Browsing by Author "Acheampong, Akwasi Afrifa"

Now showing 1 - 13 of 13
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    Assessing the quality of data from continuously operating reference stations in Ghana
    (Journal of Science and Technology, 2023) Abukari, Osman Mohammed; Acheampong, Akwasi Afrifa; Osah, Samuel; Ayer, John; 0000-0003-1640-6307
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    Comparative Evaluation and Analysis of Different Tropospheric Delay Models in Ghana
    (South African Journal of Geomatics, 2021-08) Osah, Samuel; Acheampong, Akwasi Afrifa; Dadzie, Isaac; Fosu, Collins; 0000-0003-1640-6307
    Tropospheric delay prediction models have become increasingly important in Global Navigation Satellite System (GNSS) as they play a critical role in GNSS positioning applications. Due to the different atmospheric conditions over the earth regions, tropospheric effect on GNSS signals also differs, influencing the performance of these prediction models. Thus, the choice of a particular prediction model can significantly degrade the positioning accuracy especially when the model does not suit the user’s environs. Therefore, a performance assessment of existing prediction models in various regions for a suitable one is very imperative. This paper evaluates and analyses seven commonly used tropospheric delay models in Ghana in terms of performances in Zenith Tropospheric Delay (ZTD) estimation and baseline positional accuracies using data from six selected Continuously Operating Reference Stations (CORS). The 1˚x1˚ gridded Vienna Mapping Functions 3 (VMF3) ZTD product and coordinates solutions from the CSRS-PPP positioning service were respectively used as references. The results show that the Black model performed better in estimating the ZTD, followed by Askne and Nordius model. The Saastamoinen, Marini and Murray, Niell, Goads and Goodman and Hopfield models respectively performed poorly. However, the result of the baseline solutions did not show much variation in the coordinate difference provided by the use of the prediction models, nonetheless, the Black and Askne and Nordius models continue to dominate the other models. Of all the models evaluated, either Black or Askne and Nordius model is recommended for use to mitigate the ZTD in the study area, however, the choice of the Black model will be more desirable.
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    Comparative Study of Predominantly Daytime and Nighttime Lightning Occurrences and Their Impact on Ionospheric Disturbances
    (Remote Sensing, 2022) Osei-Poku, Louis; Tang, Long; Cheng, Mingli; Acheampong, Akwasi Afrifa; 0000-0003-1640-6307
    Space weather events adversely impact the operations of Global Navigation Satellite Systems (GNSS). Understanding space weather mechanisms, interactions in the atmosphere, and the extent of their impact are useful in developing prediction and mitigation models. In this study, the hourly lightning occurrence and its impact on ionospheric disturbances, quantified using the Rate of Total electron content Index (ROTI), were assessed. The linear correlation between diurnal lightning activity and ROTI in the coastal region of southern China where lightning predominates in the daytime was initially negative contrary to a positive correlation in southern Africa where lighting predominates in the evening. After appreciating and applying the physical processes of gravity waves, electromagnetic waves and the Trimpi effect arising from lightning activity, and the time delay impact they have on the ionosphere, the negative correlation was overturned to a positive one using cross-correlation. GNSS has demonstrated its capability of revealing the impact lightning has on the ionosphere at various times of the day.
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    Congruence through repeatability of position solutions by different GNSS survey techniques
    (South African Journal of Geomatics, 2022-08) Abukari, Osman Mohammed; Acheampong, Akwasi Afrifa; Dadzie, Isaac; Osah, Samuel; 0000-0003-1640-6307
    In this study, we determined three-dimensional (3D) position coordinates for eight new Continuous Operating Reference Stations (CORS) in Ghana through three different GNSS positioning techniques. The three GNSS positioning techniques whereby the network of CORS was tied to ITRF14 and War Office 1926 datums included:1) Precise Point Positioning (PPP); 2) Precise Differential GNSS (PDGNSS), using reference stations based on ITRF14; and 3) PDGNSS, using reference stations based on War Office. The PPP solutions were computed using the Canadian Spatial Reference System Precise Point Positioning software (CSRS-PPP), available online and as an open source GNSS laboratory tool software (gLAB). The PDGNSS solutions were obtained from OPUS and AUSPOS online services, as well as from self-post-processing using Topcon Tools software v8.2.3. All solutions were computed using 24-hour data for twelve consecutive days in the month of October 2018 (GPS DoY 284 to GPS DoY 295). The quality, reliability, and acceptability of position solutions were measured by computing the average positioning error, the rate of ambiguity resolution and the repeatability ratios of the solutions. The variability of coordinate differences for each pair of different positioning techniques was computed to determine their solution congruences. Ultimately, , the average positioning errors in northing, easting, and height were 0.003m, 0.005m and 0.009m, respectively. The rate of ambiguity resolution was between 75.3% and 90.3%. Repeatability ratios ranged between 1: 68,500,000 and 1: 411,100,000. Finally, the minimum and maximum range of variability in coordinate differences for each pair of positioning techniques was 1mm to 16mm for horizontal positions and 2mm to 137mm for vertical positions.
