Space platform positioning for ground-based objects imaging using synthetic aperture radars

Space synthetic aperture radars operate in the mode of lateral view of objects located on the Earth surface. The line of sight is deflected transverse to the platform velocity vector by some angle. The usual requirement is that at the moment of antenna beam passage through the center of the synthesized aperture the line of sight should be orthogonal to the velocity vector in the Greenwich coordinate system. Thus, before the forthcoming imaging of specifically specified objects it is necessary to determine in advance those moments of time when the specified requirement will be met. In addition to this, there is a restriction that the angles of sighting should be within the predetermined interval of their change. In addition to the time instants required for imaging, the target sighting angles and the spatial orientation of the platform orthogonts and the antenna pattern scanning angles must also be estimated. This paper is devoted to the solution of these questions. The purpose of the work is to build a model for determining arrays of observation time moments, positions in orbit and spatial orientation of a space platform with SAR when imaging in the en-route mode of given objects on the Earth's surface. The model of planning in time of the process of imaging by space SAR of a given target located on the Earth surface is presented. Its position is determined by geographic coordinates in the PZ-90 geoid model, taking into account the elevation above it. Algorithms and software procedures have been developed to perform calculations using a personal computer (in the MathCad environment) of the time moments at which not only the observation of the target in its side view, but also the zero value of the center of the Doppler spectrum (Doppler centroid) of the echo signal received by the locator is ensured in the en-route imaging mode. The results of calculations are given in the map projection of platform positions and in the form of tables. They contain time moments, angles of sighting, as well as geographic latitude and longitude of the platform location on the orbit when imaging a given target in left- and right-handed modes of its observation. Building of graphical dependencies and output of tabular data take time not exceeding one second when planning the survey for a month in advance. Application of the developed algorithms and program for personal computer allows to accelerate, clarify and facilitate the time-consuming procedure of planning the survey of the Earth surface in missions of space radars with synthetic aperture.