We report here preliminary results of a mission analysis for a space weather monitoring system that provides continuous transmission of solar wind conditions 0.10 A.U. upstream from Earth. The system is based on four platforms that are phased into eccentric heliocentric orbits but, from the perspective of a fixed Sun–Earth line, the spacecraft appear to orbit Earth. This system offers a 10× improvement in reporting solar wind plasma and magnetic field characteristics beyond similar platforms located at the Lagrangian L-1 point. We describe launch and energy considerations, along with a preliminary analysis of communication requirements. The Space Weather Diamond offers significant potential for scientific insight into problems requiring coordinated observations from multiple vantage points by providing the ability to separate spatial from temporal variations. We discuss examples for payloads including both in situ and remote sensing instrumentation.

Reflected radiation by a crop canopy can be influenced by changes in the biological processes and physical attributes of the canopy. The study was conducted to characterize the effective leaf area index (LAI) and leaf angle distribution of the Streletskaya Steppe Reserve of the Russian Republic and the Konza Prairie in Kansas and to determine dependence of leaf optical properties on leaf water potentials of some dominant species in discrete wavebands in the visible and near‐infrared (NIR) (spanning 400–1000 nm range). Biophysical properties were measured in July 1991 during the Kursk Experiment (KUREX‐91) and in the summer of 1989 during the First ISLSCP Field Experiment (FIFE‐89). Leaf area index, leaf angle distribution, mean tilt angle, canopy height, leaf optical properties, and leaf water potential were measured. Generally, the KUREX‐91 steppe sites were characterized by high leaf area index and an uniform leaf angle distribution, while the FIFE‐89 prairie sites were characterized by low leaf area index and an erectophile canopy. There were differences in leaf reflectances between plant groups (grass versus forb) and between plant communities (prairie versus steppe), but not generally for transmittance. Leaf optical properties (visible and NIR) were not related to leaf water potential (over the ‐0.5 to ‐3.5 MPa range) for dominant plant species observed at the KUREX‐91 and FIFE‐89 sites. Characterization of the biophysical properties during the FIFE‐89 and KUREX‐91 experiments provide information useful in understanding the differences in canopy bidirectional reflectance observed at the two grasslands.

RÉSUMÉLes propriétés spectrales optiques (réflectance, R et absorbtance, A) ont été déterminées à partir de mesures en laboratoire de feuillage, de ramilles et d'écorce de tronc (R seulement) pour des peuplements forestiers boréaux avec des sous-étages arborescents en Saskatchewan (Canada). Ces sites incluent une plantation de pin gris (∼15 ans) (Pinus Banksiana Lamb.) et trois peuplements matures (>60 ans): pin gris, épinette noire (Picea mariana), et peuplier faux-tremble (Populus tremuloidus).Des éléments du couvert supérieur et inférieur de l'étage forestier dominant et des sous-étages ont été étudiés, de juillet à la mi-septembre 1994, dans le cadre du projet BOREAS (Boreal Ecosystem-Atmospheres Study). Dans le cas des conifères, les aiguilles et l'écorce associées à une croissance en 1994 et des périodes variant de deux à cinq années précédentes ont été examinées. Un rapport de bandes (Rapport simple, SR, le rapport proche infrarouge/rouge) et la fraction du rayonnement photosynthétiquement actif absorbé (fAPAR) ont été calculés à partir de spectres des surfaces foliaires adaxiales (au-dessus) et abaxiales (en dessous). On observe des différences significatives pour les paramètres optiques (R, fAPAR, et SR) entre les surfaces foliaires adaxiales et les surface abaxiales, et également au sein des espèces, des peuplements, des strates du couvert et au cours des saisons.Les valeurs les plus élevées de fAPAR et de SR sont associées aux surfaces abaxiales des aiguilles des conifères mais inversement avec les surfaces adaxiales des feuillus. Des relations log-linéaires séparées entre SR et fAPAR sont nécessaires pour les feuillus, le pin et l'épinette noire. Les valeurs R des ramilles et de l'écorce sont similaires à Celles du feuillage à de nombreuses longueurs d'ondes, se dégradant avec la classe d'âge dans la région du proche-infrarouge (PIR). Comparativement à l'épinette noire, les aiguilles de pin gris exhibent: une valeur de réflectance adaxiale inférieure dans le bleu (Rbleu), particulièrement au cours de l'automne; moins de saisonnalité dans les valeurs de réflectance adaxiale qu'abaxiale dans le visible (RVIS); des valeurs plus élevées de réflectance dans le proche-infrarouge (RPIR); et des valeurs plus élevées de fAPAR. Les valeurs de RPIR des aiguilles de pin gris (54 ± 3%) sont de 8–10% plus élevées que pour le feuillage du peuplier faux-tremble ou de l'épinette noire. Des profils verticaux des paramètres optiques foliaires à travers la strate du couvert (3–4 couches) révèlent des patrons nettement différents pour les trois types de communauté (peuplier faux-tremble, épinette, pin gris) pour les valeurs de Rbleu adaxiale, RPIR adaxiale et de SR abaxial. Le seul paramètre optique montrant constamment une dégradation à travers le profil vertical dans tous les couverts est la RPIR abaxiale. Ces résultats devraient fournir des caractérisations optiques utiles pour la paramétrisation des simulations du transfert radiatif en forêt boréale.

