Ondrej Fiser - Profile on Academia.edu (original) (raw)
Papers by Ondrej Fiser
2019 PhotonIcs & Electromagnetics Research Symposium - Spring (PIERS-Spring)
In this contribution we discuss the possibilities of the distrometer or videodistrometer measurem... more In this contribution we discuss the possibilities of the distrometer or videodistrometer measurement. Firstly we describe briefly what outputs are given through the videodistrometer measurement: equivolumetric rain drop diameter, fall velocity, oblateness etc. From these quantities we derive the one minute drop size distributions (DSDs) and from it pertinent radar reflectivity factors Z, rain rates R, specific rain attenuations of hypothetical microwave link A and rain kinetic energies E k . 2017 and 2018 videodistrometer data from Prague were applied. Scatterplots E k R, E k Z A-Z and A-R are shown. In two last relations the unambiguity as a function of frequency was discussed it was observed that the unambiguity is maximal at 25 GHz frequency and minimal at 100 GHz for investigated frequencies from 10 to 100 GHz. For A-R relations the unambiguity was estimated also through the coefficient of determination for the power law approximation.
Remote Sensing
We analyzed two convective storms that passed over or near the Milešovka meteorological observato... more We analyzed two convective storms that passed over or near the Milešovka meteorological observatory. The observatory is located at the top of a hill and has been recently equipped with a Doppler polarimetric X-band radar FURUNO WR2120 for cloud investigations. Our analysis was based mainly on Doppler polarimetric radar data measured in vertical cross-sections (RHI-Range-Height Indicator). Radar data was also used for classifying hydrometeors by a newly developed XCLASS (X-band radar CLASSification) algorithm. We also used rapid scan data measured by the geostationary satellite Meteosat Second Generation to validate radar measurements at the upper parts of storms. Although an attenuation correction was applied to the reflectivity and differential reflectivity measurements, the attenuation typical of X-band radars was noticeable. It was mainly manifested in the differential reflectivity, co-polar correlation coefficient and specific differential phase. Nevertheless, radar measurements...
Remote Sensing, 2020
The distribution of hydrometeors in thunderstorms is still under investigation as well as the pro... more The distribution of hydrometeors in thunderstorms is still under investigation as well as the process of electrification in thunderclouds leading to lightning discharges. One indicator of cloud electrification might be high values of the Linear Depolarization Ratio (LDR) at higher vertical levels. This study focuses on LDR values derived from vertically pointing cloud radars and the distribution of five hydrometeor species during 38 days with thunderstorms which occurred in 2018 and 2019 in Central Europe, close to our radar site. The study shows improved algorithms for de-aliasing, the derivation of vertical air velocity and the classification of hydrometeors in clouds using radar data. The comparison of vertical profiles with observed lightning discharges in the vicinity of the radar site (≤1 km) suggested that cloud radar data can indirectly identify “lightning” areas by high LDR values observed at higher gates due to the alignment of ice crystals, likely because of an intensifie...
The UWB (ultra wide band) radar output signals can be substantially affected due to electromagnet... more The UWB (ultra wide band) radar output signals can be substantially affected due to electromagnetic wave propagation through obstacles (such as walls) and multi- path effects, too. Multipath effects are analyzed and simu- lated numerically for various cases with several antenna heights and distances. Delays (due to propagation through walls and various paths of direct and reflected rays) and the ringing (similar to UWB propagation through wall) can be clearly observed and analyzed. Moreover, fre- quency spectra analyses can demonstrate both UWB in- terferences and susceptibility from electromagnetic com- patibility (EMC) viewpoint.
…, 2006
The propagation of ultra wide band (UWB) signals through walls is analyzed. For this propagation ... more The propagation of ultra wide band (UWB) signals through walls is analyzed. For this propagation studies, it is necessary to consider not only propagation at a single frequency but in the whole band. The UWB radar output signal is formed by both transmitter and antenna. The effects of antenna receiving and transmitting responses for various antenna types (such as small and aperture antennas) are studied in the frequency as well as time domain. Moreover, UWB radar output signals can be substantially affected due to electromagnetic wave propagation through walls and multipath effects.
Journal of Fundamental and Applied Sciences, Feb 24, 2018
Many countries receive the Alphasat satellite signal in both Ka and Q bands in order to analyze t... more Many countries receive the Alphasat satellite signal in both Ka and Q bands in order to analyze the random atmospheric attenuation. The received signal from the Alphasat satellite is influenced not only by the atmosphere but also by the quasi regular satellite motion causing the "cosinusoidal" signal level fluctuations of about 24 hour period. In this contribution we describe mathematical methods eliminating this fluctuation and perform tests of the suitability of particular methods. To test these methods we developed and used the signal level software simulator as we must know the "true" attenuation values while the simulated values are the "true" ones from our testing viewpoint. The tests showed that we should recommend the MY method developed at the Institute of Atmospheric Physics Prague but the differences among tested methods are low and all described methods are acceptable.
