2 - SITE DESCRIPTION
The localization of the center of each of the 4 visited 100 x 100 km^ zones is displayed in Table 1, which
indicates also the localization of the measurement areas, as measured with a GPS antenna. The latter sites are
inside the selected zones, but at some distance from the zone centers, because the practical access by car to the
selected zones turns out to be very difficult in practice. It is felt, however, that the measurement sites are quite
representative of the entire zone conditions, since the selected zones have been chosen on the basis of their
good spatial uniformity as seen by METEOSAT 4 (less than 3 % of relative spatial uniformity, see Cosnefroy et
al., 1993). Our visual impression as travelers is that the visited measurement site patterns are indeed
surrounded by similar and reproducible patterns over tens of kilometers around. There has been only one
measurement area per selected zone.
Name
zone
latitude
center
longitude
measure
latitude
ment site
longitude
Algeria 2
26,09 N
1,38 W
26,49 N
1,48 W
Algeria 5
31,02 N
2,23 E
30,69 N
2,60 E
Algeria 4
30,04 N
5,59 E
29,84 N
5,78 E
Algeria 3
30,32 N
7,66 E
29,34 N
7,33 E
Table 1 : Localization of the zone centers and of the measurement areas
All visited zones are in fact sand dunes patterns, with, however, notable differences of shape and
structure of dunar patterns between zones. Although very rare, some dry vegetation is present. The Algeria 2
site, in Erg Chech, is composed of long linear and parallel sand dunes, 50 to 100 m high, with a South West -
North East orientation. The distance between two successive dune lines is 1,5 km maximum. Between these
dunes predominantly made of sand, calcareous areas of white and grey color can be found at places. The
Algeria 5 site, in Occidental Reg, is composed of small dunes (4 to 5 m high) randomly disposed and distant of
about 30 to 50 m from each other. The Algeria 4 site, in the Western part of Oriental Erg, is composed of long
linear and parallel dunes, 100 to 300 m high, with a North - South orientation. The typical distance between
two successive dune lines is 4 km. The dunes are separated by large (~ 2 km) uniform flat grey areas, called
"regs", composed of white grey stones and particules whose size is a few mm. The Algeria 3 site, in the Eastern
part of Oriental Erg, is composed of dunes which have a pyramidal shape, 100 m high, randomly disposed. The
distance between 2 successive dunes varies from 1 to 2 km.
3. INSTRUMENTATION AND CALIBRATION
3.1. Instrument description
Three basic instruments from LOA have been used during the campaign, a ground radiometer, a sun
radiometer and a radiometer-polarimeter called REFPOL. Their spectral bands and bandwidths are reported in
Table 2.
Instrument
Spectral bands
Bandwidth
REFPOL
Ground radiometer
| Sun radiometer
450, 650, 850, 1650 nm
550, 650, 850, 1650 nm
450, 680, 870, 1650 nm
40, 40, 40, 100 nm
40, 40, 40, 100 nm
10, 10, 10, 100 nm
Table 2 - Spectral characteristics of the instruments
The ground radiometer is a 2° FOV multi spectral radiometer developed to measure the surface
reflectances in different spectral bands corresponding to the SPOT-HRV XS channels (Silicon detector) plus
one band at 1650 m (Germanium detector). This version was used to measure at nadir the reflectance of the site
in order to check the spatial homogeneity. The FOV and the dynamics of a similar instrument were modified to
obtain a sun radiometer for the measurement of the direct solar irradiance; the goal was to obtain the aerosol
optical thicknesses as inputs for TOA radiance computations. The ground radiometer is basically handheld or
can be mounted on a fixed support of height 1 meter.
The instrument REFPOL is a radiometer of FOV 2°, on which a rotating wheel equipped with 3
polarizers is adapted, and which is associated to a scanning system operating twice (one way and back) a2it
cycle scanning in a vertical plane in about 6 minutes. The instrument is mounted at 3 meters height onto an
48
