assumed at 350 km. The threshold of 0.1 has been chosen
as it characterises weak scintillation activity. At a first
glance, a crude interpretation of Fig. 3 indicates that there
seems to be some regions of the ionosphere spanned by
the Nottingham receiver that show high likelihood of
amplitude scintillation, in particular the region between
(À10; À8)°E in longitude and (47; 50)°N in latitude,
where the percentage of occurrence is almost 100%, indicating an almost omnipresent degradation of the GPS signal amplitude. Fig. 4 (top panel) shows the same
percentage of occurrence of amplitude scintillation above
0.1 as in Fig. 3, but on an azimuth–elevation map, while
the bottom panel is the same, but with the maximum of
the intensity scale adjusted to 10%. This allows an easier
comparison of the amplitude scintillation index with the
percentage of occurrence of the phase scintillation index
above 0.1 radians which is shown in Fig. 5. Figs. 4 and
5 show that both the amplitude scintillation evident in
Fig. 3 and the phase scintillation occur at low elevations.
It is also interesting to note that the area already indicated to be in the line of sight of the chimney presents
very similar values of the percentage of occurrence (about
10%). The amplitude and phase response of an electromagnetic wave to structures is determined by Fresne
fltering, which strongly suppresses the contribution for
scales larger than the radius of the first Fresnel zone rf
which can be easily calculated from the relation:
Rf ¼ kD;
where k is the wave length (about 19 cm for L1) and D is
the distance to the screen producing diffraction effects.
Considering that the chimney is about 90 m away from
the receiver antenna, about 80 m high and that the antenna
is on the roof of the former IESSG building, i.e. at about
10 m in height, the impact of the presence of the chimney
should be felt up to an elevation angle of:
a ¼ tanÀ1
ð80 À 10Þ
¼ 33:7_ ;
90
which correspond to what the azimuth–elevation maps
show.
Moreover, in Fig. 6 the dependence of the first Fresnel
zone radius on the distance is shown. The antenna to chimney distance of about 90 m corresponds to a Fresnel radius
of about 2.1 m. Considering that the radius of the chimney
varies between 3 and 2.5 m from bottom to top, it mimics