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    Deep learning model for predicting daily IGS zenith tropospheric delays in West Africa using TensorFlow and Keras
    (Advances in Space Research, 2021-08) Osah, Samuel; Acheampong, Akwasi Afrifa; Fosu, Collins; Dadzie, Isaac; 0000-0003-1640-6307
    Accessibility and precise modelling of tropospheric delay play a significant role in the precise Global Navigation satellite system (GNSS) positioning applications as well as meteorological studies and weather forecasting. However, if in the event that a GNSS Continuously Operating Reference Station (CORS) is inaccessible due to power outages, poor internet connectivity, equipment failure, and firmware issues, gaps are created in the data archive, and the quality of the tropospheric delay estimation is degraded. Over the years, several modelling approaches and methodologies have been proposed towards the precise estimation of tropospheric delay, owing to the spatiotemporal variability of water vapour content in the atmosphere. This study employs Deep learning (DL) approach with TensorFlow and Keras to develop a predictive model (DLztd) for predicting daily IGS final ZTDs over four selected IGS stations in West Africa. Daily surface meteorological parameters (Pressure (P), Temperature (T), and Water vapour partial pressure (e)), as well as daily ZTD and stations’ coordinates (latitude, and ellipsoidal height) obtained from the site-wise VMF3-ZTD products for the period 2015–2018, were used as input variables to train and test the model, while data from 2019 were used to evaluate the predictive performance of the developed model. Statistical performance indicators such as Mean Bias (MB), Root Mean Squared Error (RMSE), Mean Absolute Percentage Error (MAPE), coefficient of determination (R2), Nash-Sutcliffe coefficient of Efficiency (NSE), and the fraction of prediction within a Factor of Two (FAC2) were employed to determine the degree of agreement between the DLztd model predictions and IGS final ZTD data. The results from the various analyses indicate exceptionally good prediction capability of the DLztd model with average MB, RMSE, MAPE, R2, NSE and FAC2 of 3.25 mm, 9.62 mm, 0.30%, 0.959, 0.947, and 1.00 respectively. This demonstrates that the DLztd model provides a remarkable alternative for improving the availability of the ZTD data over the IGS stations under study should the stations' data be inaccessible or unavailable.
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    Determination of position coordinates of the new active CORS in Ghana
    (Journal of the Ghana Institution of Engineering, 2022) Abukari, Osman Mohammed; Acheampong, Akwasi Afrifa; Dadzie, Isaac; Osah, Samuel; Ayer, John; 0000-0003-1640-6307
    Global Navigation Satellite System (GNSS) technologies provide services and applications in a wide range of areas, including survey and mapping, transportation, precision agriculture, urban planning, smart mobility and smart city management, to name a few. Most of these applications rely on real-time kinematic (RTK) technology, which is typically supported by a network of continuously operating reference stations (CORS). In order to improve GNSS applications in Ghana, eight new CORS were established by the Licensed Surveyors Association of Ghana (LiSAG) in 2019 known as LiSAGNet. Accurate and precise positions of the LiSAGNet are, however, very critical for GNSS applications. Thus, the aim of this study is to determine accurate three-dimensional (3D) coordinates of the LiSAGNet using Network-based and Precise Point Positioning (PPP) techniques based on the International Terrestrial Reference Frame (ITRF). Positions of the new CORS were computed from data for 11 consecutive days using gLAB software v5.4.1 in PPP mode and the Canadian Spatial Reference System PPP (CSRS-PPP) online services as a check. Position solutions from both gLAB and CSRS-PPP were compared, which yielded coordinates variability of 0.001 m, 0.003 m and 0.029 m in the northings (N), eastings (E) and up (U) directions respectively and were therefore accepted as the final coordinates of the LiSAGNet. Positions from the PPP and Network-based techniques were also compared to determine consistencies or otherwise in the coordinates of the LiSAGNet. The study concluded that positions of LiSAGNet showed more consistency when determined by Network-based technique than when determined by PPP technique.
