Spaceborne SAR autofocus

Low Earth orbit provides an ideal operating environment for synthetic aperture radar (SAR) imaging due to well-defined platform motion. Airborne SAR platforms do not benefit from this luxury and suffer unpredictable motions causing image defocusing. Autofocus is a widely acknowledged technique for correcting these motions, causing the image to be properly focused. Accurate position and velocity information is required to process spaceborne SAR scenes with higher spatial resolutions requiring greater knowledge of the satellite's orbit. The availability of position and velocity data is dependant on the efficiency and location of the ground station and could finally lead to delays in processing times. In these cases an orbital propagation model has to be employed for immediate processing. The precision of the image is now dependent on the accuracy of the orbit model used. Using DRA patented autofocus/phase correction techniques on ERS1 raw data it is shown that the point target response of imagery obtained with no prior orbit knowledge is comparable with precision imagery. In addition the technique allows continuous strip map imagery to be produced with no discontinuities.