report of the action team on global navigation satellite systems (gnss)

Plata, Puerto Belgrano, Puerto Madryn y Puerto Deseado. From <strong>the</strong>se data <strong>the</strong> first verticalvelocity estimation <strong>of</strong> Earth’s crust in some <strong>of</strong> <strong>the</strong> tide gauge involved was computed.Ionosphere373. The ionosphere is <strong>the</strong> part <strong>of</strong> <strong>the</strong> upper atmosphere where <strong>the</strong> free electron density is highenough to disturb <strong>the</strong> propagation <strong>of</strong> radio frequency electromagnetic waves. Free electrons aremainly produced by <strong>the</strong> photoionisation <strong>of</strong> neutral atoms and molecules <strong>of</strong> <strong>the</strong> atmosphere evokedby <strong>the</strong> UV solar radiation. However, many complex physical phenomena in <strong>the</strong> solar-terrestrialenvironment participate in <strong>the</strong> production and loosing <strong>of</strong> electrons and in conditioning <strong>the</strong>irspatial distribution and temporal variations.374. The first studies that made use <strong>of</strong> GPS observations to determine empirical ionosphericmodels can be traced back to <strong>the</strong> late 1980s. In that connection, GPS can be presently considered<strong>the</strong> most powerful means to study <strong>the</strong> ionospheric wea<strong>the</strong>r. Even if <strong>the</strong> ionospheric research hasnot been <strong>the</strong> objective <strong>of</strong> <strong>the</strong> IGS, one <strong>of</strong> <strong>the</strong> IGS products provides a real topography <strong>of</strong> thismedium. The IGS maps <strong>of</strong> total electron content are <strong>of</strong> high quality and with a good globalcoverage.375. An ionospheric model has been developed to analyse <strong>the</strong> total electron content from GPSobservations and has been used to study <strong>the</strong> geographic and time variability <strong>of</strong> this parameter atregional and global scales, in quiet and perturbed geomagnetic conditions. A three-dimensionaltopography <strong>of</strong> <strong>the</strong> electron density has also been developed using GPS ground and spacereceivers.Innovation, development and transference376. In <strong>the</strong> last decade, <strong>the</strong> Argentinean geodetic reference frame has substantially evolved,despite <strong>the</strong> difficulties <strong>of</strong> developing geodetic infrastructures in parallel by <strong>the</strong> individualprovinces and <strong>the</strong> nation. At <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> 1990s, those activities were decentralised by <strong>the</strong>creation <strong>of</strong> <strong>the</strong> program called “Programa de Saneamiento Financiero y Desarrollo Económico delas Provincias Argentinas”, a program for financial monitoring and economic development <strong>of</strong> <strong>the</strong>Argentinean provinces, supported by international credit allotted to <strong>the</strong> provinces. During <strong>the</strong> lastdecade, <strong>the</strong> Ordnance Survey Offices from different provinces had invested a large amount <strong>of</strong>money to establish traditional geodetic networks, with <strong>the</strong> purpose <strong>of</strong> eliminating <strong>the</strong> problemsrelated to land registry.377. Until <strong>the</strong> mid 1990s, <strong>the</strong> geologic, geodetic and geodesic surveys and <strong>the</strong> generalcartography <strong>of</strong> Argentina were linked to <strong>the</strong> traditional geodetic network called Campo Inchauspe69. The joint effort <strong>of</strong> IGM and GESA contributed to a new national geodetic network based onGPS measurements, named POSGAR 94 (Posiciones Geodésicas Argentinas de 1994). This newnetwork is ten times more precise and a hundred times more accurate than <strong>the</strong> previous one; itwas <strong>of</strong>ficially adopted in Argentina as <strong>the</strong> geodetic and cartographic standard (Disposición 13/97del IGM).378. The IGM jointly with GESA improved <strong>the</strong> POSGAR 94 network, by linking it toSIRGAS (Sistema de Referencia Geocéntrico para las Américas) and to <strong>the</strong> international system,ITRF. GESA was responsible for <strong>the</strong> calculation <strong>of</strong> this improved network, POSGAR 98.66