Determination of the Annual Optimal Reliable Frequency for Different Transmitter/Receiver Stations Distributed over the Iraqi Territory

Abstract This paper aimed to determine the Optimal Reliable Frequency (ORF) that can maintain certain connection link between different transmitter/receiver stations laid over the Iraqi territory. Three different transmitting sites were chosen as tested stations located in the northern, central, and southern regions of Iraq. These sites are Mosul, Baghdad, and Basra, respectively. In this study, the years 2009 and 2014, which represent the minimum and maximum years of solar cycle 24, were chosen to examine the effect of low and high solar activity on the determined ORF. The datasets of the Best Usable Frequency (BUF) were calculated using the ASAPS international communication model. An analytical study was made on the generated BUF parameter dataset of the annual time for the adopted years. Also, an analytical simulation was made for the suggested ORF parameter depending on the BUF parameter datasets. The results of this simulation showed that the suggested ORF parameter can be represented by polynomial equation of different orders. The suggested formula was determined depending on different tested parameters (path length, bearing, time (day) and BUF values). For this purpose, a certain program was built using a visual basic programing language (VB6). The suggested ORF parameter was investigated for the annual time for the three sites of the years 2009 and 2014.


Introduction
The ionosphere is the upper part of the atmosphere that extends at 60-450 Km. It is a highly ionized region due to the Sun UV radiation [1]. The ionosphere consists of different layers because of different wavelengths in the Sun UV spectrum [2]. Layers of the ionosphere are different during day and night, as illustrated in Figure-1, which reflect or refract the wave depending on the wave frequency. Each layer may reflect properly a specific band frequency [1]. The radio waves at high frequency band (HFband 3-30 MHz) represent the primary communication resources that are back to Earth at distance from the transmitting station [3]. The ionosphere may act as an effective reflector on frequencies below 30 MHz. It allows radio wave propagation to distances of many thousands of kilometers, while the radio signals of frequencies above 30 MHz usually penetrate the ionosphere [1]. Sky wave propagation method can propagate signals over a great distance; and usually the high frequency (HF) band is used for sky wave propagation [4].

Ionospheric Communication Parameters
The ionospheric parameters represent important parameters in HF communications to determine the best range of reliable frequencies that are reflected from the ionospheric layers between two terminals at specific time [7], as shown in Figure-2. The MUF, LUF, FOT and BUF parameters represent the main ionospheric parameters that can determine the best communication frequencies [3]. The ionospheric parameters can be defined as follows 1. The Maximum Usable Frequency (MUF): The highest frequency at which radio wave is returned to Earth by reflection from the ionosphere, which can be used to transmit over a particular path under given ionospheric conditions at a specific time, the median value of MUF working 50% of the time [8]. 2. The Optimum Working Frequency (OWF): Also called the Optimum Traffic Frequency (FOT), which is the most practical operating frequency that one can rely on to have the least number of problems. The OWF is about 85% of the MUF [9].

The Lowest Usable Frequency (LUF):
The lower frequency that allows reliable long-range HF radio communication between two stations by ionospheric refraction. It is exceeded by the operational MUF on 10% of the specified period [9].

The Best Usable Frequency (BUF):
The frequency from the specified set with the maximum signal to noise ratio (S/N), which also satisfies the specified minimum take-off angle, the required S/N ratio, and the probability level. The BUF graph for each hour is selected by the frequency set user and determined on the basis of ionospheric conditions and the user-specified system details [10].

ASAPS HF Propagation Model
Many organizations have developed HF ionospheric communication models in order to provide more accurate usable frequencies that are maintained between transmitter and receiver stations over long distances. The performance of HF communication models has been developed progressively, starting in the 1930's with studies by radio scientists and engineers in some countries [3]. The Australian government's IPS Radio and Space Services HF propagation prediction method is known as the Advanced Stand Alone Prediction System (ASAPS). ASAPS is able to predict sky wave radio communications for the HF and VHF radio spectrum. ASAPS represents the development of the IPS Radio and Space Services of the Australian Bureau of Meteorology, merging the features of the original IPS method with the ITU-R / International Radio Consultative Committee (CCIR) models [11]. ASAPS prediction is more accurate than that of other systems and is especially accurate when driven from the T index, because this index is used to quantify the response of the ionosphere to solar activity.

