Efficient radiative transfer calculation and sensor performance requirements for the aerosol retrieval by airborne imaging spectroscopy

Detailed aerosol measurements in time and space are crucial to address open questions in climate research. Earth observation is a key instrument for that matter but it is biased by large uncertainties. Using airborne imaging spectroscopy, such as ESA's upcoming airborne Earth observing instrument APEX, allows determining the widely used aerosol optical depth (AOD) with unprecedented accuracy thanks to its high spatial and spectral resolution, optimal calibration and high signal-to-noise ratios (SNR). This study was carried out within the overall aim of developing such a tropospheric aerosol retrieval algorithm. Basic and efficient radiative transfer equations were applied to determine the sensor performance requirement and a sensitivity analysis in context of the aerosol retrieval. The AOD retrieval sensitivity requirement was chosen according to the demands of atmospheric correction processes. Therefore, a novel parameterization of the diffuse path-radiance was developed to simulate the atmospheric and surface effects on the signal at the sensor level. It was found for typical remote sensing conditions and a surface albedo of less than 30% that a SNR of circa 300 is sufficient to meet the AOD retrieval sensitivity requirement at 550nm. A surface albedo around 50% requires much more SNR, which makes the AOD retrieval very difficult. The retrieval performance is further analyzed throughout the visual spectral range for a changing solar geometry and different aerosol characteristics. As expected, the blue spectral region above dark surfaces and high aerosol loadings will provide the most accurate retrieval results. In general, the AOD retrieval feasibility could be proven for the analyzed cases for APEX under realistic simulated conditions

Impact of multiangular information on empirical models to estimate canopy nitrogen concentration in mixed forest

Directional effects in remotely sensed reflectance data can influence the retrieval of plant biophysical and biochemical estimates. Previous studies have demonstrated that directional measurements contain added information that may increase the accuracy of estimated plant structural parameters. Because accurate biochemistry mapping is linked to vegetation structure, also models to estimate canopy nitrogen concentration (CN) may be improved indirectly from using multiangular data. Hyperspectral imagery with five different viewing zenith angles was acquired by the spaceborne CHRIS sensor over a forest study site in Switzerland. Fifteen canopy reflectance spectra corresponding to subplots of field-sampled trees were extracted from the preprocessed CHRIS images and subsequently two-term models were developed by regressing CN on four datasets comprising either original or continuum-removed reflectances. Consideration is given to the directional sensitivity of the CN estimation by generating regression models based on various combinations (n=15) of observation angles. The results of this study show that estimating canopy CN with only nadir data is not optimal irrespective of spectral data processing. Moreover adding multiangular information improves significantly the regression model fits and thus the retrieval of forest canopy biochemistry. These findings support the potential of multiangular Earth observations also for application-oriented ecological monitoring

Extraction of ozone and chlorophyll-A distribution from AVIRIS data

The potential of airborne imaging spectrometry for assessing and monitoring natural resources is studied. Therefore, an AVIRIS scene of the NASA's MacEurope 1991 campaign - acquired in Central Switzerland - is used. The test site consists of an urban area, the Lake Zug with its surrounding fields, the Rigi mountain in the center of the test site, and the Lake of Four Cantons. The region is covered by the AVIRIS flight #910705, run 6 and 7 of the NASA ER-2 aircraft resulting in an average nominal pixel size of about 18 m. Simultaneous to the ER-2 overflight spectroradiometric measurements have been taken in various locations. Preselected reference targets were measured in the field with a GER Mark V spectroradiometer, and radiance measurements were taken to the lake using a Li-Cor LI 1800UW specroradiometer below and above the water surface. A comprehensive meteorological data set was obtained by joining the POLLUMET experiment which carried out measurements to investigate the summer smog in Switzerland on the same day. The quality assessment for the actual data set can be found in detail in Meyer et al. A parametric approach calculating the location of the airplane was used to simulate the observation geometry. This parametric preprocessing procedure, which takes care of effects of flight line and attitude variations as well as the pixel-by-pixel topographic corrections is described in Meyer

