GNSS Atmospheric and Multipath Delays - GNSS Remote Sensing: Theory, Methods and Applications - page 22

is the lower atmosphere below the ionosphere ranging from the surface to about
60 km, including the troposphere and stratosphere. The GNSS signals will be
refracted when going through the neutral atmosphere, with the speed change and
signal curve. The effects are called GNSS neutral atmospheric delay. Because the
neutral atmospheric delay occurs mainly in the troposphere, the tropospheric delay
is usually used instead of neutral atmospheric delay for the GNSS signal effects of
the atmosphere below the ionosphere.

2.2.2 Empirical Tropospheric Models

Several tropospheric models have been developed to compute the atmospheric
refractivity and tropospheric delay, such as Hopfield ( 1969 ) and Saastamoinen
( 1972 , 1973 ) models. While surface meteorological data should be taken into
account for tropospheric models.

2.2.2.1 Modified Saastamoinen Model

The modified Saastamoinen Model (Saastamoinen 1975 ) can be expressed as
following to calculate tropospheric path delay,

W .'; h S / P S

1; 255
T S

C 0:05 e S Btan 2 z

0:002277
cos z

L trop D C C R

(2.6)

where L trop is tropospheric path delay in meters, z is the zenith angle of
the satellite, T S , P S , and e S are the surface temperature in Kelvin, the surface
atmospheric pressure in mbar, and the partial pressure of water vapor in mbar at
the station, respectively, and W (®, h S ) D 1 C 0.0026 cos(2®) C 0.00028 h S , where h S
is the station height above sea level in km and ® is the local latitude of the station.
B and R are the correction terms.

2.2.2.2 Modified Hopfield Model

The modified Hopfield Model (Hopfield 1969 , 1971 ) can be expressed as following
to calculate tropospheric path delay,

L trop D L W C L W (2.7)

X

9

f k;i
k

D 10 6 N i r i ;

and L i .i D d; w / (2.8)

k D 1