Airborne SAR system for determining the topography of a terrain

Claim 1

A terrain-mapping airborne synthetic aperture radar system for an aircraft travelling in an x-axis direction of flight at a z-axis elevation over a target, the target being a point of terrain, the x-axis direction being transverse to a y-axis direction; the system comprising: a monopulse radar antenna including a set of radiating elements, the x-axis direction lying within the antenna plane, the antenna plane being inclined to the z-axis direction at an angle α; means for distributing radar pulses to the radiating elements in a way that radiating elements are separated into two antenna halves in the antenna plane above and below a symmetry line parallel to the x-axis direction; on a transmitting side, means for distributing radar pulses to the antenna halves such that the antenna halves transmit the pulses in phase; means for detecting echo pulses returned from the target to the radiating elements to generate return signals therefrom; first processing means for determining a target range by an echo delay of the return signals; second processing means for determining an azimuthal target bearing by detecting a doppler shift of the return signals; and third processing means for means for determining a elevational target bearing; the third processing means further comprising: on a receiving side, phase-shifting means for alternately phase-shifting by a phase angle φ, and not phase-shifting, received signals from the two antenna halves; imaging means for generating two distinct synthetic-aperture radar images, the images including an out-of-phase image corresponding to a phase-shifted sequence of returned signals phase shifted by the phase angle φ of signals from the two antenna halves and an in-phase image corresponding to a non-shifted sequence of returned signals received without phase-shifting of signals from the two antenna halves; dividing means for taking a quotient of intensities of corresponding target image pixels of selectively the in-phase image and the out-of-phase image, the quotient being a function of φ and a nadir target bearing angle θ between the z-axis direction and the target; and angle-determining means for determining from the quotient a nadir target bearing angle θ between the z-axis direction and the target, θ being equal to 90 degrees-α; whereby the ground topography may be determined as a map of target image pixels in three dimensions of range, azimuthal angle, and nadir angle.