ABSTRACT
Among other atmospheric region, ionosphere, which is the ionized region of the atmosphere, is considered to impose serious limitation on radio wave transmission while the effect of other layers, more especially, the troposphere is often treated as negligible. At higher frequency, radio waves pass through the ionosphere and are attenuated due to the free electrons present in it. However, recent studies have shown that while the ionospheric disturbances can be predicted on a global scale, tropospheric disturbances depend on geographic location due to dependence of local meteorology on surface topography and other specific location weather forcing. A number of models has been used by many researchers in the investigating of atmospheric attenuation. The objective of this thesis is to discuss the atmospheric effects on high frequency radio waves between 10 and 50 GHz propagating on earth-space in the troposphere for 12 locations in Nigeria, investigating the attenuation and losses it may come across like attenuation due to atmospheric gases, rain, clouds and tropospheric scintillation and comparing them with the model used by the researchers. Two standard elevations angle of and were used in the computation of the propagation impairment for the 12 location
TABLE OF CONTENTS
Title page i
Certification ii
Declaration iii
Acknowledgement iv
Table of Content v
Abstract ix
CHAPTER ONE
INTRODUCTION
1.1 Background of the study 1
1.2 Statement of the Problem 2
1.3 Rationale for the study 3
1.4 Aims and Objectives of the study 4
CHAPTER TWO
LITERATURE REVIEW
2.1 Attenuation by Rain 5
2.1.1 Characteristics of Rainfall in tropical region 6
2.1.2 Raindrop by size 6
2.1.3 Existing Rain attenuation models 7
2.1.3.1 Overview 7
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2.1.4 Methods of Rain attenuation measurements 12
2.2 Attenuation by Cloud 13
2.2.1 Cloud data sources 15
2.2.2 Existing Cloud attenuation models 16
2.2.2.1 Overview 16
2.3 Attenuation by Atmospheric Gases 19
2.4 Tropospheric Scintillation 21
2.4.1 Amplitude Scintillation Prediction Models 22
CHAPTER THREE
EXPERIMENTAL MODEL
3.1 Processing of Rain data 24
3.1.1 Isotherm height and Rain height 24
3.1.2 Calculation of long-term rain attenuation statistics from point rainfall rate 25
3.2 Processing of Cloud data 28
3.2.1 Procedure for evaluation of SVD, TCC and IWVC 28
3.2.1.1 Surface Water Vapour Density, ρ (SVD) 28
3.2.1.2 Integrated Water Vapour Content (IWVC) 29
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3.2.1.3 Total columnar content of liquid water in clouds (TCC) 30
3.2.2 Procedure for Evaluation of Cloud Attenuation 32
3.2.2.1 Specific Cloud attenuation coefficient 32
3.3 Processing of Gaseous Data 34
3.3.1 Gaseous Specific Attenuation 35
3.3.1.1 Dry air Attenuation (dB/km) 35
3.3.1.2 Water vapour attenuation (dB/km) 37
3.4 Processing of tropospheric Scintillation Data 38
3.4.1 Procedure for evaluating the input parameters 38
CHAPTER FOUR
RESULTS AND DISCUSSIONS
4.1 Results of Rain Attenuation 41
4.1.1 Comparing the Rain Attenuation Experimental Model with the Lognormal
Distribution Modeling by Ajayi and Olsen (2000). 45
4.2 Results of Cloud Attenuations 47
4.2.1 Comparing the Cloud Attenuation Experimental Model with the ITU-R Model. 51
4.3 Results of Gaseous Attenuation. 52
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4.3.1 Comparing the Gaseous Attenuation Experimental Model with the ITU-R Model. 56
4.4 Results of Tropospheric Scintilliation. 58
4.4.1 Comparing the Tropospheric Scintillation Experimental Model with the
ITU-R Model. 62
4.5 Results for Combined Attenuation. 63
CHAPTER FIVE
SUMMARY AND CONCLUSIONS
5.1 Rain attenuation at Frequencies between 10 to 50 GHz 67
5.2 Cloud Attenuation at Frequencies between 10 to 50 GHz 67
5.3 Gaseous Attenuation Study at Frequencies between 10 to 50 GHz 68
5.4 Tropospheric Scintillation Study at Frequencies between 10 to 50 GHz 68
5.5 Combined Attenuation Study at 0.01% Unavailability of an Average Year for
Frequencies between 10 to 50 GHz 68
5.6 Conclusion and Recommendations 69
REFERENCES 71
APPENDIX 76
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CHAPTER ONE
RODUCTION
1.1 Background of the study
