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The system noise power N is defined by 
N 7 KT, Fn B, (12) 
where k is the Bolzmann's constant, T, is the system temperature and Fp the 
receiver noise figure. 
The radiometric resolution Pg of an imaging radar is estimated by 
S 
1 + ($0.4. 
S (13) 
YN (N) 
On ^ 
as a function of the number of independent looks N, and the signal to noise 
ratio being derived from equation 11 and 12. 
4, SYSTEM EXAMPLE 
The following example of a feasible system shall indicate the general capabili- 
ties of BIPARs. 
The transmitter of opportunity could be an ECS-type communications satellite 
illuminating the Earth surface with a power density of -120 dBW per squaremeter 
or a direct TV satellite with about 10 times more output power. The transmit 
frequency is assumed to be about 11 GHz with a bandwidth of 27 MHz. 
The BIPAR uses an electronically scanning antenna of 1 m x 1 m effective area 
with a scanning angle of + 30 degrees. It flies on an aircraft at 1000 m flight 
altitude with a velocity of 100 km/h. The scan repetition rate is l Hz. It is a 
step scan with 40 positions within + 30 degrees. At each beam position 3 inde- 
pendent looks are assumed resulting in an integration time per look of about 
8.3 milliseconds. This leads to a coherent integration gain of 52 dB consider- 
ing an integration efficiency of 70 % due to the noisy scattered signal, its 
doppler spread of about 30 Hz, and misalignments within the time and doppler 
compensations. 
Typical values for the scatter coefficients might be between -10 dB and +6 dB 
(one sigma) assuming that there is not much difference between 9.6 GHz and 
1l GHz and between bistatic and monostatic systems. Under the umbrella of the 
German/Italian X-SAR project og values have been researched intensively and 
these results could be used as a guideline for a preliminary BIPAR performance 
assessment. 
Assuming furthermore a clear air propagation loss of 0.5 dB, a receiver noise 
figure of 6 dB, and a kTo of -204 dB/Ws, table 1 shows that a signal to noise 
ratio between 0 dB and 19 dB could be reached. An overall margin of 3 dB for 
other systems losses is included in this calculation. 
51