Vladimir Brazda

Enhancing the Atmospheric Visibility and Fog Attenuation Using a Controlled FSO Channel

In this letter, two methods for characterizing fog attenuation in terms of atmospheric visibility V in free space optics (FSO) communications are analyzed and compared. Both methods measure the fog attenuation based on V using a charge-coupled device (CCD) camera and a laser-diode at 0.55 μm. The methods are evaluated in a laboratory controlled FSO atmospheric chamber operating at individual wavelengths of 0.83, 1.31, and 1.55 μm using a continuous optical spectrum range 0.6-1.6 μm. The CCD technique shows great accuracy for V <; 50 m and the laser technique for the range 100 m, thus allowing enhancement of the characterization of FSO links in thick and dense fog conditions. The experimental results also show a wavelength dependency for V <; 0.5 km where the attenuation decreases from 52 dB/km at visible wavelengths to 46 dB/km at near infrared wavelengths.

Description of Alphasat satellite space motion and its consequences for signal reception

The article deals with description of spatial motion of the experimental radio-communications satellite Alphasat. The used Cartesian coordinates of the Alphasat satellite (given one week in advance in the OEM files of 5 minute resolution) are described. The coordinates conversion to the elevation and azimuth from the perspective of the receiver in Prague is added and described. This satellite motion causes some problems with signal reception - automatic antenna tracking is required or at least the antenna beam width extension. We found that the elevation of the Alphasat currently fluctuates up to 2,5°; in the case of azimuth it is up to 0.5° in Prague. The time period of these changes is 23 hours and 56 minutes. And more, also long-term trends of elevation/azimuth are described in this contribution.

Correlation of free-space optics link attenuation with sonic temperature

Abstract. 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.

Attenuation measurement on dual-wavelength optical free-space link

Institute of Atmospheric Physics measures atmospheric attenuation on experimental FSO link on 850 and 1550 nm for more than one year. Experimental site is located at meteorological observatory on the isolated mountain with frequent fog, low clouds and strong wind occurrence. Measured attenuation is correlated with the wind turbulence intensity, visibility and LWC. Simple empirical models estimating attenuation on FSO link from meteorological parameters are formulated and verified through the experiment described. The paper shows also basic statistical behavior of the longterm FSO signal level in extreme conditions.

First Ka and Q band results of atmospheric attenuation measurements using Alphasat receiver in Czech Republic

This contribution is describing interesting attenuation events in 39 and 19 GHz frequency bands using the measurement of signal of satellite Alphasat in Prague. Rain rates at two sites were measured continuously. A table of highest measured attenuation is added and discussed including thought about the rain type. The highest recorded attenuation was A19=8.1dB (A39=23dB) in 19 (39) GHz frequency bands while rain rate was exceeding 20 mm/h. An original equation for frequency scaling was formulated in the form of A39=2,7 A19 + 2,5 [dB].

Mathematical elimination of unwanted signal decrease due to satellite spatial motion

The satellite signal drop can be caused either by atmospheric attenuation (rain, clouds, snow, ...) either by unwanted satellite motion. This contribution demonstrates the software methods to extract the atmospheric attenuation from signal level record influenced by space satellite motion. Contribution is devoted to the construction of so called template (signal course in the case there is no atmospheric attenuation) using clear day signal, Fourier transform and cosine or polynomial approximation.

Comparison of parametric methods for radar signal processing

The paper deals with comparison of parametric power spectral density methods for the utilization in the meteorological radar. Type of the considered radar is pulse radar of frequency modulated pulses. We have compared duration of signal processing requested by single methods involved in the test. The methods used are the Auto Regressive (AR) model, MUSIC method, Eigen Vector algorithm and CAPON. Duration of signal processing is important for regular measurements of the radar. The best results we obtained for the AR model.

FMICW radar simulator

The main goal of the paper is to develop a Frequency Modulation Interrupted Continuous Wave radar simulator. The radar working frequency is 35.4 GHz and it is intended to detect clouds. The simulated output data are compared with output data acquired by the real radar. The real data are acquired from the observation of a static object - the chimney. Radar Cross Section (RCS) of the chimney is calculated and used for simulations. RCS value is used in the simulation for better approximation of results. Paper contains a method for detection based on Power Spectral Density.

Attenuation of optical signal in free space - Event analysis

Institute of Atmospheric Physics ASCR operates a dual-wavelength free space optical (FSO) link at meteorological observatory Milesovka (837 m.a.s.l.). Relevant meteorological quantities which attenuate the signal are also measured. To understand the physics behind also individual attenuation events are to be analyzed. This contribution focuses on interesting events with fog, rain and other meteorological phenomena which cause a high attenuation or an outage. Comparisons with theoretical models are also discussed. Widely known models describe signal behavior in individual situations, when only one type of meteorological phenomenon occurs. This paper shows an effect of different attenuation phenomena combinations.

Propagation over terrain considering refractivity profiles

The physical optics (PO) calculations have been utilized for numerical simulations of electromagnetic field above irregular ground. The PO approximation and numerical simulations using parabolic equation method are used for new comparison with findings of longstanding testing of microwave field and tropospheric refractivity profiles. It is demonstrated that the PO could offer reliable numerical simulations for low altitude field and diffraction field zone even for numerous three-dimensional distributions of the atmospheric refraction.