|Paper title||SMOS-HR (High Resolution): A proposal for a SMOS follow-on mission allowing spatial resolution improvement and RFI mitigation|
|Form of presentation||Poster|
Passive microwave observations in L-band are unique measurements that allow a wide range of applications, which in most cases cannot be done at other wavelengths: accurate absolute estimations of soil moisture for hydrology, agriculture or food security applications, ocean salinity measurements, detection and characterization of thin ice sheets over the ocean, detection of frozen soils, monitoring above-ground biomass (AGB) to study its temporal evolution and global carbon stocks, measurements of high winds over the ocean… The Soil Moisture and Ocean Salinity (SMOS) satellite, launched by ESA in 2009, which has performed for the first time systematic passive observations at L-band, has allowed to discover some of the previous applications. SMOS L-band data play a central role in the ESA Climate Change Initiative (CCI) for Soil Moisture and Ocean Salinity. Passive L-band data also contribute to the CCI Biomass. This European mission has been followed by two other L-Band missions from NASA: Aquarius and SMAP (Soil Moisture Active Passive).
In the last years, scientific and operational users were requested to contribute to a survey of requirements for a future L-band mission. One of the outcomes of this survey is that most of the applications require a resolution of around 10 km. This is the objective of SMOS-HR (High Resolution) project, a second generation of SMOS mission: the continuation of L-band measurements with an unprecedented native resolution improving by a factor of 2 to 3 that of the current generation of radiometers such as SMOS and SMAP.
In this paper, we will present this SMOS-HR project which is currently under study at CNES (the French Centre National d’Etudes Spatiales) in collaboration with CESBIO (Centre d’Etudes Spatiales de la BIOsphère) and ADS Toulouse (Airbus Defence & Space) which has been contracted by CNES for the instrument definition.
The main challenge for this CNES study is to find the best trade-off to satisfy most needs with “reasonable” mission requirements (i.e., feasible for an acceptable cost). The core mission objective for SMOS-HR is to increase the spatial resolution at least by a factor of two with respect to SMOS (< 15km at Nadir) while keeping or improving its radiometric sensitivity (~0.5-1 K) and with a revisit time no longer than 3days. Taking into account the mission and system level requirements, a new definition of an interferometric microwave imaging radiometer has been studied.
The first step has been to select the antenna array configuration: cross-shaped arrays, square-shaped arrays (which imply a Cartesian gridding), Y-shaped arrays and hexagon-shaped arrays (which imply a hexagonal gridding) have been compared. A cross shape has been selected as the best option because it allows to reduce the aliasing in the reconstructed images by adequately choosing the position of the elementary antennas along the four arms and because its accommodation is simpler than for some other configurations. The result is an instrument with 171 elementary antennas regularly spaced along the arms (~1lambda) and an antenna with an overall size of ~17 meters tip-to-tip. Then, the optimal concept for the SMOS-HR instrument consists of a hub located on the platform, carrying a dozen central antennas, and four deployable arms attached to the platform, carrying about 40 antennas each. The SMOS-HR hub gathers a Central Correlator Unit computing the correlations for all antenna pairs and generating a clock signal for instrument synchronization. The feasibility of on-board processing for Radio-Frequency Interference (RFI) mitigation has also been addressed to overcome the limitations faced on SMOS with on-ground processing. Adding this function for SMOS-HR represents another major improvement compared to SMOS (on top of the resolution improvement).
As a risk reduction activity, a breadboard of a part of the Central Correlation Unit is defined, developed and tested by ADS during this study in order to assess the achievable performances and functionalities of SMOS-HR on-board processing.
Finally, the SMOS-HR phase A has also been the opportunity to explore innovative calibration strategies based on SMOS lessons learnt.
As a synthesis, this talk will present successively:
• SMOS-HR mission and system level requirements,
• The main trade-off at instrument and sub-systems levels (antenna configuration, deployment structure, elementary antenna, on-board correlator, RF receiver, power and local oscillator distribution, calibration strategy…),
• The current results of the correlator breadboard pre-development.