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

Sea Level Measurement and InterpretationAn international working group was set up in the late1980s by the International Association for the PhysicalSciences of the Ocean, under its Commission on MeanSea Level and Tides, to recommend a strategy for thegeodetic fixing of tide gauge bench marks. Theseresulted in the ‘Carter reports’ (Carter et al., 1989;Carter, 1994). The following sections provide a summaryand describe recent developments. The reader isreferred to Neilan et al. (1998) and Bevis et al. (2002)for further details.a4.4.2 GPS MeasurementsOver the past decade, the GPS technique has developedrapidly to the extent that it is of fundamentalimportance to many areas of geophysical research (seelinks documented on the PSMSL training web page).The International GNSS Service (IGS) receives datafrom a global network of GPS stations and producesinformation on the orbits of the GPS satellites whichis significantly more precise than the ephemeridesroutinely transmitted by the satellites themselves. Thisinformation is employed by researchers to produce precisepositioning computations. GPS data from the IGSnetwork are archived at the IGS Central Bureau.bIdeally, all tide gauge sites should be equipped witha permanent continuous receiver (CGPS). However,in practice, the financial resources required are oftenlarge. Many countries adopt the procedure of installingpermanent GPS receivers at strategic tide gaugesand then densifying the network with regular GPScampaign measurements (Neilan et al., 1998). Thereis clearly an advantage in concentrating CGPS workat sites with long duration PSMSL RLR mean sea levelrecords. The GLOSS Implementation Plan refers to thisset as the GLOSS Long-Term Trends (GLOSS-LTT) network.The campaigns can then concentrate on othertide gauges in the network for which the records areshorter. The exact mix between permanent and campaignGPS tide gauges will change as the cost of GPSreceivers continues to decrease.For studies involving sea level, it is recommended thata dual-frequency CGPS receiver should be installeddirectly at the tide gauge so that it monitors anymovement of the TGBM. If the receiver is placedexactly at the TGBM, then the GPSBM and theTGBM will coincide, eliminating the need for levellingbetween the two benchmarks. The TGBM is then thefundamental point that is geocentrically located bythe GPS measurements and to which all the sea levelmeasurements are related. In practice, tide gauge sitesare not always ideal for making GPS measurements.This may be due to obscured sky visibility, excessivemultipath transmissions or because of radio interference,in which case a site should be chosen that is asclose as possible to the tide gauge. Ideally, this shouldbe within a few hundred metres.Figure 4.3 Alternative forms of GPS mounting(a) a Norwegian tide gauge with GPS antenna mountedon an adjacent platform;(b) GPS antenna on a pillar, as recommended by theCGPS@TG group (http://soest.hawaii.edu/cgps_tg).32IOC <strong>Manual</strong>s and Guides No 14 vol IV