ce
ng
of
ar
nd
of
{3
in
ar
le
Q =
>
he
ns
on
de
antenna dimension is the same for both modes, while in the
crosstrack dimension, only a limited portion of the antenna is
used for the LRNS mode, the whole area being exploited for the
HRNS mode .
Fig. 3 illustrates the conceptual scheme of the dual mode
SAR. The signals received by the two BFNs are separately pro-
cessed by two processors providing two images which can be
directly transmitted to ground processing facilities for in-
terpretation. At the same time the low resolution image is
processed on-board by a simplified KBS (SKBS) unit which iden-
tifies limited areas of interest to be imaged by the HRNS mode.
In the land and ice/ocean applications, the dual look sy-
stem can perform according two opposite working procedures. In
the first procedure the first look is accomplished by the LRWS
mode to have a quick perception of a large scene. The second
look, performed by the HRNS mode provides accurate informa-
tions on a limited area of specific interest (e.g. lakes,
catchment basin for water management and forests), where an
alarm has been delivered by the SKBS. In this case, high spa-
tial resolution is used to interpret the causes (e.g. defore-
station) of unforeseen changes in the scene (e.g. a conside-
rable variation of reflectivity in a large spatial area). In
the alternative procedure, the first look is made instead by
the HRNS mode on off-line selected areas for system calibra-
tion purposes. The second look is of LRWS type for the imaging
of large areas. The two opposite procedures of operation are
selectable according to the specific mission of the system.
In the ship application, it is preferable to have just the
conventional way of system operation i.e. first look with LRWS,
second look with HRNS aimed towards on-line selected limited
areas.
More specifically, low spatial resolution is of the or-
der of 50 m - 100 m while high spatial resolution is of the
order of 3 - 10 m. Radiometric resolution is another relevant
parameter to be specified. Also in this case, it is worthwhile
to have two opposite modes: a high radiometric resolution (i.e.
1 dB approximately) and a low radiometric resolution (in the
order of 3 dB). By physical reasoning it can be stated that
high spatial resolution is accompanied by low radiometric re-
solution while low spatial resolution may be accompanied by a
high radiometric resolution. The spatial and radiometric reso-
lutions may be traded off in several ways giving rise to /dif£f-
ferent options for each application.
The sensor should have multifrequency and multipolariza-
tion capabilities. This is particularly true for land and ice/
ocean applications where three frequencies (i.e. L, C and X)
and three transmit-receive polarization combinations (i.e. HH,
VV and HV) are recommended. This choice is motivated by the
increasing penetration capacity of the frequency moving from X,
through C, to L.. Thus multiwavelength radar response to ter-
rain cover and subsurface horizons. Concerning polarization,
like-polarization return is provided by surface and subsurface
351