Radioengineering
We draw your attention to the fact that meteorological radar does not actually measure a commonly... more We draw your attention to the fact that meteorological radar does not actually measure a commonly used quantity "radar reflectivity factor," (which is not dependent on frequency) but a different quantity called "radar reflectivity." We present the usual recalculation which is based on frequency dependency used by Rayleigh approximation of radar cross-sections (back scattering cross section of rain, cloud, fog drop). But this approximation is valid in Rayleigh region only. We concluded that for admitting error lower than 2 dB in the radar reflectivity factor determination we can use the "effective radar reflectivity factor" for frequencies up to 19 GHz only. Otherwise the error will increase. As we use (and present in this article) the Mie algorithm we can replace the Rayleigh frequency dependence estimation by more accurate radar reflectivity factor determination using the Mie scattering. The correction is presented in the form of "Correction function C" dependent on frequency and rain rate in the graphical form and polynomial approximation. Beside this we present the simplification of back scattering cross sections for Rayleigh and Optical regions and the border values of size parameter for these regions. We added the meteorological radar equation derivation. This should support the radar measurement understanding.
Optical Wireless Communications, 2016
This chapter deals with the FSO link attenuation measurements at very special (severe) climatic c... more This chapter deals with the FSO link attenuation measurements at very special (severe) climatic conditions of the mountain Milesovka observatory being appropriate for the research but being not typical one. Therefore, examples of measurements from Prague, Budapest, and Milano are added. Fog attenuation (in combination with cloud) is discussed and Kim formula predicting the fog attenuation from visibility was more or less acknowledged. More accurate results are provided for rain attenuation estimation using the Carbonneau formula. Examples discussing concurrent occurrence of rain, fog, and clouds are analyzed. In the last part, we analyze the attenuation due to atmospheric turbulences concluding that the Rytov formula overestimates the attenuation on one hand, but is in good correlation with the measured attenuation on the second hand.
Various methods for analyses of scattering are mentioned and new approximate relationships are de... more Various methods for analyses of scattering are mentioned and new approximate relationships are derived. Experimental results for thin wire and several numerical simulations of forward scattering using approximate esti- mations, physical optics and exact solutions for two-di- mensional scattering are presented both for far and near fields. That allows not only accuracy analyses but also conclusions about scattering and total fields in the pres- ence of objects, which are important for many applications such as communications, bistatic and multistatic radars and electromagnetic compatibility.
The site diversity is used to mitigate the rain attenuation on satellite links. The attenuation i... more The site diversity is used to mitigate the rain attenuation on satellite links. The attenuation is estimated through the rain rate-rain attenuation conversion based on the Assis-Einloft physical model in this study. Through the comparison of instantaneous attenuations at two receiving sites the site diversity gain is estimated. Examples of rain rate measurements in the Czech Republic followed by the site diversity gain estimation are added. This gain is greater on west-east situation of receiving sites achieving 10 dB on 0.01% exceedance level/100 km.
Comparison of hydrometeor attenuation on parallel terrestrial paths at 58 GHz and 93 GHz
2011 XXXth URSI General Assembly and Scientific Symposium, 2011
Statistics of Radio Refractivity Derived from Prague Radiosounding Data
P13.5 on Impact of Drop Size Distribution Models on Radar Measurement
BER and Availability Measured on FSO Link
Radioengineering
Propagation Characteristics and Availability Performance Assessment for Simulated Terrestrial Hybrid 850 nm/58 GHz System
Radioengineering
Optical Engineering, 2013
Our correlation analysis has shown promising and surprisingly good dependence of optical signal a... more Our correlation analysis has shown promising and surprisingly good dependence of optical signal attenuation on sonic temperature, which is shown and discussed in this study. This discovered dependence is derived from measurements (periods with fog and rain events were excluded) and presented in the form of scatter plots. It is possible to express roughly that the attenuation decreases with the sonic temperature (negative correlation coefficient). Trying to understand our results, we added the explanation of the physical quantity sonic temperature in this contribution.
