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

Sea Level Measurement and InterpretationODINAFRICA Tide Gauge Specification August 2005Peter Foden, Proudman Oceanographic Laboratory, Liverpool, UK. E-mail: prf@pol.ac.ukThe following lists technical details of UNESCO–IOCfunded radar and pressure sensor tide gauges that arebeing procured by POL for installation at sites in Africaas part of the first phase of the ODINAFRICA project. Theinstrumentation consists of four basic parts:• Sensors – the primary sensor is an OTT Kalesto radargauge measuring height above sea level, and secondarysensors are OTT PS1 pressure sensors measuringwater pressure and temperature at two points in thewater column.• Housing – the fibreglass instrumentation cabinethouses the mains power supply, 27 Ah lead–acidrechargeable battery, OTT HDR-DCP satellite telemetryunit which sends data through METEOSAT, Logosens2data logger, 2,400 baud line-powered modem andlightning suppression for the power and data lines.• Telemetry – yagi antenna and 10-m cable for datatelemetry and GPS antenna for the integrated receiversystem in the HDR-DCP.• Data logging – the principal data-retrieval system isby satellite telemetry, but the Logosens2 data loggerhas limited capacity to store the measured sensordata. Access to these data is by local operator or viadial-up modem; set-up and maintenance can also becarried out via the modem link.The radar sensor needs to be fitted to a locally-manufacturedsupport arm (specification can be supplied by POLor there is an OTT version available), a site-determinedlength of 4-core cable is provided enabling the sensorto be located a distance away from the instrumentationcabinet, if required. The pressure sensors require to befitted in a stilling well and installation kits for this purposeare included.The instrument cabinet is meant to be wall-mountedand should be inside a building that has a reliable mainspower supply and telephone connection.The HDR-DCP antenna needs to have as good a viewof the sky and horizon as possible and should bemounted away from physical obstructions, such asmetalwork and structures, where possible.Description of operationBy using the OTT HDR-DCP satellite telemetry unit,accurate real-time data are available from the integratedGPS receiver. This means that the system doesnot need to be set up after switch-on and that timemeasurement is always accurate to ±1 s.The two pressure sensors are differential (air-pressurecompensated) sensors. One measures sub-surfacepressure and the other is positioned at approximatelyMSL to provide a form of ‘B’ gauge datum controlchecking both for the first pressure sensor and for theKalesto (see explanation of ‘B’ gauges in the present<strong>Manual</strong>).The Kalesto radar sensor and the pressure sensors areinterrogated every minute by the Logosens2 data loggerand the data are recorded. The one-minute valuescan be averaged to whatever time interval is required,e.g. six-minute averages, and sent to the HDR-DCPfor transmission at the allotted time. Alternatively,data can be recovered via data modem connection.The tide gauge has a back-up supply that is capableof keeping the whole system going for approximatelytwo days without mains power.Note that this equipment is similar to that providedby IOC and POL to two sites in Mozambique, andinstalled in early 2005 by the South African NavyHydrographic Office. In that set-up, the METEOSATDCP was replaced by Orbcomm telemetry whichprovides relatively cheap data transmission but withvariable latency (see section 5, above).76IOC <strong>Manual</strong>s and Guides No 14 vol IV