1.1.1 Geophysical Measurements
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Figure 1.1 ERS-1 image Aug. 5, 1992 Brooks Range - Bathtub Ridge (Copyright ESA, 1992) |
The ENVISAT mission has both
global and regional objectives, with the
corresponding need to provide data to scientific
and application users on various time scales.
Following on from the very successful ERS-1/2 SARs, ASAR is an all-weather,
day-and-night, high-resolution imaging
instrument that will provide radar
backscatter measurements
indicative of terrain structure, surface roughness, and dielectric
constant. Important new capabilities of ASAR
include beam steering for acquiring images with
different incidence angles, dual
polarisation, and wide swath coverage.
Important contributions of ASAR to the global
mission include:
- measuring sea state conditions at various scales
- mapping ice sheet characteristics and dynamics
- mapping sea ice distribution and dynamics
- detecting large scale vegetation changes
- monitoring natural and man-made pollution
over the ocean
ASAR will make a major contribution to the
regional mission by providing continuous and
reliable data sets for applications such as:
- offshore operations in sea ice
- snow and ice mapping
- coastal protection and pollution monitoring
- ship traffic monitoring
- agriculture and forest monitoring
- soil moisture monitoring
- geological exploration
- topographic mapping
- predicting, tracking and responding to
natural hazards
- surface deformation
Some of the global and regional objectives (sea
state condition, sea ice applications, marine
pollution, maritime traffic, hazard
monitoring, etc.) require near real-time (NRT)
data products (within a few hours from sensing).
Some of these products are generated
systematically since they are assimilated by
meteorological offices (e.g. wave and wind
information from wave mode acquisitions) while
others are generated according to user requests.
Other applications (e.g., agriculture, soil
moisture, etc.) require fast turnaround data
services (within a few days).
As well as using ASAR to satisfy specific
operational and commercial requirements, major
systematic data collection programmes will be
undertaken, to build up archives for scientific
research purposes. ASAR will provide
continuity of the ERS SAR Image and Wave Modes but
with the opportunity for better temporal frequency
of coverage. The nominal spatial resolution (30
m) and swath coverage of ASAR Image Mode (100 km)
are the same as the ERS Image Mode, and ASAR will
also be in a 35-day repeat orbit. However, using
beam steering, it will be possible to obtain images
of the same area on the ground from different
orbits with different incidence angles. This gives a
revisit frequency varying from daily coverage near
the poles to weekly coverage at the equator.
ASAR has dual polarisation capabilities and a
special Alternating Polarisation Mode has been
implemented that permits half of the looks
from a scene to be acquired with horizontal and
half with vertical polarisation, thereby
considerably increasing the target
classification capability (especially if
used in conjunction with multi-temporal imaging).
Wide area coverage will be achieved by switching
between different swaths using the ScanSAR
technique. This will enable 405 km coverage
at resolutions of either 150 m or 1 km. At the 1
km resolution the data rate is low enough for
tape recording on board the spacecraft and
the recording capacity will be sufficient for
downloading low-resolution global coverage
through a single receiving station.
Incidence Angle
In contrast to the ERS SARs, which had a fixed
swath position (23° mid-swath incidence
angle), ASAR Image Mode will provide data
acquisition in seven different swath positions
(i.e., IS1 to IS7), giving incidence angles
ranging from 15° to 45°. This subject
is discussed in the section "Selectable Incidence Angles" 1.1.5.2.
Dual Polarisation
Imaging radars can transmit horizontal or
vertical electric field vectors, and receive
either horizontal or vertical return signals, or
both. The basic physical processes responsible
for the like-polarised return are quasi-specular
surface reflection and surface or volume
scattering. The cross-polarised return is
usually weaker, and often associated with
multiple scattering due to surface roughness or
multiple volume scattering. Scattering
mechanisms and the returns from different
surfaces may also vary markedly with incidence
angle. For a further discussion of this
topic see the section entitled "Dual Polarisation." 1.1.5.1.
Wide Area and Frequent Coverage
ASAR medium and low-resolution images, provided
by the Wide Swath and Global Monitoring Modes,
open up new possibilities for applications
requiring large area coverage and/or more
frequent revisit. Both modes will provide 405 km
swath coverage with a spatial resolution of
150 m for the Wide Swath and 1000 m for the
Global Monitoring. For applications where higher
resolution is necessary, better than a 5-day
repeat interval is possible for approximately
100 km x 100 km areas using Image Mode. For a
further discussion on this topic, see the
section entitled "Wide Area Coverage and
Frequency of Coverage." 1.1.5.3.
Interferometry
ASAR offers continuity with 35-day repeat images
for SAR interferometry, although interferometric
pairs will need to be taken with the same
incidence angles. The availability of ASAR
interferometric pairs acquired at higher
incidence angles will be useful for improving
the visibility of steeper slopes, and there
will also be new possibilities for
low-resolution interferometry. For a further
discussion on this topic, see the section
entitled "Interferometry." 1.1.5.4.
Wave Spectra
ASAR Wave Mode will provide wave spectra derived
from imagettes of minimum size (5 km x 5 km),
similar to the ERS AMI-Wave Mode, spaced 100
km along-track in VV polarisation. For a further
discussion on this topic, see the section
entitled "Wave Spectra." 1.1.5.5.
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