FLEX End-to-End Mission Performance Simulator Architecture Design using a Generic Reference Architecture for EO Missions
Vicent, Jorge; Sabater, Neus; Alonso, Luis; Verrelst, Jochem; Rivera, Juan P.; Tenjo, Carolina; Moreno, Jose
Laboratory of Earth Observation, University of Valencia, SPAIN
As part of the science and research element of ESA's Living Planet Programme, the FLEX mission is currently a candidate for the 8th Earth Explorer as an Opportunity mission. FLEX will be the first mission designed to globally map and monitor chlorophyll fluorescence emission originated from the core of the photosynthetic machinery in terrestrial vegetation. FLEX mission concept consist in a single platform with a hyperspectral instrument (FLORIS) acquiring in the spectral range between 500 nm and 800 nm. FLEX will fly in tandem with GMES' satellite Sentinel-3 (S3), exploiting the synergy between their data and helping in the characterization of the atmospheric state, essential for a reliable retrieval of fluorescence emission. Combining measurements from FLEX and S3 instruments, the mission shall provide accurate and reliable information on the overall photosynthetic activity.
Sun-induced chlorophyll fluorescence (Fig. 1) is a sensitive indicator of the actual photosynthesis in vegetation that can be used as a powerful non-invasive marker to track the status, resilience, and recovery of photochemical processes. Monitoring this fluorescence emission, together with atmospheric CO2 and leaf physiology, will directly provide with a measurement of the actual land photosynthetic activity. This is of particular interest for the improvement in the predictive capability of global carbon cycle models through improved parameterizations for canopy photosynthesis and the corresponding exchange processes of energy and water between the surface and the atmosphere.
In order to assess the mission performance, consolidate the technical requirements and system implementation and analyze the suitability of the developed retrieval schemes, a FLEX End-to-End Mission Performance Simulator (hereafter E2ES) for Phase A/B1 has been developed. An E2ES is a set of algorithms and computer programs, developed independently but interacting with each other, that reproduces with simulated data the expected mission planning and performances. The design of an E2ES must be done to ease its evolution during each mission Phase, reducing costly and time-consuming reengineering process caused by modifications and replacements of components and algorithms. A generic E2ES Reference Architecture (hereafter RA) is therefore needed to support the development of early Phases E2ES for new missions by following a standard procedure and promote its reusability for later mission Phases.
The work presented here introduces the design process of FLEX E2ES based on the prior development of a RA for passive optical Earth Observation (EO) mission simulators. It has been divided in three main parts: 1) conceptual development of a Reference Architecture for generic passive optical EO missions; 2) application of the RA for the particular case of FLEX E2ES and 3) demonstration of FLEX E2ES capabilities.
The conceptual development of a RA for generic passive optical EO missions was developed in the frame of ESA's project ARCHEO-E2E. A survey of past/present/planned passive optical EO missions was firstly employed, which led to the classification of missions and instruments in proposed categories. This classification identified the commonalities of passive optical missions and formed the basis for defining a high-level RA for generic EO E2ES. The proposed RA was founded on six high-level modules that can be combined to reproduce different missions of one or more platforms and instruments types. The internal architecture at lower level, by means of functional elements called Building Blocks, was also generalized despite of being highly dependent on the category of mission/instrument. The definition of common Building blocks allowed the RA to be generic, thereby maintaining the same functionalities for missions with different instrument types.
FLEX E2ES (Fig. 2) is then presented in the second part. According to FLEX mission requirements and characteristics of the instruments involved, the RA design process was applied and adapted to reproduce the particularities of the FLEX mission. A common Geometry Module was designed to provide the orbit and attitude for both FLEX and S3 platforms as well as the viewing geometry of their different instruments (i.e. FLORIS, OLCI and SLSTR). The Scene Generator Module was developed by using the state-of-the-art radiative transfer codes SCOPE and MODTRAN5, simulating high spatial/spectral resolution scenes of vegetation fluorescence, surface reflectance and top of atmosphere (TOA) radiance. Being a multi-platform and multi-instrument simulator, two parallel chains of instrument and Level-1 modules were proposed to reproduce each instrument concept. Finally, the Level-2 Retrieval Module was developed, integrating the algorithms for atmospheric correction, cloud screening and retrieval of fluorescence emission exploiting the synergies between FLEX and S3.
Finally, the third part of the work shows the potential of using FLEX E2ES to assess the performance of the mission/instrument configuration as well as the retrieval algorithms that were implemented. The employed demonstration shows the capabilities of the developed Scene Generator Module by generating a synthetic scene based on the distribution of biophysical/atmospheric parameters feeding the radiative transfer codes. The scene-generated TOA radiance signal is processed through the different instrument and Level-1 modules. The subsequent processing towards level 2 products with the implemented retrieval algorithms was analyzed by evaluating the performance of the retrieved bio-geophysical products in the Performance Evaluation Module.
In conclusion, we present the design process of FLEX E2ES, starting with the conceptual definition of a generic E2ES architecture for passive optical missions and ending with the development of FLEX mission performance simulator at both high and low levels. The presented results show that FLEX E2ES can function as a powerful and flexible tool to analyze the mission performance, helping in the development of the mission and instrument in later Phases and refinement of the current retrieval strategies. It is foreseen that FLEX E2ES will contribute to a full characterization of the instrumental and mission configuration issues that affect into the mission performance. Moreover, it is expected that the proposed RA can serve as a benchmark for the development of future passive optical EO E2ES.