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    Developing a differential GPS (DGPS) Service in Ghana
    (2008) Acheampong, Akwasi Afrifa
    The prerequisite for successful execution of projects, applications, services or products reliant on surveys or geo-information, must be a uniform and reliable coordinate reference system. Provision of control points defining these reference frames by conventional methods are expensive, tedious, limited to intervisibility between beacons and the area of survey, thus reducing the effectiveness of networks at night and in poor weather conditions. Signals from Global Navigation Satellite Systems have proven to be fast, accurate and cheaper alternative to conventional methods, but performance indices for autonomous positioning are not satisfactory and as such for high precision assignments, relative or differential positioning techniques must be employed. However, differential positioning does place additional operational demands (two GPS receivers are required with one on known point and a data link between them when DGPS is to be implemented in real-time). A base station has been established at KNUST to provide GPS logs, reference coordinates, and to disseminate differential corrections for post-processing computations. This base will serve as the second receiver required for differential operations. Using the base station, several observation sessions were conducted and processed to compute 2D transformation parameters to integrate GPS data into the National Mapping System. The parameters computed can be used within a maximum radial error of 0.58ft around campus. Further investigations were carried-out on observation times and the solutions obtained showed that at least three hours was found to be optimum time to be spent when coordinating newly established base stations.
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    Development of GNSS software for Ghana Survey and Mapping Division
    (South African Journal of Geomatics, 2022-02) Gameti, Charles; Acheampong, Akwasi Afrifa; Ayer, John; 0000-0003-1640-6307
    Processing of Global Navigational Satellite System (GNSS) data forms the basis for the usage of differential systems for obtaining spatial data. All open sources or commercial software packages developed for data processing give specific details to suit the intended purpose of the software. To obtain a uniform format for submitted survey data, Survey and Mapping Division (SMD) in various jurisdictions have specified formats for data submission for all kinds of surveys. In this regard, “GNSS Ghana” Software (GGS), a GNSS standalone Windows-based application with a modern user-friendly interface was developed for geodetic applications such as, projection and datum transformation worldwide, GNSS data post-processing of Receiver Independent Exchange Format (RINEX) files, and generating reports to meet Ghana SMD reporting standards including cadastral computations and reports for submission. To assess the developed software, GNSS data from two International GNSS Service (IGS) stations (BJCO and YKRO) were processed using GGS and three other commercial software such as GNSS Solution Software (GSS), Spectrum Survey Software (SSS), and Leica Geo Office (LGO), and the positional results compared against the existing coordinate. The results revealed that the GGS outperformed the remaining three commercial software packages with a sub-meter level of accuracy. Further assessment was conducted on datum transformation using the coordinates of 21 existing geodetic control points in Ghana. Utilizing the 7-transformation parameters of Ghana, the results gave uncertainties of [0.10ft. ± 0.99ft.] in the eastings and [0.02ft. ± 1.61ft.] in the northings with a 99% confidence level.
