Urbanization is an activity, which is an inevitable and dominant issue of our time that is increasing alarmingly. The world’s urbanization status indicates these realities. According to a United Nation report, since 2007 the world's urban population exceeds the world's rural population and the residents of the world's urban population are expected to reach 68 percent in 2055(UN 2019). Addis Ababa, the capital city of Ethiopia, is one of the fastest-growing cities in Africa and 25 percent of the urban population of Ethiopia is living in Addis Ababa (World Bank Group 2015).
Even though the driving forces of urbanization have many factors, population growth and economic developments are the leading ones. An increase in population density and demands due to economic development in urban areas are mostly related to the utilization of urban space. To fulfill both the internal and external driving needs of urban residents, the urban area expands physically (Tahir et al. 2014). The physical expansion of urban space can lead to diversified issues and effects. One of the effects is urban land cover change (Wang et al. 2007).
Urbanization is frequently observed through an increase in the land cover of built-up areas. On the other hand, the expansion of the built-up area often affects and replaces agricultural land, vegetated areas, bare surfaces, etc. The normalized difference built-up index (NDBI) and normalized difference vegetation index (NDVI) are commonly used to study the expansion and degree of change in the urban landscape(Carlson and Arthur 2000; Cui and Shi 2012; Sun et al. 2012; Bhatti and Tripathi 2014; Deng et al. 2018; Potter 2019). In other words, NDVI and NDBI are well-known remote sensing techniques that are often applied to control and identify continuously changing urban land cover, which is caused by urbanization (Karanam and Babuneela 2017).
Urbanization and associated land cover change are one of the causes of urban climate change and it is also the central concern of many findings (Fang and QuanSheng 2012; HaiShan and Ye 2013; Zhang et al. 2016). According to Adeyeri et al. (2017), urban environmental change can be addressed by studying land cover change. The land cover change caused by urban expansion is the main factor for global, regional, and local environmental effects (Odindi et al. 2015). It also affects microclimate (Lim et al. 2005; Zhang et al. 2016; Naserikia et al. 2019), the spatial distribution, amount of precipitation, atmospheric composition, near-surface humidity, energy balance, wind speed, and temperature (Gallo et al. 1999; Han et al. 2013; Pathirana et al. 2014; Tahir et al. 2014; Souza et al. 2016; Choudhary and Tripathi 2018). Deng et al. (2018) also stated that land use land cover is one of the major factors which affects land surface temperature and they have a strong correlation (Ibrahim 2017). The extent of urbanization's effect on the environment varies with the size of the city. According to Gallo et al. ( 1996), the effect of dominant land cover on climate goes up to 10 kilometers radius. The major source of all this influence in urban areas is land use/land cover change (Linh and Chuong 2015; Odindi et al. 2015).
In general, the increment of an urban built-up area leads to a decrement in urban vegetation cover, and this affects the surface temperature of the region. The impact of the change in the urban built-up area on the atmosphere is observed through released particles, moisture, and heat (Molders and Olson 2004; Odindi et al. 2015). In addition to the greenhouse gases, anthropogenic factors related to changes in land cover are also the cause of changes in the surface and lower atmospheric temperatures (Laat and Maurellis 2006). As a result of the changes in the surface and lower atmosphere, the composition of the atmosphere alters (Pielke et al. 2002). This on the other hand affects the propagation of the signal through the atmosphere.
An electromagnetic wave passing through a nonhomogeneous medium can be reflected, diffracted, dispersed, or refracted. Refraction is a directional change of propagated wave when passes from one medium to the other (Hofmann-Wellenhof et al. 2008). Atmospheric refraction is the deviation of electromagnetic propagation within the atmosphere due to the difference in air mass and density (Jin et al. 2014). Electromagnetic waves typically move at the speed of light when they are transmitted in a vacuum, but this is not always the case. The medium in which waves travel can cause them to delay.
Satellite-based positioning uses electromagnetic waves in the radio frequency range and it is a well-known fact that the use of satellite-based positioning plays a significant role in our daily lives. For the last 20 years or more, it was also used to understand the effect of the atmosphere on radio signals (Hordyniec et al. 2018). The basic working principle of satellite-based positioning is mainly dependent on the position of the satellites and the distance of the satellite from the end-user's receiver. This information is acquired by the reception of a radio signal that traveled from the satellite to the receiver, through the atmosphere. Studying how the propagation medium affects the propagated signal is necessary to acquire precise and accurate positioning. The dynamic behavior of the atmosphere is yet another factor that continuously and differently affects the GNSS signal as it passes through the atmosphere (Hordyniec et al. 2018).
