Ocean Remote Sensing Using GNSS-R - GNSS Remote Sensing: Theory, Methods and Applications - page 223

9.1.2 Atmospheric Corrections

As described in Chap. 2, the total delay suffered by microwave GNSS signals is
affected by the atmosphere: its neutral or dry-component, and the moist or water
vapor content of the troposphere, as well as the charged particles confined in the
ionosphere.
For altimetric applications of the GNSS-R, as it is for navigation and positioning
GNSS applications, these effects need to be corrected. General speaking, the
strategy to solve the atmosphere-induced delays in GNSS-R is the same as in
standard GNSS positioning. The list below highlights the main differences:

￿ The GNSS-R ray-path trough the atmosphere is two-fold: from the transmitter

down to the surface; and from the surface up back to the receiver. For receivers
installed at space-based platforms, this means that the signals cross most of the
atmosphere twice, including the ionosphere if the LEO is high enough.

￿ For space-based receivers above or on top of the ionosphere, the ionospheric

effect comes from two different regions of the ionospheric layers: the regions are
at large distances, of the order of 2H orb cos e (H orb being the orbital height of
the receiver; e the elevation angle of observation).

￿ For low-altitude receivers, the ionosphere is crossed only once, and the tro-

posphere is partially crossed twice (fully crossed on the way down, partially
crossed on the way up). This requires some degree of vertical resolution in the
atmospheric information to be used for corrections. Alternatively, a combination
of tropospheric mapping functions ( Niell 1996 ) and exponential vertical structure
of the troposphere might be a suitable approximation (e.g. Fabra et al. 2011 ;
Cardellach et al. 2011 ): then, the

comp

component of the troposphere (dry/wet)

can be estimated as comp
atm

comp
TS

C comp

SR where subindex TS and SR stand for
the atmospheric propagation effects between the transmitter and the surface and
between the surface and the receiver, respectively. These depend on the delay
produced by that same component of the atmosphere in a zenith-like ray-path
through the mapping functions M comp . In order to estimate the correction of the
first branch (transmitter-surface), information on atmospheric surface conditions
are required to compute its zenith delay from the surface level (ZD.0/):

D

comp
TS

D M comp ZD comp .0/ (9.5)

See (e.g. Davis et al. 1985 ) and Chap. 2 for ZD models. The second branch should
only account for the atmospheric layer between the surface and the receiver's
altitude h:

h
H atm ;

comp
SR

D M comp ZD comp .0/ Œ1 e (9.6)

being H atm 8 km the typical altitude of the troposphere at the experimental
location. This results in the total reflected atmospheric delay due to a given
component modelled as