Lunar Laser Ranging Instrument (LLRI): a tool for the study of topography and gravitational field of the Moon

The Lunar Laser Ranging Instrument (LLRI) developed for flight on the Chandrayaan-1 spacecraft was designed to measure the topography of the lunar surface over a 2-year period from a 100 km polar orbit of Moon. A 10 mJ diode-pumped pulsed laser together with 200 mm diameter telescope and a silicon avalanche photodiode are the principal optical assemblies of this active remote-sensing instrument. We present here science objectives of LLRI and its performance specifications along with details of instrument design. The methods by means of which LLRI performance and operability were analysed are also presented. During end-to-end testing, the integrated LLRI was operated in free space aiming at a distant target (-30 km). Test results were found to be satisfactory and well within the desired specifications.