Geodetic reference station “RIGA” J.Kaminskis, L.Lieksne, I.Širmele PERMANENT OBSERVATIONS OF GPS Observations of GPS in point “RĪGA” are made and stored since 1997. GPS data are stored with half-hour interval. Data are collected and once in an hour sent to data collection and processing centers of systems IGS and EUREF. Also data for day are sent to these centers. Data are used for scientific and practical applications in EUREF, ITRF and IGS systems. MEASUREMENTS OF GROUND WATER LEVEL As one of important parameters for the characterization of datum besides geocentric coordinates and velocity of their changes in the course of time, is determination of value of Earth gravity acceleration and control of changes in the course of time. As gravimetric point is situated about 10 meters under the level of surface in order to ensure necessary stability in urban environment with intense traffic and vibrations caused by this, level of ground water shall be determined systematically in the vicinity of point. Point “RĪGA” is datum of LR coordinate system of the Republic of Latvia and is situated in Riga botanic garden. In order to detect and control changes of coordinates of point in the result of movements of Earth’s plates, space geodesy methods that are based on processing of special measurements of geodetic and navigation satellites, which are collected for long period of time, are used. Coordinates are detected by use of GPS system in permanent or continuous mode. GPS data for twenty-four hours are regularly sent to GPS data European centre in Frankfurt. Bore-hole for determination of level of ground water Estimated ETRF2005 velocities in the N/E/U system : vN = -0.61+-0.11 mm/yvE = -0.38+-0.14 mm/yvU = 2.40+-0.37 mm/y In order to insure possibility to make regular measurements of level of ground water, in 10 December 1998 a more than 10 m deep special bore-hole was created in close vicinity of the gravimetric point. It is necessary in order to determine the volume of mass change in the vicinity of gravimetric point, which makes it possible to reduce gravimetric measurements made in different seasons to common epoch. Measurements are made with graduated tape having “water ball” hanging in its end, hollow metal detail. Measurements were made assuming level of gravimetric benchmark “B” as reference level. Benchmark is embedded in concrete cellar construction and is stable in the course of time. Distance vertically between benchmark “ B “ and main benchmark “ A “ is - 10.57 meters. Accuracy of measurements is 1 cm. GPS permanent EUREF + IGS station “RĪGA” 12302M02 This base station is unique in Latvia, because it is connected with satellite laser ranging (SLR) station, is as absolute gravimetric point and also groundwater level measurements are performed there. These measurements are stored together with measurements of other observatories of world and on this basis updated global coordinates are calculated. Measurements made in permanent point RIGA are essential contribution of the Baltic region in the maintenance of geodetic reference system in Europe and wherewith in world. Changes of coordinates of GPS station “RĪGA” in the period of time 1997 – 2007 In the beginning of 2007 (100 DY) the new receiver “BENCHMARK” and antenna failed. They were sent for repairs to producers firm. Measurements (150 DY) were continued with the old 8 channel receiver Turbo Rogue 8000. On 252 DY receiver BENCHMARK was again connected, but on 330 DY it failed again and was disconnected. Before receiver LEICA GRX 1200 PRO was set in permanent mode, pause of 20 days has arisen in measurements. The new receiver LEICA and antenna were connected on 350 DY. Benchmark of GPS permanent station “RĪGA” SATELLITE LASER RANGING Satellite laser ranging by use of very short laser light impulses is very dependent on atmospheric weather conditions. The meteorological conditions of 2007 were very unfavorable for realization of optical measurements, specially in the second half of year. For example, from 31 October till 31December there was no clear night. Weather conditions allowed to make measurements only in 89 nights, days and twilight hours. In 2007, also regular satellite laser ranging in day light was made, as far as weather conditions allowed, by use of day light measurement system established in 2004 with very low level of background noise. In the added table, summary on work amount in 2007 till 31December included is given. Satellite Pass Number of measurements Normal points FR NP Lageos-1 74 102 911 885 Lageos-2 38 26 449 372 ERS – 2 119 91 009 2 555 GFO – 1 24 9 138 343 AJISAI 53 83 872 826 CHAMP 36 7 263 633 STARLETTE 46 41 901 537 STELLA 5 2 460 56 ENVISAT 109 75 545 2 666 JASON 85 67 069 2 049 LARETS 39 17 208 366 GRACE-A 18 5 359 396 GRACE-B 23 8 128 578 TERRASARX 1 81 16 ____________________________________________________________ TOTAL: 670 538 393 12 278 The obtained results were operatively sent to global data collection and processing centers in Munich and CDDIS NASA GSFC. These data serve as input material for calculation of coordinates and other parameters of the point. Complying with requirements of ILRS central office, in 2007, also full range (FR) results of measurements were sent to EDC in Munich. Measurements of groundwater level in 2007 Measurements of groundwater level 1999-2007 GRAVIMETRIC MEASUREMENTS Realization of gravimetric measurements Gravimetric point “A” In order to realize gravimetric measurements, spring type gravimeter also is necessary. At the end of 2007, ( 14 Dec- 21 Dec), gravimetric measurements with absolute gravimeter from laboratory of Finnish Geodetic Institute were made (Jaakko Makinen). Permanent observations of GPS in station “RĪGA” in 2007 RESUME CONTACTS Using results of measurements collected during long periods of time from all observatories of this type, corresponding international organizations periodically made recalculation of Global geocentric coordinate system postulating this system as fundamental in the corresponding period. At present, such is system ITRF-2005.0 with epoch 2000.0 The new global geocentric coordinate system ITRF-2005 is published in 2006. In the result of recalculation all observatories, which have participated in the above-mentioned program, receive new coordinates of its datum in corresponding system. Thus the linking of the datum to the new system is realized and linkage of the State Coordinate System to the Global Fundamental System, which was the main purpose of the work. Director of the Department of Geodesy J.Kaminskis Janis.Kaminskis@lgia.gov.lv Deputy director of the Department of Geodesy L.Lieksne Lauma.Lieksne@lgia.gov.lv Head of Technology research and analysis I.Sirmele Divison of the Department of Geodesy Ilva.Sirmele@lgia.gov.lv Dr. phys. , Leading Researcher Head of the Fundamental Geodynamical observatory, K.Lapuska Institute of Astronomy, University of Latvia riglas@lanet.lv Laser ranging device from the left: K.Lapuska, I.Abakumovs and V.Laposka This project in Latvia is realized by specialists of Institute of Astronomy of University of Latvia Dr.phys. K.Lapuska, MSc.K. Salmins, MSc.V.Laposka and MSc. I.Abakumovs.