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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