Geodesy at a Tide Gauge Where the Tide Gauge Operator meets the Surveyor Tilo Schöne GeoForschungsZentrum Potsdam Phil Woodworth (PSMSL) & Simon Williams (POL) Correspondence: Tilo Schöne

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (0/6) “For sea level observations, a land benchmark is used as the primary reference point” (IOC Manuals and Guides No. 14, vol. IV) “TGBM’s should be connected to the International Terrestrial Reference Frame (ITRF) and monitored using GPS” (Carter et al. 1989)

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (1/6) Why and How to measure and monitor the sea level was addressed already and you are the specialists Why and how to geodetically fix the tide gauge benchmark(s) will be address in this lesson

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (2/6) Why Geodesy at Tide Gauges? –Height reference for national height systems –Reference for local mapping –Long-term consistency for law-of-the- sea (coastal/harbor navigation, claims of the continental shelf) –Scientific analyses (e.g. loading) –Predict risk of coastal flooding –…

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (3/6) Tide Gauges give values in reference to their “technical” zero Reference points (pole staff, benchmarks) are established to ensure consistency over time (e.g. replacement) For scientific application a common (and global) reference frame is needed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (4/6) Relative and “absolute” sea level –e.g. Fenoscandian uplift (PGR: +1cm/year), i.e. relative “sea level fall” is observed –e.g. German Coast subsides (PGR: -~1mm/year), i.e. relative “sea level rise” Courtesy: G. Liebsch, TU Dresden

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (5/6) Models (e.g. PGR) are insufficient to correct for local long-term vertical changes –Geological records are spanning thousand of years –Local effects may mask the regional values –Fast processes (e.g. earth quakes) and recent changes (e.g. reservoir changes) cannot be modeled

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Introduction (6/6) Neither the Sea Level nor the Land Surface are constant or fixed over short or long-term co-seismic displacements Vigny: doi: /nature03937 RLR series provided by PSMSL GPS Sea Level ?

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Summary (Introduction) Fast and efficient methods are needed for height control and monitoring of the tide gauges –Should be long-term available –Easy to operate –Cost effective –Benefitary to other fields –Should be operated by a service

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Geodetic Methods Tide Gauge Courtesy: J. Müller h Geoid Altimetry Gravity Absolute Gravity InSAR GPS Leveling

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Methods for Height Determination Space Geodetic Techniques * –GPS, (GLONASS, GALILEO) –DORIS –(SLR, VLBI) Absolute Gravity Spirit Leveling *Stable Reference / Reference Frame is needed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Microwave Techniques: Global Positioning System (GPS): USA Global Navigation Satellite System (GLONASS): Russian Pendant GALILEO: future European satellite navigation system DORIS: French doppler satellite tracking system DORIS Space Geodetic Techniques GPSGLONASSGALILEO

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov VLBI (Very Long Baseline Interferometry): observation of quasars with large radio telescopes SLR (Satellite Laser Ranging) and LLR (Lunar Laser Ranging): observation of special Earth satellites and the Moon with short laser pulses Space Geodetic Techniques LAGEOS QUASAR VLBI SLR

Stable Reference Frame (1/4) Terrestrial Reference Frames Enables measurements to be related over time One of the biggest source of errors in quantifying long-term variation in sea level 2 mm/yr origin error (Center of mass)  0.4 mm/yr in global sea level from satellite altimetry 0.1 ppb/yr ( /yr) scale error  0.6 mm/yr global sea level rate

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Stable Reference Frame (2/4) Every point on Earth is subject to vertical movements by e.g. –Post-glacial rebound –Subsidence, post-seismic effects –natural gas extraction, ground water usage, rebuilding harbors –Loading effects (atmosphere, tides)

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Stable Reference Frame (3/4) Reference is established by a series of discrete points, itself subject to –Benchmark instabilities –Measurement insufficiencies –Equipment changes –Inaccurate local ties (!)

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Stable Reference Frame (4/4) IERS combines, updates and publishes ITRF coordinates as their main product Recently the ITRF2005 became officially ITRF is a combination of VLBI, SLR and GPS on a high level of confidence

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Methods for Height Determination Space Geodetic Techniques * –GPS, (GLONASS, GALILEO) –DORIS –(SLR, VLBI) Absolute Gravity Spirit Leveling * *Stable Reference / Reference Frame is needed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS (Introduction) Technique has been established as a scientific tool since around 1993 GPS is now the most common technique for point monitoring Scientific organization (IGS/GGOS/IAG) is providing a high quality service (provision of orbits, clocks, point coordinates and velocities) (see GPS is a multi-purpose sensor

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Radar TG GPS with Choke Ring Antenna Solar Power Satellite Communication Meteo-Sensor Floating TG and Pressure TG are complementing the setup

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS inside 2-Frequency Geodetic Receiver Sampling 1Hz to 30sec

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov 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

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GFZ-AC (IGS) versus GFZ- Reprocessing Differences due to Software developments Better correction models Improved processing strategies More complete station coverage

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov TIGA Tide Gauge Benchmark Monitoring Pilot Project of the IGS op.gfz-potsdam.de/tiga

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov TIGA Pilot Project Initiated in 2001 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)

