Zhao Zhen-wei

Millimeter-wave attenuation due to fog and clouds

Millimeter wave applications are becoming increasingly considered and exploited in order to satisfy the demand for new communications systems, active and passive remote sensing, etc. Fog may be one of the dominant factors in determining the reliability of millimeter wave systems, especially in coastal areas where dense moist fog with high liquid water content and larger drop size happen frequently. Clouds are similar to fog, the attenuation due to clouds may be a factor of importance especially for microwave systems well above 10 GHz or low-availability systems.The parameters of gamma drop size distribution model of fog and clouds are derived based on the liquid water content and optical visibility, the attenuation are calculated and discussed with this model and empirical relations of the liquid water content and the visibility or other parameters of fog and clouds. A new empirical formula to estimate fog and clouds attenuation is presented based on the Reyleigh absorption approximation, which is more accurate in general and can be used in wider frequency and temperature range than other empirical formulas.

An analytic model of specific attenuation due to rain

Rain attenuation is one of the dominant factors in determining the reliability of microwave and millimeter systems; the specific rain attenuation is a fundamental quantity in calculation of rain attenuation statistics for terrestrial and earth-space paths. The power-law relationship of specific rain attenuation y/sub R/ (dB/km) and rain rate R (mm/h), y/sub R/ = kR/sup a/ is widely used and the values of k and a are usually tabulated for specific frequencies. The values tabulated are particularly convenient for estimation of specific attenuation in experimental applications or link designs at specific frequency, and the interpolated values should be used for other frequencies. However, It would be quite useful for system studies requiring calculations at many frequencies to have simple analytic expressions which give results in good agreement with the values tabulated. Regression to determine the relationships between the parameters, k and a, and frequency has been done and the regressed expressions given.

Propagation modeling of ocean-scattered low-elevation GPS signals for maritime tropospheric duct inversion

The maritime tropospheric duct is a low-altitude anomalous refractivity structure over the ocean surface, and it can significantly affect the performance of many shore-based/shipboard radar and communication systems. We propose the idea that maritime tropospheric ducts can be retrieved from ocean forward-scattered low-elevation global positioning system (GPS) signals. Retrieval is accomplished by matching the measured power patterns of the signals to those predicted by the forward propagation model as a function of the modified refractivity profile. On the basis of a parabolic equation method and bistatic radar equation, we develop such a forward model for computing the trapped propagation characteristics of an ocean forward-scattered GPS signal within a tropospheric duct. A new GPS scattering initial field is defined for this model to start the propagation modeling. A preliminary test on the performance of this model is conducted using measured data obtained from a 2009-experiment in the South China Sea. Results demonstrate that this model can predict GPS propagation characteristics within maritime tropospheric ducts and serve as a forward model for duct inversion.

Radio wave propagation characteristics measurement and modeling over the sea

Radio wave propagation test is introduced in this paper. It was held in the sea area near Qingdao city, and involved different band including L band, S band, C band and X band. Using the path loss model given in the International Telecommunication Union (ITU) recommendation propagation 1411, we derived the characteristic parameters such as path loss exponent and shadow fading for different frequencies. The statistic propagation model can be used for providing a reference for ship information communication.

The difference in variation characteristics of the F2-peak height hmF2 at the low and middle latitudes

There is a continuing requirement for simple, but representative methods for determining the height of maximum electron concentration of the F2-layer (symbol hmF2), both for communication and navigation. The techniques for estimating the F2-peak height hmF2 from M(3000)F2 are reviewed with particular stress put upon those in which the effects of underlying ionization are accounted for by a correction to M(3000)F2, formulated in terms of the ratio f0F2 and f0E. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at middle latitudes. At 21° latitude, neutral winds have a major effect on the distribution of ionization from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than summer. This paper reports on a comparison of the difference in variation characteristics of the observed monthly mean ionospheric F2-peak height hmF2 at Lanzhou (36.06°N latitude) and Haikou (20°N latitude) for both solar minimum (2005–2007) and solar maximum (2010–2011). The diurnal, seasonal, and solar variation of the hmF2 at the two sites compared with results of detailed calculations. Also, the contemporary practical implementations will be compared and contrasted in the light of this investigation and the scope for further refinements will be discussed.

Retrieving the troposphere duct by using ground-based radiometric profiler in South Coast in China

Coastal areas worldwide are known to be especially ‘rich’ in super refractive layers and waveguides that affect microwave propagation. Radiosonde data of Xiamen and Haikou in south coast in China were chosen to analysis the rule and characteristics of the troposphere duct in this paper. Based on the historical data, improved learning vector quantization neural network method (LVQ) and improved Radial Basis Function neural network method (RBF) is used to retrieving the troposphere duct. The two algorithms are presented by using the brightness temperature detected by the ground-based multi-channel microwave radiometer both in zenith and gradient angles. Feasibility of this method is demonstrated with the results by the measure data. Ground-based multi-channel microwave radiometer has been proven to be a powerful tool to sensing troposphere duct, which can perform continuous troposphere duct profiles and provide information of electromagnetism environment for the application of radar, communication and navigation systems.

The analysis of advection fog attenuation algorithms in Terahertz wave band

Terahertz wave propagation property, which contains the voltage fluctuation, the reflection effect, the dispersion effect, multipath effect, attenuation effect, etc., in random medium such as the atmosphere is one of the most essential problems to consider in the application of Terahertz wave. The absorption due to the molecules in atmosphere of the Terahertz wave is significant in the absorption peaks and relatively small in the absorption valleys and those are the wavelengths where communication applications will be operated on. The scattering of the particles in the atmosphere such as rain drop, fog, and aerosol can also affect the propagation of Terahertz wave. Attenuation of advection fog is studied because in it is the most common fog in the seaside area. Two methods for the calculation of advection fog attenuation are presented in this paper: the efficiency Rayleigh approximation and the accurate Mie theory method. Rayleigh approximation is widely used in the calculation of fog attenuation in the low frequencies, but in high frequencies such as Terahertz wave band, Rayleigh approximation could only be valid in the lower frequencies. The application regions of Rayleigh approximation under tolerant relative error are given, and this is a simple method to also simplify the process and ensure the accuracy. The specific attenuations of advection fog with different visibilities through the whole Terahertz wave band are shown in the end of this paper, and the results increase with the frequency below 3.5THz and after experiencing a slight fall down and then tend to become invariable.