Anthony Freeman - La Crescenta CA
David Blacknell - Sheffield, GB2
Shaun Quegan - Sheffield, GB2
Ian A. Ward - Chelmsford, GB2
Christopher J. Oliver - Malvern, GB2
Ian P. Finley - St. Johns, GB2
Richard G. White - Malvern, GB2
James W. Wood - Malvern Link, GB2
The General Electric Company, plc - London
The Secretary of State for Defence in Her Britannic Majesty's Government
of the United Kingdom of Great Britain and Northern Ireland - London
In a synthetic aperture radar system, information concerning the radar reflectivity of a point on the ground is placed in an image at an azimuthal (cross-range) position which corresponds to the position of zero Doppler frequency shift of the received signal, under the assumption that the antenna has followed a straight line trajectory. In practice, the antenna trajectory will undergo small deviations about this straight line path which are not taken into account in the azimuth processing and which, consequently, cause incorrect azimuthal positioning in the image of the information concerning a point target on the ground.
The invention uses the value of the slope of the frequency modulation of the received signal, calculated by means of an autofocus procedure, to determine the intercept of this frequency modulation at successive azimuth positions. This true intercept value is then incorporated in the azimuth processing, instead of the assumed intercept of the frequency modulation, allowing imagery which is free from azimuthal positioning errors to be produced.