New approach for vertical deflection determination using digital Zenith cameras

Celestial positioning has been used for navigation purposes for many years. Stars as theextra-terrestrial benchmarks provide a unique opportunity in absolute point positioning. However، astronomical field data acquisition and data processing of the collected data isvery time consuming. The advent of the Global Positioning System (GPS) has nearlymade the celestial positioning system obsolete. The new satellite-based positioningsystem has been very popular since it is quite efficient and convenient for many daily lifeapplications. Several years ago، the determination of vertical deflections (the anglebetween the true zenith (plumb line) and the line perpendicular to the surface of thereference 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):