Return pulse waveform simulation for LLRI instrument onboard Chandrayan-I

Abstract

ISRO is launching a LiDAR instrument (LLRI) onboard Chandrayan-I. The LLRI will collect topographic data of lunar surface. Flying at an altitude of 100km the LLRI will have a footprint of 100m on the moon surface. Time of travel measurement, which is fundamental for topographic coordinate computation, depends upon the shape of the return pulse. This shape in turn is a function of the characteristics of footprint, i. e. its geometry, reflectance and roughness. This paper uses a mathematical model to simulate the return waveform at the receiver for different conditions of said characteristics within the footprint. Mathematical equations are employed to generate footprints that vary in their characteristics in terms of reflectance, geometry and roughness. A footprint is divided into small bins so that each bin has uniform property. Energy distribution in transmitted pulse is considered Gaussian. Energy irradiated over footprint is approximated using the assumed distribution. For each bin the energy incident is computed and accordingly the quantum and distribution of reflected energy is determined. The final waveform is generated by integrating the energy returned from all the bins according to their time of arrival and spread. Some results are presented to show the performance of the developed system.