Net radiation (Rn) is the major source of energy for evaporating water, heating the soil and air, and photosynthesis. The objective of this study is to estimate this important parameter with various models that have been developed to estimate the radiation balance components with remotely sensed data, and readily available meteorological data. Data used in this paper were collected over grassland vegetation during the FIFE‐87, ‐88, ‐89 studies and the KUREX‐91 study. For all studies estimated values of Rn were within about 10% of measured Rn. For the KUREX‐91 study, measured and estimated Rn agreed to within about 1%. Improvement in a model(s) to estimate the reflected shortwave flux would provide an even better estimate of Rn since in all studies the reflected radiation stream was overestimated compared to the measured values. There was no clear trend for under or over‐estimation of incoming short‐wave radiation from study to study. Components of the long‐wave balance were estimated with low mean relative errors when the incoming long‐wave flux was corrected for a bias in clear daytime values. Thus, it appears feasible to use remotely sensed data to estimate the incoming and outgoing short‐wave radiation fluxes and the outgoing long‐wave radiation flux and to combine these fluxes with estimates of the incoming long‐wave radiation flux estimated from models which incorporate air temperature and vapor pressure data.

Quantifying the amount of photosynthetically active radiation (PAR) absorbed by vegetation is an essential consideration for determining useful vegetative photosynthetic capacity and surface conductance values for regional and global carbon cycle studies. This study was conducted to compare absorbed photosynthetically active radiation at the FIFE‐89 Konza prairie sites to that of the KUREX‐91 steppe grassland sites and to investigate variations in relationships between absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors. Incoming, reflected and transmitted PAR were measured from which fractions of reflected, transmitted and absorbed PAR were computed at selected FIFE prairie and KUREX steppe sites. Fractions of direct and diffuse PAR transmitted through canopies were estimated. Fractions of absorbed PAR were much lower at the FIFE sites (ranging from 0.35 to 0.65) than those at KUREX (ranging from 0.75 to 0.95) which can be explained by differences between leaf area index, leaf angle distribution, and direct and diffuse sky conditions. Scattering of PAR may be an important parameter in canopy light penetration particularly in canopies of large LAI, with non‐photosynthetically active vegetation components and illuminated at large solar zenith angles. The magnitude of spectral vegetation indices computed and plotted as a function of the fraction absorbed did not differ considerably even though LAI and the fraction of absorbed PAR did. Adjusting for background improved the distinction between spectral vegetation indices at the two experimental sites. Relationships between fraction of absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors were not consistent over illumination and view angles.

Estimates of incoming longwave radiation flux densities (R1?) obtained using several empirical and theoretical algorithms were compared to flux densities measured during the FIFE‐87 and FIFE‐88 experiments. Statistical analysis showed that all models gave values that were greater than the measured values with biases ranging from 25 to 55Wm−2. The incoming longwave radiation models were corrected to reduce or eliminate these biases and applied to independent data sets obtained during FIFE‐89 and KUREX‐91. With these adjustments, the models showed significant improvement in their ability to accurately estimate Rl?. The algorithms of Brunt and Brutsaert performed the best of all models and gave consistently good results for the FIFE‐89 and KUREX‐91 studies. Compared to the Brunt equation, the Brutsaert equation had a higher d value and smaller random and systematic errors. Both models had mean relative errors of less than 2% for the KUREX‐91 study and about 5% for the FIFE‐89 study. Outgoing (emitted) longwave radiation (Rl?) flux densities were estimated from remotely sensed surface temperature data during FIFE‐89 and KUREX‐91. Compared to measured outgoing longwave radiation flux densities (Rl?), estimated Rl? values had mean relative errors of approximately 1% and 2% for KUREX‐91 and FIFE‐89, respectively.

