Satgen330   ERS1 Volcano Monitor   by GM4IHJ                   22 July 95

In 1992 European Earth Resources Satellite ERS1 produced superb evidence
of before/after land surface deformation caused by the Landers California
earthquake of June of that year. Since then, amongst other targets, the
satellite has been monitoring Mount Etna , the active volcano in the north
east corner of the island of Sicily.

The technique used is similar to that applied to the Landers data.
Interference patterns are constructed from the differences between pairs
of synthetic aperture radar (SAR) images acquired by ESR1. Separate
interferograms are obtained showing changes in range to the volcano
surface as the satellite tracks either, north to south , looking east on
descending orbits, or south to north , looking west on ascending orbits. 
Descending orbits occur in daylight, ascending orbits occur at night. Only
images from the same family of orbits are combined.

Data quality received, suggests that night time passes give best results,
perhaps because of steadier atmospheric conditions. Areas covered in
vegetation and sites where ice and snow has gathered can obscure some of
the data but this has not seriously affected the results. Indeed ERS1
results taken in the tropics on orbits passing the Gunung Merapi volcano
on the island of Java, support the view that the rapid growth of the rich
lush tropical vegetation there does not prevent the reception of good
coherent image pairs.

Interference due to propagation disturbance in the ionosphere and the
troposphere is evident on some of the images from Mount Etna, but with the
collection of increasing amounts of image data, it is now becoming
possible to anticipate and recognise the effects of this natural
interference, thereby avoiding false assumptions based on corrupted
images. Excellent results have been obtained which show measurable
deformation of the Etna volcano coincident with its recent eruption. These
results are in general agreement with Navigation satellite Global
Positioning system GPS measurements taken on the mountain.

Readers familiar with the ominous swelling of the Mount St Helens volcano
in the United States just before its explosive eruption some years ago
will appreciate that SAR observation of this kind , and that used in the
study of the Landers earthquake, are needed at a large number of sites
around the world where there is risk from natural disasters of this kind. 

The recent launch of ERS2 , now operating in tandem with ERS1 will double
the potential of this valuable new technique.