提出了直视合成孔径激光成像雷达（SAIL）概念，发射采用两个正交偏振同轴且相对扫描的空间抛物波差的光束，接收采用自差及相位复数化探测。在交轨向产生与目标点横向位置正比的线性项相位调制，在顺轨向产生以目标点纵向位置为中心的二次项相位历程，成像处理采用傅里叶变换实现交轨向聚焦和采用匹配滤波实现顺轨向聚焦。直视SAIL与侧视SAIL一样能够使用小光学孔径在远距离实现高分辨率二维成像，但具有本质独特性即线性和二次相位项和光学足趾及其相关联的成像分辨率无论在设计和使用时都具有很大的控制调整范围，并克服了存在于侧视SAIL中的许多技术难点。直视SAIL在原理和方法上都属于光学领域。给出了直视SAIL的一般性体系结构，在数学上详细描述了包括目标信息获取和成像处理的基本原理。

A kind of down-looking synthetic aperture imaging ladar (SAIL) is proposed, with a transmitter of two coaxial counter-deflected polarization-orthogonal beams of spatial parabolic phase difference and a receiver of self-heterodyne detection combined with phase complex-valued processing. In the orthogonal direction of travel, a linear phase modulation proportional to the lateral distance of target point and a quadratic phase history centered in the longitudinal position of target point in the travel direction are collected respectively. The image is focused by the Fourier transform and matched filtering in the two respective directions. Similar to the side-looking SAIL, the suggested down-looking SAIL achieves the fine-resolution, long-distance and two-dimensional imaging with modest aperture diameters, but it has inherent feature that the linear and quadratic phase terms and the size of optical footprint together with their associated imaging resolutions are controllable and variable in a large scale no matter in use or design. And, down-looking SAIL has overcome many difficulties in side-looking SAIL. The down-looking SAIL belongs to optics in the principle and means. This paper gives the general construction of down-looking SAIL, and details mathematically the principle including the data collection and the image processing.