Joel Kuusk

Database of optical and structural data for the validation of radiative transfer models

Recent advances in airborne and spaceborne scanner technologies have been providing vast amounts of multispectral and multi-angular remote sensing data of different spatial and radiometric resolution over the Earth’s vegetation. Remote sensing of forests has been one of the major interest in the remote sensing of environment, because forests impact climate, provide different kind of resources for the economy, are related to biodiversity and, on the other hand, are threatened by several anthropogenic factors and disturbances.

Autonomous lightweight airborne spectrometers for ground reflectance measurements

A series of small, lightweight, autonomous spectrometer systems has been designed in Tartu Observatory. They are used for obtaining top-of-canopy spectral reflectance measurements for vegetation remote sensing, validation of radiative transfer models, and reflectance-based vicarious calibration of satellite and airborne sensors. The spectrometer systems are based on miniature spectrometer modules by Carl Zeiss Jena GmbH. The radiometric properties of the modules have been characterized based on laboratory measurements and appropriate data correction methods have been developed. The UAVSpec series spectrometer systems have been in operational use for several years and have been proven to be valuable tools for airborne, as well as on-ground and laboratory measurements. They are suitable for being carried by an unmanned aerial vehicle, which could make the measurement truly autonomous and independent of any service provider.

Hyperspectral reflectance of boreo-nemoral forests in a dry and normal summer

The visible-near infrared (VNIR) reflectance of several mature boreo-nemoral stands in Estonia was measured in two concurrent years which differed significantly in precipitation. Custom-designed VNIR spectrometer systems, UAVSpec and UAVSpec2, mounted onboard a helicopter, were used for the measurements. We show that broadleaved and coniferous forests react to changes in water supply differently: the relative change of the NIR reflectance was equally about 10–15% in all stands, while the change of visible reflectance of coniferous stands exceeded that of broadleaved stands. The smallest change was observed for broadleaved stands in the red reflectance, i.e. chlorophyll absorption bands. Based on our results, we conclude that the change of stand reflectance caused by weather conditions, such as drought, should be taken into account in the estimation of the optical properties of the atmosphere, needed in the atmospheric correction of optical satellite images. A widely used correction method directly benefiting from our results for boreo-nemoral forests is the dark object method.

Stray light effects in above-water remote-sensing reflectance from hyperspectral radiometers.

Stray light perturbations are unwanted distortions of the measured spectrum due to the nonideal performance of optical radiometers. Because of this, stray light characterization and correction is essential when accurate radiometric measurements are a necessity. In agreement with such a need, this study focused on stray light correction of hyperspectral radiometers widely applied for above-water measurements to determine the remote-sensing reflectance (RRS). Stray light of sample radiometers was experimentally characterized and a correction algorithm was developed and applied to field measurements performed in the Mediterranean Sea. Results indicate that mean stray light corrections are appreciable, with values generally varying from -1% to +1% in the 400-700 nm spectral region for downward irradiance and sky radiance, and from -1% to +4% for total radiance from the sea. Mean corrections for data products such as RRS exhibit values that depend on water type varying between -0.5% and +1% in the blue-green spectral region, with peaks up to 9% in the red in eutrophic waters. The possibility of using one common stray light correction matrix for the analyzed class of radiometers was also investigated. Results centered on RRS support such a feasibility at the expense of an increment of the uncertainty typically well below 0.5% in the blue-green and up to 1% in the red, assuming sensors are based on spectrographs from the same production batch.

Estimation of variable atmospheric parameters for the atmospheric correction of satellite images

ABSTRACT The information about variable components of the atmosphere (aerosol, water vapour, and ozone) during acquisition is required for the atmospheric correction of spectral images acquired by shortwave sensors of the Earth observing remote-sensing satellites. The procedure to estimate aerosol optical depth and columnar water vapour by the inversion of the atmospheric radiative transfer model 6S using moderate-resolution spectra of incident solar radiation is proposed. Comparison to the results obtained by the Aerosol Robotic Network AERONET at an AERONET site at the distance of 50 km on days when both sensors were in the same air mass shows systematic overestimation both of aerosol optical depth and of columnar water vapour if aerosol optical depth is estimated in the wavelength range of 365–425 nm and columnar water in the range of 895–985 nm using spectra of total irradiance. If more wavelengths and diffuse-to-total spectral irradiance ratio are implemented in the inversion, the bias of estimated water vapour decreases, but aerosol optical depth is underestimated. The estimates at 50 km distance are well correlated. The modelled spectral irradiance using estimated atmospheric parameters matches the measured spectra with high accuracy. In the spectral bands of the Sentinel-2 MultiSpectral Instrument (MSI), the differences do not exceed 2%.

Reflectance spectra of RAMI stands in Estonia

Simulated reflectance spectra of three mature hemiboreal forests are compared to top-of-canopy reflectance from helicopter measurements in the spectral range 400–1050 nm. Most of input parameters of the forest reflectance model FRT used in simulations have been measured in situ. The same data sets were used in the Radiation Transfer Model Intercomparison (RAMI). The reasons of the discrepancies between simulated and measured spectra are analyzed.