Seasonal study of directional reflectance properties of snow

We present an analysis of the hemispherical-directional reflectance factor (HDRF) of snow, using 16 seasonal datasets of the spectral range from 400 to 2,500 nm. The data was measured under clear sky conditions in Davos Dorf (Grisons, Switzerland, 1,560 m a. s. l.). Fieldwork was carried out on seven days between February 5 and March 30 2004 with the Swiss Field Goniometer System (FIGOS). In addition to the HDRF measurements, snow stratigraphy, temperature and density were measured, and chemical and photomicroscopical analyses of snow samples were performed. Concentration of organic and elemental carbon was determined by chemical analysis. The grain size analyses through image processing of micrographs revealed relatively small differences of 0.21 to 0.33 mm mean radius in the top layers of snow cover. Seven datasets present HDRF of wet snow surfaces with similar anisotropy at smaller sun zenith angles (qI = 3.3 to 64.5°) compared to the nine surfaces measured at larger sun zenith angles (qI = 6.5 to 75.3°). Spectral albedo is relatively constant throughout datasets of different sun zenith angles of the same day, but has large variability among measurements of different days. With increasing wavelength, it decreases significantly faster for wet surfaces than for dry surfaces. The forward scattering peak was found to strengthen with increasing sun zenith angle and with increasing wavelength for both wet and dry surfaces at wavelengths above 700 nm. Finally, a spectral wet snow determination method is performed and the cross-sensitivity to HDRF variation could be derived. The best differentiability was found for 1,380 nm. This basis work increases the understanding of snow signatures for potential imaging spectroscopy applications in alpine regions

The emergence of imaging spectrometry in Europe

In 1984 the European remote sensing research community became aware of Imaging Spectrometry. AIS, the Airborne Imaging Spectrometer of JPL had been tested from 1982 onwards and AVIRIS became operational in 1989. The paper describes how MONITEC with their FLI/PMI enabled first IS flight campaigns (EISAC-I) in Europe, which were followed by experiments with AVIRIS, CASI and later the GER-7915 called DAIS. Numerous flight campaigns were conducted by DLR, VITO, NERC, INTA and others. Commercial companies offered and are still offering flight opportunities with airborne instruments. Specim and NEO are successfully suppling commercial systems to the market. The satellite projects HIRIS, GEROS, HRIS, PRISM, SPECTRA and others resulted finally in MODIS and MERIS as well as CHRIS-PROBA and Hyperion. Future projects include among others ENMAP, PRISMA, FLEX and HyspIRI

Comparison of a hyperspectral classification method implemented in two remote sensing software packages

This paper presents a study that aims at applying, comparing and characterizing a proven method for hyperspectral land use classification that is currently integrated in two different remote sensing software packages. Recently a great deal of advances in the Geomatica Hyperspectral Analysis Package (HAP) has been made. The Spectral Angle Mapper (SAM) algorithm was used in both Geomatica and ENVI for this study and a hyperspectral dataset from the European sensor CHRIS onboard the platform PROBA-1 served as a test case. The study showed that, despite of many differences in the workflow of the two software packages, the two land use classification results of SAM turned out to be identical

Beitrag zu einer Neu-Definition der Geographie

No abstract available

Towards an automated detection of avalanche deposits using their directional properties

Snow avalanches killed more people in the Swiss alpine area during the past decades than any other natural hazard. To further improve the avalanche prediction and the protection of people and infrastructure, information about the occurrence and the distribution of avalanche activity is crucial. Nevertheless this information is missing for large parts of the Alpine area. The surface roughness of avalanche deposits differs considerably from the adjacent undisturbed snow cover and is an important factor of the directional reflectance anisotropy. The undisturbed snow-cover exhibits a strong forward scattering, while the structure of an avalanche deposit causes shadow casting and tilt effects. Therefore, the observed reflectance of avalanche deposits and undisturbed snow cover is strongly dependent on the illumination- and viewing angles. This study demonstrates the potential of multiangular remote sensing data for detecting and mapping avalanche deposits. The results indicate, that air- or spaceborne multiangular sensors are suitable for rapid detection and mapping of avalanches in inaccessible and remote regions

An algorithm for tracking APEX spectral stability by means of the In-Flight Characterization facility (IFC)

During their life span, imaging spectrometers are likely to be affected by deviations in spectral performances. Such fluctuations are mainly due to vibrations and temperature/pressure changes at the moment of launch or aging of the instrument. Prior to taking the spectrometer to the laboratory for a time- consuming re-characterization and re-calibration, it is good practice to monitor its spectral performance in- flight. For the Airborne Prism Experiment (APEX) spectrometer, this can be achieved by means of an onboard In-Flight Characterization (IFC) facility. IFC data are acquired at closed shutter with a stable input signal coming from a 75 W Quartz Tungsten Halogen (QTH) lamp. A filter wheel is interposed in the optical path leading to the detector; the spectral filters mounted on the wheel are characterized by a number of narrow spectral features. In this paper the development and tuning process of an algorithm to be used for the spectral stability monitoring of APEX is presented. The study is based on simulated IFC data and aims at identifying a spectrum-matching technique to be included in the final algorithm. In this context four spectrum-matching methods are tested in a varying range of simulated measurement conditions. We found that the methods employing the correlation coefficient and the RMSD as merit functions are more suitable and robust approaches for the estimation of the wavelength shift