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Liu J.,Texas A&M University | Yang P.,Texas A&M University | Muinonen K.,University of Helsinki | Muinonen K.,Finnish Geospatial Research Institute FGI
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2015

The Gaussian sphere has been widely used as a model to study light scattering by irregular particles; and, despite extensive numerical studies, the optical properties are not thoroughly understood. Based on Gaussian spheres and using a combination of the invariant imbedding T-matrix method and an improved geometric-optics method, the single-scattering properties (namely, the 4×4 phase matrix, extinction cross section, single-scattering albedo, and asymmetry factor) are computed in the Rayleigh to geometric optics regimes. The simulations are performed with various degrees of irregularity, and the effects of particle irregularities are investigated over a wide range of particle sizes. Furthermore, the theoretical simulations based on Gaussian spheres are used to fit the measured optical properties of feldspar particles from the well-known Amsterdam-Granada light scattering database. A mixture of several shapes is shown to closely reproduce the measured phase matrices. The results may be potentially useful for remote-sensing and radiative-transfer applications involving dust aerosol. © 2015 Elsevier Ltd. Source


Muinonen K.,University of Helsinki | Muinonen K.,Finnish Geospatial Research Institute FGI | Wilkman O.,University of Helsinki | Cellino A.,National institute for astrophysics | And 2 more authors.
Planetary and Space Science | Year: 2015

We derive initial rotation, shape, and scattering properties for asteroids from sparse and dense photometry based on the so-called Lommel-Seeliger ellipsoid (LS ellipsoid). Due to the analytical disk-integrated brightness, the LS ellipsoid allows for fast rotation-period, pole-orientation, and shape analyses, as well as efficient Markov-chain Monte Carlo solutions (MCMC). We apply the methods to simulated sparse Gaia photometry, as well as to ground-based photometry composed of dense lightcurves. For a specific Gaia simulation, we make use of a numerical reflection coefficient developed for particulate surfaces, and utilize the LS ellipsoid in the inversion of the simulated data. We conclude that, in a majority of cases, initial LS ellipsoid retrieval of the parameters is satisfactory. Finally, we formulate a single-scattering phase function that, for a spherical asteroid, results in the H,G1,G2 photometric phase function. © 2015. Source


Lyytinen E.,A+ Network | Gritsevich M.,A+ Network | Gritsevich M.,Finnish Geospatial Research Institute FGI | Gritsevich M.,Moscow State University of Geodesy and Cartography
Planetary and Space Science | Year: 2016

This paper examines the effects of the atmospheric density, more specifically, the effects that the changes of isobaric altitude have on the accuracy of a fireball's analysis. During winter, especially over high latitude regions like Finland, the true isobaric level may be more than three kilometers below the heights predicted by the US Standard Atmosphere 1976 model. It can differ even more when compared with the heights predicted by the simplified exponential scaled height model. Thus, the true mass of a fireball may differ significantly from those obtained by using either the isobaric altitudes derived from the Standard Atmosphere or the exponential atmospheric model, i.e. without taking into account the corrections for true atmospheric conditions. Likewise, the solutions for the ablation coefficient derived by the generalized models will be less accurate than those that do consider the true atmospheric conditions. This becomes especially crucial for potential meteorite droppers, i.e. low velocity fireballs, with small ablation rates and consequently, high terminal-to-initial mass ratio. We propose an atmospheric height correction method that utilizes real atmospheric data to analysis of the fireball. We demonstrate the proposed method by analyzing the data of three fireballs that were recently recorded by the Finnish Fireball Network and finally, we compare our results against those derived by using the US Standard Atmosphere 1976 model and the exponential atmosphere model. © 2015 Elsevier Ltd. All rights reserved. Source


Lehtinen K.,Finnish Geospatial Research Institute FGI | Bach U.,Max Planck Institute for Radio Astronomy | Muinonen K.,Finnish Geospatial Research Institute FGI | Muinonen K.,University of Helsinki | And 2 more authors.
Astrophysical Journal Letters | Year: 2016

Stellar occultations by asteroids observed at visual wavelengths have been an important tool for studying the size and shape of asteroids and for revising the orbital parameters of asteroids. At radio frequencies, a shadow of an asteroid on the Earth is dominated by diffraction effects. Here, we show, for the first time, that a single observation of an occultation of a compact radio source at a frequency of 5 GHz can be used to derive the effective size of the occulting object and to derive the distance between the observer and the center of the occultation path on the Earth. The derived diameter of the occulting object, asteroid (115) Thyra, is 75 ± 6 km. The observed occultation profile shows features that cannot be explained by diffraction of a single asteroid. © 2016. The American Astronomical Society. All rights reserved. Source


Saarela S.,University of Helsinki | Schnell S.,Swedish University of Agricultural Sciences | Tuominen S.,Natural Resources Institute Finland Luke | Balazs A.,Natural Resources Institute Finland Luke | And 3 more authors.
Remote Sensing of Environment | Year: 2016

Positional errors may cause problems when field and remotely sensed data are combined in connection with forest surveys. In this study we evaluated the effects of such errors on statistical estimates of growing stock volume using model-assisted and model-based estimation. With model-assisted estimation, positional errors affect the model parameter estimates for the models that are used as part of the estimation framework. In addition, positional errors affect the estimators, since the deviations between model predictions and field measurements are often larger than they would have been without positional errors. Using model-based estimation positional errors affect the model parameter estimates and thus the estimators. We compared the effects of positional errors in model-assisted and model-based estimation through Monte Carlo sampling simulation in a simulated study area resembling the forest conditions in Kuortane, western Finland. The forest population was created using a copula modelling approach based on field, Landsat and LiDAR data. We found that positional errors led to slightly biased estimators, and estimators with larger variances compared to the cases where data were perfectly geo-located. The relative increase of the variances of the estimators was of equal magnitude for model-assisted and model-based estimation, when models were developed and applied to data with geopositional errors. Further, the variance estimators were always more precise for the model-based estimators compared to the model-assisted estimators. When the models were developed based on perfectly geo-located data but applied to data with positional errors, model-based estimation was superior to model-assisted estimation. © 2015 Elsevier Inc. Source

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