VICARIOUS CALIBRATION OF MICROWAVE RADIATION IMAGER ONBOARD FENG YUN SERIES SATELLITES IN YUNNAN FOREST
Marzo 25, 2026THE CRYORAD AIRBORNE BAFFIN BAY CAMPAIGN: FIRST RESULTS AND LESSON LEARNED
Marzo 25, 2026J. R. Pardo1,2, C. de Breuck3, D. Muders4, J. A. González3, J. P. Pérez-Beaupuits4, C. Prigent2
1Consejo Superior de Insvestigaciones Científicas (Spain) – Instituto de Física Fundamental, 2Observatoire de Paris (France), 3European Southern Observatory, 4Max-Planck-Institut für Radioastronomie (Germany)
We will present the results of an extensive observational campaign carried out with the Atacama Pathfinder Experiment (APEX) aimed at measuring with unprecedented spectral resolution (better that 1 MHz) and calibration accuracy, to be described in the presentation, the 157-752 GHz atmospheric spectrum under various clear sky conditions from the high and dry site of Chajnantor (Atacama Desert in Chile, 5000 meters above sea level). APEX was, at the time of these measurements, a collaboration of the Max-Planck-Institut für Radioastronomie (MPIfR), the European Southern Observatory (ESO), and Onsala Space Observatory (OSO). These measurements have implications on testing atmospheric radiative transfer models and, therefore, are important for both astrophysics and Earth’s remote sensing:
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Studies of cold dark clouds, star-forming regions, evolved stars, galaxies, and other objects in space, through millimeter and submillimeter wavelength observations have broadened our vision of the Universe over the last few decades. These observations are usually performed from high and dry sites, where conditions allowing some sky transparency for them are found. An accurate atmospheric model is needed for planning, conducting, and calibrating the observations.
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The atmospheric millimeter and submillimeter spectrum is also important for Earth observations. Current operational meteorological applications are limited to 200 GHz. The upcoming European organization for the Exploitation of METeorological SATellites (EUMETSAT) Polar System-Second Generation (MetOp-SG), will carry an instrument, the Ice Cloud Imager (ICI), with frequencies up to 664 GHz. The main objective of ICI is to provide data on humidity and ice hydrometeors. Furthermore, the deployment of the EUMETSAT Polar System Sterna (EPS-Sterna) constellation, consisting of 6 microsatellites for launch around 2030, will enhance Numerical Weather Prediction (NWP) accuracy. Each microsatellite will carry a sounder, including channels around the H2O line at 325 GHz, providing global temperature and water vapor profiles with unparalleled coverage and revisit time. The reference radiative transfer models for these projects can be directly tested in their finest details with our Chajnantor data.
The contributors to the millimeter and submillimeter atmospheric spectrum are O2 through magnetic dipolar (M1) rotational transitions, H2O through electric dipolar (E1) rotational transitions, weaker E1 features from other “minor” atmospheric gases, such as O3, N2O, CO. HCN, HCl…, isotopologues and vibrationally excited states of some of these molecules, and, finally, non-resonant collision-induced absorption (CIA) due to several mechanisms: N2-N2, O2-O2 and O2-N2 collisions (dry CIA), and H2O-N2 + H2O-O2 collisions (foreign wet CIA), as well as H2O-H2O collisions (self wet CIA). The last one, as it involves collisions between water molecules, is almost two orders of magnitude lower than the foreign wet CIA at the dry conditions of submillimeter astronomical observatories.
A set of receivers, called SEPIA for Swedish ESO PI Instrument at APEX, are in a cryostat that can accommodate 3 receiver cartridges with tertiary optics to illuminate them inside the Nasmyth cabin A of the APEX telescope: SEPIA180 (159-211 GHz), SEPIA660 (597-725 GHz), and SEPIA345 (272-376 GHz). It was delivered by the Group for Advanced Receiver Development (GARD) at OSO in Sweden. Only one of the three receivers can be used at a given time, but the switch between receivers is very fast, taking only a few minutes. Under relatively stable atmospheric conditions this allows to scan all frequencies covered by the three receivers in just about one hour for a single air mass. The other bands, called new FaciLity APEX Submillimeter Heterodyne instrument (nFLASH), were delivered by the MPIfR Sub-mm technology division in Bonn. It is a receiver with two independently tunable frequency channels: nFLASH230 (196-281 GHz) and nFLASH460 (378-508 GHz). Both channels have two polarizations and two sidebands. The instrument is designed to work as a dual receiver to allow simultaneously observing in both 230 and 460 channels. All five heterodyne receivers are connected to modern digital Fast Fourier Transform Spectrometers (FFTSs). They are based on high-speed analog-to-digital converters (ADCs) and highly complex field-programmable gate array (FPGA) chips for signal processing. The FFTSs used for these measurements transform 4 GHz bandwidth sections of the 2 side-band (SB) 4-12 GHz intermediate frequencies (IFs) into 64k spectral channels, although the resulting kHz spectral resolution is smoothed to around 1 MHz for the final data products, which is largely enough to resolve even the narrower atmospheric lines.
For this study it was important to cover a wide range of precipitable water vapor column (PWVC) conditions, but also different seasons and times of the day. The idea was to gather a data base covering as many situations as possible in order to perform model tests and statistical analysis that would help us in refining the atmospheric model for its ESO implementation. In order to conduct the necessary observing campaigns, we applied for APEX telescope time via technical calibration proposals and, in total, we got around 40 hours of observations that were divided into several observing sessions in 2020, 2021 and 2022 with a satisfactory distribution of PWVC, diurnal and seasonal conditions.
Although some partial results have been already published, the overall data base is still under analysis and comparison with our radiative transfer model (ATM) and other models. Our presentation at Microrad 2026 will focus on the implications of this detailed analysis for both millimeter/submillimeter astrophysics and remote sensing applications.
