2. THE SYSTEM CONCEPT 
A BIPAR is a bistatic radar system combining the advantages of spaceborne and 
airborne imaging radar systems. The transmitter is located in a geostationary 
orbit with a very low probability of intervention illuminating continuously the 
area of interest on ground. The quiet receiver is carried on an airborne plat- 
form implying a high mission flexibility, high repetition capability and less 
power requirements compared to a system with spaceborne receivers. It is called 
"parasitical" because it uses non-radar transmitters of opportunity, such as 
geostationary communications satellite transmitters or transmitters of direct 
television satellites. A typical system geometry is shown in figure l. A geo- 
stationary communications or direct TV satellite is transmitting its RF down- 
link signal to Earth. Within the beam of its downlink antenna it illuminates 
the Earth surface and a certain portion of the RF energy is unavoidably 
reflected back into the air. The strength of the reflection depends on the 
reflection coefficient of the target areas on ground, such as the vegetation 
canapé, streets, houses, cars and so on. 
An aircraft with a BIPAR receiver onboard, flying within the satellite downlink 
antenna beam picks up these reflections from the ground with a scanning pencil 
beam radar antenna and provides real aperture radar images of that part of the 
Earth surface being covered by the BIPAR scan range. 
   
    
  
COMMUNICATIONS SATELLITE ANTENNA BEAM (DOWN LINK) 
7 DIRECT PATH SIGNAL 
SCATTER PATH SIGNAL 
BIPAR AIRCRAFT 
SCATTERED SIGNAL SCANNING BIPAR ANTENNA BEAM 
BIPAR RESOLUTION CELLS 
Fig. 1: Typical BIPAR System Geometry 
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