The LUNA-GLOB Mission’s Light Beacon Service to Improve the Theory of the Moon’s Motion

A. V. Bagrov, S. P. Kuzin, V. А. Leonov

Transactions of IAA RAS, issue 45, 3–8 (2018)

DOI: 10.32876/ApplAstron.45.3-8

Keywords: optical beacon on the Moon, selenodetic coordinates, onboard observations from the lunar satellite, land observations of beacons, the theory of the motion of the Moon.

Abstract

The Light Beacon Service System (LBSS) of the “Luna-25” (LUNA–GLOB) mission will not be used just in its main function to find the landed stations’ locations precisely, but for the navigation purposes as well, and also, to establish a highprecision selenographic coordinate system with 10-meter accuracy and to provide precision astrometric observations of the lunar light beacons in order to contribute in the current theory of the Moon’s Motion. We shall use a lunar satellite on the polar orbit (the “Luna-26” mission) for angular measurements from the orbit instead of surface-based angular observations. This will be achieved using a combination of self-determinations of the satellite by its on-board star sensors and the position of the bright light flashing from the beacon in the field of view of an onboard TV-camera, which is going to play the role of a goniometric device. We shall use methods of equalization obtaining a long row of the “beacon-satellite-star” measured angles in order to determine accurately both the beacon selenodetic coordinates and orbital parameters of the satellite. The positioning accuracy has to be about 10 m (if the altitude is 100 km), because the accuracy of the on-board star sensors is 6" and the TV-camera’s angular resolution is also 6". The precise enough positions of the lunar poles will be determined by the light beacon observations only. The beacon position is good for absolute latitude observations; however, “zero meridian” longitudes will be established through observations of light flashes from the laser retro-reflectors on the Moon which are going to be illuminated by our on-board projector. The established selenodetic frame will provide for the positioning of any target on the Moon surface with 10 m accuracy in the vicinity of the frame knots, and with the accuracy of less than a hundred meters further away from them. The beam of the light beacon which is pointing to the Earth will be bright enough to observe it with earth-based telescopes. It will be possible to observe this beacon and background stars near the edge of the lunar disk which will allow for precision measurements of the beacon instant positions relatively to the stars. As we know the position of the beacon relatively to the Moon masses center, we are able to calculate its instant celestial coordinates at least tenfold better than by the classical observations of stars eclipsed by the Moon.

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