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

with respect to <strong>the</strong> celestial system. The EOP provides also <strong>the</strong> tie between <strong>the</strong> celestial and <strong>the</strong>terrestrial reference frames.366. The study <strong>of</strong> <strong>the</strong> EOP constitutes an excellent challenge for geophysics since <strong>the</strong>irvariations <strong>of</strong>fer important information on <strong>the</strong> inter<strong>action</strong>s between <strong>the</strong> different components <strong>of</strong> <strong>the</strong>Earth system, that is to say, <strong>the</strong> atmosphere, hydrosphere, cryosfere and solid Earth. Itsdetermination has practical applications for navigation and satellite positioning, for <strong>the</strong> study <strong>of</strong><strong>the</strong> dynamics <strong>of</strong> <strong>the</strong> Moon by means <strong>of</strong> Lunar Laser Ranging, for <strong>the</strong> observations <strong>of</strong> extragalacticradio sources using precise VLBI and for maneuvers <strong>of</strong> interplanetary probes.367. The temporal resolution <strong>of</strong> <strong>the</strong> EOP series obtained from VLBI and GPS is better thantwo hours. Using wavelets transform, temporal variations <strong>of</strong> EOP at very high frequencies withperiods between 5 and 8 hours have been detected. A resonant effect <strong>of</strong> <strong>the</strong> atmosphere could be<strong>the</strong> excitation source, but <strong>the</strong> exact origin <strong>of</strong> such effect is still under discussion.368. A new method has been developed to combine time series <strong>of</strong> Earth rotation parameters(ERP). This method is based on an Extended Kalman Filtering considering <strong>the</strong> Earth rotationphenomena as a non-linear effect. A new multi-technique was obtained combining ERP data timeseries that were in good agreement with all <strong>the</strong> combined series calculated by <strong>the</strong> InternationalServices. An advantage <strong>of</strong> this solution was that it did not require <strong>the</strong> inclusion <strong>of</strong> any extra datasuch as atmospheric angular momentum or any smoothing process.Sea Surface Topography369. Much effort has been centred on <strong>the</strong> study <strong>of</strong> some natural phenomena that could indicatethat <strong>the</strong> physical conditions <strong>of</strong> <strong>the</strong> Earth’s system are changing. The conditions, known as “globalchanges” are: wea<strong>the</strong>r, oceanic and geophysical changes, which provoke <strong>the</strong> increase <strong>of</strong> <strong>the</strong>Earth’s average temperature, variations in average temperature <strong>of</strong> <strong>the</strong> seasons in average values <strong>of</strong>regional rains. A direct consequence <strong>of</strong> <strong>the</strong>se changes could be <strong>the</strong> mean sea level (MSL)increase. For many years <strong>the</strong> MSL was assumed to remain constant and coincident with <strong>the</strong> geoid.370. Up to <strong>the</strong> time <strong>of</strong> writing this <strong>report</strong>, it was known that both hypo<strong>the</strong>sis were wrong: onone hand, <strong>the</strong> MSL vary with time due to changes in <strong>the</strong> atmospheric pressure, <strong>the</strong> temperature,and possibly due to <strong>the</strong> global change; on <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> MSL differs from <strong>the</strong> geoid due tooceanic current, and it is consequently named sea surface topography (SST). In order to providesome examples, on <strong>the</strong> East coast <strong>of</strong> South America, maximum variations in <strong>the</strong> MSL <strong>of</strong> about -6mm/year have been detected in <strong>the</strong> Caribbean and <strong>of</strong> about +10 mm/year on <strong>the</strong> Uruguayan coast;<strong>the</strong> SST having changed approximately two meters from Caribbean to Antarctica.371. The SST and its variations can be studied from satellite altimetry data (Topex-Poseidon,ERS) or from time series <strong>of</strong> tide gauge records. The combination <strong>of</strong> both techniques is <strong>the</strong> key <strong>of</strong>this kind <strong>of</strong> study. The IGS has summoned <strong>the</strong> international community to participate in <strong>the</strong> pilotproject TIGA (GPS Tide Gauge Benchmark Monitoring; http://op.gfzpotsdam.de/staff/schoene/TIGA_CfP.pdf),whose principal objective is to establish a service <strong>of</strong>GPS stations continuously operating near <strong>the</strong> tide gauges and to detect systematic effects throughanalysis <strong>of</strong> <strong>the</strong>ir observations. These results should enable scientists to obtain reliable informationabout <strong>the</strong> vertical motion <strong>of</strong> <strong>the</strong> Earth’s crust.372. In order to decouple from <strong>the</strong> tide gauge records <strong>the</strong> movements <strong>of</strong> <strong>the</strong> Earth’s crust, fourpermanent GPS stations (PS) (from 1998) have been installed near <strong>the</strong> tide gauge <strong>of</strong> Mar del65