Seasonal Variation of Total Attenuation between Airborne Platform and Earth Station in South-West Region, Nigeria

The seasonal variation of total attenuation in the southwest region of Nigeria has been computed using eight (8) years dataset at Ku- and Ka-band of the transmitted power of the radar, transmitted antenna gain and the received antenna gain of the satellite retrieved from the archived of the GPM. The results obtained fluctuates between the seasons at Ku- and Ka-band. From the results obtained at Ku-and Ka-band, the results from analysis showed that the peak total attenuation was recorded between the early (MAM) and late wet (JJA) season when the intensity of rainfall is maximum in the South-West region. As a result, the effect on the airborne-earth station link will be severe which may further lead to signal outage. However, the state where the highest total attenuation was consistent is Lagos state. These seasons and Lagos state must be taking into consideration by engineers and radiowave propagation group when planning and sitting radiowave propagation in the study area.


Introduction
In our today's world of communication, internet connection has become paramount.The traditional system of sending mails, banking and commerce and entertainment has been modernized which forms an innovative viewpoint of information sharing and socialize way of meeting people [1].Since the advent of mobile system of communication was introduced, it has made it possible for passengers in an airplane to have access to internet connection.Survey in the United States of America has shown that 80% and above of passengers travelling onboard an airplane travelling from one location to another around the globe now makes used of smartphones and laptops [2] and would want to make do of internet access onboard to while-away time while on cruise.While on cruise, passengers can do their jobs and also socialize with friends via surfing the web, chatting e-mail service and attending meetings through video conferencing.These types of services are referred to air to ground communication (ATG).The methods of internet provision, security and navigations services to aircrafts, are through ground stations on the surface of the earth and satellites in space.However, the provision of these services for global coverage are aided by satellite link only, although, the provision of this satellite link is very expensive and the transmitted signals through the earth-space path are distorted as a result of the distance traveled by the signal [1].In the course of flight, propagation parameters affect the communication link with distinct impact.In this regards, accurate analysis of time series can make provision for simulation of the link budget.Different parameters affect aeronautical links.However, among them, the atmosphere plays a vital role as depicted in Figure 1.For earth-space link, distinct layers of the atmosphere causes attenuation effects on communication operating at microwaves frequency.The two most important effect on the microwave links are due to the troposphere and ionosphere.For the ionosphere, the effect is due to activity caused by ionization as a result of the disturbance of the sun, thereby decreases the strength of the transmitted signal below some few GHz while the troposphere is due to its layer where events caused by weather activity causes effect on signals propagation at microwave frequency operating at Ku-band and above.At microwave frequencies, rainy event and streams of air masses in the lower troposphere cause the absorption of power [3].In open literature, various authors have shown that satellite system operating at Ku and Ka-band (13.5 GHz) and above are susceptible to fading in signal strength propagated through the earth-space link or undergo path loss due to hydrometeors such as rain, ice, fog and sleet found in the atmosphere.Large losses may occur in the transmitted signal leading to total outage.Among the hydrometeors found at the atmosphere, rain is the major factor that causes fading in system of satellite communication [4][5][6].Many researchers have estimated path loss on communication between satellite in space and ground stations using distinct models in different regions of the word in the past four decades [7][8][9].However, path loss on earth-space or airborne-space link have not been given much attention.[3,10] analyses the attenuation due to the atmosphere on airborne-space path.[11] study the attenuation cause by the troposphere on communication on airborne-satellite link while [12] compared the path loss between water vapour and oxygen satellite-space path using data retrieved from NOAA.However, these works focus on satellite-space link.In this work, our focus will be on seasonal evaluation of attenuation on communication on airborne-earth station due to the troposphere as shown in Figure 1(b).

Study Area
The study area is South-West region of Nigeria which comprises of Lagos, Ogun, Oyo, Osun, Ekiti and Ondo states as shown in oceans.However, the dry season is connected to northeast trade streams of air mass flowing in from the Sahara Desert.For the purpose of this work, the wet and dry season is further divided into two; early wet such as March, April and May (MAM) and late wet such as June, July and August (JJA) and early dry such as September, October and November (SON) and late dry such as December, January and February (DJF) [15,16,17].

