Studies of Propagation Impairments For fixed Satellite Communication Links at the Microwave Frequencies in Nigeria

Studies of Propagation Impairments For fixed Satellite Communication Links at the Microwave Frequencies in Nigeria

Chapter One

Aims and Objectives of the study

The following steps were taking in achieving the objectives of the study:

Compute some propagation impairments relevant to fixed satellite communication at Ku, Ka and V band (10-50 GHz) at elevation angles of 5⁰, 55⁰andone practical elevation angle for links to recently launched Nigeria communication satellite NigComsat-1 in the 37 locations in Nigeria.
Use meteorological data from satellites and ground stations as input data into ITU-RP empirical models to generate appropriate propagation data such as, cloud attenuation, rain attenuation, gaseous attenuation, tropospheric scintillation and total propagation loss on earth-space path in each of the 37 locations.
Calculate mappings following item 2, for uplink and down link budget control during tropical rainfall along earth-space paths for fixed satellites communication service in Nigeria.
Finally, locate the highest and the lowest places affected by propagation impairments year round by dividing the country into six zones (namely SW, SE, SS, MB, NW and NE) and conduct a comparative analysis of the results with a view to recommending for the country in the future where to best site a fixed satellite base station on earth-space path (operating at Ku band and above) for deep space exploration.

Chapter Two

PROPAGATION IMPAIRMENTS AND MEASURING TECHNIQUES

Scattering and Absorption by Single Particles

Scattering is the process by which a particle (or any bit of matter) in the path of an electromagnetic wave continuously abstracts energy from the incident wave and reradiates that energy into the total solid angle centered at the particle. The particle is a point source of the scattered (reradiated) energy. For scattering to occur, it is necessary that the refractive index of the particle be different from that of the surrounding medium. The particle is then an optical discontinuity, or inhomogeneity, to the incident wave. When the atomic nature of the matter is considered, it is clear that no material is truly homogeneous in a fine-grained sense. As a result, scattering occurs whenever an electromagnetic wave propagates in a material medium. In the atmosphere the particles responsible for scattering run the size gamut from gas molecules to raindrops as listed in Table 2.0. The wide ranges of size and concentration are note worthy McCartney (1976). Figure 2.1 below describes a single scattering process.

Propagation Impairment Mechanisms

Propagation impairments of radio wave signals above 10 GHz are primarily caused by constituents in the troposphere which extends from the Earth’s surface to height of about 10 km to 20 km the vertical extent being lowest at the temperate and highest in the tropics region. Degradations induced in the Ionosphere (50-100 km) generally affect frequencies well below 10 GHz. The ionosphere is essentially transparent to radio waves at frequencies above 10 GHz. The major factors affecting Earth-space paths in the frequencies above 10 GHz are:

(a) Impairment by atmospheric gases

(b) Impairment by Cloud

(d) Tropospheric Scintillations

These are described in more details below.

Impairment by Rain

Attenuation due to rainfall plays a significant role in the design of earth-satellite radio links at frequency above 10 GHz. With the current proliferation of satellite communications systems worldwide, it becomes necessary to study the microwave attenuation by precipitation in various climatic regions. A lot of research has been carried out in several countries, such as America, Europe, and Japan on the microwave propagation characteristics and the results published in the literature (Olonio and Riva, 1998; Bowman et al., 1997; Gloaguen and Lavergnat, 1996; Li et al., 1995; Stutzman et al., 1995; Goldhirsh et al., 1992; Stutzman et al., 1990) are mainly applicable to regions of higher latitude, whereas the results available for low latitude regions in the tropical are quite limited.

Characteristics of Rainfall in Tropical Regions

The precipitation characteristics in the tropics differ appreciably from those of the temperate regions. Broadly speaking, rainfall can be classified into: Stratiform and convective rainfall. Stratiform precipitation results from formation of small ice particles joined together to form bigger nuclei. The growing nuclei become unstable and as they pass through the so-called melting layer, (extending from about 0.5 to 1km below the 0⁰C isotherm) they turn into raindrops that fall down to the earth surface, with an horizontal extent of hundreds of km for durations exceeding an hour. The vertical extent is up to the height of the bright band. Convective precipitation is associated with clouds that are formed in general below the 0⁰C isotherm and are stirred up by the strong movement of air masses caused by differences in tropospheric pressure. In this process, water drops are created and grow in size, until they fall to the earth surface. The horizontal scale is of several km for durations of tens of minutes (Ajayi, 1989). Tropical rainfall has been shown to be predominantly convective and characterized by high precipitation rates. It occurs in general, over small vertical extent and for short duration of time (Ajayi, 1993). However, during precipitation, stratiform structures develop which extend over wider areas (about 100km) with smaller intensities (0-25mm/h).

