Farnborough, U.K., gave an estimated spatial reso 
lution at 4.5 m in range and 4.0 m in azimuth. For 
the data correlated at the Deutsche Forschungs und 
Versuchsanstalt fur Luft und Raumfahrt (DFVLR), 
Oberpfaffenhofen, West Germany, the spatial resolu 
tion was in the order of 5 m or higher (Smith, op. 
cit.). 
Spatial resolution for the original optical pro 
cessed imagery at the Environmental Research Insti 
tute (ERIM), Michigan, USA, was specified as being 
in the order of 1.5 m in range and 2.1 m in azimuth 
direction (Intertech 1981), calculated on the basis 
of the properties of the imaging system. 
EXPERIMENT WITH OIL DRUMS AS SPATIAL RESOLUTION 
TARGETS 
In many radar surveys, where images are optically 
correlated, it has been found difficult to check the 
actual ground resolution of the system with respect 
to the resolution as specified by the contractor. 
The here presented experiment is based on measu 
ring the minimum distance between two point targets 
with approximately the same response strength which 
are still visible as two objects on the image. One 
option was to use a number of corner reflectors or 
Lunenburger lenses, placed in a grid with variable 
grid cell size. These reflectors are often difficult 
to procure, however and moreover, they are costly, 
especially when a survey is carried out over remote 
areas in developing countries. We therefore decided 
to test a material easily and cheaply available 
everywhere in the world. These consisted of oil 
drums, cut in half lengthwise. These half drums 
were welded together end-to-end. In this way, con 
cave upwards, they formed a perfect cylindrical 
reflector of a diameter of 0.7 m and a length of 
2.50 m. The corrugation of the drumwalls (amplitude 
3 cm) served as resonant elements and strengthened 
the return signal from these "drum reflectors" (fig. 
1) . 
Fig. 1 Drum reflectors-in the test site 
The reflectors were oriented with their length 
direction either roughly parallel or slightly obli 
que to the flight direction. Some of them were in 
clined towards the look direction under different 
angles to check the influence of orientation on the 
strength of the return signal. The spacing between 
the drums varied between 1.5 m and 13m. The comple 
te configuration is sketched in figure 2. Nine "drum 
reflectors" were situated on line (direction 83°) 
roughly parallel with the flight direction (73°). 
They have a uniform low intensity background clut 
ter from a smooth grass surface. Directly south of 
this row of drums was a 10 m wide ploughed zone 
serving as a fire divide. Three more drum reflec 
tors were situated in the grass area between the 
ploughed strip and the barbed wire fence along the 
road (fig. 1, drums 10 - 12). On the southern side 
Fig. 2 Orientation of drum reflectors in the 
test site 
of the asphalt road, there is another barbed wire 
fence, followed by a grass strip a few metres wide 
and another 10m wide ploughed fire divide. Farther 
south, there is a rather homogeneous grass cover. 
The site was flown during a mission on 23 June 
1981, With a synthetic aperture radar in X and C 
bands. Three overlapping strips cover the test 
area. They have been optically processed (high 
resolution). No digital data were obtained over 
this area. 
The northernmost strip 78 is available in X- and 
C-bands with depression angles of 32° towards the 
site. Strips 77 and 76 are available only in C-band 
because of a malfunction of the X-band during the 
flight. The depression angles are, respectively, 
42° and 70°. 
RESULTS OF MEASUREMENTS 
On all four radar strips the field of drum reflec 
tors could be easily located as bright patches by 
the high radar return. The positive film has been 
studied under the microscope with 63x enlargement 
to differentiate further detail on the transparen 
cies. 
The smooth asphalt road surface appears as a dark 
line (light tone on the negative) as could be ex 
pected because of the specular reflection. On both 
sides of the road, light toned lines occur repre 
senting the barbed wire and/or the shallow depres 
sion along both sides of the road. This is followed 
by a dark toned line with a smooth surface texture, 
caused by the 10 m wide grass strip which separates 
the fence from the ploughed fire divide. The fire 
divide appears light in tone because of the rough- 
Fig. 3 Point scattering of drum reflectors on 
strip 77c (optically correlated)