NORTH-SOUTH TRANSECT OVER AFRICA 
-5 
*10 
-15 
-20 
longitude : 15°E 
incidence : 30° ... , 
/ I I 
jU 
/ I I 
) I I 
I ' 1 
\ II 
1 
1 
1 
1 
1 
1 
1 \ 
1 
! 
J * 
- 
: 1 II 
; “ 1 1 1 1 
:'•] Il II 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
1 
- 
Sahara 
. : / i i ii 
VT 1 1 ii 
i*i i i ii 
ill ii 
1 
1 
1 
1 
1 
1 
1 
1 
Kalahari 
desert 
1 H. 1 s.s. 1 T. S. 1 w. 1 
■s. i L_i 
Tropical forest , 
1 L»— 
Tree savanna i 
1U 
desert 
25 
20 
15 
10 
5 0-5 
LATITUDE (degrees) 
-10 
-15 
- 20 - 25 
Figure 3 : Variation of the backscattering coefficient along a North-South transect in Africa for may 1992 (-) 
and September 1992 (•••)• 
II. TEMPORAL SIGNATURES OF cr° : 
Previous results highlight the capability of the ERS-1 scatterometer to monitor changes in vegetation 
development or in surface conditions. However, at this stage of analysis, care must be taken while interpreting 
the previous curves. For example, in semi-arid zones, vegetation growth starts after the first rains at the 
beginning of the wet season. Therefore, the observed evolution of the radar signal is likely to be related, at first, 
to an increase in the soil moisture content and later on to the vegetation development. 
Temporal signatures of the backscattering coefficient cr° are illustrated in figure 4 for typical vegetation 
formations of Africa. Temporal signatures were collected by the midbeam antenna at an incidence angle around 
30°. The period considered is from mai 1992 to april 1993. The tropical forest of the Congo bassin (0°, 15°E) 
shows a very stable behaviour throughout the year with a mean value of about -7 dB. Yearly variation is less 
than 1 dB. Desert areas also exhibit a very stable behaviour. Observed variations in figure 4 can be attributed to 
small variations in incidence angles. On a relatively flat surface (24°N, 4°W), the average signal (-13.6 dB) is 
nearly 7 dB below the values found over the tropical forests. On the opposite, vegetation formations growing in 
more contrasting environment e.g. in regions showing different climatic seasons present more pronounced 
behaviours. For instance, shrub savannas in the Northern hemisphere and tree savannas in the Southern 
hemisphere show very typical and different profiles which can be related to the corresponding rainy seasons. 
The peak of the radar signal is found in august for the shrub savanna (24°N, 4°W) at the maximum of the rainy 
season whereas for the tree savanna (12°S, 22°E) the peak occurs in februaiy. For both situations, the radar 
echo remains low and constant during the dry seasons. Amplitude of the radar signal is found to be of the order 
of 5-6 dB between the dry and the rainy periods. 
402