(introduction...)

Early-warning systems (EWSs) are a variant of surveillance systems, designed to anticipate a disaster and limit its effects. There is a tendency to confuse EWSs and the use of space technology (satellite photos). In fact, the concept of EWS is not new. In ancient cities crossed by rivers, the inhabitants used to measure the water level regularly in order to see when the danger level had been reached. At that point they would begin to take steps to protect themselves against the rising waters.