Comparison of the reference mark azimuth determination methods Danijel Šugar * , Mario Brkić, Drago Špoljarić University of Zagreb, Faculty of Geodesy, Zagreb, Croatia ANNALS OF GEOPHYSICS, 55, 6, 2012; doi: 10.4401/ag-5405 ABSTRACT The knowledge of the azimuth of the reference mark is of crucial importance in the determination of the declination which is defined as the ellipsoidal (geodetic) azimuth of the geomagnetic meridian. The accuracy of the azimuth determination has direct impact on the accuracy of the declination. The orientation of the Declination-Inclination Magnetometer is usually carried out by sighting the reference mark in two telescope faces in order to improve the reliability of the observations and eliminate some instrumental errors. In this paper, different coordinate as well as azimuth determination methods using GNSS (Global Navigation Satellite System) observation techniques within VPPS (High-Precision Positioning Service) and GPPS (Geodetic-Precision Positioning Service) services of the CROPOS (CROatian POsitioning System) system were explained. The azimuth determination by the observation of the Polaris was exposed and it was subsequently compared with the observation of the Sun using hour- angle and zenith-distance method. The procedure of the calculation of the geodetic azimuth from the astronomic azimuth was explained. The azimuth results obtained by different methods were compared and the recommendations on the minimal distance between repeat station and azimuth mark were given. The results shown in this paper were based on the observations taken on the POKU_SV repeat station. 1. Introduction The knowledge of the azimuth of the reference mark is of crucial importance in the determination of the geomag- netic declination. According to the definition, the geomag- netic declination is the angle between the geographic north direction and the magnetic meridian [Jankowski and Sucks- dorff 1996] i.e. the ellipsoidal (geodetic) azimuth of the mag- netic meridian. There are several azimuth determination methods: it can be calculated upon known geodetic coordi- nates of the station and target point (reference mark), by methods of geodetic astronomy, by usage of gyro-theodo- lite as well as by modern GNSS (Global Navigation Satellite System) methods. In this paper is given an overview and comparison of the determination of azimuth by GNSS methods within the CROPOS (Croatian Positioning System) system as well as by the astronomic methods. The determi- nation of azimuth by gyro-theodolite hasn't been discussed in detail because such an instrument wasn't available for the survey on the field. 2. Azimuth determination by GNSS methods The GNSS today includes different global satellite nav- igation and positioning systems: American GPS (Global Positioning System), Russian GLONASS (GLObalnaya NAvigatsionnaya Sputnikovaya Sistema), European Galileo and in future many other systems. Generally, GNSS uses two basic positioning methods: absolute and relative. Absolute method provides the coordinates with lower precision and therefore it is used in navigation, while the relative methods provide higher precision of the coordinates and are used for positioning. Furthermore, positioning methods are classified as static or kinematic depending whether the GNSS receiver during the observations was stationary or roving. Such two positioning methods (static relative and kinematic relative) were applied in this paper for the determination of the coor- dinates and subsequent calculation of the ellipsoidal azimuth. Both methods require at least two GNSS receivers simulta- neously observing the same satellites in order to allow the computation of the baseline vectors between receivers (sta- tions). Using the static relative method the baseline vectors are obtained in post-processing, while using the relative kine- matic method with ambiguity resolution in real time (RTK, Real Time Kinematic) the coordinates are determined directly on the field. The unknown coordinates of the station can be determined by adding the processed baseline vectors to the known coordinates of the reference station. If at several sta- tions with known coordinates the baseline vectors towards an unknown station were processed then the coordinates of the unknown station can be determined by the least-squares adjustment procedure [Hofmann-Wellenhof et al. 2008]. 2.1. GNSS observations during the CGRSN setup During the setup and survey of the CGRSN (Croatian Geomagnetic Repeat Stations Network) in 2004, the coordi- nates were determined by static relative positioning method [Brkić et al. 2006]. Three Trimble 4000 SSi GPS receivers Article history Received September 30, 2011; accepted April 18, 2012. Subject classification: Satellite geodesy, Geodetic (ellipsoidal) azimuth, Astronomic azimuth, Repeat station, Reference mark, GNSS, CROPOS. 1071 MagNetE workshop 2011