G33A-0664
The Global GNSS, SLR, VLBI, and DORIS Networks and their Support of GGOS: IGS+ ILRS+IVS+IDS
The global networks of the International GNSS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International DORIS Service (IDS) are part of the ground-based infrastructure for GGOS. The observations obtained from these global networks provide for the determination and maintenance of the International Terrestrial Reference Frame (ITRF), an accurate set of positions and velocities that provides a stable coordinate system allowing scientists to link measurements over space and time. Many of these sites offer co-location of two or more techniques. Co-location provides integration of technique-specific networks into the ITRF as well as an assessment/validation of the quality and accuracy of the resulting measurements. As of fall 2008, these networks consisted 410 GNSS sites, of 42 laser ranging sites, 45 VLBI sites, and 58 DORIS sites. This poster will illustrate the global coverage of these networks, highlighting inter-technique co-locations, and show the importance of these networks to the underlying goals of GGOS including providing the observational basis to maintain a stable, accurate, global reference frame.
G33A-0665
Report of Informal Working Group on Measuring and Monitoring Site Ties
Precise measurement and maintenance of the Terrestrial Reference Frame (TRF) is essential to long-term monitoring of global change. The TRF is realized through the integration of high precision GNSS, SLR, VLBI and DORIS networks. Each of these techniques has unique strengths and weaknesses. Station positions and velocities from the different techniques are combined to obtain the most accurate and robust TRF. An essential ingredient in this combination is the local eccentricity vectors at collocation sites. Hence accurate measurement of this vector is very important. At a few sites different techniques give different results for site position evolution. Determining the cause of this requires re-measuring the eccentricity vectors. The problem is further complicated because the "invariant point" for most techniques cannot be measured directly, and must be inferred. In September 2008 an informal working group with representatives from most of the space geodesy techniques looked at the application of new surveying and measuring technologies to improve the measurement and the monitoring of local ties. We looked at factors such as ease of use, ability to automate, and cost. We also studied issues such as site layout and monumentation. We report on our conclusions from this workshop and plans for future work. One goal is to perform a proof of concept experiment in the near future. We welcome input from the global geodetic community.
G33A-0666
Work out of original basis for a satellite geodetic network of Uzbekistan
Central Asia region and, particularly, the territory of Uzbekistan are non-uniformly provided high accuracy geodetic networks because there are high mountains and undulating grounds. An establishment of the Fiducial Geodetic network, based mainly on the use of space geodesy technique is a principal objective of the national program carried out in Uzbekistan. The objective is to provide a terrestrial reference frame in this region, and to help answering various question about earth deformation on one hand and hydrology on the other hand. The current investigation focuses on the initial tasks of construction of the local geodetic network, mainly constructing of the reference baseline for Uzbekistan geodetic network. The combined astronomical, geological and space results of two astronomical stations Kitab and Tashkent are used to creat geodynamical model of reference points. In this study, effort has also been made to estimate regional deformation of Tavaksay area based on the model.
G33A-0667
Simulation Analysis of the Geodetic Performance of the Future IVS VLBI2010 System
To support the design of a new geodetic Very Long Baseline Interferometry (VLBI) VLBI2010 system, the IVS has investigated the geodetic performance of a future network of VLBI2010 antennas. Monte Carlo simulations were carried out at the Goddard Space Flight Center with Calc/Solve and at the Institute of Geodesy and Geophysics (IGG), Vienna with the OCCAM Kalman filter and with precise point positioning (PPP) simulation software. The input simulated group delay noise consists of troposphere, clock, and delay measurement noise contributions. The clocks are simulated as a random walk plus integrated random walk, the wet slant path delays are generated with a turbulence model, and the measurement errors are represented by white noise. We studied four different strategies for improving geodetic performance: 1) optimized observing schedules, 2) larger number of observations due to faster antenna slewing rates, 3) improved analysis strategies, and 4) larger antenna networks. Specifically, we investigated different parameterizations of the wet delay, including gradients, spherical harmonics, shorter atmospheric estimation intervals, station-dependent correlated noise, and elevation angle weighting. Since one of the critical design specifications for the VLBI2010 antennas is slew rate, we examined the improvement of baseline length and station position repeatabilities on increasing antenna slew rates. We are also investigating the improvement of the reference frame scale and EOP resulting from increasing network size.
