International Journal of Geosciences, 2016, 7, 645-654 Published Online May 2016 in SciRes. http://www.scirp.org/journal/ijg http://dx.doi.org/10.4236/ijg.2016.75050 How to cite this paper: Elsobeiey, M. and El-Diasty, M. (2016) Impact of Tropospheric Delay Gradients on Total Tropos- pheric Delay and Precise Point Positioning. International Journal of Geosciences, 7, 645-654. http://dx.doi.org/10.4236/ijg.2016.75050 Impact of Tropospheric Delay Gradients on Total Tropospheric Delay and Precise Point Positioning Mohamed Elsobeiey 1 , Mohamed El-Diasty 1,2 1 Department of Hydrographic Surveying, Faculty of Maritime Studies, King Abdulaziz University, Jeddah, KSA 2 Engineering Department of Public Works, Faculty of Engineering, Mansoura University, Mansoura, Egypt Received 3 April 2016; accepted 15 May 2016; published 18 May 2016 Copyright © 2016 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract GPS signals are electromagnetic waves that are affected by the Earth’s atmosphere. The Earth’s atmosphere can be categorized, according to its effect on GPS signals, into the ionosphere (ionos- pheric delay) and neutral atmosphere (tropospheric delay). The first-order ionospheric delay can be eliminated by linear combination of GPS observables on different frequencies. However, tro- pospheric delay cannot be eliminated because it is frequency-independent. The total tropospheric delay can be divided into three components. The first is the dry component, the second part is the wet component, and the third part is the horizontal gradients which account for the azimuthal dependence of tropospheric delay. In this paper, the effect of modeling tropospheric gradients on the estimation of the total tropospheric delay and station position is investigated. Long session, one month during January 2015, of GPS data is collected from ten randomly selected globally dis- tributed IGS stations. Two cases are studied: the first case, the coordinates of stations are kept fixed to their actual values and the tropospheric delay is estimated twice, with and without tro- pospheric gradients. In the second case, the station position is estimated along with the total tro- pospheric delay with and without tropospheric gradients. It is shown that the average bias of the estimated total tropospheric delay when neglecting tropospheric gradients ranges from −1.72 mm to 2.14 mm while the average bias when estimating gradients are −0.898 mm to 1.92 mm which means that the bias is reduced by about 30%. In addition, the average standard deviation of the bias is 4.26 mm compared with 4.52 mm which means that the standard deviation is improved by about 6%. Keywords Precise Point Positioning, Electromagnetic Waves, Tropospheric Delay, Tropospheric Gradients