Technical notes: A detailed study for the provision of measurement uncertainty and traceability for goniospectrometers
Peltoniemi, J.I. ; Hakala, T. ; Suomalainen, J.M. ; Honkavaara, E. ; Markelin, L. ; Gritsevich, M. ; Eskelinen, J. ; Jaanson, P. ; Ikonen, E. - \ 2014
Journal of Quantitative Spectroscopy and Radiative Transfer 146 (2014). - ISSN 0022-4073 - p. 376 - 390.
reflectance distribution function - laboratory goniometer system - bidirectional reflectance - diffuse-reflectance - spectro-goniometer - test field - light - polarization - snow - calibration
The measurement uncertainty and traceability of the Finnish Geodetic Institutes¿s field gonio-spectro-polarimeter FIGIFIGO have been assessed. First, the reference standard (Spectralon sample) was measured at the National Standard Laboratory of MIKES-Aalto. This standard was transferred to FGI¿s field reference standard (larger Spectralon sample), and from that to the unmanned aerial vehicle (UAV), reference standards (1 m2 plates). The reflectance measurement uncertainty of FIGIFIGO has been estimated to be 0.01 in ideal laboratory conditions, but about 0.02–0.05 in typical field conditions, larger at larger solar or observation zenith angles. Target specific uncertainties can increase total uncertainty even to 0.1–0.2. The angular reading uncertainty is between 1° and 3°, depending on user selection, and the polarisation uncertainty is around 0.01. For UAV, the transferred reflectance uncertainty is about 0.05–0.1, depending on, how ideal the measurement conditions are. The design concept of FIGIFIGO has been proved to have a number of advantages, such as a well-adopted user-friendly interface, a high level of automation and excellent suitability for the field measurements. It is a perfect instrument for collection of reference data on a given target in natural (and well-recorded) conditions. In addition to the strong points of FIGIFIGO, the current study reveals several issues that need further attention, such as the field of view, illumination quality, polarisation calibration, Spectralon reflectance and polarisation properties in the 1000–2400 nm range.
Intercomparison of modis albedo retievals and in situ measurements across the global fluxnet network
Cescatti, A. ; Marcolla, B. ; Santhana Vannan, S.K. ; Roman, J.Y. ; Moors, E.J. - \ 2012
Remote Sensing of Environment 121 (2012). - ISSN 0034-4257 - p. 323 - 334.
reflectance distribution function - broad-band albedo - surface albedo - boreal forests - climate-change - vegetation - products - feedbacks - validation - instrument
Surface albedo is a key parameter in the Earth's energy balance since it affects the amount of solar radiation directly absorbed at the planet surface. Its variability in time and space can be globally retrieved through the use of remote sensing products. To evaluate and improve the quality of satellite retrievals, careful intercomparisons with in situ measurements of surface albedo are crucial. For this purpose we compared MODIS albedo retrievals with surface measurements taken at 53 FLUXNET sites that met strict conditions of land cover homogeneity. A good agreement between mean yearly values of satellite retrievals and in situ measurements was found (r2 = 0.82). The mismatch is correlated with the spatial heterogeneity of surface albedo, stressing the relevance of land cover homogeneity when comparing point to pixel data. When the seasonal patterns of MODIS albedo are considered for different plant functional types, the match with surface observations is extremely good at all forest sites. On the contrary, satellite retrievals at non-forested sites (grasslands, savannas, croplands) underestimate in situ measurements across the seasonal cycle. The mismatch observed at grassland and cropland sites is likely due to the extreme fragmentation of these landscapes, as confirmed by geostatistical attributes derived from high resolution scenes.