J. Earth Syst. Sci. (2020) 129:220 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-020-01477-y (0123456789().,-volV)(0123456789().,-volV) Investigation of anomalous propagation conditions in Central and West African stations using high-resolution GPS radiosonde observations 1,6, 2 3,4 3 SAMUEL KAISSASSOU *, A LENOUO ,RSTANESSONG ,KAMSU TAMO , 3 3,5 6 AVONDOU ,WPOKAM and J KANKO 1Department of Electrical and Telecommunication Engineering, National Advanced School of Engineering, University of Yaounde 1, P.O. Box 8390, Yaounde, Cameroon. 2Department of Physics, Faculty of Science, University of Douala, P.O. Box 24157, Douala, Cameroon. 3LEMAP, Department of Physics, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon. 4Faculty of Agronomy and Agricultural Sciences, School of Wood, Water and Natural Resources, University of Dschang, P.O. Box 786, Ebolowa, Cameroon. 5Department of Physics, Higher Teacher’s Training College, University of Yaounde 1, P.O. Box 47, Yaounde, Cameroon. 6Department of Meteorology, Climatology, Hydrology and Soil Sciences, National Advanced School of Engineering, University of Maroua, P.O. Box 46, Maroua, Cameroon. *Corresponding author. e-mail: [email protected] MS received 13 December 2019; revised 23 May 2020; accepted 10 July 2020 A comprehensive study on the anomalous propagation (AP) conditions occurring over the central and west African stations was made from 2 years (January 2005–December 2006) high-resolution data measured by GPS (Global Positioning System) radio survey observations. Through data quality control and diagnostic analysis, the probability of AP occurrence and characteristic quantities of the three typical anomalous propagation conditions were given. The sub-refraction, super-refraction and ducting cases were investigated statistically using the vertical proBle of modiBed refractivity gradient. Strong diurnal variation in the percentage occurrence of the AP had its peak during the wet months, while the dry months had the lowest values. From 0600 to 1800 local time (LT) at day (1800–0600 LT at night), the total percentage occurrence of super-refraction, sub-refraction and ducting were 82.5% (78.5%), 11% (15.5%) and 6.5% (6%), respectively. Besides statistical results, local meteorological conditions prevailing over central and west Africa have also been discussed. Keywords. Anomalous propagation; west-central Africa; duct; radiosonde. 1. Introduction the tropopause. The AP conditions can conduct to a propagation of nonstandard electromagnetic Atmospheric refraction has a significant impact on telecommunication systems such as RADARs, the propagation of electromagnetic waves. The LIDARs and other wireless tools. It has been shown discovery of this eAect dates as far back as the that, the electromagnetic waves are bending emergence of the radio wave technology (Freehafer because of the spatial stratiBcation of the refractive 1988). We noticed that anomalous atmospheric index of air sometimes alter the wave direction and conditions for propagation occur generally below intensity. 220 Page 2 of 16 J. Earth Syst. Sci. (2020) 129:220 Amongst those damages are multipath fading height dependence (Fink et al. 2010; Cheng et al. and interference, diAraction due to the Beld 2015). Unfortunately, in west-central African obstacles and so on (Lavergnat and Sylvain 2000; regions, the radio survey stations (Faccani et al. Zribi et al. 2016). Thus awareness of anomalous 2009) are limited and their geographic locations propagation will significantly improve the opera- are not homogeneous. Thus, AP information that tional performance of telecommunication systems. generally requires high-resolution observation Information about radio propagation measure- database resource has almost been inexistent in ments in tropical and sub-Saharan regions are recent years. The AMMA (African Monsoon not well known. Some eAorts based on observa- Multidisciplinary Analysis) campaign was laun- tional data such as in-situ measurements are ched to enhance data in these regions. Further- needed to provide scientiBc explanation and to more, campaigns such as AMMA and other data satisfy engineers’ need. EAorts are also needed for measurements are expected to build a credible the conceptualization of telecommunication tools database in this part of the world. As mentioned such as RADAR or satellite basement in the earlier, neither the climatology nor the meteorol- region. AP detection has been scrutinized in dif- ogy of anomalous propagation in west-central ferent regions of the world based on the vertical Africa has been documented and discussed satis- proBle of atmospheric parameters. Most of the factorily due to unavailability of high-resolution previous works on anomalous propagation condi- data to date. Therefore, the main objectives of tions, at the troposphere, in west-central Africa our current work are to provide an overview of (Parker et al. 2008; Guy et al. 2011; Kaissassou the eAects of anomalous propagation as well as et al. 2015a,b) relied on accurate radio survey the place and time of its occurrence. This paper