In recent years

spaceborne/airborne bi-static synthetic aperture radar（SA-BSAR） has been proposed as a new kind of microwave remote sensor.The development of SA-BSAR is still at an early stage since there are many challenges faced ahead.Due to the huge difference between the satellites’ and the aircrafts’ velocities and flying altitudes

there are obvious differences between SA-BSAR and the earlier bi-static SAR systems which have symmetrical structures

such as the spaceborne bi-static SAR and the airborne bi-static SAR.Three basic and critical issues in SA-BSAR are discussed

including geometry

Doppler properties and signal model.In the discussion of geometry

the proper coordinate systems are first established to describe the movement of the satellite and the aircraft.Next

the appropriate flight path for the aircraft is quantitatively analyzed in detail from many aspects

including: how to improve the range resolution

how to improve the azimuth resolution

how to receive the direct path signal more conveniently

and how to make the spatial synchronization much easier.The following conclusions are drawn: the aircraft should fly on the same side of the imaging scene with the satellite;the difference angles among the platforms’ course angles can not be too large;the aircraft must be illuminated by the satellite’s main beam within the imaging time;the aircraft must appear at the right position just in time to cooperate with the satellite to implement the spatial synchronization.In the discussion of Doppler properties

several analytical formulae for computing Doppler parameters are first derived using the method of vector analysis.Afterwards

to make the formulae more practical

the derived expressions are transformed to an easy-to-use form that is expressed by orbit elements of the satellite

satellite antenna pointing angles and kinematical parameters of the aircraft.In the discussion of signal model

the model for SA-BSAR is first established based on the model for mono-static SAR and then the great difficulties in designing the imaging algorithm are discussed.Through the analysis for the established model

the following conclusion is drawn: the traditional Range-Doppler（RD） algorithm

after properly modified and extended

can also be applied to the imaging of SA-BSAR when some special conditions are simultaneously met.These special conditions include: the synchronization scheme based on wide-beam illumination is employed;the satellite and the aircraft fly in parallel

and both the transmitter and the receiver are operated in side-looking modes.In order to validate the above conclusions

two simulation experiments were performed.The simulation results verify the accuracy of the derived formulae for the calculation of the Doppler parameters

and also verify the applicability of the RD algorithm when some special conditions are met.On the basis of the research conducted in this paper

the imaging algorithm

when the synchronization scheme based on antenna steering is employed and the flight path of the aircraft is arbitrary

can be further designed in the next step.