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

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov CTA DGFI ULR Geoscience Australia EUREF GFZ TIGA Network TOS fully acceptedProposed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS: Recommendations Each long-term and reliable tide gauge should be equipped with continuous GPS If (for practical or budgetary reasons) impossible, establish a pillar for regularly repeated GPS campaigns Single/Dual receiver concept –Primary GPS + additionally GPS marker within less than 10 to 50 m Best, on top of the system (even if obstructions) Continuously operating –Secondary GPS on stable ground Continuously operating

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS: Recommendations Dual frequency receivers and Choke Ring Antenna 30 Sec sampling (1Hz if hazardous area) Short latency for data transmission No obstructions around, use safe places, and no radio interference Prevent multipath, e.g. from roofs, walls or the pillar itself Leveling benchmark at the GPS pillar and regular leveling to other benchmarks

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS: Recommendations Supply meta data (tide gauge data, meteorological data, equipment changes, unusual impacts, reconstruction works) For guidelines how to build reliable GPS pillars refer to the IGS web page GPS does NOT “solve everything for nothing”. Keep your first-order leveling capability, before you retire -> teach!

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Methods for Height Determination Space Geodetic Techniques * –GPS, (GLONASS, GALILEO) –DORIS –(SLR, VLBI) Absolute Gravity Spirit Leveling * *Stable Reference / Reference Frame is needed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Absolute Gravity: Introduction The “Carter reports” on sea level monitoring recommended the use of Absolute Gravity (AG) as a complimentary technique to CGPS for VLM monitoring. Results from AG measurements at UK tide gauges are presented

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Absolute Gravity G = Gravitational Constant = 6.67x m 3 kg -1 s -2 M e = Mass of the Earth R e = Radius of the Earth How do we Measure “ g ”? Using free fall methods. Measure x and t of a mass in free fall (in a vacuum) and use the above equation to get g motion ofequation the gives timerespect to with twiceintegrating gttvxx g x  ..

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Absolute Gravity This machine has a precision and accuracy of about 1-2  gal (1  gal =10 -8 ms -2 ) (1  gal  5mm vertical motion)

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Absolute Gravity So how does measuring g give us changes in vertical land movement? This assumes there is no additional mass : “Free Air Model” If additional mass is involved then This is the “Bouger Model” mper ms Differentiate the basic equation with respect to distance       e R g R GM g R g R ee e e e  

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov. 2006

Methodology Measure for at least 3 days at each site, at least once per year (hopefully!) Data from each day are processed separately and correction made for solid-earth tides, ocean-loading effects, atmospheric pressure, polar motion and comparator response. Single admittance factor and local pressure data are used to correct for atmospheric pressure. Gravity gradients for Newlyn and Aberdeen determined using a relative spring gravimeter

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Results

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov GPS versus Absolute Gravity and Vertical Land Movement

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Absolute Gravity: Conclusions AG measurement spanning 5-6 years can be used to measure vertical land movements at tide gauges. Absolute Gravity is a useful complimentary technique to CGPS. Good site selection is essential to AG measurement success. Given a few more years of measurement, results can be used to test competing GIA models.

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Methods for Height determination Space Geodetic Techniques * –GPS, (GLONASS, GALILEO) –DORIS –(SLR, VLBI) Absolute Gravity Spirit Leveling * Stable Reference / Reference Frame is needed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Leveling (Introduction) Leveling is needed for local surveys (stability, re-installation, equipment changes) Time consuming and expensive Only for short distances Sub-mm accuracy achievable Local set of benchmarks is already available (TG operator duties)

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Leveling (Technique) Addressed later

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Leveling (Benchmarks) Brass or aluminium casting Set in bedrock Easy to access Well documented (meta data) Should be made public to scientists Additional markers if unstable or destroyed

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Leveling (Recommendations) Preferable 5+ leveling markers on stable ground within 5 to 500 m Connection to the GPS benchmark Re-leveling at least annually (more frequent if geological situation is instable), Documentation (!) For extended surveys use first order leveling (0,2mm/km) For points more that 1km away achieving the required accuracy is unlikely !!

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Some References Carter, W.E., Aubrey, D.G., Baker, T.F., Boucher, C., Le Provost, C., Pugh, D.T., Peltier, W.R., Zumberge, M., Rapp, R.H., Shutz, R.E., Emery, K.O. and Enfield, D.B Geodetic fixing of tide gauge benchmarks. Woods Hole Oceanographic Institution Technical Report, WHOI-89-31, 44pp. Carter, W.E. (ed.) Report of the Surrey Workshop of the IAPSO Tide Gauge Bench Mark Fixing Committee. Report of a meeting held December 1993 at the Institute of Oceanographic Sciences Deacon Laboratory. NOAA Technical Report NOSOES pp. Neilan, R., Van Scoy, P.A. and Woodworth, P.L. (eds) Proceedings of the Workshop on methods for monitoring sea level: GPS and tide gauge benchmark monitoring, GPS altimeter calibration Workshop organized by the IGS and PSMSL, Jet Propulsion Laboratory, March pp. Bevis, M., Scherer, W. and Merrifield, M Technical issues and recommendations related to the installation of continuous GPS stations at tide gauges. Marine Geodesy, 25,

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium Nov Some Web sites op.gfz-potsdam.de/tiga soest.hawaii.edu/cgps_tg igscb.jpl.nasa.gov