Enhancing the Atmospheric Visibility and Fog Attenuation Using a Controlled FSO Channel
IEEE Photonics Technology Letters, 2013
Proceedings of ERAD, 2002
The influence of the DSD on rain attenuation computation is studied in the 10-150 GHz frequency r... more The influence of the DSD on rain attenuation computation is studied in the 10-150 GHz frequency region. The influence of particular rain drop sizes (diameters) is shown concluding that the prevailing contribution is caused by drops of the diameter close to 0.7-1.5 mm when the importance of small rain drops is increasing with the increasing frequency. A 2 step model is suggested not to be able to derive the attenuation only but also to model the rain drop spectra accurately. Conclusions are derived from the typical DSD (which was measured in the Czech Republic and corresponds to the 2.17 mm/h rain rate). The convenience of DSD analytical approximations (DSD models) is also discussed.
EURASIP Journal on Wireless Communications and Networking, 2011
Long-term monthly and annual statistics of the attenuation of electromagnetic waves that have bee... more Long-term monthly and annual statistics of the attenuation of electromagnetic waves that have been obtained from 6 years of measurements on a free space optical path, 853 meters long, with a wavelength of 850 nm and on a precisely parallel radio path with a frequency of 58 GHz are presented. All the attenuation events observed are systematically classified according to the hydrometeor type causing the particular event. Monthly and yearly propagation statistics on the free space optical path and radio path are obtained. The influence of individual hydrometeors on attenuation is analysed. The obtained propagation statistics are compared to the calculated statistics using ITU-R models. The calculated attenuation statistics both at 850 nm and 58 GHz underestimate the measured statistics for higher attenuation levels. The availability performance of a simulated hybrid FSO/RF system is analysed based on the measured data.
Modeling of 830 nm FSO Link Attenuation in Fog or Wind Turbulence
This study is focused on the experimental research of the atmosphere impact on FSO link attenuati... more This study is focused on the experimental research of the atmosphere impact on FSO link attenuation. Experiment is performed on a mountain observatory Milesovka of severe weather conditions. The empirical relationships on 830 nm only between FSO attenuation A on one hand and atmospheric visibility V or wind turbulent energy ET on the other hand are presented and discussed. In the fog case it was found A= 401.4V-0.5738- 1.462 dB/60m (V in meters) while for the attenuation due to the wind turbulence ET it holds A = 2.112 - 2.213 exp(-0.2867ET) dB/60m (ET in m2s-2, it was derived for 830 nm wave length). A discussion concerning problems of the “clear air attenuation,” atmospheric turbulences and sun shine impact, which was observed as a maximum 2 dB attenuation for a few minutes with a period of 24 hours is added.
2019 PhotonIcs & Electromagnetics Research Symposium - Spring (PIERS-Spring)
In this contribution we discuss the possibilities of the distrometer or videodistrometer measurem... more In this contribution we discuss the possibilities of the distrometer or videodistrometer measurement. Firstly we describe briefly what outputs are given through the videodistrometer measurement: equivolumetric rain drop diameter, fall velocity, oblateness etc. From these quantities we derive the one minute drop size distributions (DSDs) and from it pertinent radar reflectivity factors Z, rain rates R, specific rain attenuations of hypothetical microwave link A and rain kinetic energies E k . 2017 and 2018 videodistrometer data from Prague were applied. Scatterplots E k R, E k Z A-Z and A-R are shown. In two last relations the unambiguity as a function of frequency was discussed it was observed that the unambiguity is maximal at 25 GHz frequency and minimal at 100 GHz for investigated frequencies from 10 to 100 GHz. For A-R relations the unambiguity was estimated also through the coefficient of determination for the power law approximation.
Remote Sensing
We analyzed two convective storms that passed over or near the Milešovka meteorological observato... more We analyzed two convective storms that passed over or near the Milešovka meteorological observatory. The observatory is located at the top of a hill and has been recently equipped with a Doppler polarimetric X-band radar FURUNO WR2120 for cloud investigations. Our analysis was based mainly on Doppler polarimetric radar data measured in vertical cross-sections (RHI-Range-Height Indicator). Radar data was also used for classifying hydrometeors by a newly developed XCLASS (X-band radar CLASSification) algorithm. We also used rapid scan data measured by the geostationary satellite Meteosat Second Generation to validate radar measurements at the upper parts of storms. Although an attenuation correction was applied to the reflectivity and differential reflectivity measurements, the attenuation typical of X-band radars was noticeable. It was mainly manifested in the differential reflectivity, co-polar correlation coefficient and specific differential phase. Nevertheless, radar measurements...
