1 - INTRODUCTION 
The experience gained with orbiting SAR-sensors (SEASAT, SIR-A, SIR-B, ALMAZ, ERS-1 and 
JERS-1), demonstrates the Earth surface observation capabilities and proved its independence of 
weather conditions and day/night time. 
All above-mentioned systems are of conventional design and use passive antennas. Princi 
pally, their viewing geometry is fixed, (with the exception of SIR-B, which used an elevation mount 
to change the off-nadir angle). 
Meanwhile, technological developments of monolithic microwave integrated circuits (MMIC) 
for transmit /receive modules allow the design of antennas as active phased arrays. These are 
systems which include phase controlled transmit/receive modules (T/R-modules) distributed across 
the antenna surface, making it possible to change beam direction, shape of beam (beam width), 
and therefore, also the antenna gain. 
In the paper an X-band SAR system is outlined which takes advantage of the capabilities of 
a full fledged active phased array. Expected performance is discussed. 
2 - ASSUMPTIONS FOR SAR SYSTEM LAYOUT 
Many land application scientists request revisiting times for areas of interest in time intervals shorter 
than a week. 
Computations of data-take opportunities for a SAR capable of selecting a viewing angle 
between 20° and 50° off-nadir showed a coverage cycle of about five days for a sun synchronous 
orbit of 440 km altitude and 97° inclination. This means that areas of interest may be covered 
approximately every five days, but under different viewing angles. 
For civil applications, a pixel size of 10 m times 10 m for a 4 look image seems to be 
adequate, but in SCANSAR operation the pixel size will be increased approximately in proportion 
to the increased swath width. 
The system bandwith will always be adapted in such a way that the pixel size will remain 
independent of the off-nadir angle. This means that the band width will be varied (Fig. 1) with 
1/sine, (e = incidence angle, which is slightly larger than the off-nadir angle due to the earth surface 
curvature). 
Furthermore, for the various viewing geometries (off-nadir angle) and strip mapping, the 
antenna gain will be adapted by changing the phase distribution function across the antenna in 
the elevation plane in order to fulfill the mi nimum requirements for the necessary antenna area A , 
which can be estimated by 
where v 
f 
H 
€ 
A 
v 
10 -H 
sine 
cos 2 e 
satellite velocity, 
frequency, 
satellite altitude, 
incidence angle (against nadir). 
(1)