G33A-0668
VLBI2010 Demonstrator Project
The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in
twenty-four hours. Knowing that spatial and temporal fluctuations in the atmosphere delay are a major
component of the error in position determination, the VLBI2010 committee has carried out a large number of
simulations to arrive at design goals for the antenna system. These goals are fast slewing antennas and
high delay precision per observation. With existing and anticipated data recording capabilities, these
translate to an antenna diameter of 12 m or larger and a per-observation delay precision of approximately 4
psec.
The major innovation for the VLBI2010 concept that allows the use of relatively small antennas to achieve
these goals is the proposal to observe in four frequency bands, instead of the two currently used, in order to
gain the higher precision of phase delays compared to the group delay. The other advance that enables the
use of small antennas is the significant increase in data acquisition rates that has been made possible by the
development of disk-based recorders and digital back ends. To evaluate this concept, a prototype of the
feed-to-recorder system has been implemented by the Broadband Development Team* on two
antennas, the 5 m MV-3 antenna at Goddard Space Flight Center near Washington, D.C., and the 18 m
Westford antenna at Haystack Observatory near Boston. The system includes a broadband feed and low
noise amplifiers covering the range approximately 2 GHz to 13 GHz, all cooled to 20K; a newly developed
phase calibration generator; a flexible local oscillator (LO) that allows selection of any band in the range of
the feed/LNAs; Digital Back End; and a disk-based recorder capable of a sustained rate of 2 gigabits per
second (gbps). Four sets of the LO/DBE/recorder chain are used at each antenna to give a total record rate
of 8 gbps.
The systems have been successfully used in the band 8.5 to 9 GHz with one set of the recorder chain.
Observations demonstrating the full four-band configuration are planned for October. In this talk the results
of these tests, the improvements that are anticipated for the operational VLBI2010 network, and the status of
other developments in the next generation of geodetic VLBI systems will be presented.
* Bruce Whittier, Mike Titus, Jason SooHoo, Dan Smythe, Alan Rogers, Jay Redmond, Mike Poirier,
Chuck Kodak, Alan Hinton, Ed Himwich, Skip Gordon, Mark Evangelista, Irv Diegel, Brian Corey, Tom Clark,
Chris Beaudoin (in reverse alphabetical order)
G33A-0669
Construction of a New Geodetic VLBI Station in Korea
In Korea, KVN (Korean VLBI Network) project dedicated to radio astronomy is being promoted by Korea Astronomy and Space Science Institute (KASI), and three 21-m diameter antennas equipped with 22/43/86/129 GHz receivers have been already constructed at Seoul, Ulsan, and Jeju. On the other hand NGII (National Geographic Information Institute, Korea) has planned to construct their own antenna dedicated to geodetic VLBI measurements to maintain the Korean Geodetic Datum accurately. It also aims at a fundamental station in East Asia and will contribute to the better definition of the ITRF there. Grand design named KVG (Korea VLBI project for Geodesy) realizing NGII's plan has been proposed by Ajou University with collaborations from NICT (National Institute of Information and Communications Technology, Japan) and GSI (Geographical Survey Institute, Japan). KVG project got a national budget for construction formally this year (2008), and it has entered the three-year term of construction and development phase since October, 2008. A VLBI antenna will be constructed in Sejong city (about 120km south-southeast from Seoul) and its construction will be completed in 2011. The antenna is designed based on the VLBI2010 except for receiving frequencies and the diameter where VLBI2010 is the guideline for next generation's geodetic VLBI system compiled by IVS (International VLBI Service for Geodesy and Astrometry). Newly constructed antenna can receive 2/8/22/43 GHz band simultaneously in order to carry out geodetic VLBI observations not only with current geodetic VLBI stations equipped with 2/8 GHz receivers but also with KVN stations equipped with 22/43 GHz receivers. The antenna is also designed to be able to introduce a broadband feed and receivers in the future according to VLBI2010's suggestion. The diameter of antenna is chosen to be 22-m, which is larger than the VLBI2010's recommendation (13-m), to make astronomical VLBI observations of weak radio sources with KVN antennas. This diameter also enables us to conduct geodetic VLBI with a small mobile antenna in future. High sensitivity observations using such a larger antenna will contribute to the improvement of ICRF.
