409 
A more detailed analysis shows the required X-band antenna area as function of the incidence 
angle (Fig. 2). It should be noted that 55° incidence angle corresponds to 50° off-nadir angle, and 
therefore, the required antenna area is approximately 8 m 2 . 
The mentioned antenna gain (beam width) adaptation (Fig. 3 and Fig. 4) always leads to 
the optimum possible swath width for the chosen off-nadir angle (Fig. 5). 
It should be mentioned that for system considerations, a S/N » lOdB for the average 
backscatter coefficients of various matters (grass, shrubs, snow etc.) has been selected, in order to 
estimate the required average transmitter power. However, the resultant radar transmitter power 
will be defined by the number of T/R-modules times the transmitter power of each module. 
3 - ANTENNA CONCEPT 
As mentioned above, a minimum antenna area of 8 m 2 is required for X-band SAR operation at 50° 
off-nadir and a satellite orbit of 440 km altitude. 
Taking a 4 look mode and 10 m resolution as a baseline, the real aperture length should be 
about 4 m (to 3,2 m) according to 
(0.6 to0.8) N y ’ 
where r az stands for the azimuth resolution and N for the number of looks. 
The factor (0.6 to 0.8) depends on the weighting function used for the synthetic azimuth 
antenna pattern. For this case an antenna length of 4 m has been selected, and therefore, the 
antenna width is 2 m. 
A candidate for the antenna radiator is an interesting center fed slotted waveguide design for 
dual polarization radiation which has recently been developed by SAAB ERICSSON SPACE (Fig. 
6). The upper waveguide radiates in horizontal polarization (vertical slots on the small wall side) 
and excitation of the slots is generated by the wires transversing the inner part of the waveguide in 
diagonal directions. The vertical polarization will be radiated by the horizontal slots of the ’’broad” 
wall side of the (lower) folded waveguide. The waveguide has been folded in order to reduce the 
broad wall dimension, which makes it easier to achieve a scanning capability of about ±20° (without 
grating lobes). The scanning range depends on the distances of neighbouring radiating elements of 
same polarization, which can be expressed by [1]: 
s 1 
A < 1 + | sin#| 
where s = distance of neighbouring radiating elements (same polarization), 
A = wave length, 
0 = scan angle. 
Each of the waveguide pairs will be fed by an individual T/R module for horizontal and 
vertical polarization (Fig. 7). As indicated in the block diagram, the phase adjustment and the 
polarization switch are located at the low power input in order to avoid losses between the trans 
mitter (HPA) and the radiating waveguide, which are, respectively, a low noise amplifier (LNA) and 
a slotted waveguide.