Objective   Inverse Synthetic Aperture LADAR (ISAL) is an active imaging detection method. Its working principle is consistent with that of Inverse Synthetic Aperture Radar (ISAR). The signal works in the laser band (μm level), which obtains range high-resolution information by actively transmitting large broadband laser signals, and obtains azimuth high-resolution information through the virtual aperture formed by the movement of the target relative to the emitter. Relevant studies have evaluated and explored the ISAL imaging of GEO space targets, and there are also corresponding researches that have preliminarily analyzed the parameters of space-based SAL imaging. However, there is still a lack of detailed analysis and evaluation of ISAL imaging mode in LEO space. In this study, the performance analysis and feasibility of using space-based ISAL for skimming and flying-around imaging modes of LEO targets is explored to provide a basis for the study of space-based ISAL imaging of LEO targets.

  Methods  Two kinds of observation modes, skimming and flying-around imaging modes, are set up. The skimming imaging mode is to use the natural rendezvous of ISAL payload satellite and target satellite for imaging. This imaging mode has many situations, such as co-orbital skimming and hetero-orbital skimming (Fig.1). Flying-around imaging mode means that during the target satellite rotating around the earth for one circle, ISAL satellite also orbits the target satellite for one circle (Fig.4). The key system indicators such as imaging resolution, coherent accumulation time (Fig.6-7), minimum pulse repetition frequency for unambiguous azimuth imaging (Fig.8) and SNR (Fig.9) for the two kinds of observation modes are compared

  Results and Discussions   For the skimming imaging mode, all performance indicators of all rendezvous scenes under given parameters are within the range of both forward and reverse scenes on the same plane. The smaller the relative angular velocity is, the smaller minimum pulse repetition frequency for unambiguous azimuth imaging (PRF) and the higher the signal-to-noise ratio (SNR) are required, but coherent accumulation time required for imaging is increased, especially the revisit period will be greatly increased. Compared with the skimming imaging mode, the flying-around imaging mode can realize the continuous observation of the target, and the flying-around period is short, and can quickly obtain more abundant target information; Although the coherent accumulation time is longer, it is only 130-190 ms in the range of 300-2 000 km orbit height, which is completely feasible for engineering applications; The minimum unambiguous PRF is reduced by half and the SNR is higher.

  Conclusions  The feasibility of using space-based ISAL to scan and fly around LEO targets is explored, and the key indicators of the system, such as imaging resolution, coherent accumulation time, minimum pulse repetition frequency for unambiguous azimuth imaging are analyzed and compared with the assumptions that meet the constraints of engineering application. Within the range of observation and simulation calculation of LEO targets, all indicators of the grazing imaging mode and the flying-around imaging mode are feasible for engineering application, and the flying-around imaging mode is applicable in rapid, high-resolution and all-directional continuous observation of important targets and high-value assets. The skimming mode is suitable for traversing and imaging the LEO target at the altitude of close orbit, so as to establish the feature library of the target.