High-resolution radar as well as hyperspectral optical imagery
was acquired during flights across two test areas in southern
Tunisia. Meanwhile ground teams precisely documented ground
vegetation and terrain at sampling sites within these test
areas, with samples taken to local laboratories for detailed
analysis. And ESA's Envisat, ERS-2 and Proba satellites acquired
images of these sites around the same time. The aim was to scale
up the findings from the ground, and at the same time to use
this 'ground truth' to calibrate satellite imagery with reality
on the sandy arid ground as well as seeing what can be learnt
about the water beneath it. The Sahara has altered through the
ages: during the last Ice Age, 10 000 years ago, there was
savannah here with rivers, lakes and plentiful rains. That
landscape has vanished now, but the rains from that period
progressively percolated beneath the ground to be collected in
layers of water-bearing rock known as aquifers. This 'fossil
water' is today used in North African