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Water vapor is
considered as one of the most important greenhouse gas in the atmosphere of our
planet, this parameter that has a fast spatial and temporal variation is a key
factor in the study of the weather forecast and the climate system of the earth
1. In the other way, the delay caused by the earth’s troposphere is one of
most influential errors who can disturb the GNSS signals and limit the
performance of the systems. The troposphere delay introduces an error on the
transmitted satellites signals from about 2.5m at the zenith to over 20m at lower
elevation angle, below then 15°. This delay due to the troposphere which is
considered for a long time as a critical problem for the precise GNSS
applications is now defined as a key parameter in the estimation of the IWV
distribution, which is defined as the total atmospheric water vapor contained
in an area of 1m2 123.  

With the technological
development of the GNSS systems in addition to many signals exist as open
service to the GNSS users, many algorithms and methods was developed to determine
the IWV value using ground-based GNSS. Those algorithms used for monitoring the
water vapor variation has received a great attention and experienced rapid
development, now it can provide a high resolution in any time, location and all
weather condition. The studies presented by many researchers group have been
achieved encouraging results and they conclude that the results obtained by the
GNSS sensing has the same level of accuracy (with 13-mm
root-mean-square (RMS) )as the traditional method of sensing using
Radiosonde measurements and microwave radiometers. In addition, the low cost
and the portability of the GNSS base-ground make of it an ideal method of the
atmosphere sensing specially for researchers in developing countries 45.

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Nowadays, with the
rapid deployment of GNSS monitoring station, a remarkable development in GNSS
network has been established, this network can be used in several applications
as the prediction of weather. The IGS real-time service (RTS) was officially
launched on April 1st 2013 and provides real-time products for GPS. In
this paper we present an algorithm for the IWV estimation, we used the precise
IGS ephemeris and the GPS correction to calculate the ZTD values. The current
accuracy level of precise GPS orbits from the IGS is sufficient to provide estimation
of ZTD with accuracy in the order of 1 mm. In this work the zenith total delay
calculated using a mapping function is the sum of Zenith Hydrostatic Delay and
Zenith Wet Delay, the ZHD will be estimated using one of the models developed
as the Saastamoinen model or UNB3 model, in the end the relation between the
ZWD and IWV will be establish to estimate the IWV value 467.

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