G33A-0670
Height signals determined from SLR, absolute gravity and GPS observations at the Space Geodesy Facility, Herstmonceux, UK
In the framework of the Global Geodetic Observing System, geodetic sites offering multi-technique capabilities are becoming increasingly important. Maintenance of the terrestrial reference frame, for example, benefits greatly from SLR, GNSS, DORIS and VLBI observations co-located and precisely linked via accurate site ties. Of prime importance is the generation of accurate, bias-free observations and a knowledge of the stability of the sites, since any local un-modelled deformation will directly influence the integrity of the global reference frame. The UK Space Geodesy Facility operates a very precise, long-established Satellite Laser Ranging (SLR) station at Herstmonceux in southern England. It also operates two GNSS receivers, one of which has contributed to IGS for over 14 years and, since late 2006, an Absolute Gravimeter (AG) which provides a weekly time-series. The local water table is monitored at high rate via a borehole in close proximity to both the SLR and the gravimeter room and precise temperature, pressure and humidity devices operate autonomously. In this poster we investigate time series of local height variations determined by analyses of the independent techniques of SLR, AG and GPS since July 2006. The geodetic results suggest annual vertical variations of amplitude about fifteen millimeters, whereas the absolute gravity values show very little power at annual frequency but systematic variations of order four microgals. Of particular interest is the hydrological signals in the gravimeter data and we will aim to use the space geodetic results to separate them from local vertical signals.
G33A-0671
Vertical Crustal Motion of Taiwan Determined from Tide Gauge and Altimeter Data
Taiwan is located at the western bordure of the convergent boundary between the Philippine Sea plate and Eurasian plate. Processings of plate collision and mountain building is obvious and occur uncountable earthquakes here. In this study, we applied the method of comparing the tide gauge (TG) and altimeter (ALT) data to resolve land vertical motion at 20 TG sites along Taiwan coast. Sea surface height measured by altimetry is related to an absolute reference frame, while TG is related to the coast where they are attached on land. The difference of time series Alt(t)-TG(t) contains information about vertical crustal motion. This method has been used to many applications, such as the global image of vertical ground motion [Cazenave et al., 1999; Nerem and Mitchum, 2002], the tectonic activity around Mediterranean [Garcia et al., 2007], and glacial isostatic rebound occurred in Scandinavian Peninsula, Great Lakes region, and Alaska [Kuo, 2004], etc. In this study, the tide gauge records are adopted from the Marine database of Central Weather Bureau, and altimetry data are the Geophysical Data Record (GDR) which conducted from the tracks of TOPEX/Poseidon(1992 - 2002) and Jason-1 (2002 - Present), charging from JPL¡¦s Physical Oceanography Distributed Active Archive Center (PO.DAAC). We calculate the linear trend of the ALT-TG record for each of the TG locations. Seasonal signals, primarily periodic tides are removed from both TG and ALT data as the beforehand treatment for linear regression. We can therefore figure out a whole image of vertical motion along the Taiwan coast. The majority of these movements tend to descend in the latest decade. However, some TG stations reveal an extreme large uplift rate which may be affected by local effects. Sites in the eastern Taiwan generally have a smaller descending rate in comparison to the western sites. Furthermore, our estimations in the southwestern Taiwan show a clear land subsidence from -6.2mm/yr to -34.3mm/yr. Another focus of our study is to determine the coseismic motion in the TG sequence. Step-function with least- square method is used to measure the magnitude of transient motion probably caused by earthquakes neighboring to TG station. In terms of geometrical spreading of seismic energy, the determined coseismic transient motion should be function of hypocentral distance and earthquake size. A forward rupture modeling is also taken to predict coseismic vertical offset with the preset parameters of focal mechanism solution. Further application of the determined vertical offset from TG sequence can help to discuss the coseismic movement of historical earthquakes, which occurred in the period prior to the modern rapid response of coseismic measurement.