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    Electrification planner for Ghana using open-source WebGIS
    (Journal of the Ghana Institution of Engineering, 2022) Boamah, Edward Oppong; Acheampong, Akwasi Afrifa; 0000-0003-1640-6307
    Energy system planning provides information, such as electrification rate and access, essential to match demand and supply of energy. As countries strive to rapidly grow their economies and increase the living standards of its citizens, energy planning system is essential to keep track of assets, accessibility, adequacy, availability, and distribution of energy resources across locations targeted for development. In Ghana, the Ghana Energy Development and Access Project (GEDAP) was mandated to provide up to 100% electrification rate to the citizens by 2020. While this time has elapsed, public information system showing the spatial distribution and statistical analysis of electrification rate and access in communities and local administrative areas remain scanty. Such decision support system, which can inform energy investment decisions and policy formulation by local and international investors is not readily available, impeding on the Government of Ghana’s (GOG) electrification expansion efforts. It also hinders the nation in attaining the United Nation’s (UN) Sustainable Development Goal (SDG) 7. Thus, the aim of this study was to develop a decision support system on electrification rate in Ghana. The study used energy access data and open-sourced Geographical Information System (GIS) to map the spatial distribution and provide statistical analysis of electrification rate in the country. The resulting information was connected to a WebGIS that can provide access to query, manipulate, and visualize electrification rate in the counting. The developed system estimated that, presently, Ghana has an electrification rate of 85.16% as of November 2020. This information, and the system in general, will aid decision makers to make swift decision and provide geospatial evidence-based report in achieving 100% electrification rate in the country
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    Evaluation of Zenith Tropospheric Delay Derived from Ray-Traced VMF3 Product over the West African Region Using GNSS Observations
    (Advances in Meteorology, 2021) Osah, Samuel; Acheampong, Akwasi Afrifa; Fosu, Collins; Dadzie, Isaac; 0000-0003-1640-6307
    The growing demand for Global Navigation Satellite System (GNSS) technology has necessitated the establishment of a vast and ever-growing network of International GNSS Service (IGS) tracking stations worldwide. &e IGS provides highly accurate and highly reliable daily time-series Zenith Tropospheric Delay (ZTD) products using data from the member sites towards the use of GNSS for precise geodetic, climatological, and meteorological applications. However, if for reasons like poor internet connectivity, equipment failure, and power outages, the IGS station is inaccessible or malfunctioning, and gaps are created in the data archive resulting in degrading the quality of the ZTD and precipitable water vapour (PWV) estimation. To address this challenge as a means of providing an alternative data source to improve the continuous availability of ZTD data and as a backup data in the event that the IGS site data are missing or unavailable in West Africa, this paper compares the sitewise operational Vienna Mapping Functions 3 (VMF3) ZTD product with the IGS final ZTD product over five IGS stations in West Africa. Eight different statistical evaluation metrics, such as the mean bias (MB), mean absolute error (MAE), root mean squared error (RMSE), Pearson correlation coefficient (r), coefficient of determination (r 2 ), refined index of agreement (IAr), Nash–Sutcliffe coefficient of efficiency (NSE), and the fraction of prediction within a factor of two (FAC2), are employed to determine the degree of agreement between the VMF3 and IGS tropospheric products. &e results show that the VMF3-ZTD product performed excellently and matches very well with the IGS final ZTD product with an average MB, MAE, RMSE, r, r 2 , NSE, IAr, and FAC2 of 0.38 cm, 0.87 cm, 1.11 cm, 0.988, 0.976, 0.967, 0.992, and 1.00 (100%), respectively. &is result is an indication that the VMF3-ZTD product is accurate enough to be used as an alternative source of ZTD data to augment the IGS final ZTD product for positioning and meteorological applications in West Africa.
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    Homogenizing coordinates through the use of the active CORS1 in Ghana
    (South African Journal of Geomatics, 2023-08) Abukari, Osman Mohammed; Acheampong, Akwasi Afrifa; Osah, Samuel; Ayeh, John; 0000-0003-1640-6307
    In this study, the course towards determining the homogeneous three-dimensional (3D) coordinates of the newly established active Continuously Operating Reference Station (CORS), based on ITRF2014 in Ghana, is revealed. The aim is to address coordinate inconsistencies and inhomogeneity in the published positions of the new active CORS in Ghana. In order to attain homogeneity, the coordinates of two primary control points, GCS 305 and GCS 306, were obtained using AUSPOS online services via email. These were subsequently used as reference stations to compute the position of the LISAG_KUMASI CORS. Adjustments to the position coordinates were performed using Topcon Tools v8.2.3 software at a 1mm standard deviation. The adjusted coordinates of LISAG_KUMASI were used as the reference points to compute the positions of the LiSAGNet CORS in differential mode by using 24 hour data for 11 consecutive days. The GPS data covered DoY 284 to DoY 295 in 2021. The final positions of the CORS, computed by this approach, indicate some differences from the officially published coordinates of the same CORS, confirming the suspicion of inhomogeneity in the source coordinates used in determining the coordinates of the local CORS. Furthermore, a test network, consisting of five COR stations, was designed and used to address the coordinate inconsistencies in the officially published coordinates. Using the officially published coordinates as reference inputs, the ROVER I station was fixed by three different CORSs, thus resulting in average coordinate variabilities of 2.78m and 0.80m in the northing (N) and easting (E) directions, respectively. Through substitution, the coordinates computed in this study as reference inputs, namely, the ROVER I station, were fixed by the same three CORSs, thus resulting in a coordinate variability of 0.002m and 0.006m in the northing (N) and easting (E) directions, respectively. The analysis revealed inconsistencies and inhomogeneity in terms of the officially published coordinates. It is, therefore, recommended that the officially published coordinates of the CORS be replaced by the adjusted homogeneous and consistent values determined through the approach adopted in this study.