Data Collection
The long-term data (eight years) required (March 2014 to December 2021) for the computation of the seasonal total attenuation for South-West region consisting of six stations such as Lagos, Ogun, Oyo, Osun, Ekiti and Ondo states used for the determination of the seasonal trends was obtained from the archive of the Global Precipitation Measuring Mission (GPM).The daily dataset obtained were first and foremost checked for outliers and noise for expulsions.After which the radar received power of the dataset was obtained on a daily, monthly and finally on a yearly basis from the raw dataset in excel format before the total attenuation was computed using Equations 1-6.The value of the total attenuation computed was subtracted from the received power at Ku-and Ka-band using Equation 1.

()
Where EIRP , r G and PL , is the equivalent isotropic radiated power (dB), received antenna gain (dB) of the satellite, and pathloss (dB), respectively.The equivalent isotropic radiated power is computed using Equation 2 [15]; Where  (m) and D (m) are the wavelength and slant range or distance (m) from the surface of the earth to the satellite at Ka and Ku band, respectively.The wavelength of the satellite is computed for Ka and Ku-band, using the relationship between the speed of light, frequency and wavelength, given by Equations 4; Where v is the speed of light ( 83 10  ms -1 ),  and f are the wavelength and frequency (Hz) of the satellite at Ka and Ku-bands, respectively.The longest beam (slant range (m)) from the earth station to the satellite estimated from Pythagoras theorem using Equation 5; The total attenuation was estimated using Equation 6; is the computed or average attenuation (dB) and measured A is the measured attenuation (dB).Note, all parameters used in these computations are converted to their standard unit before the computations are carried out.

Seasonal Variation of Total Attenuation at Ku-band
The summary of the results of the seasonal analysis of the total attenuation at Ku-band for South-West region is presented in Figure 3.The results reveal the seasonal analysis at Ku-band from 2014-2021.In 2014, the highest and lowest attenuation was observed in Lagos and Ekiti states with a value of 0.6518 dB and 0.1764 dB in the late wet (JJA) and late dry (DJF) season, respectively.In 2015, the early (SON) dry season with a value of 0.6682 dB registered peak total attenuation in Ogun state due to a slight increase in the value while Osun state recorded the least total attenuation with a value of 0.4462 dB in the late dry (DJF) season.In 2016, the total attenuation value decreases slightly with maximum value recorded in the late wet (JJA) season with a value of 0.5045 dB in Lagos state.However, the late wet (DJF) season recorded the minimum total attenuation in Osun state with a value of 0.2608 dB.In 2017, the total attenuation value increases to a value of 0.5180 dB in Lagos state recorded in the late dry (DJF) season while least attenuation value of 0.2583 dB occurred in Osun state in the early wet (MAM) season.
In 2018, the maximum total attenuation value further increases to 0.5230 dB in the late wet (JJA) season in Lagos state while minimum value of 0.2622 dB occurred in the late wet (JJA) season at Osun state.In 2019, Lagos state was also observed to have the peak total attenuation of 0.4915 dB in the late wet (JJA) season and least value of 0.2573 dB registered in the in the early wet (MAM) season.In 2020, the highest total attenuation increases to a value of 0.5751 dB which was recorded in the late wet (JJA) season while the lowest value of 0.2704 dB was registered in the early wet (MAM) season in Ondo state.In 2021, it was further observed that Lagos state recorded the maximum total attenuation in the late dry season (JJA) with a value of 0.5751 dB.However, the minimum value of 0.2688 dB occurred in the early wet (MAM) in same state.The results from analysis showed that the peak total attenuation was recorded between the early (MAM) and late wet (JJA) season when the intensity of rainfall is maximum in the South-West region.The state where the highest total attenuation was consistent is Lagos state.The implication of this during the wet season is that, rainfall is the major contributor to the total attenuation between the airborne platform and ground station in the South-West region between 2014-2021 is rainfall which may leads to the unavailability of signal between the path link [10,11,[19][20][21].It is recommended that Engineers must take this into consideration when planning radio wave propagation between an airborne platform flying through this region communicating with a ground station.