G33A-0672
Hydrology and Noise Affecting Land-Based Gravity Measurements
To observe secular land movements of the order of a few millimetres per year, a very precise instrument with long-term stability is required. This can be achieved using absolute gravimeters. Repeated absolute gravity (AG) measurements have now been performed at several sites to estimate tectonic deformation. It is often assumed that these time series contain only white noise. However, many geodetic data sets suffer from large temporal correlations into the data. One common statistical model for many types of geophysical signal (which may contribute to the noise) maybe described as a power-law process. Accounting for the type of noise is not only very important when estimating gravity variations and their uncertainties, but also to mitigate this noise. Generally speaking AG measurements still suffer from a sparse spatial and temporal resolution, and are not yet available in data centres. To better understand the structure of the noise affecting ground- based gravity measurement at different locations, we take profit of 19 superconducting gravimeter (SG) continuous time series provided by the GGP data centre. Presently, the quality of hydrological models is not sufficient to correct the ground-based gravity time series, but it is possible to study the spectral behavior of the hydrological effects down to the decadal time scale. Such study allows one to test whether the colored noise of the gravity signal can be attributed to hydrological causes. The spectra of the SG time series are compared with the predicted gravity effect calculated using water storage estimates from the Milly and Shmakin Land-Energy balance (LaD) model (1° X 1°, monthly). For most of the GGP stations, we evidence a good agreement for periods longer than 100 days, though they are located in quite different climatic regimes.
G33A-0673
Recent variation in the Earth Dynamic Oblateness, J2, from SLR data
Satellite Laser Ranging (SLR) data tracked by the ILRS network have recorded the global nature of long- wavelength mass redistribution occurring within the Earth system for more than three decades. Studying the variations in J2 has provided a clear vision of the large-scale mass redistribution with a long-term signature within the Earth system from analysis of SLR data. Early analysis of 28-year time series of monthly SLR estimates of J2 [Cheng and Tapley, 2004] has indicated that in addition to the secular, 18.6 year tidal and seasonal variations, the J2 has undergone significant interannual variations with time scales of ~4-6 years and decadal variation with a period of ~21 years. Two large interannual variations are related to the strong El Niño-Southern Oscillation (ENSO) events during the periods of 1986-1991 and 1996-2002. Recent analysis including an additional five years of data suggests that the Earth has undergone another 3 fluctuations cycles starting from middle of 2002 and variation with a period of~11 years. A Because of the significant aliasing effects in the GRACE data derived J2 coefficient and the J2 variations from SLR data is the most accurate measurement for the application of GRACE product to extract the signal of mass variations in ocean and hydrological. This paper presents detail analysis for the variations in J2 from analysis of multiple geodetic satellites over the period from 1976 to 2008, and a comparison with the monthly solutions from GRACE measurements.
G33A-0674
Global Marine Gravity and Bathymetry at 1-Minute Resolution
We have developed global gravity and bathymetry grids at 1-minute resolution. Three approaches are used
to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw
waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40%
and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a
reference field to provide a seamless gravity transition from land to ocean. Third we have used a biharmonic
spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard
gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in
areas with rugged seafloor topography. The largest errors occur on the crests of narrow large seamounts.
The bathymetry grid is based on prediction from satellite gravity and available ship soundings. Global
soundings were assembled from a wide variety of sources including NGDC/GEODAS, NOAA Coastal Relief,
CCOM, IFREMER, JAMSTEC, NSF Polar Programs, UKHO, LDEO, HIG, SIO and numerous miscellaneous
contributions. The National Geospatial-intelligence Agency and other volunteering hydrographic offices
within the International Hydrographic Organization provided global significant shallow water (< 300 m)
soundings derived from their nautical charts. All soundings were converted to a common format and were
hand-edited in relation to a smooth bathymetric model. Land elevations and shoreline location are based on
a combination SRTM30, GTOPO30, and ICESAT data. A new feature of the bathymetry grid is a matching
grid of source identification number that enables one to establish the origin of the depth estimate in each grid
cell. Both the gravity and bathymetry grids are freely available.
