Browsing by Author "Osah, Samuel"

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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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    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 Gnss Suite for Geodetic Applications
    (2013-10-15) Osah, Samuel
    Global Navigation Satellite System (GNSS) is one of the most innovative and practical technology developed today. Since its inception it has grown to provide not only world-wide, all-weather navigation, but precise position determination capabilities to all manner of users especially for surveying and geodetic applications. In surveying and mapping, this represents a revolutionary departure from conventional surveying procedures, which rely on observed anglesand distances,for determining point positions. The scope of GNSS is enormous: from data acquisitions and processing, to detailed Computational algorithm in position estimations. Many processing software or applications both scientific and commercial have been developed for GNSS post-processing. These software are developed for general applications or standards and might not meet a country‟s mapping specifications particularly Africa. This means, that the inherent accuracy of a GNSS receiver can be enhanced or degraded depending on which software is used to do the post-processing. Besides, these software tend to be limited to the institutions that wrote them and use of them can require a considerable investment in time to understand the software and how best to use it under various circumstances. Expert training is often recommended by the distributors. In the light of these setbacks and limitations, a GNSS suite, ‘GeoSuite’for geodetic applications such as, GNSS data post-processing, Datum transformationandDirect and Inverse geodetic computationsforGhana,has been developed.The suite is a standalone windowbase application with a user-friendly interface written in Matlab.TheGNSS data post-processing application of the Suite is based on both code andphase observables, and on double differences between simultaneous observations using Rinexfile as an input GNSS raw data.In addition, the Datum transformationapplicationof the Suiteis also centered on the WGS 84 and War Office ellipsoidsusing the 3,5, 7 and 10 transformation parameters, publishedby the Survey and Mapping Division of Ghanaas default transformation parameters or user-defined transformationand ellipsoidparameters. The suite was tested on field measurements and existing coordinates,and from the results obtained it was concluded that, GPS data can be successfully processed using the developed suite for survey workin Ghanaand can perform any form of transformation on the WGS 84 and the War Office ellipsoids or user-defined ellipsoid.The Suite can easily be adapted for other countries particularly within the Africacontinent.
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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.