Tide gauge registrations show the sum of both: sea level variations and vertical movements of the Earth`s crust where the instruments are embedded. In order to estimate reliable secular sea level changes, the effects of vertical land motions must be determined and reduced from the tide gauge measurements. With this purpose, the International GNSS Service (IGS) established the TIGA project (GPS Tide Gauge Benchmark Monitoring Pilot Project) in 2001 and converted in IGS Working Group in 2010. DGFI contributes to this Working Group operating TIGA observing stations and working as TIGA Analysis Centre. In cooperation with local authorities, several continuously observing GPS stations at (or close to) tide gauges around the Atlantic were installed and put into operation. The GPS data at these sites are acquired and provided to TIGA and the international GNSS community. The processing carried out in DGFI is based on the double difference approach (using the Bernese Software) and addresses in particular the accuracy of the height component.
Until now, DGFI processed a network distributed along the Atlantic, including some IGS fiducial stations to refer station positions and velocities to the geocentric reference frame ITRF. With the objective to support the determination of a global vertical reference frame (see Vertical Reference Systems), DGFI decided to extend its computations to a global network including about 180 stations. To evaluate the long term stability of the velocity estimates, daily normal equations between June 1996 and December 2010 are being computed using the IGS reprocessed products (IG1) and absolute phase centre corrections. The main analysis is oriented
- to determine sea level changes with respect to the geocentric reference system (ITRS/ITRF), and
- to compare time series derived from GNSS positioning with those obtained from the combined analysis of satellite altimetry data and tide gauge registrations.