Recent Progress on Best Practice Protocols for Vegetation-Oriented Sensors Characterization, Calibration and Validation

Dear Colleagues,

The dynamic response of vegetation to changing environmental conditions can be revealed if sufficient and accurate information is acquired on vegetation status, health, and photosynthetic activity. A well-known proxy of photosynthesis is sun-induced fluorescence (SIF), which represents a very low proportion of the energy received by vegetation from the total quantity of sunlight (just a few mW m⁻² sr⁻¹ nm⁻¹ from the total signal). This is a signal of extremely low magnitude, and its measurement is very sensitive to uncertainties that could lead to erroneous results. Some of these uncertainties can be related to instrumentation performance and use under field conditions. In recent years, ESA prepared a new mission, FLEX (Fluorescence Explorer), dedicated to globally measuring the SIF emission from terrestrial vegetation; it will be launched in 2025. To be of societal benefit, space-based observations must be validated and vegetation fluorescence better understood. To do this, it is necessary to have data from multiple fixed locations close to the ground as well as measurements in the spatial domain. This necessitates the direct comparison and analysis of measurements from multiple instruments and platforms. The essential base for making measurements comparable across scales is well-described data products, which include information about the features of the equipment employed as well as uncertainty tracing from the raw data to the SI units.

Fiducial Reference Measurements (FRMs) are a suite of independent, fully characterized, and metrologically traceable ground measurements that follow the guidelines outlined by the Quality Assurance Framework for Earth Observation (QA4EO) of the Committee on Earth Observation Satellites (CEOS). Suites of FRM-like activities are currently running for several satellite products, such as FRM4VEG, FRM4SOC, FRM4SST and FRM4ALT (meaning FRM for vegetation, satellite ocean color, sea surface temperature, and altimetry, respectively).

This Special Issue is linked to the COST Action "Optical synergies for spatiotemporal SENsing of Scalable ECOphysiological traits" (SENSECO) WG4 activities, and its purpose is to collect all procedures that could assist in preparation of achieving “FRM4FLEX”-like status for products of the local, regional and global vegetation photosynthetic activity.

The remote sensing community is invited to submit their works related to any methodological procedures that could be used to facilitate future validation campaigns for satellite missions (FLEX, Sentinel) having the investigation of terrestrial vegetation behavior as a goal. Any works on topics including, but not limited to the following, are welcome:

• Novel vegetation-oriented spectroradiometer development and improvement, including their calibration and characterization;
• Best practice for traceable laboratory characterization, calibration and validation of optical sensors used in field measurements for vegetation photosynthetic activity monitoring;
• Comparisons including calibration sources, laboratory and in situ-based vegetation sensors measurements;
• SI traceability and end-to-end uncertainty budgets—from calibration to field measurements;
• Best practice to achieve high-quality field measurements;
• Measurements from permanently deployed sites and their data quality monitoring;
• Methods showing how data from multiple sensors can be combined to provide a correct overview of vegetation photosynthetic activity at local, regional or global scale.  

Dr. Laura Mihai
Dr. Aga Bialek
Dr. Alasdair MacArthur
Guest Editors

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