Determination of astro-geodetic deflections of the vertical using digital Zenith camera system

Current studies on geodesy mainly focused on precise geoid modeling techniques in order to use GNSS technology efficiently. Analog instruments and the limitations on time determination methods reduced the usage of astro-geodetic observations during the past two decades. The studies performed during the analog era showed that the deflections of the vertical components can be derived with an accuracy of ±1 arc-sec using traditional optical-mechanical direction measuring systems and time recording devices. Geoid studies at Austria, Switzerland, and Germany integrated various data sets to their national geoid models including astro-geodetic deflections of the vertical. The contribution of astro-geodetic data set to these models considerable and improves the accuracy of the models. In order to determine a cm-geoid using astro-geodetic method, 5-10 points/1000 km2 are suggested. Recently, Turkish National Geodesy Commission coordinates a new project that is still in progress for height modernization in Turkey, which is aiming to achieve 1-cm Turkish geoid model. GPS/Levelling geoid of Istanbul has been defined in 2005 for Istanbul metropolitan area covering 6000 km2 using 1005 geoid determination points. The absolute accuracy of this geoid was published as ±3.5 cm. In order to accomplish a high precision geoid model, homogenous data derived from various techniques such as GPS/Levelling, precise surface gravity observations, airborne gravity, and satellite data have to be used including astro-geodetic measurements. Astro-geodetic deflections of the vertical are crucial measurements in geoid determination studies and density investigations. Invention of CCD sensors and the widespread usage of GNSS, lead major developments in both astronomical and geodetic applications. Traditional optical-mechanical instruments updated with CCD sensors, GNSS devices, and precise tiltmeters. As a result, digital zenith camera systems were designed beginning from the 2000s and in this way the determination of astro-geodetic deflection of the vertical components became more efficient. This study covers two main parts. The first part, introduce the DZCS designed in this study. The DZCS is introduced in detail, including the selection of the system parts, hardware and software design, and integration of the system components. The observations were performed in 4 different positions of instrument with 5-10 (20-40 images) single observations, which is defined as one series. During the test observations, CCD and telescope calibrations, shutter latency measurements, suitable epoch determination, and image quality test were also performed. The system designed in this study is developed as a transportable instrument with the combination of a Schmidt–Cassegrain type 8'' apertured telescope, a CCD camera with high quantum efficiency between the visible light range of the electromagnetic spectrum, digital tiltmeters, and a GPS receiver. The components of the system are controlled by a computer, and data flow designed as real time and automatically. Second part of this study introduces the test observations and their results. Zenithal star field is captured with Telescope/CCD integrated system, and the epoch information is related with the images using GPS time. Digital tiltmeters are used in order to adjust the system towards its zenit direction. The first observations of the DZCS used in this study, are performed at the main station that is located at the campus of Bogazici Unversity Kandilli Observatory and Earthquake Institute. Images of zenithal star field are processed in order to align the axes of the system components for the optimum solution. Potential stars captured on the images are registered using centroiding algorithms such as point spread function fitting, and moment analysis. Reference stars in the CCD images were identified using GSC (Guide Star Catalog), UCAC2 (U.S. Naval Observatory CCD Astrograph Catalog) and UCAC3 star catalogs using the parameters of the DZCS (focal length, sensor size, pixel scale). Coordinates of the reference stars are defined on the reference epoch (J2000.0) in the star catalogs. Apparent topocentric coordinates of the stars are calculated using earth rotation parameters, (time, polar motion, presession, nutation), proper motions of stars, atmospheric pressure, temperature, geodetic coordinates of the station, and ephemerides information of the bodies in the solar system. Observations that were made under high humidity and unsuitable meteorological conditions are eliminated from the results. Coordinates of the zenith point is determined using projective transformation of zenithal stars on the CCD images. Average coordinates of the zenith point are calculated applying the tilt corrections recorded from the four different instrumental position, and using the geodetic coordinates of the observing station, the deflections of the vertical components were calculated. The precision of processing star images is about ±0.1-0.2''. Levelling precision of the system is reached to ±0.4'' and the system is averagely leveled within a circle with 10'' radius. The precision of latitude determination is ±0.19'' and longitude determination is ±0.28'' at the test station. The system is also tested on a network located at the Anatolian part of Istanbul. The observations are performed and calculated at the test network, which has an average of 20 km baselines with 4 points including the test station. At the test network the root mean square of the average value of the North-South, and East-West components of the vertical deflections are calculated as 0.35'' and 0.37'' respectively. Deflection of the vertical components are compared with the values that are calculated using global geopotential models (EGM08, GGMPlus). Deflections of the vertical of the test network are in good agreement with deflections of the vertical calculated EGM08 and GGMPlus global models. Correspondence of the models with observed deflections of the vertical is about ±1'' excluding the latitudes of the stations which are near coastlines. The GPS/Levelling and DZCS derived heights of the test network are also compared in this study. The analysis of the height differences of the stations calculated using different weighting approaches, the root mean square of determining the geoid undulations for 1 km length is calculated as ±14.7 mm/km for PΔN = 1/Skm and ±2.9 mm/km for PΔN = 1/S2km. The geoid height differences calculated at the test network using least squares adjustment, depending on the geoid height of the Kandilli station, and the root mean square of the sample of system observations with PΔN = 1/Skm is ±17.6 mm/km and with PΔN = 1/S2km is ±3.6 mm/km. The maximum root mean square of the geoid heights after the adjustment process is ±57.1 mm for PΔN = 1/Skm and ±54.6 for PΔN = 1/S2km The results obtained at the main station and the test network indicate that zenith camera observations can be used to improve reliability of GPS/Levelling geoid, and to eliminate the systematic errors in Leveling networks. The results show that the system has the capability to contribute the modernization of the national vertical datum studies in Turkey. As a result, in this dissertation through using various instrumentation a DZCS is designed and used to determine precise deflection of the vertical components. The results are compared with GPS/Levelling and global geopotential models, optimum observation and processing methods are investigated. The necessary hardware and software improvement suggestions also discussed as a result of this study. The improvement of the system is still an ongoing process. This study was supported by TUBITAK Scientific and Technological Research Projects Funding Program with the grant number 111Y125.