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Lunar Laser Ranging Instrument (LLRI)
The elevation map of the Moon prepared using the laser ranging instrument carried onboard Chandrayaan-1 spacecraft will help in studying the morphology of large basins and other lunar features, study stress, strain and flexural properties of the lithosphere and when coupled with gravity studies, would be able to find the density distribution of the crust.
Scientific Objective:
To provide ranging data for determining the height difference between the spacecraft and the lunar surface.
Payload Configuration Details:
LLRI works on the time-Of-Flight (TOF) principle. In this method, a coherent pulse of light from a high power laser is directed towards the target whose range is to be measured. A fraction of the light is scattered back in the direction of the laser source where an optical receiver collects it and focuses it on to a photoelectric detector. By accurately measuring the roundtrip travel time of the laser pulse, highly accurate range/spot elevation measurements can be made.
LLRI consists of a 10 mJ Nd:YAG laser with 1064 nm wave source operating at 10 Hz pulse repetition mode. The reflected laser pulse from the lunar surface is collected by a 200 mm Ritchey-Chrétien Optical receiver and focused on to a Silicon Avalanche Photodetector. The output of the detector is amplified and threshold detected for generating range information to an accuracy <5m. Four constant fraction discriminators provide the slope information in addition to range information. The different modes of operation of LLRI and the range computations from the detector output are controlled and computed by a FPGA based electronics. The processed outputs of LLRI will be used for generating high accuracy lunar topography. The payload mass is 11.37 kg with base plate.
LLRI payload is developed by ISRO
 
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Last Update
11 Nov 2008