scattering while cross-polarized return is mainly due to the 
volume scattering. 
Another sensor parameter to be defined in relation to the 
application is the swath width which is of the order of 100 - 
200 Km for the low resolution mode and 10 - 30 Km for the high 
resolution mode. The narrow swath has to be selectable within 
the wide swath by means of beam steering and/or beam forming 
techniques. Incidence and squint angles are additional  parame- 
ters defining the geometry of the remote sensing system. The 
relative squint angles of the lst look beam with respect to 
the beam of the 2nd look determine the amount of time avai- 
lable to process, on a real time basis, the data of the 1st 
look and then to select the area of interest to be imaged with 
the 2nd look mode. 
Another parameter related to the response time of the 
system is the time interval between the successive overpasses. 
It ranges from few days for land application, to a day for ice/ 
ocean monitoring and to few hours for ship/ocean application. 
The latter requirement may require more than one satellite to 
be fulfilled. 
Table 1 summarizes the sensor requirements for the three 
different applications and, where possible, for the st and 
2nd look modes. According to these requirements a preliminary 
calculation of the SAR parameters has been made for the two 
modes and the three operating frequencies. The results are 
summarized in Tables 2 and 3 for the LRWS and HRNS modes,  re- 
spectively. 
4.3 SYSTEM OPTIONS GENERATION AND PRUNING 
Several options derive from the different algorithms to 
obtain the high resolution and low resolution images (range - 
azimuth correlation function) and from the different algo- 
rithms to switch between the lst and 2nd mode (i.e. SKBS) and 
to understand the image information content. Another source of 
system options is the technology (i.e. device and computer) to 
implement the system. As a consequence, an expanding tree of 
system options can be drawn representing all the possible al- 
ternatives to the remote sensing problem. A major objective of 
the feasibility study is represented by these options and as- 
sessment of their technical feasibility, technical risk, time 
required to implement, industrial fall out and cost. In order 
to exploit at best the limited research effort, the majority 
of options will be pruned leaving few actractive alternatives 
to be analyzed more deeply, designed and tested during the 4 
years study. 
4.4 CRITICAL TECHNIQUES AND TECHNOLOGIES 
Crucial to the system options generation is the  identifi- 
cation of algorithms for the pixel extraction (range -  azimuth 
raw data compression) and image understanding. The state of 
work concerning these topics is briefly mentioned in the en- 
suing subsections 4.4.1 and 4.4.2. 
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