| The SMOS mission, which just celebrated 8 years on orbit, was the first to carry a two dimensional interferometer operating at L-band. Since 2009 it has been continuously providing brightness temperature measurements, in full polarisation and for a wide range of incidence angles.
These measurements have been used succesfully to derive estimation and monitoring of soil moisture and ocean salinity, with unprecedented accuracy.
Closely following in SMOS footsteps, Aquarius and SMAP were launched in 2011 and 2015. Based on a different technology (real aperture instead on interferometry), they have both provided most valuable results, specially benefiting from better radiometric accuracy and radio frequency interferences (RFI) mitigation capability, that was not available for SMOS.
Still, all these instruments are limited by a spatial resolution of few tens of kilometers. In order to be able to make use of these data in hydrological models, and for many other applications, like the survey of water resources at the scale of irrigated zones, a better spatial resolution must be achieved. Typically it should be improved by an order of magnitude.
This is the main goal of a future mission, that should be able to provide brightness temperature measurements at the 4km spatial resolution, with radiometric accuracy around 1 K, and significant range of incidence angles.
But the design of that future mission, as described in , needs to cope with major trade offs. For real aperture instruments, the size of the reflector would make it rapidly unpractical, and for interferometers the size of the baselines and number of antennas required to achieve both spatial resolution and radiometric accuracy also ends with a serious size problem. The possibility to use a multi satellite instrument, already presented in , faces another important challenge: the possibility to operate unconnected interferometer in orbit.
To solve this issue, CNES initiated a feasibility study for a demonstration mission, relying on at least two nanosatellites flying in close range formation (around 40m between satellites) and transmitting signals acquired by similar detectors onboard all satellites to ground to allow synchronisation and correlation computation. The objectives of this mission are to demonstrate our ability to make use of these measurements to compute visibilities, usable to reconstruct brightness temperature maps. It should also demonstrate our ability to cope with synchronisation problems between the detectors, accurate relative localisation, and RFI mitigation.
The outcome of this phase 0 study will be presented in this paper, along with preliminary studies results, based on the simulator developped in this context and described in .
 - Soldo, Y., Cabot, F., Rougé, B., Kerr, Y., Al Bitar, A., & Epaillard, E. (2013). SMOS-NEXT: A New Concept for Soil Moisture Retrieval from Passive Interferometric Observations. European Astronomical Society Publications Series, 59, 203-212
 - E. Anterrieu, F. Cabot, A. Khazaal and Y. H. Kerr, "On the Simulation of Complex Visibilities in Imaging Radiometry by Aperture Synthesis," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 10, no. 11, pp. 4666-4676, Nov. 2017.