http://topex.ucsd.edu
G33A-0675
Processing Marine Gravity Data Around Korea
In Korea currently 4 research ships are under operating in Korea, after the first research vessel equipped shipborne gravity meter was introduced in 1990s. These are Onnuri(launch 1991) of KORDI(Korea Ocean Research & Development Institute), Haeyang2000(launch 1996), Badaro1(launch 2002) of NORI(National Oceanographic Research Institute) and Tamhae2(launch 1997) of KIGAM(Korea Institute of Geoscience and Mineral Resources). Those of research vessel, Haeyang2000 have observed marine gravity data over 150,000 points each year from year 1996 to year 2003. Haeyang2000, about 2,500 tons, is unable to operate onshore so NORI has constructed another 600 tons research ship Badaro1 that has observed marine gravity data onshore since year 2002. Haeyang2000 finished observing marine gravity data offshore within Korean territorial waters until year 2003. Currently Badaro1 is observing marine gravity data onshore. These shipborne gravity data will be very useful and important on geodesy and geophysics research also those data can make a contribution to developing these studies. In this study NORI's shipbrne gravity data from 1996 to 2007 has been processed for fundamental data to compute Korean precise geoid. Marine gravity processing steps as followed. 1. Check the time sequence, latitude and longitude position, etc. of shipborne gravity data 2. Arrangement of the tide level below the pier and meter drift correction of each cruise. 3. Elimination of turning points. 4. The time lag correction. 5. Computation of RV's velocities, Heading angles and the Eötvös correction. 6. Kalman filtering of GPS navigation data using cross-over points. 7. Cross-over correction using least square adjustment. About 2,058,000 points have been processed with NORI's marine gravity data from 1996 to 2007 in this study. The distribution of free-air anomalies was -41.0 mgal to 136.0 mgal(mean 8.90mgal) within Korean territorial waters. The free-air anomalies processed with the marine gravity data are well matched with the submarine topography as the terrestrial free-air anomalies are closely related with inland topography. In the case of East Sea, the free-air anomalies is low in high depth comparing with other area, whereas the characteristics of shallow sea are shown well on free-air anomalies in depth around 100m, in Yellow Sea. The free-air anomalies are relatively changed in the same manner that submarine topography undulates like the inland free-air anomalies changing.
G33A-0676
Seafloor geodetic reference station branched from submarine cable
We launched a project supported by the Japan Society for the Science Promotion as the Grants in Aid for Scientific Research. In this project, we are aiming at developing new-generation seafloor geodetic observation system that conquers difficulties inherent with the current system. Central idea of this project is to utilize techniques of underwater robot (Autonomous Underwater Vehicle) and submarine cable to make measurements in place of using the research vessels. Combination of underwater robot and submarine cable make it possible to provide permanent seafloor reference point, to conduct the observation with selecting favorable condition of sea and GPS satellite distributions, to make much more frequent observations and to enable flexible planning of observation in response to sudden geodetic events. Prototype of the on-board system which should be installed on an AUV was finished. Several trials had been done with the system in the sea. The results from them showed that the new on-board system will reach to the higher level in performance than the current system in the near future. And then we started to dedicate ourselves mainly to developing new seafloor transponder. The current seafloor transponder system is stand-alone one which runs on internal batteries. We expect five to ten years for the lifetime of the current seafloor transponder, even though it depends on how often we perform measurements with the transponder. Replacement of the seafloor transponder will be needed when we target seafloor crustal deformation that has long time cycle more than several decades. Continuity of seafloor geodetic observation will be stopped. New seafloor transponder which we have been developing is one which can be connected to a submarine cable by wet-mate connectors. Power is supplied through submarine cable and then the new seafloor transponder will be a permanent reference station for seafloor geodetic survey. Submarine cable can supply accurate GPS time (1pps) and clock to the transponder as well as power. The new cable transponder can realize acoustic ranging between sea surface and bottom with much higher accuracy than the current system. Submarine cable system off Toyohashi (Tokai-SCANNER) in central Japan is located on the source region of the huge repeated earthquakes. It is under the control of the JAMSTEC and used for long-term geophysical monitoring. This cable is one to which we are planning to connect newly developed seafloor transponder. The cruise of JAMSTECfS ROV "KAIKO 7000 II" and R/V "KAIREI" is scheduled to deploy and connect the seafloor transponder to the Tokai-SCANNER cable system from Sep. 18, 2008. We will report the overview of the cable transponder system and its deployment procedure in this presentation.
