The method of determining the orientation of dynamic coordinate system in relation to HCRF

Abstract

© SGEM 2019. As known, all GIS (geographic information system) coordinate systems tied to the dynamic coordinate system. In this paper, the method of determining the orientation of the dynamic coordinate system in relation to the HCRF (Hipparcos Celestial Reference Frame) system is described. The lunar occultation observations (LOO) were used for this purpose. The photoelectric method of recording an LOO that allows obtaining LOO moment with an accuracy up to 0.001s (i.e. 100 times more accurately than by visual method) is the most accurate. Apart from the tasks of space geodesy, photoelectric observations allow carrying out other interesting researches, too. These tasks include the determination of amendments to orbital longitude and latitude of the Moon and the ones to ephemeris time. The photoelectric observations of LOOs are valuable materials for solving some astrophysical tasks. If one records changes of magnitude at the moment when a star is occultated by the Moon, then one may obtain diameter of the occultated star through the spectrum of those changes. Other important problems are detecting double and multiple systems of stars and measuring angular distances between their components. Using the method of lunar LOO and photoelectric recording, one may observe double stars, that are 100 times closer, than with any other method. Recording changes of a star's magnitude forms the main task of the photoelectric observation method. If a star has a large diameter, then the curve of magnitude change differs from the curve of a point light source. The value of that difference depends on the diameter of the star in a certain way, and the intensity drop lasts for 20 to 40 milliseconds. Thus, the problem is to record a very quick process of a star's brightness fluctuations. However, when taking photoelectric observations of LOO, it is necessary to record changes of a small light flow from a star in the presence of the bright Moon. Distinguishing light from the star among the background formed by the Moon is the main task which should be solved when reducing the LOO. A comparison between the dynamic coordinate parameters of the system’s orientation in relation to the HCRF using the method developed in this paper and the values produced by other authors demonstrates a good agreement within the limits of computational error for the parameters obtained.