Test and Results
In the present work, a study was made to determine the OTF value that can maintain a certain communication link between different transmitter/receiver stations laid over the raqi territor he se e te sites are osu 43 119 36 335 agh a 44 401 33 341 , an asra 47 780 , which are distributed respectively on the northern, central, and southern regions of the study area. The selected sites were selected to represent transmitter stations. Eighty different locations were selected for each station (8 directions and 10 distances) to represent receiver stations. The calculation of the geographic coordinates was performed by building a program using Matlab programming language. The calculations were made depending on the values of the geodesic parameters which were adopted in this research (path length 100 200 300 …, 1000) km and bearing 0 45 90 … 315 egree , calculated using equations (1) and (2). Samples of the geographical coordinates (latitude and longitude) for the locations of the receiving stations distributed over the studied area are illustrated in Table-1. where (λ ) and (φ, ) are the longitude and the latitude values of the receiver and transmitting stations, respectively, is the distance between two points along a vertical of latitude, and is the distance between two points along a parallel of longitude; B: Bearing; R: Radius of Earth.  In this work, the values of the BUF parameter were studied statistically by analyzing the generated dataset from the ASAPS model for the annual time period of the selected years of solar cycle 24. The effects of path length (100-1000) km, bearing (0 0 -315 o ), day time (0-23), and BUF parameter are represented using a simple mathematical model for the annual variation. The datasets of the ionospheric parameter ORF were generated for minimum and maximum years of the solar cycle 24 by program which was built up (implemented) using visual basic programing language (VB6). The annual expressions that describe the relations between the parameter (ORF) and the time, path, season, and bearing are where d: Path Length (Km), B: Bearing (Degree), t: Time (UT), and n: The polynomial order. Many tests have been performed to determine the orders (n 1 , n 2 , n 3 ) of the polynomial equation (7). These tests include the calculations of squared correlation coefficient (R 2 ), mean difference (MD), mean square deviation (MSD), and mean absolute deviation (MAD) between the BUF and ORF ionospheric parameters. Depending upon the calculation result, the suitable polynomial coefficients set that reflects a close behavior of BUF and ORF is attained to be in the of order of n 1 =2, n 2 =2, n 3 =10.

Discussion and Conclusions
In this work, an analytical simulation for the ORF parameter behavior was achieved over the Iraqi territory for an annual period of the years 2009 and 2014. The results of the ORF parameter for Mosul station for the year 2009, as shown in Figures-4, reveal values that vary with the path length parameter; the ORF values were increased with increasing the path length value, reflecting a directly proportional relationship, reaching a maximum value at 1000 km. The highest ORF values occurred at noontime, then they started to decline until reaching their minimum values at sunset. This might be due to the changes in the electron density of the ionosphere layers. Also, was noticed that the ORF parameter showed differences in behavior at different bearing angles, possibly due to the effect of the sunrise and sunset phenomena. The variation of the ORF parameter at different bearing angles had its highest value at (180 o ), may be because of the impact of thermal and geographical equators on the structure of the ionosphere layer at this bearing value. For Baghdad station (central region) in the year 2009 and the minimum solar cycle 24, it was observed, as shown in Figure-5, that the ORF parameter had approximately the same behavior and variation as that shown for Mosul station, except that it showed higher values. It was also noted from the results of the minimum solar cycle for the year 2009 for Basra transmission station (the southern region), shown in Figure-6, that the behavior of the ORF parameter had the same behavior in general for the stations of Baghdad and Mosul, with slightly higher value. This might be attributed to the effect of the proximity of the city of Basra to the thermal and geographical equator, compared to the locations of Mosul and Baghdad. The results of the variation values of the ORF parameter in the maximum solar cycle for the year 2014, illustrated in Figures-(4-6), showed almost the same behavior as that for the year 2009, but with more fluctuations and higher values. This might be due to the effect of high solar activity on the behavior and structure of the ionosphere layer.
Based on the previous discussion, it was noted that 2009 showed a more stable variation than that observed in 2014. The ORF parameter showed higher values in the southern region (Basra site) and these values decreased towards the northern region (Mosul site). The higher ORF parameter values for the three tested sites at the annual time of the minimum and maximum years 2009 and 2014 of solar cycle 24 were 8.656 and 11.475 for Mosul station, 8.963 and 11.922 for Baghdad station, and 9.918 and 12.493 for Basra station, respectively. According to the above discussion, our conclusions are summarized in the followings: 1. Radio wave communications depend on different parameters, including path length, bearing angle between the transmitting and receiver stations, time of the day, and minimum and maximum of the solar cycle. 2. The values of the ORF ionospheric parameter increase with the increase of path length (distance) between the transmitting and receiving stations. 3. The value of the ORF parameter varies with the bearing between the transmitting and receiving stations. This behavior may be due to the geographical location and closeness to the thermal and geographical equators. 4. The ORF values were higher at the maximum solar cycle (2014) than the minimum solar cycle (2009), while the year 2009 (minimum solar cycle) showed more stable variation than the year 2014 (maximum solar cycle). 5. The results of the annual variation of the optimum reliable frequency showed almost the same behavior with different values during various path length, bearing, and solar activity. 6. The values of the ORF parameter increase in the morning time, during sunrise, until they reach their highest values at midday (in the afternoon time), then they start to decrease at sunset due to the decrease in the electron density of the ionosphere layer. 7. The ORF values increase as the equator approaches, since the ORF parameter values in Basra station (southern region) were higher than those for Baghdad (central region) and Mosul (northern region) stations.