RONIM Data Acquisition and Transmission
The MARELTA tidal measurement data loggers used by SHOM on the sites of the RONIM network have been specially developed by the company ELTA. This unit digitally acquires the raw measurement from the water level sensor.
The data logger processes and formats each data item for transmission and archiving. This involves:
- referencing the measurement with respect to the chart datum;
- correcting any linearity and offset errors.
The referencing with respect to the chart datum is carried out by introducing a measurement corresponding to the levelling of the sensor's reference point. The measurement is corrected by applying coefficients calculated during a calibration, in accordance with the procedures in force in SHOM.
The unit carries out two types of acquisition, one "raw" measurement every second and one averaged measurement, which represents a digital filtering of the swell or choppiness noise by averaging one data series per second.
The unit's time check is fundamental for dating the tidal measurements and in case of the clock drifting. The clock is automatically reset thanks to a GPS receiver (in the case of sites equipped with satellite transmission facilities) or by a telephone link with SHOM. The unit is also used to transmit special alarms in the event of a failure or alert (water height thresholds exceeded, mains power outage, memory capacity exceeded, etc.), These alerts are transmitted instantaneously to SHOM via the telephone network.
An atmospheric pressure sensor (Vaisala PTB210A) is also fitted inside the unit and measures the atmospheric pressure every hour.
The entire system is connected to the mains or supplied by solar panels. In the event of a power outage, batteries will supply the system for at least 5 days, thus minimising the number of gaps in observations.
Data transmission and archiving
Depending on the applications, various types of transmission and archiving methods are used:
10-minute tide gauge data
Every 10 minutes, the unit acquires a series of measurements over a period of two minutes. The series is averaged to produce digitally-filtered water height data. These, as well as the hourly atmospheric pressure measurement are filed on the unit's memory board (capacity approximately 1 year's data) and transmitted regularly to SHOM via the telephone network. Supervision software then processes and archives the data.
1-second tide gauge data
The measurements taken every second are not archived by the data logger but are transmitted to SHOM in real-time via the Internet and, where applicable, to other organisations (certain tide gauges are directly linked to tsunami alert centres). Depending on the location of the tide gauges and the transmission possibilities, the data are sent by ADSL telephone line or GPRS cellular network. Dedicated software and a computer server at SHOM validate, archive and disseminate this real-time data to other portals (UNESCO, PREVIMER).
Data transmission by satellite
Certain sensitive sites are also equipped with satellite data transmission facilities providing a back-up to the real-time internet link or allowing it be replaced, in the case of particularly isolated and inaccessible sites, by telephone networks. SHOM's tide gauges transmit to meteorological satellites supporting the DCS (Data Collection System) designed for recovering and re-broadcasting data derived from meteorological platforms.
These satellites are the Meteosat (EUMETSAT – Europe), GOES (NOAA – USA) and GMS (Japanese Meteorological Agency) geostationary satellites. In this case, the water height data are transmitted by the acquisition unit to a beacon which transmits a message to the satellite containing the water height data. The message transmission rate is determined by the agency in charge of the satellite, ranging from 6 to 60 min. The transmitted data are measurements taken every minute and usually averaged over 15 seconds. The messages received and broadcast by the satellite are then available to the accredited bodies on the GTS (Global Telecommunications System) managed by the WMO (World Meteorological Organization). The RONIM network also strives to follow the recommendations issued by international institutions.
To find out more ·
- Poffa N., S. Enet, J.-C. Kerinec et l'équipe RONIM (2012). Évolution instrumentale des marégraphes du réseau RONIM. 12ème journées nationales Génie Côtier - Génie Civil, Cherbourg, pp. 611-618. DOI:10.5150/jngcgc.2012.066-P
- Poffa N. et l'équipe du projet RONIM (2011). Évolution instrumentale des marégraphes du réseau RONIM. Annales hydrographiques 2011, 6ème série, volume 8, n° 777, ISSN : 0373-3629.
Last updated: 12/12/2012