measurements. also aims to understand impacts of anomalous Previous studies include the works of Falodun propagation conditions and how frequent these and Okeke (2012) which were based on radio wave events occur, which is fundamental for the propagation measurements in Nigeria. Falodun understanding of radar data information and also and Ajewole (2006) have also published an article for the weather forecasting model (Mesoscale on the temporal variability of the refractive index Atmospheric Simulation System and WRF) in Akure city (7.15°N, 5.12°E), Nigeria. They validation, and avoiding misinterpretation of described seasonal variations of the vertical gra- erroneous data. dient index and made some correlation between In addition, it might serve to promote aware- the dry and rainy seasons in Akure. Attempts are ness amongst scientists and operational workers made to evaluate such phenomena in terms of using microwave propagation like telecommuni- statistics of duct conditions over the Wallops cation tools (Kaissassou et al. 2015a, b). The data Island, Virginia (Babin 1996) and using radio used here, collected from the AMMA campaign, survey observations in Barcelona, Spain (Bech were obtained using high-resolution balloon GPS et al. 2002). Other authors like Dalmaz (1977) radio surveys. These radio surveys were launched studied the inCuence of temperature on the several times daily (0000, 0600, 1200, and 1800 refractivity at Goztepe; Barla (1986) noticed UT) in order to provide a vertical atmospheric spatial and seasonal variability in Turkey; Mentes proBle for parameters such as pressure, tempera- and Kaymaz (2007) carried out the statistics of ture, relative humidity, and wind speed. GPS surface duct conditions over Istanbul, Turkey radio surveys have some advantages over classic (41°N, 29°E). The statistical study of microwave radio surveys: they have higher vertical resolu- refractivity is accurate for forecasting AP condi- tion, which facilitates the detection of small AP tions like ducting. These results can be more dimensions. significant if work is done on a larger scale and for Section 2 enumerates the data and methodology. a longer period. An eAort to identify the meteorological conditions Investigations of AP occurrences across the prevailing in west-central African regions is being world were made public recently (Ao 2007; presented in section 3. Section 4 treats the seasonal Mentes and Kaymaz 2007; Lopez 2009; Kaissas- variations of AP over west-central Africa. In sec- sou et al. 2015a,b) using different techniques such tion 5, we discuss the impact of atmospheric as radio survey for an eAective detection of AP. refractivity on propagation of waves. Section 6 The radio survey measures atmospheric parame- summarizes the work followed by a brief ters (viz., temperature, pressure, humidity) with conclusion. J. Earth Syst. Sci. (2020) 129:220 Page 3 of 16 220 2. Data and methodology Furthermore, in the wet season during certain periods (15 July–15 September 2006 called special 2.1 Data observing periods (SOP3)), the number of surveys increased and became regular on a daily basis. This During the AMMA campaign, over 19 stations in was in order to monitor the diurnal cycle of the west-central African cities regularly released high- unusually intense monsoon (Parker et al. 2005). In resolution radio survey balloons (Vaissala RS-80, fact, the AMMA 2006 intensive observing periods RS-92, and MODEM). Figure 1 shows their geo- (IOPs) involved launching of many radio surveys graphic locations. This current study makes use of several times daily (up to 4 times). Thus, the radio 2 years data (January 2005–December 2006). Most survey network widened with new stations (Parker of these radio surveys were launched around 00:00 et al. 2008). and 12:00 LT. Temperature (T), pressure (P), There was collection and storage of radio relative humidity (RH), and horizontal wind pro- surveys in their original forms with a temporal Bles have been collected in different days, months resolution, averaging 5 sec, and a vertical reso- and seasons. lution of about 5 m less than the standard sur- Figure 1 illustrates the spatial distribution of vey of the World’s Meteorological Organization GTS radio survey network and the percentage (WMO). of surveys received at each station, valid at night Due to the increase in the number of daily (1800–0600 LT) and in the day (0600–1800 LT). It launches, several data from the same stations also shows that during the campaign, many dor- were available in the 3 or 6 hrs window, that mant radio survey stations were re-activated, while enable the quantity and the quality of data used other unreliable stations were renovated. In addi- in this study. Data has been separated into two tion, new stations were installed in the Gulf of subgroups: daytime data and night-time data. Guinea from the coast (Douala-Cameroon: For a better description of the vertical atmo- 04°010N, 09°420E; Abidjan-Ivory Coast; 05°150N, spheric structure, we divided the atmosphere into 03°560W) to inland (Niamey-Niger: 13°290N, large numbers of vertical levels.