Manual on sea level measurement and ... - unesdoc - Unesco

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Sea Level Measurement and Interpretationgravity and GPS are both approaching the equivalentaccuracy of 1 cm that is required for measuring verticalcrustal movements.Absolute gravity measurements are normally made in aconvenient building that provides reasonable temperaturecontrol. This site then needs to be connected to theTGBM and the local benchmarks using high precisionlevelling. Corrections for ocean tide loading and attractionare particularly important at or near coastal sites, asis the additional ocean tide attraction due to the elevationof the site.Owing to the higher cost of absolute gravimeters comparedto GPS receivers, the number of tide gauges beingmonitored is likely to be a small sub-set of the tide gaugeswith GPS. It has been recommended that the measurementsof absolute gravity should be concentrated at keytide gauges in the GLOSS-LTT network, where they willbe most useful in contributing to meeting the challengeof determining vertical crustal movements to an accuracyof better than 1 mm per year. Absolute gravity measurementsat the GLOSS tide gauges at Newlyn and Lerwickhave recently shown that there are systematic errors inCGPS, which lead to errors in the vertical rates determinedfrom GPS (Teferle et al., 2006). This shows the importanceof independent techniques for identifying systematicerrors and the reduction of these errors is currently animportant topic of research in the GPS community.4.4.5 Geocentric Co-ordinates and Vertical LandMovements of Tide Gauge BenchmarksFrom 2001 to the end of 2005, the International GNSSService (IGS) set up a pilot project called TIGA, whichis processing and analysing CGPS data from over 100tide gauges around the world in a consistent globalreference frame. The web site (http://adsc.gfz-potsdam.de/tiga/index_TIGA.html) should be consulted for informationabout the stations and the results that are beingobtained. The GPS global sea level monitoring networkwill be a fully integrated component of the InternationalGNSS Service – International Earth Rotation Service (IGS/IERS) International Terrestrial Reference Frame (ITRF). Theproducts from this network are the co-ordinates andvelocities of the benchmarks at tide gauge stations. ThePermanent Service for Mean Sea Level (PSMSL) archivingsystem has been designed to bank the vertical crustalvelocities derived from selected IGS solutions, along withexplanatory information, including the names of expertswho can be contacted by users of the system.34IOC <strong>Manual</strong>s and Guides No 14 vol IVFigure 4.4 Schematic of an absolute gravimeter.
Sea Level Measurement and Interpretation5. Real Time Data Transmission5.1 IntroductionSea level data acquired by a tide gauge may berequired in ‘real time’, ‘near real time’ or in ‘delayedmode’ depending on the application. For example, astorm surge or tsunami warning system may requirethe data to be transmitted to the competent authoritiesin a very short time. On the other hand, for some scientificresearch, it is often only necessary to recover thedata annually, in which case it can be stored locally andrecovered during a site visit, either by downloading thedata to a PC or by extracting and replacing a memorycard. In any case, it is expedient to adopt such a localprocedure, even if a communication link is in operation,to prevent loss of valuable data.The method of communication depends largely on thedistance the data have to be transmitted. For short links(e.g. harbour operations), a radio link is often convenient.For countrywide links, Subscriber Trunk Dialingor dedicated telephone lines of the Public SwitchedTelephone Network (PSTN) are an effective medium.Where fixed lines are not practical, the growth in theuse of Mobile Phone Links using General SwitchedMessaging (GSM) technology and General Packet RadioSystem (GPRS) protocols has extended the potential forlong-distance communication. Both the fixed and mobiletelephone systems give access to the Internet through anInternet Service Provider (ISP) which can greatly enhancethe transmission of data. For example, many of the GPSstations of the global network of the International GNSSService, which has some similarities to the global tidegauge network in terms of number of sites and amountof data to be transmitted, report through the Internet.All the forms of telephony are merging into one, withtelephone links provided by a supplier for which theconnection method is transparent to the user. After thetsunami of 26 December 2004, India has implementeda real-time coastal sea level data transmission by meansof GPRS with continuous connection to Internet, withmuch lower costs than previous experiments based onSMS and Data Call Services (Antony Joseph, personalcommunication).For more remote areas, the use of mobile satellite linksis an alternative. There are now upward of 30 orbitingsatellite systems in operation dedicated to datatransmission, some on a global basis. Mobile satellitesystems (MSS) may be classified according to orbit altitudeas follows:GEO – geostationary earth orbit, approximate altitude:35,000 kmMEO – mid-altitude earth orbit, approximate altitude:10,000 kmLEO – low earth orbit, approximate altitude:
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