The propagation of a signal from the satellite to the user’s receiver is delayed by particles in the atmosphere. The neutral part of the atmosphere, which is also known as the troposphere, contains dry gas and water vapor. The delay due to the neutral atmosphere is called tropospheric delay and it causes the geometric lengthening of propagated signals (Jin et al. 2007). The tropospheric delay plays a vital role in studying weather, climate, and the vertical motion of atmospheres (Jingyong et al. 2005; Jin et al. 2007; Jin et al. 2009; Hordyniec et al. 2018). The land surface temperature directly or indirectly affects atmospheric composition. Vegetation cover and built-up area are on the other hand related to land surface effects (Malik et al. 2019) and they are interrelated. Because of these, it is logical to presume the indirect effect of urban expansion on the propagated GNSS signal. Therefore, a study was conducted using data from the IGS station ADIS, which is located on the premises of the Institute of Geophysics, Space Science, and Astronomy (IGSSA) of Addis Ababa University (AAU), as well as remotely sensed data on the expansion of the built-up area, changes in vegetation cover, and land surface temperature of Addis Ababa city, used to investigate this presumption. The location of the study area is shown in Fig. 1.
Data sources
To test the presumption that urbanization might have an impact on the positioning of points using the GNSS technique four data sets were used in this study. One is Landsat data, which is used to measure the physical expansion of the study area. The other data set is MODIS data, which is used to quantify changes in vegetation coverage and to investigate the Land Surface Temperature of Addis Ababa city. The Up directional of the GNSS data from the IGS site ADIS was used to observe the long-term variation in positioning.
Landsat Data
Landsat, the former Earth Resource Satellite Technology (ERST) is designed to study the earth's surface. Since 1972 Landsat satellite has continuously provided information about the changes occurring on the earth’s surface in a multispectral image (USGS 2019). It delivers moderate-resolution multispectral data of the earth's surface on a global scale. In this study, Landsat 5 TM (Thematic Mapper), Landsat 7 ETM+(Enhanced Thematic Mapper Plus), and Landsat 8 OLI(Operational Land Imager) products were used to measure the urban physical expansion of Addis Ababa city, from 2005 to 2019. To overcome the problem of cloud cover and avoid any seasonal effects, the images used were from January. In rare cases, it was necessary to search for cloud-free data from December to April, to get cloud-free images.
MODIS Data
Landsat has a good spatial resolution to measure land cover change. However, it has a limitation on temporal resolution and is highly affected by clouds. Moderate Resolution Imaging Spectroradiometer (MODIS) sensors enable measuring surface change by providing daily worldwide observation (Weng 2011). MODIS data will improve our insight into the earth’s surface variability at a larger scale and process global changes, such as in oceanography, biology, and the atmosphere(Gao 2009). From those vast application areas and data sources of MODIS, in this study vegetation index and Land Surface Temperature (LST) were used to study urban vegetation cover and land surface temperature of Addis Ababa city, from 2005 to 2019.
The vegetation index (VI) is one of the MODIS data sets, which is used to study vegetation covers of the earth. It empirically measures vegetation coverage at the land surface. In this study, the Terra MODIS Vegetation Indices (MOD13Q1), with 16 days temporal and 250m spatial resolution was used. The other MODIS data used in this study is LST. LST is one important measure of surface energy (Assiri 2017). It gauges the temperature of the earth's surface at a specific location. MODIS provides LST data daily in global coverage. In this study, MODIS land surface temperature and emissivity MOD11A1, version 6 product, with 1km spatial resolution was used to generate information about the LST of Addis Ababa city. On some specific days, there were however missing data. Both NDVI and LST were downloaded from NASA’s FTP server.
GNSS Data
The positional information generated from GNSS data can be computed at different levels using different types of approaches. In those areas of application that demand very high accuracy, it is common to use a differential technique in the data processing. This on one hand requires the availability of reference stations and an accurate reference location. The IGS is one of the pioneering organizations, which stands to deliver quality reference locations and data. IGS is dedicated to advancing society and science by providing high-precision-free GNSS data (Johnston et al. 2017).
ADIS is one member station of the IGS network, which is run by the Institute of Geophysics, Space Science, and Astronomy (IGSSA) of Addis Ababa University (AAU). Since 2007 ADIS station played its role by providing continuous data with 30-second temporal resolution. In this work, the data from ADIS is used to analyze the effect of urbanization and the associated climate change on the GNSS positioning. In addition to ADIS other 11 IGS stations (HERS, HYDE, MAL2, MAS1, NKLG, NOT1, RAMO, REUN, TEHN, and YIBL) data were used during the processing to precisely generate the daily solutions. The GNSS data was processed using the GAMIT scientific software (Herring et al. 2010) of the Massachusetts Institute of Technology (MIT). Although GNSS data for ADIS was available since July 2007, this research used data from 2008 to 2019 to avoid the effect of station monument settlement, which is common at the early stage of any station operation. This precaution is made, as the expected long-term station coordinate change is in the order of a few millimeters. All the required information for data analysis was downloaded from the archives of IGS and IGSSA.