Seasonal Variation of Total Attenuation at Ka-band
The summary of the results of the seasonal analysis of the total attenuation between an airborne platform and ground station for South-West region at Ka-band is presented in Figure 4.The Figure reveals the results of the seasonal analysis of the total attenuation from 2014 to 2021.In 2014, the highest total attenuation was recorded in the late dry (DJF) season with a value of 0.8618 dB in the late dry (DJF) season in Ogun state while Lagos state registered the lowest total attenuation with a value of 0.3708 dB also in the late dry (DJF) season of the year.In 2015, the peak and the least total attenuation decrease slightly to a value of 0.7016 dB and 0.2609 dB in the early wet (MAM) and early dry (SON) season of the year in Lagos state.In 2016, the maximum and minimum total attenuation increases slightly to a value of 0.7355 dB and 0.3290 dB in the early dry (SON) and late wet (JJA) season, respectively also in Lagos state.In 2017, the peak total attenuation decreases slight to a value of 0.6829 dB in the early wet (MAM) season of the year observed in Ogun state.However, Lagos state also recorded the least total attenuation with a value of 0.3654 dB in the late wet (JJA) season of the year.
In 2018, there is a slight deviation in the highest and lowest total attenuation with a value of 0.5815 dB and 0.2669 dB registered in the late dry (DJF) and late wet (JJA) season in Ondo state and Lagos state, respectively.In 2019, the peak total attenuation was recorded in Osun state with a value of 0.5414 dB in the early dry (SON) season while the least total attenuation occurred in Ondo state with a value of 0.3067 dB in the late dry (DJF) season of the year.In 2020, the maximum total attenuation increases sharply to a value of 0.6251 dB in the late wet (JJA) season and the minimum value of 0.3252 dB occurred in the early wet (MAM) season of the year both in Osun state.In 2021, the highest total attenuation further increases to a value of 0.6498 dB in the late wet (JJA) season of the year in Osun state.However, the lowest total attenuation was recorded in the late dry (DJF) season of the year with a value of 0.3557 dB in Ogun state.Generally, the results from analysis showed that the highest total attenuation was mostly recorded between the early (MAM) and late wet (JJA) season when the intensity of rainfall is at its maximum.Also, the states where the maximum total attenuation is consistent in the South-West region are Ogun state, Lagos state and Osun state.This implies that the major contributor to the total attenuation between the airborne platform and ground station in the South-West region between 2014-2021 is the presence of rainfall influenced by the Tropical Maritime air mass that flows in from the Atlantic Ocean, which may lead to the unavailability of signal between the path link.This is an indication that the average seasonal variation obtained has higher total attenuation values in the wet season and lower values in the dry season which is one among many other features of the tropical climate [19,21].

Conclusion
The seasonal variation of total attenuation in the South-West region of Nigeria has been computed using eight ( 8) years dataset at Ku-and Ka-band of the transmitted power of the radar, transmitted antenna gain and the received antenna gain of the satellite retrieved from the archived of the GPM.It was observed that the highest total attenuation was recorded between the early (MAM) and late wet (JJA) season when the intensity of rainfall is at its maximum.Also, the states where the maximum total attenuation is consistent in the South-West region are Ogun state, Lagos state and Osun state although, little attention also needed to be given to other states in the region at Ka-band because the higher the frequency the greater the attenuation effect.However, at Ku-band, the results from analysis showed that the peak total attenuation was also recorded between the early (MAM) and late wet (JJA) season when the intensity of rainfall is maximum in the South-West region.The state where the highest total attenuation was consistent is Lagos state.The seasons must be taking into consideration by engineers and radiowave propagation in planning and sitting radiowave propagation must especially in Lagos station in the study area.

Figure 1 :
Figure 1: Communication link between: (a) airborne-ground station and airborne-space station (b) airborne-ground station with impact of the atmosphere [13].

Figure 2 .
The coordinate of southwest region lies between longitude 2 o 12 1 E and 6 o E and latitude 6 o 21 1 N and 8 o 38 1 N [14] with a land mass of 77,818 km 2 .The area is bounded by Delta and Edo state in the East, Kogi and Kwara state in the north, Republic of Benin in the west and the Gulf of Guinea in the south.The Southwest region has a tropical climate characterized by rainy (wet) and dry seasons with temperature ranging 21 o to 34 o C while annual rainfall lies between 1500 and 3000 mm.The wet period is connected with southwest streams of monsoon wing flowing in from the Atlantic IOP Publishing doi:10.1088/1755-1315/1342/1/0120363

Figure 2 :
Figure 2: Map of Nigeria showing the stations in the study area [14].

6 Figure 3 :
Figure 3: Seasonal variation of total attenuation for South-West region of Nigeria at Ku-band from 2014-2021

Figure 4 :
Figure 4: Seasonal variation of total attenuation for South-West region of Nigeria at Ka-band from 2014-2021