AN OVERVIEW OF FANTASIOR, A FLEXIBLE ANTENNAS ARRAY FOR SYNTHESIS IMAGING IN OBSERVATIONAL RADIOMETRY

E. Anterrieu¹, L. Yu², P. Gonzalez², R. Contreres², N. Mézieres², G. Le Fur², O. Fouquet³, M. Goujon³

¹CNRS, ²CNES, ³INGEspace

Digital beam forming and synthetic aperture interferometry are signal processing techniques that mix the signals collected by an array of elementary antennas to obtain high-resolution images with the aid of a computer. The latter approach is routinely exploited by SMOS, the Soil Moisture and Ocean Salinity mission, whereas the former one has never been implemented operationally in imaging radiometry. Although the two paradigms use the same signals and share the same goal, they are not necessarily two sides of a same coin as there is no evidence that they provide the same information in terms of retrieved brightness temperatures. Numerical simulations have shown a certain benefit of beam forming compared to interferometry regarding the effects of a disparity in the voltage patterns of the elementary antennas on the reconstruction floor error [1]. However, no experimental study has confirmed, nor contradicted, this observation, as a consequence of a lack of an instrument allowing the comparison. Within the frame of FRESCH [2], the Fine Resolution Explorer for Salinity, Carbon and Hydrology project currently undergoing pre-phase 0 studies under ESA contract for the next generation of imaging radiometers from space, it is of great importance to have this capability to conduct innovative investigations that combine numerical simulations and experimental measurements in order to compare interferometry and beam forming at different levels such as onboard calculations, ground processing and of course instrumental performances.

The concrete answer to this need is FANTASIOR: a Flexible ANTennas Array for Synthesis Imaging in Observational Radiometry. It is a phased array of dual-polarized elementary antennas operating in the L-band, capable of digitizing and recording few seconds of the signal captured by each antenna. As a consequence, FANTASIOR will be flexible from the paradigm point of view in as much as these signals will be combined appropriately only in deferred-time with the aid of a computer. On one hand, pairs of signals will be mixed in a multiplicative way to reproduce cross-correlations and to obtain complex visibilities, as in any synthetic aperture interferometer. On the other hand, the signals will be merged all together additively to obtain a map of antennas array temperatures, thanks to beam steering technique. Moreover, any hybridization of these two approaches will also be conceivable. Whatever the paradigm, these raw data will be inverted and the brightness temperature maps thus obtained will be compared with each other and also confronted to the ground truth. Of course, it will be possible to introduce errors in the digitized signals to perturb synchronization, phase, amplitude or polarization and to investigate how they propagate in every approach. It will also be easy to degrade the sensitivity of the instrument as well as its resolution and to conduct comparative studies at this level.

FANTASIOR is the instrumental side of R&D activities initiated at CESBIO with the strong support of the French Space Agency (CNES). It is a low-cost (~200 k€) “operational breadboard” under construction in the premises of INGEspace, a company specialized in precise digitization and real-time processing of high-frequency signals with FPGA for defense and space applications. FANTASIOR relies on a sparse square array of 32 elementary antennas operating in the 1400-1429 MHz band and regularly spaced every 151 mm (it is upgradable to 64 elementary antennas for filling the square array). Consequently, when it is mounted on top of a crane at 20 m above ground surface, the swath of the synthesized field of view is about 18 m and the expected spatial resolution in the Nadir direction is about 2 m. Regardless of any unforeseen event that might slow down the activities, the delivery of FANTASIOR is scheduled for Q4’25. Every elementary antenna will be characterized in the anechoic chamber of CNES during Q1’26 so that the 32 elementary patterns will be compared to those simulated with ANSYS HFSS during the design studies phase. The first synchronized recordings of the 32 elementary signals will be carried out in the aftermath of this characterization, in the same controlled environment, to perform end-to-end calibration tests with non-directional beacons [3]. Furthermore, the behavior of FANTASIOR in the protected band 1400-1427 MHz devoted to imaging will be confronted to that in the service band 1427-1429 MHz used for calibration. If everything goes right, the first images from outdoor acquisitions are expected for Q3 or Q4’26 over an experimented field operated by the French National Research Institute for Agriculture and Environment (INRAE), well in-time for contributing to the submission of FRESCH to Earth Explorer 13.

This contribution aims at giving an overview of FANTASIOR, as detailed as possible, and at discussing the preliminary results available at the time of the conference.

1. https://doi.org/10.3390/rs14092285

2. http://dx.doi.org/10.1109/IGARSS53475.2024.10641846

3. https://doi.org/10.3390/rs17061098