This paper proposes cross-sensors benchmark of Level 2 products. In addition to the standard comparison methods, we introduce a method based a modified version of the gamma index to spatially identify spectral singularities between products. This method proved to be suitable for fitness-for-purpose assessments given that we can choose various criteria such the uncertainty values and spatial resolution.

We first present results for assessing the surface spectral consistency between Sentinel-3 synergy and Proba-V. The proposed methodology evaluates for spectral band similarity which takes into account the individual pixel reflectance magnitudes and spatial variance from various sensors. In this study we explore examples applied to imagery capture simultaneously by the sensors sampled over different land surface types (agricultural, mountainous, forest and desert areas). The matching spectral bands between instruments are tested and their spectral (in)consistencies identified. In addition, impacts on OLCI FAPAR and Copernicus Global Land Service FAPAR is illustrated.

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For over 20 years, the processing facility at VITO has been processing, archiving and disseminating satellite data derived from SPOT4-Vegetation1, SPOT5-Vegetation2 and PROBA-V in order to serve a large user community providing vegetation information products with nearly daily global coverage. Since these data are widely used to monitor environmental change and the evolution of vegetation cover at global scale, the continuity of the SPOT-Vegetation and PROBA-V time series is important for a wide range of users.

The Sentinel-3 synergy “VGT-like” (SYN VGT) products were defined to provide continuity to the SPOT-Vegetation and PROBA-V time series. Therefore, the consistency between SYN VGT products and PROBA-V Level2 products needs to be assessed. This work is performed in the frame of the Sentinel-3 Mission Performance Center (S3-MPC).

In first instance, this is done through direct comparison of almost-simultaneously acquired Sentinel-3 SYN VGT products and PROBA-V 1km resolution top-of-atmosphere reflectances, at segment level. Previous analysis on a limited number of S3A_SY_2_VGP segments (IPF baseline 2.26), made available to expert users in 2017, has shown relatively high correspondence for Blue, Red and near-infrared (NIR) bands (systematic differences below 3%), but large systematic differences (up to 17%) for the shortwave-infrared (SWIR) band, related to inaccuracies in the Sentinel-3A SLSTR radiometric calibration. In addition, issues were raised on errors in the VGT flag information and NDVI value inconsistencies.

By the end of 2018, new Sentinel-3 SYN VGT products will be made available: the upcoming new processing baseline includes an upgraded reprojection method, better aligned compositing methods compared to the PROBA-V processing chain, and improved quality flagging information. The inter-comparison will be therefore reiterated on this new dataset. The analysis will focus on the calculation of measures of similarity (geometric mean regression parameters, R²) and measures of difference (overall, systematic and unsystematic differences, mean bias error, etc.) between paired observations. Further analysis includes the inter-comparison of Sentinel-3 SY_2_VG1 and SY_2_V10 with PROBA-V daily and 10-daily maximum NDVI value composite products, and time series comparison including external data (e.g. from MODIS and METOP-AVHRR).

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The Proba-V satellite, launched in May 2013, is successfully operated by ESA after more than 5 years in space with excellent end-to-end performances and no sign of degradation. The main mission payload, the Vegetation sensor, was conceived as a miniaturized successor of the former VGT-1 and VGT-2 instruments, carried on board the SPOT-4 and SPOT-5 satellites and operated by CNES since 1998. The first objective of the Proba-V mission was to guarantee continuity to the SPOT-VGT long-term data series of global/daily observations of landmasses and coastal areas at 1 km spatial resolution in the visible, near- and short-wave infrared. The continuity requirement was crucial to bridge the gap until the launch and operations of the Sentinel-3 twin satellites constellation. In order to fulfill this goal, the Proba-V mission had to provide products of (at least) the same quality of the SPOT-VGT sensors, both in terms of radiometric and geometric accuracy and long-term stability.
Nowadays, after more than 5 years in space, we can safely say that the Proba-V mission successfully fulfills its primary goal and that the Vegetation sensor considerably outperforms the accuracy and continuity requirements originally set. The radiometric performances are well within 5% in absolute value and 3% relative, despite the lack of on-board calibrator device. The geo-location accuracy, of the order of 60 m at nadir, allows in particular, exploiting the observations capability of the central camera, providing to the users exceptional imagery at 100 m spatial resolution with 5-days revisit time. This level of accuracy was fundamental for ensuring continuity to the vegetation user community, in particular to the Copernicus Global Land Service. Furthermore, the 100 m products open the way to new applications of Proba-V data, notably for regional monitoring over fragmented landscapes, such as rural fields, coastal and mountainous areas, inter-tidal zones.
Looking ahead, the main Proba-V goal of gap-filler to Sentinel-3 will be fully completed by end of 2019, with potential further continuation up to Spring 2020. This includes one year of overlap between the two missions to guarantee a smooth transition and proper inter-calibration. By that time (Spring 2020) the mission overpass, which is not maintained and it is slowly drifting toward early morning, will be around 9:30 AM, meaning that the illumination conditions will start to be less favorable and the long-term variation of the solar zenith angle could potentially induce spurious trends in the derived bio-physical products. The current approach is to start at this time a new mission phase of limited duration (2 years), during which, innovative mission concepts will be explored and verified. Several proposals and concrete projects started already at ESA, this includes the launch of a CubeSat companion satellite carrying a spare camera equivalent to the one on-board Proba-V for extending the swath, and the acquisition of special measurement campaigns, such as multi-view angle and yaw manoeuvre, exploiting the Proba-V platform agility. The objective of this extended phase is twofold, on the one side, to guarantee the continuation of 100 m imagery over sample target sites, on the other side, to foster the development of advanced studies for better characterizing surface anisotropy over specific Cal/Val sites. This extended phase will include as well special acquisitions, such as Moon and Antarctica, which are of extreme value respectively for radiometric calibration and for cryosphere applications.
The aim of this paper is to present the current status of the Proba-V mission, to review the performances of the Flight and Ground Segment, to illustrate the current algorithm and data quality status and the plans for improvements, and most importantly to inform the user community on the strategy and roadmap being implemented at ESA for mission extension beyond Spring 2020.