G33A-0677
Continuous Seafloor Vertical Deformation Measurements: a Prototype Pressure Reference System for Correcting Instrumental Drift in Geodetic Pressure Gauges
Resonant quartz gauges measuring ambient seawater pressure are used to observe vertical deformation on submarine volcanoes, with sub-cm resolution. However, instrument drift can be of the same order as deformation between eruptive events, of order 20-30 cm per year. Continuous measurements are thus limited to events of relatively short duration. One possible solution for geodetic pressure gauge drift is to periodically measure a constant reference pressure. This signal can be generated with an automated piston gauge calibrator, in which a mass is mounted on a precisely machined piston in a pressurized cylinder; the pressure is increased until the mass-piston assembly floats freely in the cylinder. At this point, the weight of the mass-piston assembly equals the pressure multiplied by the area of the piston. A laboratory prototype has been constructed and tested. An autonomous run over 50 days duration demonstrated a standard deviation of the generated reference pressures of 0.52 mbar, equivalent to a depth uncertainty of 5 mm. Planning and design efforts are ongoing for a more refined and seafloor ready instrument for deployment on Axial Volcano on the Juan de Fuca ridge. This instrument would be capable of multi-year autonomous deployments and would support on-going continuous and survey-mode vertical deformation measurements.
G33A-0678
Development of the Acoustic Ranging System Toward the Cable Network System
We have developed a short-range seafloor ranging system as a possible future application to the DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) cable system. Direct acoustic ranging is a simple way to monitor local crustal deformation, and various groups have successfully detected deformation on the seafloor using such a system. We aim to monitor the splay faults in the rupture area of the Tonankai earthquake in Nankai subduction zone (Park et al, 2002). Slip along the active splay faults may be an important mechanism that accommodates the elastic strain caused by relative plate motion. We plan to deploy PXPs (precise acoustic transponder) across the splay faults in the Kumano-nada area to measure the horizontal crustal movement to the accuracy required to detect VLF (Very Low Frequency) earthquakes. On the first step we collected 660 ranging data in this one-day experiment. The round trip travel time shows a variation with peak-to-peak amplitude of about 25 mm in the range. It was confirmed that most of the variation could be explained by the change in sound speed estimated from measured temperature and pressure. The remaining fluctuation in the acoustic measurements is +/-2 mm. On the next step, we carried out the experiment to estimate the accuracy of the long period. It collected the data during four month because it had made a trouble. Therefore we can recovery only two slave PXPs. We try to recovery two master PXPs using Hyper dolphin on September 2008. We report on the experiment with a direct acoustic ranging system and estimate the accuracy of the acoustic measurements for the middle period.
G33A-0679
Development of a new tubular buoy for seafloor geodetic observation with cabled system
Seafloor geodetic observation using the GPS/acoustic technique is now a practical tool to monitor crustal
movement beneath the ocean. The DONET project, which plans to install a seafloor cable system and is
promoted by MEXT, Japan is a promising candidate to achieve semi-realtime and long-term seafloor
observation. However there still remain some technical problem for the system. One of the largest problem is
the stability and safety of a mooring buoy for long-term. A box-type small buoy as a surface platform, which is
out present system, heavily oscillates with ocean swells especially for short waves and has large friction
against ocean current within water. To overcome this problem, we have developed a new buoy as a
prototype of long-term mooring system. Since the new buoy is tubular in shape and more than 4 m height, its
attitude at the sea surface is quite stable. A typical oscillating period is improved from 2 sec for the box-type
buoy into more than 6 sec for the new tubular buoy. Further more, effect of multiple reflection against sea
surface or bottom of the buoy is almost disappear due to its acoustic transducer locates at greater depth
(around 3 m). We have confirmed that observed acoustic signals of the reply from seafloor transponders are
quite clear and has small and symmetric siderobes in the correlogram, which is important for proper
traveltime detection. The new buoy is designed as low power and remote controlled system, which is also
suitable for future cable connection.
Using this new system, we have conducted a test survey for several hours in Kumano-nada, along Nankai
trough, where the seafloor cable is planed to be laid. At this site we have placed a seafloor transponders'
array with five instruments rather than usual three transponders. This will help to estimate lateral gradient of
sound speed structure in the ocean, which degrade positioning accuracy in the past survey and analytic
styles. In this talk, we present the initial result of the survey using the new buoy.
