Getting ready for VLBI observations to the moon

Abstract

Geodetic Very Long Baseline Interferometry (VLBI) is based on the observation of extra galactic radio sources, primarily quasars. In recent years, the concept of equipping artificial and natural celestial bodies, such as satellites and the Moon, with VLBI transmitters has emerged. This concept opens up new perspectives in the fields of geodesy, astronomy and spacecraft navigation. This thesis deals with the required models for the analysis, described by the precise positioning of a VLBI transmitter at the location of the Apollo 15 reflector on the Moon. The determined position is evaluated by analysing Lunar Laser Ranging (LLR) measurements. LLR measures the flight time of laser light pulses to travel from the LLR station to the LLR reflector on the lunar surface and back. Due to the considerable distance between the objects involved, the laser pulses are affected by several delays. These must be considered in the LLR analysis. In particular, this thesis focuses on the two dominant propagation delays, namely the atmospheric delay and the Shapiro time delay. The atmosphere causes a light propagation delay of 2 to 9 m. The Shapiro time delay can be subdivided into two components. The Sun is responsible for delays in the range of 6 to 8 m, while the Earth causes delays of a few centimetres. After modelling these delays, the residuals from 2002 to 2021 (no measurements available for 2006, 2007 and 2008) lie between -1 and +1 m.The second part of this thesis presents a visibility analysis of the VLBI transmitter on the lunar surface. For this purpose, modifications are made to the VLBI scheduling software VieSched++. The visibility is evaluated based on the elevation and the number of stations from which the VLBI transmitter is simultaneously visible. It was found that the number of hours per day during which the VLBI transmitter is visible varies considerably depending on the location of the VLBI station. Furthermore, the VLBI transmitter's visibility duration depends on the Moon's position. Therefore, to conduct successful VLBI observations to the Moon, it is essential to create an appropriate observation plan involving a globally distributed network of VLBI stations.