New approach for vertical deflection determination using digital Zenith cameras

Celestial positioning has been used for navigation purposes for many years. Stars as the extra-terrestrial benchmarks provide a unique opportunity in absolute point positioning. However, astronomical field data acquisition and data processing of the collected data is very time consuming. The advent of the Global Positioning System (GPS) has nearly made the celestial positioning system obsolete. The new satellite-based positioning system has been very popular since it is quite efficient and convenient for many daily life applications. Several years ago, the determination of vertical deflections (the angle between the true zenith (plumb line) and the line perpendicular to the surface of the reference ellipsoid) often required 2-3 h or even more, using conventional astrogeodetic instrumentation such as analogue zenith cameras or astrolabes.The invention of the electro-optical devices at the beginning of the 21st century was really a rebirth in geodetic astronomy. Today, digital cameras with relatively high geometric and radiometric accuracy have opened a new insight into satellite attitude determination and the study of the Earth's surface geometry and physics of its interior, i.e. computation of astronomical coordinates and the vertical deflection components. The Digital Zenith Camera System consists of a zenith camera equipped with a CCD imaging sensor, which is used for the determination of astronomical latitude, , (the angle between the plane of the earth's equator and the plumb line (direction of gravity) at a given point on the earth's surface) and longitude,, (the angular distance of a point on the celestial sphere from the great circle perpendicular to the ecliptic at the point of the vernal equinox, measured through 360° eastward parallel to the ecliptic). By means of the positions of stars on the celestial sphere which are defined by equatorial coordinates . The equatorial coordinates can be linked to the astronomical parameters by GAST (Greenwich Apparent Siderial Time):
The second component is a GPS receiver which is used for time tagging of the exposure epochs as well as for determining geodetic latitude and longitude  of the camera. Vertical deflections at the surface can be obtained by combining both components:
In the automatic star detection, high precision and reliable in the extraction of the star centers from the captured images and relating them to the astronomical coordinates is the most important point.In this study, the star's centers were extracted by an advanced image processing technique with sub-pixel precision. Relating the parameters of the presented technique to the star's Mag was one of its exclusive properties. Using the theory of coherent motion, the corresponding stars were detected first, and the outliers were removed by the MSAC algorithm afterwards. The suggested method was applied to the images taken by a TZK2-D camera which consisted of two major components: A zenith camera equipped with a CCD was used for the determination of the plumb line and a GPS-receiver for precisetiming and measurement of the ellipsoidal coordinates. Validations showed that the adopted approach in this study is highly capable of yielding reliable results.