Geoscience Reference
In-Depth Information
4.5 Beam blockage
Radar beam can be blocked by ground targets, i.e. places where the beam hits terrain. A
geometrical approach is applied to calculate the degree of the beam blockage. This approach
is based on calculation what part of radar beam cross section is blocked by any
topographical object. For this purpose a degree of partial beam blocking (
PBB
) is computed
from a digital terrain map (DTM). According to Bech et al. (2003, 2007), the
PBB
is calculated
from the formula:
2
y
a
2
2
2
ya
y
a
arcsin
a
2
PBB
(4)
2
a
where
a
is the radius of radar beam cross section at the given distance from radar,
y
is the
difference between the height of the terrain and the height of the radar beam centre. The
partial blockage takes place when -
a
<
y
<
a
, and varies from 0 to 1 (see Figure 1).
Fig. 1. Scheme of partial beam blockage
PBB
calculation using Bech et al. (2007) algorithm.
Quantity
y
in Equation 4 and Figure 1 is calculated as an altitude obtained from DTM for
pixel located in radar beam centre taking into account altitude of radar antenna, the Earth
curvature, and antenna elevation. Then the correction of partial beam blocking is made
according to the formula (Bech et al., 2007):
B
1
(5)
cor
ZZ
10 lg
(1
)
10
The correction is introduced if the
PBB
value is lower than 0.7. For higher
PBB
values “no
data” (Bech et al., 2007) or reflectivity from neighbouring higher elevation (Ośródka et al.,
2012) may be taken. A quality of blocked measurement dramatically decreases and can be
expressed by:
1
0
PBB
PBB
a
QI
(6)
PBB
PBB
a
where coefficient
a
can be set as 0.5 (Fornasiero et al., 2005) or 0.7 (Bech et al., 2007; Ośródka
et al., 2012). If reflectivity in a specific gate has been replaced by reflectivity from higher