Satellite communication is normally thought of as a robust means of communication, not sensitive to environment impacts. This perception is not totally accurate. Satellite communication can be and is affected by the environment in which it operates. Space environment effects on satellite communication can be separated into (1) effects on the space element (i.e. the satellite), (2) effects on the ground element (i.e. the Earth station), and (3) effects on the signals propagating through the Earth’s lower and upper atmosphere. The propagating signal may be affected by its passage through the ionosphere (upper atmosphere) or the troposphere (lower atmosphere). These effects depend significantly on frequency, but include signal absorption, scintillation, Faraday rotation and bandwidth decoherence. Geographic location and signal propagation path can also determine the extent to which the signal is affected. The ionosphere is a region of the upper atmosphere that extends from 70 to 500 km in altitude. It is a region where some of the atoms have had their outer electrons removed by extreme ultra-violet (EUV) and X-ray radiation coming from the sun. The atmosphere is thus said to be partially ionized – hence the name, ionosphere. The ionosphere usually consists of four layers but two layers are more important in radio propagation.
Troposphere is the lower most layer of the atmosphere adjacent to the earth’s surface and contains approximately 80% of the atmosphere’s mass and 99% of its water vapour and aerosols. This layer extends up to about 14-17 km in the tropical region and becomes shallower towards polar region. It is the layer that determines the daily weather cycle of the earth. Within the
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troposphere there is generally a gradual fall in temperature with increasing altitude and most of the turbulent mixing occurs in this region. The refractive index of the air in the atmosphere plays a dominant role in microwave signal propagation over earth-space path. Tropospheric weather conditions, occurring in the lower 10 km of the atmosphere, can also cause losses in signals propagating between satellite and ground stations. Beam spreading is more pronounced at (Very High Frequency) VHF and (Ultra High frequency) UHF. Water vapour is particularly damaging to signals above about 2GHz, causing absorption of signals which becomes greater as frequency increases. K-band signals (10-20 GHz) are particularly susceptible, and precipitation in the vicinity of satellite ground stations can cause total loss of signal. Again, signals with low elevations are more affected than those propagating near the zenith, because the wave has to follow a longer path through the atmosphere. At frequencies above 20 GHz, we start to encounter resonant absorption at specific frequencies. Oxygen in particular, will absorb electromagnetic energy only at certain well-defined frequencies. These frequencies correspond exactly to the energies required to lift the oxygen atoms into higher energy states. Satellite communication links are designed to avoid these well-known frequency bands
1.2 Statement of the problem
Radiowave propagating between terrestrial links and earth-space links are adversely affected by atmospheric gases, rain, clouds and tropospheric scintillation. The problems become more acute for systems operating at frequencies above 10 GHz. Nigeria is located in the tropics unlike the temperate environments such as Europe and North America. The effects of the troposphere on radiowave propagation will be most severe in the tropics because of high frequency of occurrence of rainfall, high temperature, high relative humidity, and high rain intensities which
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lead to a more severe signal attenuation and outage than what is experienced in the temperate region.
1.3 Rationale for the study
There is very little information on propagation studies on earth-space communication links in Nigeria. Where there is information, there are only few that cover the effect of atmosphere. The one that was carried out in Ilorin was to experiment the effect of rain on Ku band frequency and satellite communication in the tropical region (Adimula, et.al.,2005). The present study is therefore aimed at investigating the effects of rain, cloud, gases and tropospheric scintillation on earth-space path at 12 stations in Nigeria by using satellite data for the period 2009 to 2013.
1.4 Aims and objectives of the study
The aim is to investigate the effect of attenuation on earth-space path and the following steps were taken in achieving the objectives of the study:
1. Compute some propagation impairments relevant to fixed satellite communication at elevation angles of 5 , 55 in the 12 locations in Nigeria.
2. 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.
3. Finally, compare the result to the model used by other researchers and verify if they match well or not.
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