Imaging model is the basis for most geometric processing of space-borne SAR imagery

including image rectification

radargrammetry and image geolocation.This paper develops a parametric imaging model for SAR

which explicitly describes the relationship between the 3D cartographic coordinate of a ground target and the 2D image coordinate of the corresponding pixel.The parameters of imaging model consist of near range

image starting time

Doppler parameters and orbit parameters.After quantitatively evaluating the influence of the Earth perturbations on satellite orbit

we present an improved satellite orbit model

which uses less parameters and is able to fit the space orbit at nearly the same accuracy compared with classical model.As an application of the imaging model

an orbit refinement algorithm using ground control points（GCPs） is presented.For this purpose

the imaging model is rewritten to incorporate only 9 orbit parameters

and then linearized through differentiating with respect to the orbit parameters.For each GCP

the image coordinates are expressed as the function of the orbit parameters and two error equations are formed.The orbit parameters can then be iteratively determined through solving a linear equation set consisting of the error equations from all GCPs under least square rule.The orbit refinement algorithm is tested on a RADARSAT SGF（SAR Georeferenced Fine resolution product） scene.5 GCPs are identified from 1∶50000 scale topographic maps to refine the orbit parameters coming with the SGF image.The accuracy of the refined orbit is demonstrated through geometric correction of the SGF image.While the SGF data is poorly corrected using the original orbit data

it is precisely corrected with refined orbit parameters.