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European Space Agency

Press Release

All that glitters: The first ERS/Envisat interferogram

25 April 2003
 
With its casinos and neon lights, Las Vegas may be the Glitter
Capital of the world, but two ESA radar images of the city have come
together in another kind of glitter showcase. 
 
In an ESA first, radar images from the Envisat and ERS-2 satellites
were processed by researchers at the German Aerospace Centre (DLR)
and combined to produce a topographic analysis of the area in and
around the US city.

The analysis matched data acquired in 1999 by the synthetic aperture
radar (SAR) onboard ERS-2 with imagery acquired last year by
Envisat's Advanced SAR to produce the accompanying interferogram.
Although it might look like something hung on a university student's
wall, an interferogram is actually an image generated by analysing
the differences between two radar signals taken over the same area
on Earth.

This one shows an area of approximately 30 x 35 km around Las Vegas.
The city is visible in the centre of the image as an area of bright
colours, with the city's major thoroughfares appearing as straight
dark lines. The image was produced by the Oberpfaffenhofen Remote
Sensing Technology Institute of the German Aerospace Centre (DLR)
under an ESA project to study the uses of joint ERS and Envisat data.

Each colour cycle in the image, from blue to red to yellow,
represents an elevation change of 8.4 metres. The effect produces
bands of colours showing the topography of the city and surrounding
area. 
 

Originally thought impossible
 
Producing an interferogram with data from these two satellites was
not thought initially to be feasible. Although creating
interferograms from SAR images is not new, they usually are generated
from images acquired by the same or compatible radars.

What makes this interferogram stand out, according to Nico Adam, the
DLR scientist who headed the effort, is that the SARs on ERS-2 and
Envisat operate at slightly different frequencies, enough to
complicate the joint processing of data from the ERS and Envisat
sensors. 

"This is the first time that the mixing of Envisat and ERS data is
performed because it requires specific geometric conditions which
have been met for this demonstration," Adam said. 

The result highlights the ingenuity of European scientists and
provides evidence that new ideas for future satellites can be found
by doing with the existing ones what was thought to be impossible.

"The practical demonstration of the radar cross-interferometry was of
scientific and operational interest," Adam added. "It guarantees the
continuity in the monitoring of the Earth's surface. Long time-span
data series can be analysed; hence, very slow geophysical effects can
be monitored and unnoticed effects can be explained even after
years."

ERS-1 was launched in 1991 and operated up to 1999. ERS-2 was orbited
in 1995 and is still in operation. Envisat was launched last year
carrying a suite of 10 sensors to monitor various aspects of the
Earth's environment. 

The interferometric use of ERS and Envisat imagery will be the
central topic to be discussed later this year at the Fringe 2003
Workshop, 2-5 December, at ESA's ESRIN facility outside of Rome. All
scientific investigators carrying research and applications
development in interferometry using ESA data are expected to be in
attendance.

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