Accurate geoid height differences computation from GNSS data and modern astrogeodetic observations

Abstract

This work presents the revival of the astrogeodetic observation procedure by means of modern instrumentation and processing. The determination of astronomical coordinates and the components and of the deflection of the vertical is achieved with an accuracy that reaches ±0.01. The used method brings to the fore the classical geodetic astronomy actually aided by modern technology. The system of NTUA is used [Lambrou (Development of a methodology for astrogeodetic determinations, using digital geodetic instruments. Ph.D. Thesis, 2003)]. This system consists of a first order total station connected with a GNSS receiver in order to register time with 1 ms. The same method is applied by changing the instrumentation. An advanced “imaging” total station is used, providing a direct connection to the internet, to a world time server, in order to obtain the necessary accurate time information. This total station runs windows CE and moreover it allows carrying out the observations on a digital screen, as the telescope is not deemed essential anymore. The sightings may be also carried out remotely via a PC’s screen situated anywhere and connected to the internet. Alternatively, in order to bypass the need of accurate time information, the Laplace equation can be used. Thus the determination of is achieved. As a limitation, this procedure requires visibility between the points. By using the corresponding geodetic coordinates ®, oe and azimuth, which are all provided by the GNSS measurements, the deflection of the vertical components are computed. Thereinafter the geoid undulation differences N are calculated, with few mm accuracy. By following this procedure, the results are independent from the definition of the local height datum. For this reason, it can improve the global geoid models as it enriches their data in areas characterized by large geoid variations due to rough topography. Finally, this procedure could be used for the unification or the definition of a unified height datum, for a country.