10 (M2K2), and by now all are using the M10/M20 technology. Most of these sites produce two M10 or M20 radiosoundings per day (Dupont et al., 2020).