THE CIMR-AIR INSTRUMENT – AN AIRBORNE RADIOMETER SYSTEM TO SUPPORT THE COPERNICUS IMAGING MICROWAVE RADIOMETER MISSION

J. Kainulainen¹, T. Saarinen¹, J. Uusitalo¹, T. Häkkilä¹, T. Yesilyurt¹, J. Lahtinen¹, J. Lemmetyinen², K. Rautiainen², M. Mäkynen², P. Ahola², R. Forsberg³, M. Suess⁴, C. Donlon⁴

¹Harp Technologies Oy, ²Finnish Meteorological Institute, ³Technical University of Denmark, National Space Institute, ⁴European Space Agency

The Copernicus Imaging Microwave Radiometer (CIMR) mission of the European Space Agency (ESA) comprises two identical remote sensing satellites that will be launched sequentially, with expected launches in 2029 and 2035. They will provide a continuum of passive microwave remote sensing data of the Earth over a time period of ten to fifteen years, providing information on diverse geophysical variables over both oceans and land surfaces, including sea surface temperature, sea ice concentration, sea surface salinity, soil moisture, and terrestrial snow water equivalent, among others. The mission payload is a multifrequency radiometer instrument operating at L, C, X, K, and Ka bands and performing measurements with a ground resolution varying from <5 km (Ka-band) to <60 km (L-band).

Development of an airborne instrument with the main characteristics similar to the CIMR payload instrument was initiated in 2023 by ESA with Harp Technologies Oy as the prime contractor. The airborne CIMR-AIR system is to be used in field experiments in various environments for gathering data for geophysical retrieval algorithm development, and for CIMR calibration and validation activities.

The system consists of the CIMR-AIR instrument, a Level 1 & Level 2 (L1&2) Processor, and supporting equipment such as laboratory and field calibration loads and hardware for transportation and installation. Further, the CIMR-AIR instrument consists of five fully polarimetric radiometers, visual and thermal infrared cameras observing the radiometer’s footprint area, infrared thermometer for sky radiation measurements, power management system, aircraft-independent attitude and heading reference system for accurate position and footprint position determination, and an operator’s display unit. The main radiometric characteristics the CIMR-AIR radiometers are similar to those of the CIMR satellite radiometer. The main characteristics of CIMR-AIR are shown in Table I.

The CIMR-AIR instrument is operated as a side-looking instrument hosted in an aircraft with adequately large (approximately >1.4 x 1.4 m) side door opening. The mechanics enable adjustment of the instrument’s installation angle, allowing the nominal beam angle relative to the ground (the observation angle) to be maintained, also during flight experiments involving circular flight with a constant aircraft roll. The radiometer’s modular design also allows for other mechanical installations, through updates to the mechanical interfaces. The current design is done for Twin Otter aircrafts, which are commonly used type of aircraft in scientific field experiments. An illustration of the design, mounted on a Twin Otter’s side cabin door opening, is shown in Figure 1.

The L1&L2 Processor, developed by Finnish Meteorological Institute (FMI), is an offline software tool, which processes raw radiometer measurements into L1A, L1B, and L2 products. The L1A data are time oriented engineering data that are corrected for instrument non-idealities. L1B data are temporally aggregated Stokes vectors geo-referenced in local ground coordinates and polarization. In L2 processing, the L1B data are processed to Sea Ice Concentration (SIC) and Sea Surface Temperature values (SST) in order to provide a first estimate of these geophysical variables. The L1A, L1B, L2 data as well as all instrument data necessary to reprocess the data are delivered to user in netCDF format.

Currently (September 2025), the CIMR-AIR system development is in the middle of manufacturing, assembly, integration, and testing phase. The integration is expected to be finished in early 2026, and the first flight experiment is currently planned for spring 2026.

In the conference presentation we elaborate the details of the system and explain the timely status of the development.