Measuring the Ocean Surface with GNSS-controlled buoys | GNSS-controlled buoys

The picture shows the tsunami buoy after deployment. On top are antennas for the GNSS and satellite communication surrounded by solar panels for power generation.
GNSS Tsunami buoy after deployment (Note: The GITEWS tsunami buoys are no longer in operation as part of the InaTEWS system.) (T. Schöne)
The picture shows part of the ship's bow with the GNSS antenna and two radar sensors. In the background the sun is setting over the lake.
GNSS- und Radar sensors during a ground truthing cruise at Issyk Kul (Kirgizstan) (T. Schöne, GFZ)
The picture shows a GNSS buoy on the Issyk-Kul in the center, which is towed on a line behind the ship. The GNSS antenna can be seen on the buoy. In the background, the shore and the mountains of the Tien Shan can be seen.
GNSS buoy during the measurement campaign on Lake Issyk Kul (Kyrgyzstan) (T. Schöne, GFZ)

Detecting tsunamis requires real-time off-shore sea level measurements near the source. Based on our experience in GNSS operated at buoys for radar altimetry calibration, we have further developed and adapted this technology for the Indonesian Tsunami Warning System GITEWS. Under this project, we have built, delivered, and operated a number of buoys in Indonesia, offshore Sumatra and Java. 

The 7-meter-high buoys, with a 2-meter diameter, were anchored at a depth of 80 to 150 nautical miles offshore from the Sumatra and Java coasts, in waters up to 6,000 meters deep. Each buoy is equipped with a high-precision GPS (Septentrio AsteRX), a motion sensor to account for tilting, and three dipping sensors for heave compensation. A wide range of meteorological sensors measure supplementary scientific data. The solar-powered buoy is designed to withstand the harsh offshore environment and can be moored in water depths down to 6000 m (depth rating of the anchor releaser). An embedded PC104 inside the buoy works for system management and data storage. All attached sensors are operated and maintained using specially designed software modules. Data is stored on-board for delayed transmission. 

Data transmission is the most critical part of off-shore installations. We use three types of communication equipment: INMARSAT's BGAN network, Iridium satellite phones, and INMARSAT's Asia-based PASTI phone service. This ensures uninterrupted access to the buoy and continuous data transmission. The access is designed as a two-way communication. In the event of a tsunami threat, the buoys are equipped with either self-activating capabilities or a tsunami warning system triggered in Jakarta. 

Meteorological data (e.g., air temperature, air pressure, wind speed) is measured every minute. GPS data is sampled at 1 Hz, but down-sampled to 1/3 Hz. In case of a tsunami alert, the most recent hour of GPS data plus continuous 2’ updates are transmitted to the Tsunami warning center. The data processing uses a kinematic solution using baseline approaches. The resulting time series of buoy coordinates are corrected for dipping and tilting, and are smoothed to reduce for the sea state. This time series is then analyzed for tsunami signatures and used in the decision process at the Tsunami Warning Center.

As a further development using GNSS technologies on floating platforms a river buoy has been developed for the Project WISDOM in cooperation with GFZ’s section 5.4. The lightweight and easy-to-handle buoy is moored at the Mekong River for stage monitoring. The buoy transfers GNSS data with local HF communication to a base station with a GNSS reference station. Both GNSS data sets are transferred by local communication and processed in a differential post-processing mode. 

Another application of GNSS technology to determine water levels is at Lake Issyk Kul in Central Asia. Here we use GNSS on buoys and on a ship to measure the lake level and then apply this information to monitor radar altimetry.

Project partner:

Project Duration:

  • GITEWS: 2005 – 2011
  • PROTECS: 2011-2014
  • WISDOM: 2007-2010
  • POF Programme and GCOCA

Funding:

  • BMBF
  • Haushold, ACROSS Initiative of the HGF

Project-related publications:

  • Schöne, T., Pandoe, W., Mudita, I., Roemer, S., Illigner, J., Zech, C., & Galas, R. (2011). GPS water level measurements for Indonesia's Tsunami Early Warning System. Natural Hazards and Earth System Sciences (NHESS), 11(3), 741-749. doi:10.5194/nhess-11-741-2011
  • Flüh, E. R., Schöne, T., Weinrebe, W. (Eds.) (2006): FS Sonne Fahrtbericht / cruise report SO186 B, C & D GITEWS: German Indonesian Tsunami Early Warning System; Jakarta-Jakarta & Jakarta-Singapore & Singapore-Singapore, (IFM-GEOMAR report; 5). https://gfzpublic.gfz-potsdam.de/pubman/item/item_234266
  • Apel, H., Hung, N. G., Thoss, H., Schöne, T. (2012): GPS buoys for stage monitoring of large rivers. ‐ Journal of Hydrology, 412‐413, 182‐192 DOI: 10.1016/j.jhydrol.2011.07.043
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