Results of long-term measurements of the refractive index structure constant in the boundary layer are introduced. The measurements were made on a 150-meter-high lattice mast equipped by nineteen meteorological sensors and one pressure sensor at the bottom of the mast. The Kolmogorov statistical theory of turbulence was used to calculate the refractive index structure constant C<sup>2</sup><sub>n</sub>, allowing us to present annual cumulative distribution functions (CDFs) and seasonal quantiles. The quantiles of measured height dependence of the refractive index structure constant are also shown and compared with existing models (Hufnagel/Andrews/Phillips, SLC Day and Gurvich). Parameters of a linear model were calculated to fit the measured median height profile of the refractive index structure constant with the uncertainty of measurements also being addressed.
Atmospheric hydrometeors such as rain and fog may cause attenuation of an optical signal and degrade the performance
of free-space optical (FSO) systems. For efficient design of the FSO links, attenuation characteristics must be predicted
by sufficiently reliable models that have been tested on experimental data. A long term experiment on the FSO links
operating at 850 and 1550 nm wavelengths is conducted in Prague. The path lengths are 100 and 853 m. Received power
fluctuations on the FSO links and relevant meteorological quantities such as rain intensity and liquid water content of fog
are measured simultaneously. The relationships between the physical parameters of hydrometeors and path attenuation
are analyzed and compared with theoretical relations derived using the Mie scattering theory together with some natural
assumptions about the physical properties of scattering particles such as droplet size distribution of different types of
hydrometeors. Long term statistics of attenuation are obtained and availability performance of the experimental FSO
links is assessed. The method for predicting attenuation statistics based on physical and statistical models is introduced
and the errors of the proposed models with respect to measured data are analyzed. The models are compared with the
existing empirical relationships derived from other FSO experiments and differences are discussed.
Recent measurement campaigns in Prague, Czech Republic have recorded the specific attenuation caused by fog in terrestrial free-space optical (FSO) links along with visibility, liquid water content (LWC), and integrated particle surface area for operational wavelengths of 1550 and 830 nm. Data recorded during 5 months of measurements has been analyzed in an attempt to validate and improve the empirical models for LWC and visibility-based attenuation and the probability density function (PDF) estimation of the received signal strength (RSS). The results strongly suggest that further attempts in refinement of the empirical modeling for visibility-based attenuation or LWC-based attenuation shall move towards the establishment of a global data bank of fog attenuation measurements in different climatic regions. This data bank can then help develop generic models which could lead to better system design of future terrestrial FSO links. The PDF estimation of RSS puts forth the Kumaraswamy distribution as the best fit for settled continental fog conditions.
Attenuation of electromagnetic waves at 860 nm and 58 GHz due to scattering in rain is measured on the terrestrial paths simultaneously with rain intensity. The relationship between rain intensity and rain attenuation of light is analyzed using the Marshal-Palmer drop size distribution model and the Mie scattering theory. The pertinent power law model is derived from drop size distribution parameters. An empirical composite power law model is proposed and fitted to the dataset extracted from rain attenuation events recorded during a seven-year experiment in Prague. Observed average rain attenuation of light at intensities larger than about 5 mm/h is lower than attenuation predicted by standard models. Rain attenuation measured at 58 GHz is larger than attenuation at 860 nm for rain intensities in the range from 0.1 to 100 mm/h.
The paper deals with the results of a propagation study on a fixed hybrid Free Space Optical (FSO) and Radio
Frequency (RF) system operating in 850 nm / 58 GHz bands. Propagation models for the availability assessment of both
FSO and RF links were examined against a comprehensive database of meteorological attenuation events. The
influences of individual hydrometeors were analyzed and the availability performances of the simulated FSO/MMW
hybrid link were evaluated. The study pointed out that visibility and rainfall measurements can be only used for the raw
assessment of availability performance due to the concurrent occurrence of different attenuation effect.
The specific results of a propagation study carried out on a free space optical link working at 850 nm on an 853 m long
terrestrial path over a 4-year period from August 2004 to July 2008 are presented. The records of both the observed fade
events and the concurrent meteorological conditions were processed. Individual recorded attenuation events were
classified according to the types of individual hydrometeors (rain, rain with hail, snow and fog) that occurred. Fog
attenuation events were analyzed in detail and were subdivided into the attenuation events due to fog only, and to
combinations of fog with rain, fog with snow, and fog with rain and snow, respectively. The cumulative distributions of
attenuation due to fog events for four individual year periods, the entire 4-year period, the individual worst months and
the average worst month from the four-year period are presented. It is shown that not only attenuation due to fog only but
also attenuation due to fog combined with other hydrometeors may have a significant impact on the attenuation of the
optical beam. The assessments of availability performance and error performance parameters are given. The use of
measured visibility data for the calculation of attenuation is discussed.
Results of concurrent measurements of both attenuation due to rain at different frequencies on terrestrial line-of-sight paths and rain intensity measured in many countries all over the world are included i the data bank of the Study Group 3 ITU-R. These data can be accepted for the testing of various methods presented occasionally in professional journals and conferences for the calculation of rain attenuation distribution from measured rain intensity data. At TESTCOM, software was developed which can be used for choosing the best method for rain attenuation distribution calculation. Data from 62 terrestrial line-of-sight paths collected in 16 countries were used for 36 testing methods. Statistical parameters of testing procedure were calculated and encountered risks are described. The real proposal of cumulative distributions of attenuation due to rain at 19 GHz frequency band in the Czech Republic based on both the 5-year results of observation at TESTCOM and the above mentioned software is shown in detail and discussed. Rain attenuation calculation methods based on measured propagation and rain intensity data are of fundamental importance for the realistic calculation of new both the Point-to-Point and the Point-to-Multipoint systems.