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The Proba-V Mission Exploitation Platform (MEP) is a collaborative working environment where users can have direct access to the complete Proba-V and SPOT-VGT archive with the required resources, tools and ancillary data for processing and data analysis. The benefit of Proba-V MEP for data exploitation activities and Cal/Val purposes is being demonstrated in a series of scientific projects. This paper presents the results of a project, which made full use of the MEP for the systematic global validation of Proba-V Top-of-Canopy (TOC) reflectance products. The final goal of the project is to estimate uncertainty at global scale and investigate spatial and temporal consistency of Proba-V TOC products currently provided to users.
The validation of surface reflectances is a very challenging task, owing to the lack of spatial and temporal representative ground truth data. The common approach, which is currently used for Sentinel-2 and Landsat-8 missions, is to use AERONET ground-based network. The advantage of this network is to be globally spread, automatic and fully traceable in terms of calibration and processing accuracy. The rationale is to build a synthetic surface reflectance dataset over specific region of interest centred around the AERONET stations; this dataset is computed by correcting satellite TOA reflectances using the AERONET atmospheric state variables (for aerosol and water vapour) and an accurate Radiative Transfer Model (RTM). These synthetic surface reflectances can be eventually treated as an independent validation dataset to be used to estimate uncertainty of the Proba-V TOC products.
The use of the Proba-V MEP for this exercise allows accessing the full archive at all resolution together with the complete AERONET validation data, which was uploaded on the platform for the purpose. Furthermore, a fully customizable development environment was available on the platform, including a set of libraries, which are relevant for Cal/Val, in particular for fast identification of match-up, for mapping and plotting. A Python program was developed with the possibility to access a cluster of CPU nodes to cope with computation issues linked to running complex RTM.
The project started in September 2017 and significant results on the AOT estimate assessment and RTM accuracy have been reached. The final goal is to define a range of validity where the accuracy is not fulfilling the requirements and potentially to derive a set of per-pixel flags to enhance products metadata quality information. The same validation exercise without the MEP would have required an enormous investment on the user side, e.g. to download and store the ground-based dataset, to harvest the full Proba-V archive, to look for space and time collocation, and finally to perform the validation analysis. All these activities can now be performed within the MEP in a systematic and autonomous way allowing near-real time estimation of the end-users products quality at global scale.

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The Proba-V ESA mission, launched in May 2013, has the primary objective to ensure continuity to the SPOT-VGT long-term data series bridging the gap to Sentinel-3 for global daily monitoring of land surfaces. Despite being specifically designed to serve the land vegetation community, the Proba-V mission has proven to provide valuable imagery at different spatial resolution (1km, 333m and 100m) for serving other applications, namely, coastal and inland water, biosphere, and cryosphere monitoring. As part of the effort of extending the applicability of Proba-V imagery to other scientific domains, the acquisition planning was recently updated to include also acquisition of Antarctica continent. The rationale was to provide complementary imagery in the visible, near- and short-wave infrared, in order to contribute to the international effort in monitoring short and long-term changes of ice sheet masses over this continent. To this end, the added value of Proba-V imagery is the large swath (2285 km) and the spatial resolution of 100 m, enabling to repetitively cover the full Antarctica continent and to monitor medium scale surface topography changes.
The first Proba-V Antarctica campaign was performed between November 2017 and March 2018 and the campaign was repeated starting from November 2018. The products were provided to the users as Top-Of-Atmosphere (TOA) reflectances at different spatial resolutions (1km, 333m and 100m) in polar stereographic projection. Owing to the peculiarity of the imaging conditions, in particular the extreme solar zenith angles, the operational cloud/snow/ice screening and atmospheric correction algorithms could not be reliably applied to this dataset. A high level of expertise is therefore required on the user side in order to efficiently exploit these products. In order to lower the bar of expertise and ease access to this data, a dedicated study started at ESA with the final goal to provide guidelines for users and dedicated higher-level products, such as multi-temporal cloud-free surface reflectances.
The present paper describes the current status of this study and highlights the value of this new dataset. The products analysis is performed accessing the Proba-V Mission Exploitation Platform (MEP) archive and focusing on products at 100 m spatial resolution. As a first step, the satellite imagery is extensively used to characterize the surface glaciology and topography, fracture patterns, surface wind patterns, rifting and calving of ice shelves. In this context, one of the first highlight was that Proba-V was able to track the Larsen C’s changes, in particular, the motion of the A-68 iceberg, which calved away from the Larsen C ice shelf on July 2017, as largely documented with a sequence of images looking at the tracking changes of iceberg. As second step, dedicated cloud-screening approach is under evaluation, using the short-wave infrared band to discriminate cloud from ice surfaces. The final goal is to generate a multi-temporal seasonal compositing of the Antarctica continent at different spatial resolution (1 km, 333 m and 100 m). As an outlook of the project, the feasibility of retrieving key snow properties will be assessed, in particular snow grain size and snow albedo.

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