Sensors on satellite platforms with extreme view angles have been increasingly used to analyze regional and global vegetation cover and productivity because of frequent observations. This study, using experimental and theoretical methods, analyzed variations in vegetation indices with sun-view geometry as a means of understanding the sensitivity of relations between vegetation indices and the biophysical properties, the leaf area index (LAI), and the instantaneous fraction of absorbed photosynthetically active radiation (fAPAR). Canopy bidirectional reflectance factors (BRFs) of an alfalfa crop were measured and simulated at a variety of solar and view zenith angles. Also, fAPAR, LAI, and leaf optical properties were measured. Measured and simulated canopy reflectances agreed generally within 1% (absolute). Normalized difference and simple ratio vegetation indices (NDVI and SRVI, respectively), derived from BRFs, varied with view and solar zenith angles. The minimum for near-infrared (NIR) BRFs and relatively high red BRFs generally occurred near nadir, resulting in some of the lowest vegetation index values. Highest VI values were generally obtained at forward view angles. Variation of NDVI with sun-view-geometry was greatest at LAIs <2, whereas the range in SRVI was greatest for LAIs>2. Measured reflectances indicate that relations between NDVI and LAI and between SRVI and fAPAR were curvilinear across all solar and view zenith angle combinations in the solar principal plane, whereas relations between SRVI and LAI and between NDVI and fAPAR varied from linear to curvilinear. Analyses revealed that vegetation indices at large view zenith angles were poorly correlated with fAPAR, whereas those at small zenith angles were strongly correlated. In general, vegetation indices were more sensitive to fAPAR than to LAI, which is attributed to the fact that fAPAR is a radiation quantity, whereas LAI is nonlinearly related to radiation. Regression of fAPAR with VI values derived from combinations of red and NIR BRFs from similar and nonsimilar directions indicates that the highest correlation is in near-nadir and backscatter directions. However, further investigation into variations of relations between remotely sensed observations and canopy attributes and into the usefulness of off-nadir in extracting information is recommended. (C) Elsevier Science Inc., 1997.

Leaves of the dominant grass species of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site reflect and transmit radiation in a similar manner to other healthy green leaves. Visible reflectance factors (RFs) and transmittance factors (TFs) were lower for older leaves than younger leaves except during senescence, when RF and TF values were higher. Near‐infrared (NIR) RF values increased and TF values decreased with leaf age, with the reverse occurring as the leaf underwent senescence. Leaf optical properties were not found to be dependent on leaf water potential in the range from −0.5 to −3.0 MPa. Canopy bidirectional reflectance factor (BRF) values generally increased with increasing view zenith angle (θυ). Maximum values were in the backscatter direction, whereas BRF values in the visible region were lowest at oblique off‐nadir θυ in the forward scatter direction and at or near nadir in the NIR region. Solar principal plane BRF values varied most at large solar zenith angles (θs). Visible and mid‐infrared canopy BRF values decreased and NIR BRF values increased with leaf area index (LAI). Soil BRF distributions in the solar principal plane varied slightly with θs and θυ and varied considerably for wet and dry surfaces. Spectral vegetation indices (SVIs) varied with θs and θυ; values were lowest in the backscatter direction and highest in the forward scatter direction. The fraction of absorbed photosynthetically active radiation (APAR) increased with increasing θs. APAR had a strong linear relationship to nadir‐derived SVI values but not to oblique off‐nadir‐derived SVI values. The relatively small dependence of off‐nadir SVI values on θs should allow daily APAR values to be estimated from measurements made at any time of the day.

The estimation of incoming and outgoing radiation streams using bidirectional spectral reflectances and bidirectional thermal emittances is described. Good agreement between measured and modeled estimates of the radiation balance is obtained. The data used were collected over selected grassland sites on the Streletskaya Steppe Reserve in Russia in July 1991. Results from this study compare well with results obtained during the first ISLSCP field experiment (FIFE) study in Kansas in 1987-89.

Instantaneous fractions of absorbed photosynthetically active radiation (APAR) were measured at the Streletskaya Steppe Reserve in conjunction with canopy bidirectional-reflected radiation measured at solar zenith angles ranging between 37 and 74 deg during the Kursk experiment (KUREX-91). APAR values were higher for KUREX-91 than those for the first ISLSCP field experiment (FIFE-89) and the amount of APAR of a canopy was a function of solar zenith angle, decreasing as solar zenith angle increased at the resrve. Differences in absorption are attributed to leaf area index (LAI) and leaf angle distribution and subsequently transmitted radiation interactions. LAIs were considerably higher at the reserve than those at the FIFE site. Leaf angle distributions of the reserve approach a uniform distribution while distributions at the FIFE site more closely approximate erectophile distributions. Reflected photosynthetically active radiation (PAR) components at KUREX-91 and FIFE-89 were similar in magnitude and in their response to solar zenith angle. Transmitted PAR increased with increasing solar zenith angle at KUREX-91 and decreased with increasing solar zenith angle at FIFE-89. Transmitted PAR at FIFE-89 was considerably larger than those at KUREX-91.