Linking GPS to Tide Gauges and Tide Gauge Benchmarks Tilo Schöne GeoForschungsZentrum Potsdam Understanding Sea-level Rise and Variability, WCRP Workshop, UNESCO, June 2006, PARIS

Why the Vertical * Long-term stable and consistent frame to relate ALL globally distributed sea level related measurements Long-term stable and consistent frame to relate ALL globally distributed sea level related measurements Short-term control in earthquake-prone areas Short-term control in earthquake-prone areas Provide pointwise constraints for e.g. uplift modelers Provide pointwise constraints for e.g. uplift modelers * Workshop related issues

From Tide Gauges to First (known) continuous measurements in Europe by the Royal Society of London (founded in 1660) Description of a “floating tide gauge device” by Athanasius Kircher in the horologium aestus marini (around 1665) After 1880 increase in number of tide gauges … After 1980 the GPS system was developed Since 2000 increase in number of GPS equipped tide gauges

From the past to the present GAVDOS Project Mertikas et al., SPIE conference, 2002 A. Ott (Kempten) (1885)

Tide Gauges and GPS Tide gauges have a long technical and measurement history Tide gauges have a long technical and measurement history Well maintained and with a stable local network of benchmarks Well maintained and with a stable local network of benchmarks Local water level changes only, even with ties to 1st order leveling networks Local water level changes only, even with ties to 1st order leveling networks Sea level and changes required in long-term stable global reference frames Sea level and changes required in long-term stable global reference frames GPS is the easy-to-use technique to solve for this problem, but … GPS is the easy-to-use technique to solve for this problem, but …

IGS-GPS network and IGS network: Geographically balanced Some clustering in populated areas Optimized point distances Strict requirements on latency (minutes to a few days) Sites along coastlines Many clusters in Europe, Japan, USA Short GPS baselines, sometimes multiple receivers at one site Many remote and manually operated sites © IGSCB However, the IGS is the best source for GPS time series for many sites

Problems with IGS-solutions and more generally More tide gauges equipped with GPS than available within IGS (completeness) More tide gauges equipped with GPS than available within IGS (completeness) Strict timeliness in GPS data provision (latency) Strict timeliness in GPS data provision (latency) Currently, accuracy and continuity of GPS time series is limited (accuracy of the vertical) Currently, accuracy and continuity of GPS time series is limited (accuracy of the vertical) Tide gauge / GPS operators are of different agencies Tide gauge / GPS operators are of different agencies Frequent reprocessing of GPS and inclusion of high-latency data is still a challenging task Frequent reprocessing of GPS and inclusion of high-latency data is still a challenging task

GFZ-AC (IGS) versus GFZ- Reprocessing Differences due to Software developments Better correction models Improved processing strategies More complete station coverage

TIGA Tide Gauge Benchmark Monitoring Pilot Project of the IGS

TIGA Pilot Project Initiated in 2001 Initiated in 2001 Goals are Goals are –Establish, maintain and expand a global network –Compute precise station parameters for the stations with a high latency –Reprocess all previously collected GPS data at stations, if possible back to 1993 –Promote the establishment of links to other geodetic sites which may contribute to vertical motion determination (DORIS, SLR, VLBI, AG)

TIGA Pilot Project 102 out of 280 stations contribute 102 out of 280 stations contribute Processing latency 460 days Processing latency 460 days Six analysis centers provide solutions Six analysis centers provide solutions Forward (since GPS week 1121) and backward processing on a best-effort basis Forward (since GPS week 1121) and backward processing on a best-effort basis Backward processing to 1993 Backward processing to 1993 Preparations for the combination started at GFZ; will provide a combination solution on a regular basis Preparations for the combination started at GFZ; will provide a combination solution on a regular basis

CTA DGFI ULR Geoscience Australia EUREF GFZ TIGA Network TOS fully acceptedProposed

GFZ: AC versus TIGA ITRF96 ITRF97 IGS00 TIGA Start RMS North/East/Height Residuals [mm]

Comparison of TIGA Solutions Number of Points RMS North/East/Height Residuals [mm]

TIGA: Combination for 2002 < 3 mm < 7 mm Daniela Thaller, GFZ

TIGA: Combination for 2002 Daniela Thaller, GFZ

Lessons learned (+) * Station repeatability (7mm) same level as IGS Station repeatability (7mm) same level as IGS Reduced height variability for the reprocessed data Reduced height variability for the reprocessed data More consistent, no discontinuities at ITRF-changes More consistent, no discontinuities at ITRF-changes High latency of processing allows more stations to contribute High latency of processing allows more stations to contribute Massive contribution to the new ITRF Massive contribution to the new ITRF Loading effects must be evaluated in detail Loading effects must be evaluated in detail Data useful for e.g. uplift modelers Data useful for e.g. uplift modelers * Workshop related issues

Lessons learned (-) * Solution becomes outdated (e.g. absolute pcv’s), but cleaned input data are kept Solution becomes outdated (e.g. absolute pcv’s), but cleaned input data are kept No consolidated and agreed data base on station discontinuities is available No consolidated and agreed data base on station discontinuities is available Few areas are very little represented Few areas are very little represented Meta information (especially ties) are not well documented or updated Meta information (especially ties) are not well documented or updated Loading effects are affecting GPS time series different ashore and inland Loading effects are affecting GPS time series different ashore and inland Distance between GPS and TG benchmarks are to large to be representative and accurate Distance between GPS and TG benchmarks are to large to be representative and accurate * Workshop related issues

Schedule for TIGA Starting Summer 2006, GFZ will provide a first set of coordinates/velocities for all TIGA sites based on a combined solution of the recent reprocessing Starting Summer 2006, GFZ will provide a first set of coordinates/velocities for all TIGA sites based on a combined solution of the recent reprocessing Reprocessing with absolute pcv’s and based on ITRF2005 Reprocessing with absolute pcv’s and based on ITRF2005 Studies on optimal combination of TIGA solutions Studies on optimal combination of TIGA solutions Independent checks of the results and intercomparison to TG, AG (by the community) Independent checks of the results and intercomparison to TG, AG (by the community) The goal is to turn TIGA into an IGS service The goal is to turn TIGA into an IGS service It’s a good time to open your now!

What will you get from us? Coordinates and velocities (focus on the vertical) for the GPS sites at or near tide gauges (and other GPS sites) Coordinates and velocities (focus on the vertical) for the GPS sites at or near tide gauges (and other GPS sites) Reference sites Reference sites and when? End of 2006, beginning 2007 End of 2006, beginning 2007

What do we need from you? Provision of GPS data at tide gauge sites on a regular basis Provision of GPS data at tide gauge sites on a regular basis Local ties between the GPS and tide gauge benchmark Local ties between the GPS and tide gauge benchmark Maintenance of the “system” Maintenance of the “system” Feedback! Feedback!

Thanks to the six analysis centers the six analysis centers –EUREF (etg) –Geodätisches Forschungsinstitut München (dgf) –GeoForschungsZentrum Potsdam (gft) –Geoscience Australia (aut) –Universities Canberra, ANU, and Tasmania (cta) –University La Rochelle (ulr) the numerous operators of tide gauges and GPS stations the numerous operators of tide gauges and GPS stations the surveyors carrying out the leveling the surveyors carrying out the leveling the IGS for providing support and the infrastructure the IGS for providing support and the infrastructure