Chapter Three

RESULTS AND DISCUSSIONS

Validation of TRMM Data with Measured Rainguage Data in Nigeria

Available rain gauge data from January 1991 to December 2000 were collected at 14-locations for the purpose of validating TRMM data which became available to the scientific community in January 1998. Ground data from January 1998 to December 2000 were selected to validate the closeness of the TRMM satellite data with the rain gauge data at 14-locations, which cover the entire six regions in Nigeria. Table 3.1 shows the Seasonal Variation of Mean Precipitation, Mean Bias Error and the overall correlation coefficients of TRMM 3B43V6 Data with the ground data collected at National Meteorological Centre Oshodi Lagos.

Table 3.1 shows the seasonal percentage mean bias error of TRMM 3B43V6 data with the ground data. In December, January, February (DJF) Mean bias error greater than ±50% occurred in ten locations, except Ado-Ekiti, Ibadan, Ikeja, and Calabar where the two data perfectly agree. While in March, April, May (MAM) bias error greater than ±50% occurred only at four locations, Minna, Kano, Jalingo and Maiduguri, there was perfect agreement between the two data in ten locations. In June, July, August (JJA) (the peak of rainy season) there was perfect agreement between the two data in thirteen locations, the seasonal mean bias error greater than ±50% occurred only in Kano. While in September, October, November (SON) the bias error greater than ±50% occurred only at four locations, Adoekiti, Yenagoa, Kaduna, and Jalingo. There was perfect agreement between the two data in ten locations.

The results suggest that in MAM, JJA and SON there was good agreement between the satellite data and the ground data, but in DJF (dry season) there was little agreement between the two data except for the South-West region. Adeyewa and Nakamura (2003) had observed similar seasonal percentage mean bias error of TRMM 3B43V6 data with ground data over the six major climatic zones in Africa:

Chapter Four

SUMMARY AND CONCLUSIONS

Propagation impairments of radio wave signals due to rain, cloud, gas and tropospheric scintillation on earth-space path have been studied for frequencies between 10 to 50 GHz at the two standard elevation angles of 5⁰, 55⁰ as well as some elevation angles for links to the Nigeria communication satellite, (NigComsat-1) for 37 stations in Nigeria. For the computation of the propagation impairments, the recent recommendations of International Telecommunication Union Radiowave Propagation Study Group Three (ITU-RP SG3) empirical models have been used. The ITU-RP models were chosen because they are based on recent measurements around the world and the models showed good agreement when tested with measured data for all latitudes in the probability range 0.001 to 5% unavailability in an average year.

CHAPTER FIVE

Conclusion and Recommendations

The knowledge of various propagation impairments such as rain, cloud, gas and tropospheric scintillation and the combined effects is of ultimate importance for fixed satellite communication service at frequencies above 10 GHz for planning, budgeting and predicting the transmission and reception of radio waves signals on earth-space path. The present studies indicate the significance of the influence local and regional climatic factor on microwave signals on earth-space path. The predicted attenuations show the various effects of local weather on propagation of radiowave signals for major cities in Nigeria. The results suggest that the southern part of Nigeria experience the highest propagation impairment while the Northern part experience less impairment.

Overall, Sokoto and Katsina appear as good locations to site fixed satellite earth stations (operating at Ku band and above) for deep space exploration as the results obtained showed consistently that the two locations are less affected by all propagation impairments investigated.

Secondly, for a complete understanding and local modeling of various propagation parameters relevant to the study of impairments of radio waves signals in the earth-space path at the 37 stations, it is recommended that an Agency be set up to fund propagation research by providing necessary instrumentation such as: satellite beacons (receivers), radiometers, distrometers and Automatic weather station for radio scientists at each of the 37 stations to measure propagation impairments on earth-space path monitored from the recently launched Nigeria communication satellite (NigComsat-1). NigComsat-1 has a life span of 15-year, a period long enough for the accumulation of propagation data for the country to produce its own local propagation models. This will help in quick integration and expansion of telecommunication services in Nigeria. It will also help in the assessment of the standards of telecommunication equipment shipped into the country.

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