Remote Sensing, 2020
The distribution of hydrometeors in thunderstorms is still under investigation as well as the pro... more The distribution of hydrometeors in thunderstorms is still under investigation as well as the process of electrification in thunderclouds leading to lightning discharges. One indicator of cloud electrification might be high values of the Linear Depolarization Ratio (LDR) at higher vertical levels. This study focuses on LDR values derived from vertically pointing cloud radars and the distribution of five hydrometeor species during 38 days with thunderstorms which occurred in 2018 and 2019 in Central Europe, close to our radar site. The study shows improved algorithms for de-aliasing, the derivation of vertical air velocity and the classification of hydrometeors in clouds using radar data. The comparison of vertical profiles with observed lightning discharges in the vicinity of the radar site (≤1 km) suggested that cloud radar data can indirectly identify “lightning” areas by high LDR values observed at higher gates due to the alignment of ice crystals, likely because of an intensifie...
The UWB (ultra wide band) radar output signals can be substantially affected due to electromagnet... more The UWB (ultra wide band) radar output signals can be substantially affected due to electromagnetic wave propagation through obstacles (such as walls) and multi- path effects, too. Multipath effects are analyzed and simu- lated numerically for various cases with several antenna heights and distances. Delays (due to propagation through walls and various paths of direct and reflected rays) and the ringing (similar to UWB propagation through wall) can be clearly observed and analyzed. Moreover, fre- quency spectra analyses can demonstrate both UWB in- terferences and susceptibility from electromagnetic com- patibility (EMC) viewpoint.
…, 2006
The propagation of ultra wide band (UWB) signals through walls is analyzed. For this propagation ... more The propagation of ultra wide band (UWB) signals through walls is analyzed. For this propagation studies, it is necessary to consider not only propagation at a single frequency but in the whole band. The UWB radar output signal is formed by both transmitter and antenna. The effects of antenna receiving and transmitting responses for various antenna types (such as small and aperture antennas) are studied in the frequency as well as time domain. Moreover, UWB radar output signals can be substantially affected due to electromagnetic wave propagation through walls and multipath effects.
Journal of Fundamental and Applied Sciences, Feb 24, 2018
Many countries receive the Alphasat satellite signal in both Ka and Q bands in order to analyze t... more Many countries receive the Alphasat satellite signal in both Ka and Q bands in order to analyze the random atmospheric attenuation. The received signal from the Alphasat satellite is influenced not only by the atmosphere but also by the quasi regular satellite motion causing the "cosinusoidal" signal level fluctuations of about 24 hour period. In this contribution we describe mathematical methods eliminating this fluctuation and perform tests of the suitability of particular methods. To test these methods we developed and used the signal level software simulator as we must know the "true" attenuation values while the simulated values are the "true" ones from our testing viewpoint. The tests showed that we should recommend the MY method developed at the Institute of Atmospheric Physics Prague but the differences among tested methods are low and all described methods are acceptable.
Radioengineering
We draw your attention to the fact that meteorological radar does not actually measure a commonly... more We draw your attention to the fact that meteorological radar does not actually measure a commonly used quantity "radar reflectivity factor," (which is not dependent on frequency) but a different quantity called "radar reflectivity." We present the usual recalculation which is based on frequency dependency used by Rayleigh approximation of radar cross-sections (back scattering cross section of rain, cloud, fog drop). But this approximation is valid in Rayleigh region only. We concluded that for admitting error lower than 2 dB in the radar reflectivity factor determination we can use the "effective radar reflectivity factor" for frequencies up to 19 GHz only. Otherwise the error will increase. As we use (and present in this article) the Mie algorithm we can replace the Rayleigh frequency dependence estimation by more accurate radar reflectivity factor determination using the Mie scattering. The correction is presented in the form of "Correction function C" dependent on frequency and rain rate in the graphical form and polynomial approximation. Beside this we present the simplification of back scattering cross sections for Rayleigh and Optical regions and the border values of size parameter for these regions. We added the meteorological radar equation derivation. This should support the radar measurement understanding.
Optical Wireless Communications, 2016
This chapter deals with the FSO link attenuation measurements at very special (severe) climatic c... more This chapter deals with the FSO link attenuation measurements at very special (severe) climatic conditions of the mountain Milesovka observatory being appropriate for the research but being not typical one. Therefore, examples of measurements from Prague, Budapest, and Milano are added. Fog attenuation (in combination with cloud) is discussed and Kim formula predicting the fog attenuation from visibility was more or less acknowledged. More accurate results are provided for rain attenuation estimation using the Carbonneau formula. Examples discussing concurrent occurrence of rain, fog, and clouds are analyzed. In the last part, we analyze the attenuation due to atmospheric turbulences concluding that the Rytov formula overestimates the attenuation on one hand, but is in good correlation with the measured attenuation on the second hand.
