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

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Sea Level Measurement and InterpretationA radar signal with a frequency of approximately 1–30GHz is sent from the antenna to the water surface.After reflection at the water surface, the return signal isreceived, with a time lag. The characteristic of the radargauge is written in Table 1.Table 1. Characteristics of the tested radar gauge.Rad_C Rad_BRad_D Rad_AMethod [-] Pulse Pulse Pulse FMCWMicrowavefrequency[GHz] 26 26 5.8 8.5-9.9The Rad_A gauge works with the frequency modulatedcontinuous wave (FMCW); all the other devices work withthe pulse method, at different frequencies. As all radargauges have outliers in the data, the recorded time-serieshas to be examined in this way and the data have to besmoothed. The exponential smoothing, with a weightingfactor of 0.001 at a sample rate of 1.3 Hz achieved thebest results. Figure 2 shows the measured error probabilitiesof the radar gauges and the official gauge.In conclusion, the radar gauges Rad_C and Rad_B are themost suitable devices for measuring water level. The erroris in the same range as that of the official gauging station.The Rad_A sensor is the only one with another measuringprocedure. The pulse method (see Table 1) for the waterlevel measurement is more suitable. Rad_D, which hasalso been tested since summer 2003, had problems withits data communication system. Therefore the analysisshown in Figure 2 was not done. Furthermore, comparedto the classical gauge, the maintenance for the radargauge is much less and the latter has delivered data sincesummer 2003. Even various environmental factors, suchas bird excrement or saline air, have no influence at all onthe measurement.ReferencesBarjenbruch, U., Mai, S., Ohle, N., Mertinatis, P. 2002.Monitoring water level, waves and ice with radargauges. Proceedings of the Hydro 2002 Conference,Kiel, Germany. pp.328-337BfG (Bundesanstalt für Gewässerkunde). 2002. Testvon Radarsensoren im Küsten- und Küstenbereich,interner Bericht. BfG-1276, Koblenz, Germanyerror probabilityerror probabilityσµ σµerror probabilityσµerror probabilityerror probabilityσµFigure 2. Comparison of the measured differences between each gauge and the Magnetostrictive-Sensor for aparticular day.66IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and InterpretationExperience with SRD Tide Gauges and the Reasoningbehind a Change to Radar Tide GaugesRuth FarreSouth African Navy Hydrographic Office, E-mail: hydrosan@iafrica.comHistorical summaryThe SAN Hydrographic Office started installing itsown float-actuated ‘Kent’ gauges in 1958. As thesegauges aged, additional types, also float-actuated,were installed; they comprised LEA gauges, similar tothe Kents, Italian SAIP gauges in Saldanha and Simon’sTown, and OTT gauges in Port Elizabeth and WalvisBay. These gauges proved to be reliable, but by themid-1980s, the Navy’s Tide Gauge (TG) network wasageing. Spares for the Kents were becoming difficultto come by and replacement on a significant scale wasurgently needed.A project was initiated with the CSIR, Stellenbosch,to develop an accurate, modern Acoustic Water LevelRecorder. These came into service in 1990 and eightwere installed throughout RSA and Namibia. Thesegauges were a failure. They were erratic, difficult totune and grossly inaccurate. Virtually no usable datawere obtained and the gauges were finally abandonedbetween 1996 and 1998.Specifications for the replacement gauges were drawnup in conjunction with IMT (Institute of MaritimeTechnology) in 1995 and were tendered for in 1996.The tender was won by Messrs SMD Electronics CC,with their SRD acoustic gauges meeting our requirements.These promised well and were installed, somewhathurriedly, in 1996, mounted on tubes, as recommendedby the manufacturers. The supplier stated thatthe gauges were calibrated in the factory and weretherefore self-calibrating. It was immediately foundthat the gauges were very inaccurate, well outside thespecification. With assistance from IMT, remedial measureswere taken and the gauges were calibrated byIMT and remounted without tubes. The HydrographicOffice had to devise a method for check-calibratingthese gauges in situ. Their performance has subsequentlybeen just acceptable, but not of the accuracydesired. Their reliability has varied. Maintaining correctreadings and downloading the data are problematical.An OTT Kalesto Radar tide gauge was tested by theHydrographic Office in Simon’s Town at the beginningof 2002. The results obtained from this test werereaffirmed by a test done at IMT in September 2002.The results from these tests indicated that the Kalestogauge performs with a higher degree of accuracy andstability than has been encountered in the past.Currently the SA Hydrographic Office maintains 10 tidegauges along the South African coastline. The networkis as follows:• Port Nolloth – A Kalesto gauge is installed. AHARTRAO GPS Rx is also fitted.• Saldanha Bay – At present there is an SRD gaugeinstalled.• Cape Town – At present there is an SRD gaugeinstalled.• Simon’s Town – At present there is an SRD and aKalesto gauge installed. A HARTRAO GPS Rx is alsofitted.• Mossel Bay – At present there is an SRD gaugeinstalled.• Knysna – At present there is an SRD gaugeinstalled.• Port Elizabeth – A Kalesto gauge is installed• East London – A ““Kalesto gauge is installed.• Durban – At present there is an SRD gaugeinstalled.• Richard’s Bay – A Kalesto gauge is installed. AHARTRAO GPS Rx is also fitted.The Hydrographic Office has purchased new Kalestoradar gauges with the intention of installing them untilsuch time as the entire South African tide gauge networkhas been upgraded.SRD gaugesAn independent study was done on the SRD gauge, byIMT, after the SAN Tidal Superintendent expressed concernover the accuracy and quality of the data beinggenerated by the SRD tide gauge network. The studyshowed that transducers housed in tubes producedlarge errors due to temperature gradients that formedin the tube. The solution to this was to remove ormodify the existing tubes to maintain a thermally wellmixed air column around the transducerThe accuracy of the two gauges used in the study wastested: one was found to have a systematic error ofabout 6 mm per metre over and above a fixed offsetof about 24 mm. The other was found to have a systematicerror of approximately 10 mm per metre witha fixed offset of about 36 mm. This accuracy problemwas taken up with the manufacturers to establish whythe claimed accuracy of 0.05% over the 2- to 10-mworking range could not be achieved with the unitsIOC <strong>Manual</strong>s and Guides No 14 vol IV67
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