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    Regression models for predicting daily IGS zenith tropospheric delays in West Africa: Implication for GNSS meteorology and positioning applications
    (Meteorological Applications, 2021) Osah, Samuel; Acheampong, Akwasi Afrifa; Fosu, Collins; Dadzie, Isaac; 0000-0003-1640-6307
    The ability to precisely and accurately model and predict tropospheric delay is essential for precise global navigation satellite system (GNSS) and meteorological applications. The International GNSS Service (IGS) provides highly accurate and highly reliable daily time series zenith tropospheric delay (ZTD) products for all its member sites using data from each IGS site. Nevertheless, if for reasons such as poor internet connectivity, equipment failure, and power outages the IGS station is inaccessible, gaps are created in the data archive, resulting in degrading the quality of the ZTD estimation, as well as inhibits the quality of precipitable water vapour (PWV) estimation, needed for precise positioning applications, meteorological studies, and weather forecasting. To address this challenge, five regression models are proposed in this study to model and predict daily ZTDs using daily datasets from four IGS stations in West Africa over a period of 5 years (2015–2019). The site-specific Vienna Mapping Functions 3 (VMF3) products (ZTD, pressure, temperature, water vapour partial pressure) and stations' coordinates (latitudes and longitudes) are used as the predictors, while the IGS final ZTD product as the response variable in fitting the models. Several performance measures are calculated to compare the predictive performance of the models. The results show that the five regression models performed outstandingly and agree very well with the IGS-ZTD data, and hence provide a useful alternative for ZTD predictions and also in the event the West African IGS stations' ZTD data are unavailable. Nonetheless, the support vector regression model outperformed the remaining four models.
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    Retrieval of integrated water vapour from GNSS signals for numerical weather predictions
    (2016-10-25) Acheampong, Akwasi Afrifa
    Atmospheric Water vapour is an important greenhouse gas and contributes greatly in maintaining the Earth's energy balance. This critical meteorological parameter is not sensed by any facility in Ghana contributing weather data to the Global Telecommunication System of WMO. This thesis presents a highly precise tool for water vapour sensing based on the concept of Global Navigation Satellite Systems (GNSS) meteorology and tests the computed results against global reanalysis data. Conventional approaches used to sense the atmospheric water vapour or precipitable water (PW) or Integrated Water Vapour such as radiosondes, hygrometers, microwave radiometers or sun photometers are a ected by meteorological conditions, expensive and have coverage limitations. However, GNSS meteorological concept o ers an easier, inexpensive and all-weather technique to retrieve PW or IWV from Zenith Tropospheric Delays over a reference station with very high temporal resolutions. This study employed precise point positioning (PPP) techniques to quantify the extent of delays on the signal due to the troposphere and stratosphere media where the atmospheric water vapour resides. The KNUST GPS Base station was used to log dual-frequency signals for approximately 260 days between the months of February 2013 to December 2014. Stringent processing criteria were set using an elevation cut-o of 5 o , precise orbital and clock products, Antex les, nominal tropospheric correction and mapping functions. The delays which were originally slanted are mapped unto the zenith direction and integrated with surface meteorological parameters to retrieve PW or IWV. This research work investigated the applications of ground-based GNSS to meteorology and gives all correction models implemented in PPP and for Tropospheric delay estimation.The gLAB software was used for ZTD computations. PW values obtained were compared with ERA-Interim, Japanese Meteorological Agency Reanalysis (JRA) and National Centres for Environmental Prediction (NCEP) global reanalysis data. Correlation analysis were run on computed PW from logged GNSS datasets and downscaled reanalysis data. The obtained results show stronger correlation between the retrieved PW values and those provided by the ERAinterim. The computed amount of ZTDs varies perfectly with weather pattern in the country. Again, a linear-model was derived that could predict PW based on ZTD with standard errors of 6.01 mm for JRA, 5.40 mm for ERA-Interim and 6.34 mm for NCEP reanalysis data. Finally, the study results indicate that with a more densi ed network of GNSS base stations the retrieved PW or IWV will greatly improve numerical weather predictions and more speci cally precipitation forecasting in Ghana.