http://www.aob.geophys.tohoku.ac.jp/dmg/gpsa/
G33A-0680
CGPS Implementation and Lidar/Laser Altimeter Experiences at l'Estartit, Ibiza and Barcelona Harbours for Sea Level Monitoring
In the framework of a Spanish Space Project, the instrumentation of sea level measurements has been improved by providing the Barcelona site with a radar tide gauge and with a continuous GPS station nearby. The radar tide gauge is a Datamar 3000C device and a Thales Navigation Internet-Enabled GPS Continuous Geodetic Reference Station (iCGRS) with a choke ring antenna. It is intended that the overall system will constitute a CGPS Station of the ESEAS (European Sea Level) and TIGA (GPS Tide Gauge Benchmark Monitoring) networks. Puertos del Estado (Spanish Harbours) installed the tide gauge station at Ibiza harbour in January 2003. The station belongs to the REDMAR network, composed at this moment by 21 stations distributed along the whole Spanish waters, including also the Canary islands. The tide gauge also belongs to the ESEAS (European Sea Level) network. At the Barcelona harbour they have installed a radar tide gauge near a GPS station belonging to Puerto de Barcelona. L'Estartit floating tide gauge was set up in 1990. Data are taken in graphics registers from each two hours the mean value is recorded in an electronic support. L'Estartit tide gauge series provides good quality information about the changes in the sea heights at centimeter level, that is the magnitude of the common tides in the Mediterranean. Two airborne calibration campaigns carrying an Optech Lidar ALTM-3025 (ICC) were made on June 16, 2007 with a Partenavia P-68 and October 12, 2007, with a Cessna Caravan 208B flying along two ICESat target tracks including crossover near l'Estartit. The validation of this new technology LIDAR may be useful to fill coastal areas where satellite radar altimeters are not measuring due to the large footprint and the resulting gaps of about 15-30 km within the coastline. Measurements with a GPS Buoy at l'Estartit harbour were made during the June experience and a GPS reference station was installed in Aiguablava. On October 12, 2007, another LIDAR campaign was made at night at the same time of the ICESat overflying. A description of the actual geodetic CGPS infrastructures at Ibiza, l'Estartit and Barcelona is presented as their applications to sea level monitoring and altimeter calibration.
G33A-0681
Is coastal mean sea level at tide gauges sites identical to the global mean based on satellite altimetry? A comparison over 1993-2007
Using tide gauges records from the Global Sea Level Observing System network, we investigate the mean tide gauge-based sea level evolution between January 1993 and December 2007, a time span over which precise altimetry data are available. Mean sea level calculated over this time span from a set of carefully selected 91 tide gauges records, exhibits a positive trend of +3.1 +/- 0.2 mm/yr. This value is in good agreement with the rate of global mean sea level rise of +3.4 +/- 0.3 mm/yr, derived from Topex/Poseidon and Jason-1 satellite altimetry over the same period. Therefore coastal and global sea level rose at comparable rates during the last 15 years. We thus find no evidence of different trends at the coasts and in terms of global mean. However coastal mean sea level displays large interannual fluctuations associated with ocean-atmosphere natural variability, in particular ENSO.The large interannual variability of the tide gauge- based mean sea level is in good agreement with the variability of coastal mean sea level calculated from satellite altimetry data along the global coastline. Interannual sea level variability, enhanced along coastlines, is smoothed out when global averages are considered. Therefore the large interannual variability observed in tide gauge-based mean sea level is not only caused by insufficient tide gauge sampling but also by physical differences between open ocean and coastal processes.
G33A-0682
Geographical Variability in the Oceanic Response to Atmospheric Pressure Fluctuations, as Inferred from 'Dynamic Barometer' Green's Functions
A decade ago, a theoretical approach was developed (Dickman, 1998 J. Geophys. Res.) for determining the dynamic response of the oceans to atmospheric pressure variations -- a response nicknamed the 'dynamic barometer' (DB) -- and the effects of that response on Earth's rotation. This approach employed a generalized, spherical harmonic ocean tide model to compute oceanic Green's functions -- the oceans' fluid dynamic response (at specific frequencies and harmonics) to unit-amplitude pressure forcing -- and then construct rotational Green's functions, representing the rotational effects of that response. When combined with the observed atmospheric pressure field, previously decomposed into spherical harmonic coefficients at each time interval, the rotational Green's functions would then yield the effects of the DB on Earth's rotation. The Green's functions reflect in some way the geographical and spectral sensitivity of the oceans to atmospheric pressure forcing. We have formulated different measures of that sensitivity. In this talk, we will use them to identify geographically sensitive and insensitive regions of the oceans both as a function of frequency and versus time. The Green's functions computed in 1998 were limited to pressure forcing on time scales of at least a few days, because of the failure of the underlying tide model to accurately predict short-period tidal effects on Earth's rotation (long-period tidal predictions were much more successful). Recent improvements in the tide model will allow us to relax that limitation, and eventually employ higher-frequency Green's functions for a more complete picture of the oceans' sensitivity to atmospheric pressure forcing.
