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Sentinel-2 satellite data enables multispectral monitoring of the earth at a high temporal revisit rate. Combining this information with a network of optical ground measurements enables a more detailed and a more complete understanding of terrestrial ecosystems. However, independent optical ground measurements often lack consistency, especially when comparing different sites in geographically remote locations. Using the very high temporal and spectral resolution offered by the automated field spectrometer systems FloX and RoX (Fluorescence Box and Reflectance Box, respectively, JB-Hyperspectral Devices GmbH, Duesseldorf, Germany), we investigated continuous time series ranging over three years and in ten different locations across Europe, Africa, America and Asia. The continuous records of ground-measured reflectance were first validated against Sentinel-2 top of canopy (TOC) reflectance to evaluate the consistency of the in-situ network. Our results suggest a good agreement of ground-measured reflectance with Sentinel-2 TOC reflectance in vegetation and snow with R2 around 0.79 in the 833 nm band and R2 up to 0.94 in the bands around 559 nm and 492 nm, demonstrating good consistency across the network. Spatial misalignment of Sentinel-2 pixel-sizes with respect to the different footprint sizes of the ten automated spectrometers on the ground, atmospheric uncertainties, sub-optimal instrument setup and spatial-temporal variable landscape heterogeneity were identified as the most relevant sources of uncertainties in the network. Comparing the Normalized Difference Vegetation Index (NDVI), Transformed Chlorophyll Absorption in Reflectance Index (TCARI) and Enhanced Vegetation Index (EVI) between ground and satellite revealed a decreasing agreement with increasing complexity of index formulation. The best agreement between satellite and ground was exhibited by NDVI with R2 around 0.96 and relative error of 4.3% investigating vegetation and snow across all ten sites. Fu

The upcoming Fluorescence Explorer (FLEX) satellite mission aims to provide high quality radiometric measurements for subsequent retrieval of sun-induced chlorophyll fluorescence (SIF). The combination of SIF with other observations stemming from the FLEX/Sentinel-3 tandem mission holds the potential to assess complex ecosystem processes. The calibration and validation (cal/val) of these radiometric measurements and derived products are central but challenging components of the mission. This contribution outlines strategies for the assessment of in situ radiometric measurements and retrieved SIF. We demonstrate how in situ spectrometer measurements can be analysed in terms of radiometric, spectral and spatial uncertainties. The analysis of more than 200 k spectra yields an average bias between two radiometric measurements by two individual spectrometers of 8%, with a larger variability in measurements of downwelling radiance (25%) compared to upwelling radiance (6%). Spectral shifts in the spectrometer relevant for SIF retrievals are consistently below 1 spectral pixel (up to 0.75). Found spectral shifts appear to be mostly dependent on temperature (as measured by a temperature probe in the instrument). Retrieved SIF shows a low variability of 1.8% compared with a noise reduced SIF estimate based on APAR. A combination of airborne imaging and in situ non-imaging fluorescence spectroscopy highlights the importance of a homogenous sampling surface and holds the potential to further uncover SIF retrieval issues as here shown for early evening acquisitions. Our experiments clearly indicate the need for careful site selection, measurement protocols, as well as the need for harmonized processing. This work thus contributes to guiding cal/val activities for the upcoming FLEX mission.

The results of the fifth quarter year's effort of a 24-month program on the analytical and experimental characterization of the physical properties of selected liquid propellants are presented in three phases. In Phase I, a continuous review of the literature was conducted to ensure the acquisition and documentation of the latest possible propellant properties data for evaluation and possible inclusion into a propellant properties handbook. Phase II experimental efforts have resulted in the measurement of chlorine trifluoride vapor pressure in the high-temperature region and an estimation of critical properties; sonic velocity in chlorine trifluoride and chlorine pentafluoride under pressurized conditions; nitrogen gas solubility in chlorine pentafluoride; specific heat of saturated liquid chlorine trifluoride; thermal conductivity of MHF-5; and viscosity of chlorine pentafluoride. Phase III efforts included the evaluation and assembly of all data generated in Phases I and II, a curve fit of specific heat data on solid chlorine trifluoride, and a report of unpublished data on the normal boiling point of chlorine pentafluoride and chlorine pentafluoride storability.

This interim report describes the work performed under Contract NAS3-7949 to advance the state-of-the-art of resin matrices required for improved ablative materials for use with high energy fluorine-containing liquid propellant systems during the period 10 January through 1 July 1966. This is being accomplished through experimental studies on consecutive material development tasks initiated during this period involving synthesis and property determinations, resin formulation optimization and composite processing parameter studies. In future work these tasks will be continued and the objective will be attained through synthesis scale-up and fabrication of ablative composites tasks. This program is an extension of initial studies conducted under Contract NAS3-4188. Synthesis and property determinations have been directed to preparation and evaluation of new modified polyimide resins based on new anhydride monomers, bis(5-tetrahydrophthalic anhydride) sulfone and bis(p-3, 4-dicarboxy anhydride phenoxy phenyl) sulione, which offer improved processability. Optimization of resin systems, identified in Contract NAS3-4188 as advanced resin materials is being carried out to improve physical, chemical and processing characteristics. Composite processing studies have been conducted to determine the parameters which affect the fabrication of graphite and silica reinforced polyamide-imide and modified polyimide resins. Preliminary testing of the resistance of neat resins and composites containing state-of-the-art and advanced resins in fluorine/hydrogen and FLOX/methane combustion environments shows that (1) 'stiff' polymers appear to be superior to other types of organic resins, and (2) different reinforcement are preferred in each propellant combination combustion environment- - graphite in fluorine/hydrogen and silica in FLOX/methane.

This report describes the work accomplished in demonstrating the feasibility of applying the main-tank injection (MTI) concept to high-energy propellants including hydrazine, mixed hydrazine, thixotropic hydrazine, nitrogen tetroxide, storable fluorine derivatives, and fluorine. The MTI process provides propellant tank pressurization by the controlled injection of a hypergolic reagent to generate the required gas pressure resulting from the reaction with the propellant. The technical approach included determination of theoretical pressurization system performance with the new propellants based on previous experience, selection of suitable reagents, and demonstration of process feasibility by tests performed in a 5 1/2-cu-ft test tank at 25, 150, and 300 psig. Process feasibility for all the propellants except for Alumizine at the higher pressures. No testing was completed using common ullage. Although pressurization process control was adequately demonstrated during a nominal 2 1/2-minute propellant outflow, the reagents selected are not necessarily the optimum candidates. Furthermore, the original and post test theoretical predictions of pressurization system performance have been invalidated by actual test results.