Detection of tsunamis is requiring near real-time sea level measurements close to their source areas. Here, traditionally, tide gauges are the preferable source for information. Based on our experience in both sea level measurements with tide gauges and GNSS technology, our Section has installed a network of GNSS-controlled tide gauges around the Indian Ocean (GITEWS). In addition, several tide gauges without GNSS have been installed too.

The GNSS-controlled tide gauges are based on our ROMPS concept. Each station has different tide gauge sensors, a GNSS receiver, meteorological devices, and may have a seismometer connected. An embedded computer manages all attached hardware and provides the platform for data acquisition. Up to three different tide gauge sensors are collecting sea level data preferable every 20 seconds. An on-site algorithm is analyzing the incoming data stream for outliers and is estimating the excess between the actual measurement and a predicted tidal value. If the excess exceeds a threshold, the station is transmitting automatically all recently acquired data to a warning center. For tide gauges in Indonesia, a dedicated tsunami warning center (InaTEWS) can activate the station at any time. In addition, all sea level data is transmitted independently and parallel to IOC for international data exchange. 

A meteorological compact sensor samples a range of values (e.g., air temperature, air pressure, wind speed, rain, or humidity) every minute. This data is used in short-term for correcting sea level data for inverse barometric effects, which may hamper the detection or falsification of sea level excesses. On the long-term these measurements can be used to study surge effects or long-term trends in sea level. 

The 2004 tsunami in Indonesia caused significant vertical displacement along the northern coast of Sumatra, prompting a swift shift in sea level measurements. To ensure precise monitoring, all our tide gauges in Indonesia are equipped with GNSS, controlling both the vertical and horizontal positions. Our tide gauges' GNSS data is analyzed in the event of a strong earthquake to determine vertical displacement and constrain earthquake source parameters. 

These, and the other GNSS-controlled tide gauges, serve additionally to international projects and initiatives, like GGOS, GCOS or GLOSS.

For our sea level research, the tide gauge data is used for monitoring the stability of radar altimetry measurements, study local and regional sea level change, and contribute to our sea level hazard monitoring program.

Project partner:

• GITEWS & PROTECS (https://www.gitews.de)
• Badan Informasi Geospasial (Indonesia) (https://www.big.go.id) 

Project Duration:

• GITEWS: 2005 – 2011
• PROTECS: 2011-2014

Funding:

• BMBF

Project-related publications:

• Schöne, T., Illigner, J., Manurung, P., Subarya, C., Khafid, Zech, C., & Galas, R. (2011). GPS-controlled tide gauges in Indonesia - a German contribution to Indonesia's Tsunami Early Warning System. Natural Hazards and Earth System Sciences (NHESS), 11(3), 731-740. doi:10.5194/nhess-11-731-2011
• Aarup, T., Wöppelmann, G., Woodworth, P. L., Hernandez, F., Vanhoome, B., Schöne, T., & Thompson, P. R. (2019). Comments on the article “Uncertainty and bias in electronic tide-gauge records: Evidence from collocated sensors”. Measurement, 135, 613-616. doi:10.1016/j.measurement.2018.12.007
• Fenoglio-Marc, L., Schöne, T., Illigner, J., Becker, M., Manurung, P., & Khafid (2012). Sea Level Change and Vertical Motion from Satellite Altimetry, Tide Gauges and GPS in the Indonesian Region. Marine Geodesy, 35, Suppl. 1, 137-150. doi:10.1080/01490419.2012.718682