Various methods for analyses of scattering are mentioned and new approximate relationships are de... more Various methods for analyses of scattering are mentioned and new approximate relationships are derived. Experimental results for thin wire and several numerical simulations of forward scattering using approximate esti- mations, physical optics and exact solutions for two-di- mensional scattering are presented both for far and near fields. That allows not only accuracy analyses but also conclusions about scattering and total fields in the pres- ence of objects, which are important for many applications such as communications, bistatic and multistatic radars and electromagnetic compatibility.
The site diversity is used to mitigate the rain attenuation on satellite links. The attenuation i... more The site diversity is used to mitigate the rain attenuation on satellite links. The attenuation is estimated through the rain rate-rain attenuation conversion based on the Assis-Einloft physical model in this study. Through the comparison of instantaneous attenuations at two receiving sites the site diversity gain is estimated. Examples of rain rate measurements in the Czech Republic followed by the site diversity gain estimation are added. This gain is greater on west-east situation of receiving sites achieving 10 dB on 0.01% exceedance level/100 km.
Comparison of hydrometeor attenuation on parallel terrestrial paths at 58 GHz and 93 GHz
2011 XXXth URSI General Assembly and Scientific Symposium, 2011
Statistics of Radio Refractivity Derived from Prague Radiosounding Data
P13.5 on Impact of Drop Size Distribution Models on Radar Measurement
BER and Availability Measured on FSO Link
Radioengineering
Propagation Characteristics and Availability Performance Assessment for Simulated Terrestrial Hybrid 850 nm/58 GHz System
Radioengineering
Optical Engineering, 2013
Our correlation analysis has shown promising and surprisingly good dependence of optical signal a... more Our correlation analysis has shown promising and surprisingly good dependence of optical signal attenuation on sonic temperature, which is shown and discussed in this study. This discovered dependence is derived from measurements (periods with fog and rain events were excluded) and presented in the form of scatter plots. It is possible to express roughly that the attenuation decreases with the sonic temperature (negative correlation coefficient). Trying to understand our results, we added the explanation of the physical quantity sonic temperature in this contribution.
Enhancing the Atmospheric Visibility and Fog Attenuation Using a Controlled FSO Channel
IEEE Photonics Technology Letters, 2013
Proceedings of ERAD, 2002
The influence of the DSD on rain attenuation computation is studied in the 10-150 GHz frequency r... more The influence of the DSD on rain attenuation computation is studied in the 10-150 GHz frequency region. The influence of particular rain drop sizes (diameters) is shown concluding that the prevailing contribution is caused by drops of the diameter close to 0.7-1.5 mm when the importance of small rain drops is increasing with the increasing frequency. A 2 step model is suggested not to be able to derive the attenuation only but also to model the rain drop spectra accurately. Conclusions are derived from the typical DSD (which was measured in the Czech Republic and corresponds to the 2.17 mm/h rain rate). The convenience of DSD analytical approximations (DSD models) is also discussed.
EURASIP Journal on Wireless Communications and Networking, 2011
Long-term monthly and annual statistics of the attenuation of electromagnetic waves that have bee... more Long-term monthly and annual statistics of the attenuation of electromagnetic waves that have been obtained from 6 years of measurements on a free space optical path, 853 meters long, with a wavelength of 850 nm and on a precisely parallel radio path with a frequency of 58 GHz are presented. All the attenuation events observed are systematically classified according to the hydrometeor type causing the particular event. Monthly and yearly propagation statistics on the free space optical path and radio path are obtained. The influence of individual hydrometeors on attenuation is analysed. The obtained propagation statistics are compared to the calculated statistics using ITU-R models. The calculated attenuation statistics both at 850 nm and 58 GHz underestimate the measured statistics for higher attenuation levels. The availability performance of a simulated hybrid FSO/RF system is analysed based on the measured data.
Modeling of 830 nm FSO Link Attenuation in Fog or Wind Turbulence
This study is focused on the experimental research of the atmosphere impact on FSO link attenuati... more This study is focused on the experimental research of the atmosphere impact on FSO link attenuation. Experiment is performed on a mountain observatory Milesovka of severe weather conditions. The empirical relationships on 830 nm only between FSO attenuation A on one hand and atmospheric visibility V or wind turbulent energy ET on the other hand are presented and discussed. In the fog case it was found A= 401.4V-0.5738- 1.462 dB/60m (V in meters) while for the attenuation due to the wind turbulence ET it holds A = 2.112 - 2.213 exp(-0.2867ET) dB/60m (ET in m2s-2, it was derived for 830 nm wave length). A discussion concerning problems of the “clear air attenuation,” atmospheric turbulences and sun shine impact, which was observed as a maximum 2 dB attenuation for a few minutes with a period of 24 hours is added.