G33A-0683
Rotationally Acceptable Ocean Tide Models
In 1998, I developed a theoretical approach for predicting the dynamic response of the oceans to atmospheric pressure variations -- a response I called the 'dynamic barometer' (DB) -- and the effects of that response on Earth rotation. The response was determined using a spherical harmonic ocean tide model, generalized to allow for arbitrary atmospheric pressure forcing. The tide model itself had been frictionally calibrated to insure agreement with observations of long-period tidal effects on polar motion; but it severely under-predicted short-period tidal effects, so calculations of the DB response forced on time scales of a few days or less were avoided. In order to be able to extend the DB calculations to short time scales, an effort has been made to improve the tide model's dynamics at higher frequencies. The first goal was to increase the resolution of the model ocean, whose earlier versions included coastlines and ocean bathymetry expanded at most to spherical harmonic degree and order 48. However, even with high-resolution oceans (coastlines and bathymetry to degree and order 180) the model's predictions of diurnal and semi-diurnal tides, and their effects on rotation, were changed only to a minor extent. In contrast, modifying the physical parameters of the model has a significant impact on the resulting tide characteristics. It turns out that the frictional parameters employed in the original tide model corresponded to an extreme 'high-friction regime' in which short-period tidal effects on Earth rotation are appreciably suppressed. Overall agreement with other researchers' predictions of short-period tidal effects, e.g. for the diurnal tide O1, is easily achievable through a frictional re-calibration of the tide model. This general agreement is possible with both assimilated tide models (such as TPXO.6.2, Egbert and Erofeeva 2003) and pure hydrodynamic tide models (such as Seiler 1991; rotational effects tabulated by Gross 1993). This 'rotational agreement' will allow the effects of the oceanic DB response to be extended to diurnal frequencies. It also suggests that numerical ocean tide models, in which frictional calibration using the major semi-diurnal tide M2 is performed at the outset, might be more successful in predicting long-period tides if they consider alternative friction coefficients for the latter.
G33A-0684
Empirical predictions of UT1 and polar motion using least squares fit and ARIMA
Based on the spectra of the UT1 or polar motion (PM), we fit the observed data to a combination of polynomial and sinusoids using least-squares techniques, and then predict by extrapolating using the coefficients estimated. We use both the observed data and the differences between the successive data in the least square fit. The residuals are handled using the ARIMA model. This way, the predictions are made in several steps. The atmospheric angular momentum (AAM) is incorporated in tow ways: One is the multivariate ARIMA, and the other is empirical. The results are compared with the IERS predictions.
G33A-0685
The causes of the prediction errors of the x, y pole coordinates data
The precise transformation between the celestial (ICRF) and terrestrial (ITRF) reference frames for the time moment of observation needs the precise earth orientation parameters (EOP) data and their predictions. The prediction errors of x, y pole coordinates data or UT1-UTC data increase with the prediction interval length and usually for a few days in the future is several times greater than the observational error. It's well known that the short period and seasonal components of x, y pole coordinates data are excited by equatorial components of the atmospheric, ocean and less by the hydrologic excitation functions. These fluid excitation functions as well as their sums were used to compute the x, y pole coordinates model data using numerical integration of differential equation of polar motion. The IERS x, y pole coordinates data and the x, y pole coordinate model data were then predicted by the combination of the least-squares model extrapolation and autoregressive prediction. Comparison of the prediction errors computed at different starting prediction epochs for the IERS x, y pole coordinates data and the x, y pole coordinates model data, has enabled examination of the influence of the irregular oscillations in the fluid excitation functions on the prediction errors of x, y pole coordinates data. It was shown that the increase of x, y pole coordinate data prediction errors for prediction interval length up to 50 days in the future are caused mostly by irregular short period oscillations seen on time-frequency spectra of the model data computed from the atmospheric and ocean excitation functions.